Radio World Engineering Extra 263 - October 19, 2020

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IN THE FIELD

ENGINEERING EXTRA

OCTOBER 21, 2020

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In-Depth Technology for Radio Engineers

Co-locating an AM With a Cellular Tower Page 6

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a Getty Im

Hybrid Synchronization in the Sunshine State

t free ges/ rm sfo r an

The author creates a unique quasi-SFN using Intraplex gear

BY HAL KNELLER

The author is chief engineer of WSRQ LECOM Radio.

In 1992, WZZS, Zolfo Springs, Fla., was launched as a Class A FM on 106.9 MHz. This is approximately 40 miles inland (east) of Sarasota/Bradenton Florida.

Due to the extremely flat topography and the subtropical climate, auto reception was audible up and down the coast though the Sarasota area.

In 2007, W295BH, a 250-watt FM translator, commenced operation in the Sarasota area on the same frequency. While it has moved several times over the years, it never was a real problem for WZZS other than eliminating the fringe coverage, which was well Hal Kneller

outside the Class A FM 60 dBu contour.

The same could not be said in reverse. WZZS inflicted considerable interference on the translator, even well inside the translator’s 60 dBu contour. Motorists driving through Sarasota and Bradenton would often hear WZZS “pop through” while listening to the W295BH translator.

Areas east of Sarasota, which remained still within the W295BH 60 dBu contour, were especially inundated with interference from the co-channel Class A FM. This was due to both WZZS’s higherpower penetration into the inland Sarasota area, and the much lower power of the 106.9 translator in that

same part of the market.

These unpleasant experiences lasted the better part of 10 years, until Lake Erie College of Osteopathic Medicine (LECOM) entered the picture.

The college purchased WSRQ(AM) 1220 and W295BH in December 2018, along with another translator on 95.9 (W240DP) located further south of Sarasota.

The new owner desired to expand its reach into the growth areas of Sarasota and Manatee counties, and eliminate the interference caused by

★

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ENGINEERING EXTRA Vol. 44 No. 24

FROM THE TECH EDITOR

October 21, 2020

www.radioworld.com FOLLOW US www.twitter.com/radioworld_news www.facebook.com/RadioWorldMagazine CONTENT Managing Director, Content Paul J. McLane, paul.mclane@futurenet.com, 845-414-6105 Senior Content Producer — Technology Brett Moss, brett.moss@futurenet.com Technical Advisors Thomas R. McGinley, Doug Irwin Technical Editor, RWEE W.C. “Cris” Alexander Contributors: Susan Ashworth, John Bisset, James Careless, Ken Deutsch, Mark Durenberger, Charles Fitch, Travis Gilmour, Donna Halper, Craig Johnston, Alan Jurison, Paul Kaminski, John Kean, Peter King, Larry Langford, Mark Lapidus, Jim Peck, Mark Persons, Stephen M. Poole, James O’Neal, Rich Rarey, Jeremy Ruck, John Schneider, Randy Stine, Tom Vernon, Jennifer Waits, Chris Wygal Production Manager Nicole Schilling Managing Design Director Nicole Cobban Senior Design Director Lisa McIntosh and Will Shum ADVERTISING SALES Senior Business Director & Publisher, Radio World John Casey, john.casey@futurenet.com, 845-678-3839 Publisher, Radio World International Raffaella Calabrese, raffaella.calabrese@futurenet.com, +39-320-891-1938 SUBSCRIBER CUSTOMER SERVICE To subscribe, change your address, or check on your current account status, go to www.radioworld.com and click on Subscribe, email futureplc@computerfulfillment.com, call 888-266-5828, or write P.O. Box 282, Lowell, MA 01853.

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An example of a site that is designed, built and maintained well. Oftentimes, broadcast transmitter sites are anything but.

Does Your Site Have Good Bones?

Pay attention even to small things for the health of your site

BY CRIS ALEXANDER

From time to time, my travels take me to transmitter sites of radio stations that are not part of our company, because I have been asked to come in and do some antenna work or perform a due diligence inspection.

Some of these sites are designed, built and maintained well, a credit to those responsible.

Other times, sadly, what I find is anything but.

Many of these facilities are run down, overgrown and infested with insects and rodents. Some are downright hazardous to be in or even around.

The list of issues can be quite long, including weeds, brush or even trees grown up in the tower base areas; base fencing that, if it exists at all, is damaged or deteriorated; transmission lines falling off the ice bridge or supports; tower paint that has faded far out of tolerance and/or is flaking off; infestations of mice and other rodents in transmitter building and equipment; evidence of snakes, spiders and flying, stinging insects in residence … and on and on.

Sometimes, the equipment and facility wiring is in bad shape.

I see electric panels with the covers removed and wiring exposed, audio and remote control wiring is hanging loose with connections twisted together. I’ve seen transmitters with side and back panels removed and interlocks jumpered out so that lethal voltages are exposed. I’ve seen phasor cabinets with their doors removed and RF components exposed for anyone to touch.

How does this happen?

STATE OF DECLINE Sometimes the answer is obvious. The station is in a state of decline, barely hanging on in a small community with little business and competition coming from (continued on page 4)

THIS ISSUE OCTOBER 21, 2020 Hybrid Synchronization in the Sunshine State. . . . . . . . . . . . . . . . . . . . . . . . 1 Does Your Site Have Good Bones?. . . . . . . . . . . . . 3 Co-locating an AM With a Cellular Tower. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Repack Impact: How Has It Affected Wireless Mics? . . . . . . . . . . . . . . . . . . . . 10 Build an Unbalanced-toBalanced Adaptor. . . . . . . . . . . . . . . . . . . . . . . . . 20

Radio World (ISSN: 0274-8541) is published bi-weekly with additional issues in February, April, June, August, October and December by Future US, Inc., 11 West 42nd Street, 15th Floor, New York, NY 10036-8002. Phone: (703) 852-4600, Fax: (703) 852-4583. Periodicals postage rates are paid at New York, NY and additional mailing offices. POSTMASTER: Send address changes to Radio World, P.O. Box 282, Lowell, MA 01853.

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GOOD BONES (continued from page 3)

several other directions. In those cases, there’s often no money to spend on maintenance, and the situation becomes one of self-perpetuating decline.

Other times it’s not so much a matter of money as of resources. A solo engineer, employee or contract, is stretched between many facilities, oftentimes scattered over a large geographical area, competing for attention and each receiving very little.

And sadly, at times it is what I would characterize as indifference. A person charged with the care and feeding of the site or station just doesn’t care and does the bare minimum to get by. The station is on the air, and nobody sees the transmitter site but the engineer; so no one is the wiser that there are big problems there resulting from neglect or worse.

Fortunately, these situations are by far the rare exception, but they do exist.

In a lot of cases like those I’ve described, the bones of what used to be are still visible.

At one time, it was a very nice, wellengineered facility that was undoubtedly well maintained, the pride and joy of the engineer given charge of it.

And unless there are structural or other issues that go beyond cosmetics, there often is hope for such a run-down and neglected facility. It may never shine as in its glory days, but it can be a safe, functional, well-maintained, reliable transmitter site.

