Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: Your proposal should work.

Posted by Charles Peterson (A) on December 31, 2005 at 15:51:50

In Reply to: Your proposal should work. posted by Al Sekela on December 30, 2005 at 11:21:23:

Al Sekala wrote:
However, the larger problem is likely to be noise induced in the interconnect cable. This noise includes electrostatic and electromagnetic induction. A higher amplifier input resistance will increase the voltage developed by magnetic induction of current in the circuit comprising the crossover's output (the driving stage output impedance), the cable, and the amplifier input. This noise can be minimized by reducing the amplifier input resistance to a value as low as is compatible with good operation of the driving circuit, as you also recognize.

Thank you! Yes, I agree that induction noise is reduced by making the power amp input impedance lower, and that's exactly what I intend to do. Unlike the amp designer, who seeks maximum compatibility, I don't have to make a compromise based on lack of knowledge about the source component whose output impedance could be anywhere between 1 ohm and 20,000 ohms.

Now I'm still a bit confused about where the 80uV of noise when the amp input pot is wide open comes from. Is that all induction noise from the RCA jack, chassis wiring, etc? Or is it resistance noise because when no input is connected the "pot" itself becomes the lowest impedance "source," rather than just a relatively harmless shunt across a much lower impedance (hopefully) source.

BTW, this afternoon I was testing out an Acurus L-10, which has a discrete transistor circuit. At 2 volts output, loading down the output to 100 ohms makes barely a difference in the distortion measured from 20-10Khz. Only at 7Khz it rise an extra 0.003% above the residual of my Fiji card (which is 0.002% THD at 1Khz, rising to about 0.005% at about 7Khz). At 1000 ohms, it's still riding right on top of the residual, so I think it would be comfortable driving a 1000 ohm load, if not lower. It would probably meet minimal "hifi" standards driving less than 100 ohms, though I'm not going to go there.

Charles

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