The requirement is to design a specialist one-off audio amplifier to drive a loudspeaker at the end of a long cable for experimental purposes. The voice coil is nominally 15 ohms with a pure resistance of10 ohms. The loop resistance of the cable will be somewhere between 2 and 4 ohms. It would be desirable to have a damping factor of 10 or better, so I need to reduce the effect of the cable resistance in some way.
I have considered the use of line transformers or a four-wire feedback system, but neither of them is practical in this particular case. As this amplifier will form part of a fixed installation and will only ever be used to drive the one loudspeaker, I am now looking into the possibility of giving the amplifier 3 ohms of negative output impedance.
The simple solution is to use an off-the-shelf unit for the power amplifier, the TDA7295 should be more than adequate. With a 0.5 ohm resistor in the return loudspeaker wire, I can measure the output current and derive a suitable voltage for adding to the input signal.
The practical situation generates at least two problems which the simple theory does not take into account:1) The capacitance between the conductors of the long loudspeaker circuit will give the effect of a falling loop impedance as the frequency increases, leading to ultrasonic instability.
2) The long unscreened run of loudspeaker wire, which might be in close proximity to mains cables and dimmable lighting circuits, may pick up interference by capacitive coupling, which will then be injected into the feedback point and amplified.
Both of these might be overcome by a capacitor across the 0.5 ohm resistor, the value being chose to only take effect above the highest wanted audio frequency.
The problem is that I have to construct this equipment without access to the loudspeaker or wiring, and it then has to be certain of working correctly the first time it is installed. For this reason I would be very grateful if anyone has had any practical experience of anything similar and could let me know of the snags they have encountered and the solutions which worked.