STATE OF HEALTH There’s a psychological element to

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October 21, 2020

such a situation that goes well beyond the physical condition of the site.

The worse shape a facility is in, the harder it is for even the most dedicated engineer to gin up any level of concern.

It looks hopeless, so in his or her eyes, it really is. At some point, the roof will fall in and nature will reclaim the place, leaving little or no trace that it was ever there… or so it seems.

But clean the place up, plug the holes, remove the critters and their leavings, and that same engineer starts to feel better about the place, becoming hopeful. Maybe he or she even begins to take pride in it.

In years past, an FCC agent in charge

way, I suspect they would agree.

So there may be another benefit to sprucing up a run-down site.

I mentioned the self-perpetuating nature of neglect. It also works in the other direction, although it requires some input of energy. If a facility is in good shape, most engineers will want to keep it in good shape or even improve it.

We like to feel good about the facilities we maintain; and like it or not, those facilities do reflect on us.

The point is that if you have in your area of responsibility a site that is in some state of neglect or deterioration, you can very likely reverse it, without

I’ve seen transmitters with side and back panels removed and interlocks jumpered out so that lethal voltages are exposed. of a western field office was the selfdescribed “master of the ten-minute inspection.” He was a great guy, super to work with and always helpful and courteous.

If he walked into a transmitter site and found it clean and well-maintained, he wouldn’t look very hard for minutia or hidden violations. It was his view that if the company and engineer in charge cared enough to keep the place clean and maintained, it would very likely be in compliance with the rules.

He was right. While I can’t speak for current district Enforcement Bureau people, who might not admit to it any-

HAVE YOU READ THESE EBOOKS?

spending a lot of money. That reversal will pay big dividends, both in your own attitude, in the longevity and reliability of the facility and even in the sound and performance of the transmitter signal.

Start with a “to do” list developed by taking an objective walk-through of the site. What are the problems and issues?

Take the list and prioritize it logically. For example, if there is a rodent or insect infestation, first figure out where/ how they are getting in and deal with that before you start to work cleaning up the rodent or insect mess or you’ll have to do it twice.

A good trick is to go to the site at

night, and turn on the lights inside the building, tuning house or ATU. Then go outside and shut the door, with all outside lights off.

Wherever you see light escaping, even a pinhole, is a potential entry point for critters and bugs. Of course there may be entry points beneath, perhaps at a conduit or telco cable entry penetration where the light trick won’t help you, but those should be readily identifiable.

Plug the holes, then suit up (Tyvek suit, gloves, mask and eye protection) and start cleaning. Remove the bigger stuff by hand, then use the shop vac, then go to (safe) solvent cleaners/disinfectants and paper towels.

The work may take some elbow grease, but when you’re done, you’ll be amazed how much better the site looks and feels!

After that, focus on prevention. I’ve found mothballs help keep the critters away (they help keep me away, too, but it’s a small price to pay). Clear vegetation from the building or ATU cabinet, and put out an insecticide barrier, replenishing it regularly. An herbicide can also be applied to keep vegetation from growing back up close to the structure and providing critters a closeby habitat.

A lot of sites, both AM and FM, have good bones. Even if they have fallen into neglect, they can often be restored to a condition that an engineer can be proud of, despite the equipment being decades old. In most cases, the result is well worth the effort.

Cris Alexander, CPBE, AMD, DRB, is tech editor of RW Engineering Extra.

He is the director of engineering for Crawford Broadcasting.

Radio World ebooks explore trends that are changing the nature of radio and audio engineering. Here are excerpts from three recent issues. Find these and dozens of other free ebooks at radioworld.com/ebooks.

Roz Clark of Cox Media and the NAB Radio Technology Committee, talking about virtualization of the air chain: “Once we can get a process into the world of software and it can be networked and connected together, it lends itself to, ‘Okay, where is this software? Where does it live? Does it live in a purpose-built box or does it live in a server amongst other instances of the software?’ And once it’s into that ‘virtual environment,’ the location of it becomes optional, depending on available networking. … We’re doing a tremendous amount of it on the business side but less on the broadcast side at the moment, mainly because of the networking and perceived reliability requirements.”

The ipDTL service has been enjoying more attention during the pandemic but has been around for several years. It comes from In:Quality, which “operates a worldwide network for the real-time transmission of professional audio.”

Users include the BBC, New York Public Radio, NPR and Global Radio.

The service is based on the open source Opus codec. Founding Director Kevin Leach says ipDTL runs smoothly on any modern computer: “If you can browse the internet smoothly on your computer, then you can run a stable ipDTL connection.”

A subscriber of the service can send a link to another location, where that link is opened in a browser and a bidirectional studio quality audio link would be established. (Written by Paul Kaminski)

Some stations take the “auxiliary system” approach and leave the aux transmitter connected directly to the auxiliary line and antenna and perhaps even fed with the backup STL, audio processor and PPM encoder.

That has the advantage of “one-stop shopping” when anything goes wrong — just switch transmitters and you’re back on. But that approach often exacts a significant penalty in sound quality and coverage.

A better approach is one that is flexible, where the aux transmitter can be connected to either main or aux antenna, fed by either main or aux audio processor and either STL. (Written

by Cris Alexander)

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IN THE FIELD

Co-locating an AM With a Cellular Tower

RADIOWORLD ENGINEERING EXTRA

October 21, 2020

A paper by Jim Dalke and Stephen Lockwood tells how KARR did it

BY TOM VERNON

Much of AM radio has been in dire straits for some time. It has seen profits decline when faced with new competition from streaming broadcasters and better-quality FM signals.

At the same time, many stations are losing their long-established transmitter sites and being forced to relocate. For them, the options usually don’t include finding another high-quality site, but instead locating a space where they can put a minimal signal into their city of license.

It’s no wonder that the number of licensed AM stations is declining. Over the past decade, their numbers have decreased by 224 — to 4,680 as of September 2020 — while both commercial and NCE FM totals have increased, according to FCC data.

The driving force behind many AM relocations is the rising cost of real estate in what was once farmland or a swamp in the middle of nowhere.

Environmental restrictions involving aesthetics, radiofrequency exposure and local zoning restrictions can also be contributing factors.

For some stations, co-locating with other AM broadcasters on a common tower and connecting via a diplexer or triplexer can work, provided the common location will provide adequate service.

Another option is “Co-locating AM Transmitter Facilities With Cellular Monopole Towers,” which is the title of a paper by James A. Dalke of Dalke Broadcast Services, Inc. in Bellevue, Wash., and Stephen S. Lockwood, P.E., of Hatfield & Dawson Consulting Engineers in Seattle. It was to be presented at the spring NAB Show this year before world events intervened.

DIFFICULT RELOCATION The plight of many AM broadcasters is typified by KARR, 1460 kHz, licensed to Kirkland, Wash.

On Feb. 28, 2014, after nearly 50 years of broadcasting, KARR went dark and seemed to have little prospect of returning to the air.

The station had begun broadcasting in 1965 as a 5 kW daytimer with a threetower array in the rural residential Rose Hill area of Kirkland, Wash., across

Lake Washington, east of Seattle. When it was built, Kirkland’s population was about 10,000 and the six-acre transmitter site was on farmland.

