Technical ZOOM Why’s and what’s of adjustable output impedance power amplifiers by Tiziano Morganti
An ideal audio power
amplifier is a device that will generate an output voltage that is an exact replica of the input voltage, multiplied eventually by a constant value (the gain), and regardless of the output current delivered to the load. This is a voltage controlled ideal voltage generator, with zero output i mpedance. The output current doesn't cause any voltage drop.
This is the desired behaviour of most audio amplifiers, and K-Series is better than others, with its sub-milliohm output impedance, thousands damping factor and
150 A peak capability.
What happens if we add a resistor in series with the output of the amplifier? The damping factor falls down from thousand to units, bass reflex enclosures start to sound boomy, and transients are less tight and definite. Nothing desirable at all... But do this happen? Yes, in at least two cases: long cable runs, and loudspeaker voice coil heat-up.
So, why did we add an adjustable “emulated” resistor in series with the
negative
output of our K-Series? Because we can make its value to compensate parasitic cable and/or hot voice coil effect, in order to maintain the correct damping factor.
Applications of this unusual feature: feature:
How we implemented this resistor? The KDSP board acquires the output
voltage and current signals, sampled at 96 kHz just like the main input audio signal. A fraction of the output current signal is added to the signal output by the KDSP. For example, suppose that for a given amplifier output current I, we add a contribution of 2 times the current to output voltage. With no load, and a 0 V input, we will obtain a 0 V output. If now we sink a 1 A current from the output, we will see that the output voltage will raise up to 2V, thanks to the “I” contribution. We built a generator which output voltage increase with output current, a typical negative resistance behaviour. Positive resistance, voltage drops with current, negative resistance, voltage increase with current... Changing the factor from 2 to another value, changes the emulated resistor value (2 factor => -2 ohm, -6 factor => +6 ohm).
Most of the i mplementation effort was taken to guarantee the stability of the system despite of ADC/DAC and DSP latencies, and to manage situations where user settings caused and unstable behaviour of the whole negative output resistance amplifier + load
Did you ever seen a -4 ohm output impedance amplifier with a 2 ohm load? system without causing any destructive oscillation...
cable loss compensation voice coil increase in resistance due to heating compensation, even dynamically adjusted with the estimated voice coil temperature damping factor “creative” adjustment, to create a more dry and damped or boomy bass response active loudspeakers with reduced loading volume: a negative resistance has the effect of reducing the Qes of the driver, allowing for smaller enclosures for the same driver More? Perhaps...The
programmable output impedance for power amplifiers is an outstanding and unique feature, internationally patented by Powersoft.
W W W . K- SE RI ES .I T - W W W .P O W E RS OF T- A U D I O.C O M