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Reducing Noise by Choosing Impedances

Posted by Charles Peterson (A) on December 29, 2005 at 17:15:59

In recent measuring, I noticed that the noise output of my Parasound HCA-1000A power amp varies considerably, about 7dB IIRC, with the position of the input attenuator. With no actual input, what this is really varying is the input impedance. Of course you get the lowest noise with shorted input, but that isn't useful. What might be useful is finding the best input impedance for matching to the driving component, and replacing the pot (I always hate those) with a metal film resistor and handy "mute" switch.

But this raises I lot of issues which I'd like to understand better.

Generally power amps have a relatively high input impedance (such as 33K or even 100K) to be compatible with any possible "preamp" (which might also be, as in my case, an electronic crossover). This is unnecessarily high for most modern preamps, which might do fine with a load of a few hundred ohms. But if the power amp input impedance is too low, this might lead to excess distortion and frequency response errors. So there's a bit of a tradeoff between low noise and low distortion-and-flat frequency response.

Now in my case the driving component is a dB systems electronic crossover (which, BTW, I think sounds and works fine...though I do have ideas to make it better). The last component in the dB is a 5K output pot to set the level. To avoid having this work like a low pass filter, I keep it turned up all the way. But it demonstrates that, in principle, the circuit is fairly happy driving about 5K, and possibly 5K in parallel with the expected amplifier impedance of maybe 10K. Actually, I'm a little worried that the 5534 based circuit shouldn't be loaded down significantly more than 5K. It might even perform slightly better if the total load were 10K.

So to minimize the actual load on the driving circuit, I was thinking, why not scrap the output pot (and its 5K load) altogether! Then if I made the input resistor on the amplifier 5K (replacing it's 50K pot), that could be no worse than it was before in terms of the total load, and now it would be at the best location to reduce the hum, noise, and RF pickup from the cable and the amplifier input itself.

What is wrong with the source component not having an internal load at all, other than being careful not to remove cables during operation (which you can't really do anyway)?

Alternative, I could replace the 5K pot with a higher value resistor, say 50K. That would provide the "safety" function of having some onboard load resistor, but it would still allow me to move the entire 5K of loading to the amplifier, which would in effect dampen all incoming noise, including hum and RF pickup from the cable.

But then, I would have the problem of the 50K resistor itself contributing noise.

But if there's no resistor at all at the crossover (or preamp) output, isn't that the same as having a defacto 10-100M resistance (air, vinyl insulation, etc)? So shouldn't THAT be even noisier? That has me very confused. The absense of a resistor might be noisier than a resistor itself.

Charles

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