In the 40-plus years since, the area has developed into a dense residential area, with the transmitter site one of the only significant areas zoned for residential development but not developed. The population of Kirkland is now 85,000.

The property had become far more valuable for residential use.

Because of the residential development, relocating the transmitter site became increasingly problematic. The Class B license requires co-channel protection for KUTI, a 5 kW Class B in Yakima, Washington, a hundred miles southwest of KARR, and adjacent channel 1 kW Class C stations KONP, Port Angeles, 70 miles northwest, and KSUH, Puyallup, 34 miles south.

Protection required for these stations required a traditional array of least three towers on six plus acres of suitable land.

In order for KARR to return to the air, a more non-traditional solution needed to be explored. Fortunately, there was help available from the FCC. I n the commission’s ongoing “AM revitalization” effort, three rules in particular affected the KARR relocation.

The FCC relaxed the minimum daytime Community of License (COL) coverage standard from 80 percent to 50 percent of the COL, and for nighttime operations, eliminated the coverage standard for existing AM stations, and reduced the standard to 50 percent for new stations.

The FCC found that these changes would make it easier for stations to cover a community that has expanded beyond the reach of existing facilities, as well as relocate antenna facilities to improve coverage.

Second, the FCC eliminated the “ratchet rule,” which required AM stations to reduce their signal strength when they make facility changes to modify their signal. They found that the rule has not achieved its intended goal of reducing interference.

The commission has generally granted waivers when a station was forced to relocate because of circumstances out of its control, such as the loss of transmitter site lease; the recent rule change (continued on page 8)

Fig. 1: The cell tower used for the KARR project.

On Feb. 28, 2014, after nearly 50 years of broadcasting, KARR went dark and seemed to have little prospect of returning to the air.

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AM (continued from page 6)

simplified the relocation application.

Third, the rules allow authorized AM owners to acquire and move an FM translator up to 250 miles from the AM station’s location to rebroadcast the AM station programming.

While the new rules have enabled an FM translator to rebroadcast KARR, it does not provide any advantage in relocating the AM transmitter facilities.

The AM broadcaster with an associated translator must still maintain the licensed AM broadcasting facility.

SLANT AND SHUNT Cellular monopole towers with a slant or shunt feed have been a solution for AM broadcasters facing the same challenges as KARR, and the slant wire method has been the subject of several research papers.

Ten years ago Ben Dawson of Hatfield & Dawson Engineers in Seattle authored a paper titled: “The Slant Wire Shunt Fed Monopole: A Neglected but Invaluable Technique.” It is available on the Hatfield & Dawson website at https://tinyurl.com/rwee-slant.

In this paper, he concluded that the slant wire feeds are simpler electrically than other ways of feeding grounded based structures. The slant wire technique also imposes far less structural load and is less susceptible to weather related damage in hostile climate conditions.

Dawson also concluded that the shunt fed monopole provides convenient impedance matching, good bandwidth, and efficient radiation patterns.

A viable ground system is also a key component of AM antenna systems.

For many years, it has been apparent that the standard ground system for AM radio using 120 quarter-wavelength ground wires was excessive to providing an effective ground system. This was covered in a paper by Dawson & Lockwood. Using NEC-4 it was possible to model ground system with each individual ground wire.

The FCC has allowed a simplified model for a ground system using an equal area model, which uses a circular model that has an area that is equal to the irregular area of the property. This is useful if the tower is offset from the center of the land or there are other obstructions that require use of a shortened ground system. Where a more detailed simulation is desired, the location of each ground wire can be calculated and the antenna system can be modeled to learn the effects of the compromised ground.

As part of the AM revitalization proceeding, the antenna efficiency requirements have been reduced. These chang-

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October 21, 2020

remain a viable option to continue to provide service, and may be the only feasible option for some licensees.

Fig. 2: A slantwire was used to shunt feed the cell tower with KARR’s 740-watt signal.

The FCC has allowed a simplified model for a ground system using an equal area model, which uses a circular model that has an area that is equal to the irregular area of the property. es have enabled more options for using electrically short antennas and sites that have less area to provide a standard ground system.

Many of the techniques used in colocations of other radio facilities with AM stations have been standard processes, and there are many techniques used to accomplish this. These changes, along with the change in FCC policy to allow slant wire feeding of an AM antenna, allows for use of a tower without a base

insulator or a skirt wire arrangement.

Slant wire feed systems were somewhat common before 1960. After that, FCC policy did not allow use of this feed system because it was assumed that there was some asymmetry of the field produced from a slant wire shut fed tower.

These policy changes have greatly expanded the options for dislocated AM stations. While these facilities are a compromise from an ideal facility, they

BACK ON THE AIR Utilizing the slant wire feed and improved ground system modeling, Jim Dalke, the current owner of KARR, along with Stephen Lockwood of Hatfield & Dawson, obtained the construction permit to install the Hatfield & Dawson-designed slant wire fed cellular monopole. Unfortunately, the COVID crisis hit before construction was complete. Installation will continue as soon as restrictions are lifted. The station is licensed to operate with 740 watts daytime, and provides good coverage for the Kirkland area.

In order to obtain approval from the commission, the engineering application had to demonstrate that the facility meets the FCC requirements for a nondirectional antenna.

This is best done by using NEC-4 to model the ground system to calculate the efficiency and radiation pattern above the horizon. All AM antennas have radiation patterns that produce fields that are directed above the horizon. The antenna system must also comply with the f(θ) curves provided in the familiar Fig. 8 of FCC rules (47 CFR § 73.160 and 73.190).

This also will model the effects (if any) of the slant wire feed system on pattern circularity. As pattern circularity was better defined in moment method rule making process and the use of moment method for re-radiation analysis from other communication towers, this is defined at ± 2 dB. The tower can be modeled to include the cellular antenna platforms and other changes from a uniform cross section guyed tower. This helps to account for any additional “top-loading” that these fixtures provide. There could be situations where the sectionalized tower and top-loading flags used in the FCC database to describe unusual antenna might be employed. This was not the case for KARR, as the modeled radiation pattern was enclosed within the f(θ) curves for a tower height of 80.2°. The modeled elevation pattern in several pertinent azimuths is compared to the standard pattern and f(θ) curves to assure that it does not exceed these limits.

The H&D drawing shows how the shunt feed is connected between the transmitter and the 150-foot monopole antenna. The cellular antenna array at the top actually provides some “top loading.”

While the AM coverage from the new site will be significantly less than the original site abandoned in 2011, KARR has an FM translator associated with the AM license under the FCC’s revitalization rules.

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MICROPHONES

RADIOWORLD ENGINEERING EXTRA

October 21, 2020

Lectrosonics’ D Squared fourth generation is also representative of new high-tech wireless systems designed to help users cope with limited spectral resources.

Repack Impact: How Has It Affected Wireless Mics?

Manufacturers and users must cope with loss of spectrum

BY JAMES E. O’NEAL

Ever since the use of wireless microphone technology exploded several decades ago, virtually all TV broadcasters — large and small — “cut the cable,” deploying wireless mics both in the field and in the studio. Ditto with radio remote broadcasts, churches, live music, Broadway and Hollywood adopting wireless mics for sound reinforcement and motion picture production.

Paradoxically, as the popularity and use of these devices has risen, the amount of permissible spectrum for their deployment continues to shrink, beginning with the TV channel repack some 10 years ago which made the 800 MHz spectrum “off-limits,” and continuing today with the cap being lowered to 608 Mhz. The FCC issued a “heads-up” notice addressing this as the most recent TV spectrum auctions were ending, advising that wireless mics could still be used in 600 MHz spectrum as long as there was no interference to TV broadcasters or wireless broadband operations. The com-

mission had also established a mid-2020 deadline for cessation of wireless mic operations for a complete cessation of wireless mic operations in the 600 MHz spectrum deeded over to the telecoms.

WILL THERE BE ANY WHITE SPACES LEFT? With the deadline past, wireless mic users face the loss of more than 80 MHz in the region between 608 and 692 MHz

is in addition to the 100 MHz or so that was reallocated for wireless broadband use when TV Channels 52 to 69 “went away” in the aftermath of the 2009 transition to DTV. The FCC is still allowing unlicensed wireless mic operation on any unoccupied UHF (or VHF) TV channel, but the gotcha is that with the current consolidation of stations into an even smaller neighborhood, there are fewer

With the current consolidation of stations into an even smaller neighborhood, there are fewer and fewer unoccupied channels, especially in metropolitan areas, where the great majority of wireless mic users are found. where mics had been allowed to operate (minus, of course, spectrum occupied

by TV stations which were operating in that range). This lost chunk of spectrum

and fewer unoccupied channels anymore, especially in metropolitan areas — where the great majority of wireless mic users are found.

Admittedly, a wireless mic requires a lot less operational spectrum than a TV station, but users are feeling the pinch nonetheless.

“The reallocation of the UHF Band for mobile and other services is the single largest regulatory challenge that has faced the wireless microphone industry globally,” said Mark Brunner, vice president of global corporate and government relations at Shure. “Reduction of TV band spectrum has occurred while demand by productions for wireless microphones and related pro audio equipment has steadily increased. The TV spectrum has been the primary home for wireless microphones since the beginning, and this spectrum is ideally suited to the RF performance characteristics desired by technical crews.”

Karl Winkler, vice president of sales and marketing at Lectrosonics, echoed Brunner’s observations. “The popularity and utility of wireless microphones has not waned at all despite the loss of a significant portion of the spectrum,” he said. (continued on page 14)

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October 21, 2020

WIRELESS (continued from page 10)

“The convenience, freedom of set design and movement, and the ability to rapidly respond to evolving news situations have all contributed to this popularity. Thus, the loss of spectrum has forced the industry to respond with newer technologies and features, along with the need to offer assistance to customers who have been displaced.”

Winkler noted that this assistance has been in the form of rebates, tradeins, special pricing and other incentives being offered by wireless mic manufacturers, and observed that while broadcasters were being compensated for the changes forced by the reduction in spectrum, this was not the case with mic users.

“The government has not offered any compensation for wireless users the way they have for broadcasters to repack or go off the air,” he said.

Joe Ciaudelli, director of spectrum affairs within Sennheiser’s Research and Innovation division also weighed in on the issue.

“The repurposing of spectrum from traditional over-the-air broadcast to mobile broadband is an issue of distribution of content. It slices the distribution ‘pie’ into smaller slices.

Conversely, the content creation ‘pie,’ which requires wireless microphones and similar devices, continues to experience exponential growth. Furthermore, productions continue to advance in technical sophistication.”

This has put engineers in a very difficult position, according to Ciaudelli.

“Many producers and directors don’t know the details of the spectrum crunch and regulatory changes,” he said. “They understandably are focused on nontechnical aspects of the production.

They just expect the engineers to make it work, without realizing the magnitude of the challenge the engineers face.”

HOW BAD IS THE PROBLEM? While an exact headcount of wireless microphones, IFB systems and belt-

Mark Brunner, vice president of corporate and government relations at Shure.

Jim Dugan, president of wireless microphone company Wisycom USA and Jetwave Wireless.

pack intercoms in the United States isn’t available, James Stoffo, the chief technology officer at wireless intercom manufacturer Radio Active Designs, offered what he believes to be a good working estimate of the number of such systems that are now competing for spectrum.

“My partner Geoff Shearing and I went to the FCC on multiple occasions on behalf of the broadcast and production communities, and tried to enlighten them as to how many wireless microphones and intercoms were out in the United States,” said Stoffo.

“When we first went, the FCC thought there were less than 1,000 wireless in the entire country. Actually, at that time there were closer to 2.5 million devices, and that caused a couple of jaws to drop.”

Stoffo, who has been in the wireless business since the 1980s and has served

as RF coordinator for numerous large events for years, noted that even three decades ago, the number of wireless systems in use was close to 2 million and has been growing steadily.

“At the NBC studios in New York in one building there are probably 500 devices,” he said. “In Broadway theaters — in just a three-block radius — there are probably 1,500 wireless devices.

And in Disney World alone there are almost 2,000 devices.”

Obviously, the diminishing spectrum for these devices is a huge issue and one that is not going to go away. So, with the loss of the 600 MHz TV channels, can there still be a “business as usual” attitude among users?

CAREFUL PLANNING Jim Dugan, president of wireless microphone company Wisycom USA and Jetwave Wireless, a firm

Joe Ciaudelli, director of spectrum affairs, Sennheiser’s Research and Innovation division.

Karl Winkler, vice president of sales and marketing at Lectrosonics.

James Stoffo said that even three decades ago the number of wireless systems was close to 2 million and that it has been growing steadily since.

James Stoffo, CTO, Radio Active Designs.

Wisycom’s dual-channel wireless mic receiver, the MRK980, is designed to help operators maximize operating spectrum.

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WIRELESS (continued from page 14)

that specializes in wireless technology deployment, has been involved in many large-scale wireless system deployments during his three decades in the business. These include ABC’s “Dick Clark New Year’s Eve Times Square” broadcast, along with many other big-ticket sporting and music events, presidential inaugurals and more. He was asked about the prospects of being able to “go wireless” at future events, given the contracting spectrum situation.

“It’s all about coordination,” said Dugan, pointing out that he acts, on a volunteer basis in conjunction with the local Society of Broadcast Engineers, as frequency coordinator for special events

RadioWorldMagazine

EDUCATION IS KEY Loss of spectrum because of the repack is not the biggest problem facing the wireless mic arena, however, according to Dugan.

“I think it’s spectral efficiency and spectrum management [band planning],” he said. “This problem shows up differently for different verticals. If you’re an ENG operator you want to have 20 or 30 MHz between transmit and receive frequencies for talent. In broadcasting bigger installations, band planning is very important. A lot of people turn these things on and don’t realize that they interact.

“I tend to look at everything in terms of inbound and outbound frequencies,” he added. “People seem to understand this the least. There’s really a big gap in the understanding of ‘best practices’ by a lot of operators out there. It can’t

Loss of spectrum because of the repack is not the biggest problem facing the wireless mic sector, said Jim Dugan. It’s spectral efficiency and band planning. in Washington, D.C., and also works to coordinate wireless device use for many of the really big events just mentioned.

Dugan cites a recent NHL All-Star game as a good example of the challenges of frequency coordination. “There were 500 frequencies in coordination — really a lot of stuff going on,” he said.

“We’re in the process of revisiting this whole usage and coordination model right now.”

Dugan is also involved in coordinating the annual New Year’s Eve broadcasting activity taking place in and around Times Square in New York. He observed that in addition to the mainline multiple TV networks doing remotes in the area, there are also musical acts going on in the area — all involving wireless communications.

“During the past several years we’ve been looking at six TV networks with probably 500 wireless mics,” Dugan added. “There have also been maybe 10 wireless cameras in use too, along with the thousands of people with their cell phones generating RF noise. This is a really good test of the technology.”

Dugan starts notifying the networks in October, but adds that once the coordination plan was established it has pretty much stayed the same over the years.

This Sennheiser SK6000 bodypack transmitter is typical of the very large number of wireless devices vying for space in an ever-shrinking RF spectrum landscape.

be a situation anymore where ‘planning’ is accomplished by just issuing a purchase order for wireless mic equipment.”

While it appears that postrepack wireless mic usage can continue — along with overthe-air TV broadcasting — there will be fewer and fewer parking slots.

This just might be a good time to check out your old triedand-true corded microphones and cable sets to make sure they’re serviceable, just in case the wireless broadband folks start eyeing some of those remaining parking places.

October 21, 2020

SO, WHAT EXACTLY IS LEFT SPECTRUM-WISE? With the loss of spectrum above TV Ch. 36 (602 to 608 MHz), what’s left in which wireless mics, wireless IFB systems and beltpack intercoms can operate?

According to the FCC, this depends. Rules allow such unlicensed devices to freely navigate both the VHF and (remaining) UHF spectrum, provided there is no licensed entity operating there. With the repack forcing some very close channel adjacencies, and even movement from UHF down to low VHF slots (there will even be 11 stations operating on Ch. 2 when the dust settles, according to www.rabbitears.info), broadcast spectrum that might be used for wireless mics and similar devices seems to be in short supply, especially in larger markets.

The FCC, in a recently issued bulletin (“Operation of Wireless Microphones”), rather optimistically states: “Many frequencies in the TV bands that had been available for wireless microphone use prior to the auction will continue to be available after the transition period.”

Notwithstanding the reality of the highly-compacted post-repack spectrum, there is a glimmer of hope in that operation of these devices will still be permitted above 608 MHz within “certain frequencies” in both the wireless broadband’s 614–616 MHz “guard band” (just above TV Ch. 37) and in its “duplex gap” (657–663 MHz for unlicensed mics) and (653–657 MHz) for licensed devices.

However, the commission’s memorandum does not specify what the “certain frequencies” may be.

(Definitely off-limits are the 617–652 MHz and 663–698 MHz regions.)

The commission has also required that anyone selling or leasing wireless mic gear capable of operation in these latter two spectrum blocks were to inform customers that they “must cease operating on these frequencies no later than July 13, 2020.”

The FCC memorandum also recognizes some “bands outside the TV bands for wireless microphone use.”

Specifically, these are the 169– 172 MHz region, some 900 MHz spectrum, as well as 1,435–1,525 MHz in the UHF band, and 6,875– 7,125 MHz in the SHF (super high frequency) microwave region.

A license is required to “fire up” a wireless device operating on any of the frequencies in these ranges;

however, this may not be the easiest thing to get unless you belong to a certain group.

This is spelled out clearly in FCC rules dealing with low-power auxiliary stations (LPAS) licensing and operation, with only large entities such as TV and motion picture production companies, cable TV operators, performance venues, or professional sound reinforcement companies that “routinely use 50 or more” such low power devices eligible for a license.

If you don’t fall into this group of “super users,” the commission also sanctions wireless mic operation in some other spectrum blocks, but these are shared, and are not always the best neighborhoods.

They include 902–928 MHz (also referred to as the 33-centimeter band), and 1,920–1,930 MHz, along with some additional territory in the microwave region (portions of the 2.4 GHz and 5 GHz spectrum bands.)

If you opt for 33-centimeter spectrum, you’ll be sharing it with industrial, scientific and medical (ISM) RF equipment users, as well as radio amateurs. The 1,920–1,930 MHz block is shared with licensed “Fixed and Mobile” users, as well as some cordless telephones. Also, the commission forbids the plunking down of conventional FM wireless mics (or other communication devices) in this region. As it’s shared, any unlicensed device must perform continuous monitoring and only produce RF where no other signal is detected, which requires a specialized class of wireless mics. You’re also going to have some company in the 2.4 GHz and 5 GHz regions too, with such things as ISM devices, cordless phones, Wi-Fi and Bluetooth transmitters, and even garage door openers and microwave ovens vying for spectrum.

(Editor’s Note: The spectrum usage information presented here only summarizes applicable FCC rules and regulations at the time the article was prepared, and is not intended to replace careful reading and observance of the complete rule set. Also, as the ongoing pandemic continues to alter private sector and governmental processes and activities, please refer to the FCC’s website for the most up-todate information on wireless microphone and any other spectrumrelated matters.)

October 21, 2020

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◗MARKETPLACE Multitrack Madness: The PodTrak P4 is a new portable multitrack recorder from Zoom North America aimed squarely at podcast production.

It retails for $199.

Weighing about half a pound, the recorder combines a mixer, mic inputs, headphone outputs, sound pads and Bluetooth in a unit measuring about 4.6 x 6.1 inches The unit records to SD cards up to 512 GB or can work as a two-input, two-output audio interface, connecting to a computer. Up to eight tracks can be recorded in 16-bit /44.1 kHz audio WAV format. All input sources can be recorded simultaneously on separate tracks.

The P4 provides four 48V phantom-powered XLR inputs, each of which have its own mute button and control knob providing gain up to 70 dB. Correspondingly, there are four 3.5 mm-1/8-inch stereo mini headphone outputs with individual volume controls as well, as there is a mix-minus feature to help prevent echo and feedback.

Remote interviews via phone can be recorded l via a TRRS cable or via a USB cable attached to a computer running conferencing software. Users with the optional Zoom BTA-2 Bluetooth Transmitter/Receiver can wirelessly connect a smartphone to the P4 in order to record remote guests; an Apple Lightning to USB camera adapter is required for iPhone users.

For users who want to work in music, ads, jingles, stingers and the rest, four assignable stereo sound pads allow them to trigger 11 onboard sounds, as well as load in their own audio for triggering by the pads.

The P4 can run up to four hours on a pair of AA batteries or can be powered externally by a USB Type C cable as well as an AC adaptor.

Info: https://zoomcorp.com

HD Radio Tool: 2wcom’s latest is the HDR-CC, an HD Radio capture client for the delivery of additional HD Radio channels.

The company says that by using the unit, users can set up the importer IP address and directly connect audio to the small box’s XLR connectors.

The box is able to accept one digital or analog stereo audio channel and provide it to an importer. The unit also sends the compressed audio via IP using an HD Radio codec. The HDR-CC can thus be located in a different location than the importer.

Due to HDR-CC’s sound processing capabilities the loudness is almost the same as on the main program.

In addition, 2wcom says the unit simplifies audio switching for emergency alerts. Utilizing a new feature Xperi has implemented in Generation 4 importers, a single HDR-CC is able to provide the entire emergency alert for all supplemental channels on the transmitter.

The company said that the only setup required is an AES audio connection to the capture client and a GPI to trigger the alarm. When the alarm is triggered the HDR-CC logs into the importer and replaces all supplemental channels (HD2–HD4) with the alarm program. After the GPI is released the HDR-CC logs out and the importer continues with normal operation.

Info: www.2wcom.com

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SYNCH

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October 21, 2020

Harris (GatesAir) Z5CD transmitter that integrated an older exciter that was unable to properly match modulation components to the W295BH transmitter from a different manufacturer.

Without full matching, there was no way to properly and fully synchronize.

Thus, it was proposed to replace the old Harris exciter in the Z5CD with a new GatesAir FAX-50 exciter, and install a GatesAir Flexiva FAX 1 kW air-cooled transmitter/exciter at W295BH. It was determined that the existing Wheatstone FM-55 audio processor with its integrated stereo generator could be retained, along with the SmartGen RDS encoder from Deva Broadcast. Both devices were moved from the W240DP Nokomis site to the WSRQ studio in preparation for the networking and transmission upgrades.

Meanwhile, the Flexiva FAX 1 kW system replaced the translator for

(continued from page 1)

WZZS. LECOM accomplished both objectives by acquiring WZZS in December 2019.

The initial simulcasting efforts fell short of the intended goal. With no simple way properly to time the audio using the existing setup, along with a multipath-like (simulcast interference) sound in many important areas, it became clear that only full synchronization of the 106.9 signals would cure the problem.

With the help of Tony Gervasi, Intraplex specialist and sales manager for GatesAir, a proposal was delivered for an intelligent IP networking system with built-in synchronization. The goal was to establish a composite multiplex signal over IP to the three FM transmit-

WheatNet, which has a standard analog composite multiplex output and a digital output that can be either AES/EBU (AES3) or AES192.

We chose to use the AES192 interface to the IP Link MPXp. We are told that there is a several dB of separation advantage and a slight loudness advantage by going this route.

This is not anything like the old AES3 interfaces to an exciter, where overshoots were common and perceived loudness was reduced. Performance has proven to us that the modulation control is very good over AES192. The RBDS encoder is also connected to an SCA input on the studio IP LINK MPXp codec, which means we can deliver a fully-processed stereo signal with embedded RBDS over IP. The Intraplex codecs have some “secret sauce” not fully known to us, but one thing we learned is that AES192

CONNECTING THE LINKS Next came the decision on which Intraplex IP Link codecs to use, most of which have the option to add Intraplex SynchroCast software.

The decision came down to either using AES192, with each site utilizing its own stereo generator and RBDS encoder, or some form of multiplex composite over IP, either analog MPX or digital AES192.

To function properly, the system would require one codec at the send site, and one at each of the three FM sites; for the AM transmitter, we planned to continue using the existing Intraplex

Wheatstone IP-12 IP Console

Harris Z5CD

Wheatstone FM55 Processor GatesAir FAX1K

GatesAir MPXp

BW Broadcast Transmitter

GatesAir FAX50

GPS GatesAir MPXp w/Synchrocast

GatesAir MPXp

GatesAir MPXp

w/Synchrocast

w/time stamped audio synchrocast

GPS STUDIO

Images subject to copyright

Fig. 1: System Block Diagram

ter sites in a four-way simulcast with WSRQ (AM), using fiber connectivity to the public internet.

PREPARATIONS The initial work required an evaluation of infrastructure to support the new networking strategy.

One positive was that the existing fiber connections (and related Service Level Agreements) to all FM locations were adaptable to the new system. It was also possible to retain the existing GatesAir 950 MHz HD Link feeding the AM transmitter.

Less adaptable were the existing exciters and stereo generators — together anyway — which were not matched and would be unable to achieve the required synchronization.

The initial work involved extending the existing VPN to include the Zolfo Springs transmitter site that would become WSRQ(FM). That site used a

W295BH 106.9

W295BH, and an existing BW Broadcast translator for W240DP on 95.9 FM was determined as adaptable to the new system. We were able to retain that transmitter since it lives on a different frequency, and its internal audio processor was engaged while the new system was being set up.

A single-frequency network with several transmitter sites would require matching exciters at each location to avoid synchronization issues. This is because when dealing with microseconds as it relates to the path of the radio signal, the differential delay in an unmatched signal through using different exciters will introduce unpredictable latency.

For this project, it was only required to use the same exciter on all 106.9 systems, making this a rarer kind of synchronized network than what we more typically see with a complete SFN architecture.

WSRQ-FM 106.9

HD Link 950 MHz STL. We looked at three IP Link codecs, all of which support digital AES192 AES composite transport. After consultation with GatesAir’s Tony Gervasi, we elected to use AES192 distribution.

We selected the Intraplex IP Link MPXp codec model, which has a builtin feature set that includes GPS lock for synchronization, and optional LiveLook software for advanced network analytics. This gave us the flexibility of using both AES192 as well as the analog MPX composite using BNC connectors.

We needed the analog MPX composite capability to feed our emergency backup transmitters /exciters, so the IP LINK MPXp offered the best of both worlds.

The audio chain at the studio consists of a Wheatstone WheatNet system that feeds IP audio to compatible devices on the network. The Wheatstone FM-55 processor is fed as a blade (node) on

W240DP 95.9

as a standard is not highly efficient. The IP Link MPXp repacks the data much more tightly, and can deliver our complete signal in an uncompressed-linear mode at the astounding data rate of 1.64 Megabits per second. Conventionally, a stereo uncompressed linear AES3 signal requires about 1.5 Mbps, but here we have the entire baseband in a neat little package.

Redundancy is important, and the use of fiber and public internet for connectivity can provide for two parallel networks to move program audio. Using GatesAir’s Dynamic Stream Splicing (DSS) feature, we can run four streams across the two networks. At the present time we are installing a second ISP connection over a cable modem to back up the fiber network.

If the primary on the fiber fails or experiences packet losses on one provider, the software repairs the stream by utilizing redundant packets from the

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October 21, 2020

other. If there is only one connection available, two streams are sent with a slight delay on the second one (we are using 500 mS) and rarely have an audible dropout.

The system employs GPS lock at the studio and all FM sites, with the two 106.9 transmitters also utilizing the Intraplex SynchroCast feature. The Nokomis/Venice W240DP translator, on a separate frequency, receives the same composite signal with audio timed to synchronize with the 106.9 signals.

This is critical since there is some overlap in coverage.

The Dynamic Stream Splicing feature is configured with between 180 and 500 microseconds of delay between the streams as they move across all FM sites to address that overlap. Since the W240DP translator operates on a different frequency, it does not require that we carrier lock the signal but the audio is synchronized.

Since going live, weeks go by before a single drop is noticed, which is a testament to the robust nature of Dynamic Stream Splicing feature. On rare occasions, ISP issues have created some problems, but the software still produces a “hitless” experience.

The Intraplex package also included Intraplex LiveLook feature for real-time network analytics. This feature also provides historical reporting on the quality of the IP connection between the different sites, including email notifications of connectivity issues. LiveLook provides insight on network behavior over time, which also delivers predictive capabilities for network troubleshooting.

NETWORK DESIGN CRITERIA There are some compromises with our system’s design from the ideal.

We cannot control the overlap areas completely, or offer precision timing over such a wide area. We also cannot pinpoint correction to all of our interference areas.

The presence of unpopulated (farmland/ranchland) areas between the sites has proven beneficial, and we were able to force most of the interference over that area. In an ideal SFN, we would use directional antennas, and perhaps even use multiple transmitter sites, to create a more robust, interference-free network. This is not a main station and booster scenario, however; this is a main station and translator situation. The math behind this calculates the “launch delay” at W295BH, because the signal takes longer to reach the areas of interference from the Zolfo Springs site due to distance.

Our “sweet spot” from an interference standpoint was to be the licensee’s dental college in Lakewood Ranch, a suburb east of Sarasota subject to considerable interference. The distance

W295BH 106.9 180µs Launch Delay

LECOM Dental

Area of noticeable interference WSRQ-FM 106.9 Studio

W240DP 95.9

Area of noticeable interference

Fig. 2: Propagation Paths

Firgure 2

from Zolfo Springs to that site is 63.43 km. We calculated that the signal from W295BH would reach that point 180 microseconds before the Zolfo Springs signal would arrive.

We set the launch delay from W295BH at 180 microseconds so the signals arrive in-phase and fully synchronized in every way, at our reference point. The signal can be heard at that location if either transmitter is turned off, but with very similar field strengths. Therefore, it is also an ideal test for perfect synchronization. When signal strength is nearly equal, timing is extremely critical. As the ratio of D/U increases, timing will become less critical until there is at least 20 dB difference.

The calculations proved correct: That site receives a clean signal with no multipath-type interference. With matched equipment at both the translator and the main FM station, the modulation components are identical, and in the end, we have a robust solution.

The big question: Would this timing work out for the rest of the area to the west where either signal may be heard, depending upon location?

STRONG RESULTS As Fig. 1 shows, the IP Link MPXp feeds a Flexiva FAX 1 kW translator at W295BH. At the former WZZS site,

now WSRQ(FM), a FAX50 exciter feeds the legacy Harris Z5CD transmitter. At W240DP, the audio is also time-stamped and transmits in audio synchronization with the 106.9 signals, but again the carrier is not locked

by GPS. Fig. 2 shows the propagation paths between the translators and the Class A FM. As mentioned, LECOM operates a dental college in Lakewood Ranch, east of Sarasota. Lakewood Ranch is a “boom town” and has had huge growth over the last 10 years, which is projected to continue. It was also an area prone to extreme chronic interference, especially under certain seasonal weather conditions.

We were able to have enough control to be able to lock in Lakewood Ranch and still not have any noticeable interference in the immediate Sarasota/ Bradenton area from the WSRQ signal from 40 miles away.

East of Myakka State Park there is an interference zone that lasts about five miles before the WSRQ signal overrides the translator’s interference, traveling east. The good news is the interference area is programmed to be over “noman’s land.” It is agricultural in nature and is not a heavily traveled commuter route. The timing could be adjusted either way to move this interference slightly, as needed.

The only other area of interference which could not be controlled in this scenario is south of Venice (see Fig. 1) where the signal strength of the 106.9 signals is very close to equal. However, this is right in the heart of the W240DP translator signal that simulcasts the 106.9 signals.

What is management saying about the results?

Operations & IT Manager Charlie Halley said, “I used to pull my van into my driveway inside the City of Sarasota.

On one end I heard our station, and the other end I heard WZZS. Easy to tell because they were Spanish, we are Classic Hits. Now it’s seamless, end to end.” Charlie also appreciates having the insights provided by the LiveLook software, to keep an eye on the quality of the network.

General Manager Jim Schaffner adds, “I am very pleased with the station’s expanded reach and the sync of the two stations. And now our coverage is extended to over 60 miles inland, giving us several additional counties in our primary coverage area.”

Hal Kneller owned and operated radio stations from 1986 to 2014 and has served on numerous industry organizations such as NRSC and NAB technical committees. He is a member of the SBE National Certification Committee and Past Chapter Chair of SBE 90, Southwest Florida.

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October 21, 2020

Build an Unbalanced-to-Balanced Adaptor Buc Fitch provides an easy solution to an everyday engineering problem

BY CHARLES S. FITCH, P.E.

Our recent contribution to the Workbench column in Radio World about our home-brew construction of an XLR-3 cable tester (https://tinyurl. com/rw-xlr) produced a notable, positive reader reaction.

It seems the “maker instinct” exists in almost all of us in this profession.

Although I’d promised in our next article to jump cold into a complex follow-up project of a step oscillator, our distinguished editors thought something as useful as the cable tester but slightly more complicated might be a better next step … sort of an organic progression.

Advancing somewhat by inculcating some simple active circuitry would also be apropos. Staying in the universe of audio, our project in these pages is an elegantly straightforward unbalancedto-balanced audio converter.

As is often said, the beauty and joy of technical standards is that there are so many to choose from!

This dilemma holds true for the world of audio as well. We often need to make a marriage in our stations between the near-universal broadcast impedances and signal levels with the ubiquitous

“consumer” output levels.

Audio from handheld digital recorders, air tuners, computer sound cards etc. needs to be taken into the broadcast plant.

The situation is complicated by differences in impedances between devices, not to mention the unique characteristics of odd sources such as “pro” audio signals with their slightly higher levels.

ON WITH THE SHOW We’ve attempted to accomplish several goals here. In our example unit, we’ll use a similar enclosure to the one from our cable tester article so that, at least on our workbench, our new tools will have that matching look.

We’ll provide some flexibility in uses and setup. We’ll keep complexity and fabrication to a minimum and consequently, minimize the cost, especially as we may be making many of these.

We’ll provide enough design description and suggestions for changes and customization so you can make the project truly your own and as you need the device to work in your plant Let’s get oriented. Take a look at the schematic.

Addressing just levels, consumer

Breadboarding the circuit.

audio from something like a CD player is expected to peak at 0.447 volts with an RMS (Root Mean Square) value of 0.316 volts, which is well within the handling capability of an operational amplifier (op-amp).

If our audio is headed to a typical 600-ohm standard broadcast mixer input of +4 dBm, the level at that point will need to be a peak of 1.736 volts or 1.228 volts RMS. As such, the RMS voltage gain will need to be about 3.9, or 11.8 dBV. Fig. 1: Project Schematic

This project is divided into two parts, one for the signal flow area and the other for power supply, connectors and enclosure.

CIRCUIT DETAILS Like most electronic circuits, one can generally divide the beast into main activity and support sections. The main in this case is the signal flow area; the support is essentially the power supply, connectors and enclosure.

Start with main. In our circuit, the unbalanced signal enters the converter and encounters a 10k ohm shunt resistor to ground, which is included as the industry-suggested load impedance that provides the flattest signal response transfer. The 220 uF series capacitor isolates the audio, stabilizing the gain of the first active stage to follow.

The op-amp sections in this device are configured in the inverting format, providing great flexibility and stability.

Many readers will remember that most of my previous audio processing work in these pages used the classic 1458 op-amp, as I had been given and used over 100 of them. Now that I’ve been given an even larger number of the similar but higher performing 5532 opamps, we’ll start using this new group to a similar depletion.

The 5532 is a self-compensated, dual-section unit. The first op-amp section provides two functions for us. The first is gain, and with that in mind, our required gain might be that voltage gain of 4 noted above to achieve line level output, but could also be a negative gain to help us get down to mic level.

TO INVERT OR NOT TO INVERT A fundamental facet of a non-inverting op-amp (the alternative arrangement of the inverting circuit configuration used herein) is that one cannot achieve (continued on page 22)

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ADAPTOR (continued from page 20)

negative gain, which is gain less than 1.

The inverting configuration, used here, can accomplish negative gain similar to an attenuator.

The second purpose of this stage is to create one of the two output lines of our active balanced output. As our op-amp is inverting the input signal, the AC audio voltage here would be the inverse polarity of the input. So, we have marked the line – (negative).

The complimentary output line (+) is generated by the second of the IC sections configured once again as an inverter.

We need the mirror imagine of the other line, so we invert with a gain of one. A gain of one is achieved by using the same value of resistors in all positions. Incidentally, the value annotated on our schematic of three 3.3k

October 21, 2020

Final assembly.

was determined by the circumstance of being given over 100 3.3k resistors via the largess of a generous friend and that these 3.3k’s were 1% tolerance. This close resistance matching is quite helpful in achieving that perfect gain of one if you’re a real op-amp purist.

One can use three of just about any value, but something between 1k and 10k, at least in my experience, seems to work best.

As noted, we have created a very usable balanced output with the two opamp sections.

A fundamental characteristic of all op-amps is a very low output impedance such that these active devices approach being a perfect current source.

The author has been blessed with a great number of knowledgeable and generous instructors, and I’ve honored them often in these pages. From the wisdom of my mentors has come a world of wise words. One of my favorite quips,

on this very subject, is: “Because of that low impedance, an op-amp can almost drive anything right down to a short … if there is even a little bit of resistance in that short.”

With this in mind, to create a source impedance for best and flattest power transfer, we insert resistors in each of

the output of these op-amps to match the impedance of the input to follow, hence, the 300 ohm resistors in the + and – sides, which provide a 600 ohm source.

Shunting these individual resistors with 100 ohms results in 75 ohms on each side, dropping the source impedance to 150 ohms and mimicking a mic output.

Parts List Qty.

Part

Description/Notes

1

5532

Dual section op-amp – one used in each converter

1

DIP Socket

8-pin DIP socket for 5532 IC

2

10 kΩ

½ watt resistor

5

3.3 kΩ

½ watt resistor

2

300 Ω

½ watt resistor

2

100 Ω

½ watt resistor

1

80 Ω

½ watt resistor

1

10 kΩ

20-turn miniature trim potentiometer

1

220 uF

10-volt electrolytic capacitor

3

330 uF

25-volt or greater electrolytic capacitor

1

Phono Jack

RCA proprietary female skirt-mount connector

1

XLR-3 Male

Skirt type

1

DC-DC Converter

Murata model NMA0515SC – 5 volts in with + & - 15 volts out

X

DIP Switch

A switch with at least three sections per converter; A single 8-section switch was used here for two converters on one PCB

X

0.01 uF

Ceramic or mylar capacitors 25 volts or greater used for decoupling as needed

1

Bridge Rectifier

1-amp better than 25-volt PCB type

1

7805

5-volt positive three pin regulator preferably in a TO-220 case

x

DC Source

Wall wart or similar between 8 and 24 volts – 250 mA or better

1

DIN DC Jack

Skirt mounted power jack to match your wall wart

x

PCB Proto Board

Protoboards with DIP format sized to fit in the BUD box were used here

1

Enclosure

BUD box or similar packing as needed

Optional

The finished project.

1

Isolation Transformer

Triad Magnetic model TY-145P 600 to 600 ohm

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FLEXIBILITY The changing of the output resistance between these two choices is accomplished by a DIP switch on each resistor. Another DIP switch changes the gain from a nominal 10 dB to –40 dB, bringing the output level down to about microphone level.

The gain of an inverting op-amp is set by the relationship of the input and feedback resistors. The 10k 20-turn trimmer fine adjusts this relationship, pretty much allowing you to land the gain (and maybe even the fader position) exactly where you want it.

Bench testing indicates that my finished version has a remarkably flat frequency response. The value of the input cap was chosen to improve/maximize low frequency coupling. THD was excellent with the worst case no higher than 0.4%.

The abundance of 0.01 uF caps are just good engineering practice and, once again, enabled by being given a big bag of beautiful 50-volt mylars.

(Buc, don’t you ever have to buy anything?)

If full isolation of the output is needed, a transformer can be connected to this balanced output, and a typical transformer with nominal 600 ohm impedance on each side is listed at the end of the parts list.

Fig. 2: Power Supply Schematic

POWER SUPPLY AND OTHER SUPPORT ITEMS Most audio sources in one’s radio station in 2020 are stereo, and so for that reason, our constructed box has two channels.

We’ve chosen to use a wall-wart from the salvaged/harvested box of about 8 volts DC output as our power source.

Anything between 8 and 24 volts should work. This DC could be applied directly to the 5-volt three-terminal regulator, but since the power might come from an AC or reverse polarity source in the future, a bridge rectifier was included. If away from hard power, one could even use the DC from a 9-volt battery as a power

TECHMART

source. Using this scheme, one has infinite options.

If you’re permanentizing your converters, a transformer with simple regulator circuit is located in an isolation box at the lower corner of the schematic as an alternative to the above concept.

A DC to DC converter follows and that takes the regulated 5 volts in and converts this to + and – 15 volts for use by the opamps. The particular converter chosen has 33 mA capability for each rail, which is more than adequate. In development, the worst-case current demand noted was about 8 mA for each 5532.

The + and – 15-volt operating point is a foible of the author as it seems to me to

produce a slight but measurable improvement in S/N. Further, this 30-volt differential easily allows a +10 output if needed.

The abundance of 0.01 caps are for RF elimination. They also reduce any penchant for oscillation along the power supply rails.

RCA (phono) connectors were used on the unbalanced side as this is the industry standard. XLR-3 males were used on the output in anticipation that we will be using high-level broadcast inputs primarily.

Now that you have the “big picture,” it’s up to you to build one that is flexible, custom, useful and reflective of your own genius.

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2020 TECHMART ADVERTISING RATES: 3” X 2” TECH-MART BOX $95 NET PER INSERTION* (multiple boxes available at discounted rates)

Contact John Casey at john.casey@futurenet.com to reserve your space.

Space reservation deadline for the next RWEE, November 25 2020 issue is December 16, 2020

October 21, 2020

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