passive preamp

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This is a highly opinionated comment, from me:

There is concern in the high performance audio community that potentiometers introduce a lot of their own special kind of non-linearity into the signal chain. Rightly or wrongly, the construction of a pot - with a band of resistive material, impressed on it with a multi-prong 'wiper', which taps the resistance in a relatively continuous smooth range of values - is also subject in surprisingly short time to noise, both in turning, and just "sitting still".

There is also (I think less than wisely) concern that the ganged-potentiometers in a typical stereo-capable single-shaft volume control are not necessarily very well matched in their tracking of resistance, and therefore volume-per-channel. This can be pretty easily measured with a simple digital volt-ohm meter: hook up to measure resistance, and turn the knob to various positions, measuring the shunt resistance per channel. Often differences exceeding 2% are measured.

So the thinking goes ... if the whole point of high-end audio is to present the most balanced and natural sound-stage through your amplifier - which incidentally is almost always "trimmed" for very-high linearity on each channel - then 2% resistance-ratio difference ... is quite a kick in the pants to the underlying ideals and theories of exactness.

Enter, the 24/36/48 step attenuators - which are just switches, with a whole lot of precision resistors on board. Now, with precision resistors, it is possible to get channel-to-channel attenuation matched closer than 0.1% ... it all depends on what you want to spend for the precision resistors and big old switch.

Personally, from listening over a lifetime to zillions of uber-high-end systems, some with really nice pots, some with crappy pots, some with sort-of-OK-pots, some with click-attenuators, some with entirely solid-state encoder-attenuators ... all I can say is that a 2% difference is inaudible. Not just "barely audible", but utterly inaudible. Truly so.

Consider the math of it.

2% "more" or "less" than a reference signal is either 0.98x or 1.02x. Do the decibel calculation:

20 x log10( 0.98 ) = -0.176 dB
20 x log10( 1.02 ) = +0.172 dB

Which of course makes sense: people can not hear 0.2 dB of difference in a signal. The dB scale was chosen, in fact, so that "1 dB" was just about the minimum difference in sound level that people can discern. I admit there are "golden-ears" out there than can definitely hear 0.5 dB and 0.4 dB in A/B switching tests ... but insofar as I know, there is no evidence that anyone can hear the difference below about 0.2 dB or so. So... +/- 0.17 dB is quite acceptable.

HOWEVER - before you finish with any conclusion, remember that those wiper-on-resistance-strip pots also can become noisy, both sitting static (for old pots) and when being changed. There have been plenty of ideas to mitigate this - but mostly it comes down to sealed pots (get rid of most of the dust), use of resistive polymers (for the resistance strip, eliminating dust), and having a multitude of "in parallel" wiper contact-fingers, which together eliminate any scratchiness, as well as average out the resistance-strip resistance, somewhat increasing precision.

Now you know entirely too much.

Go forth and either buy NICE sealed pots, or NICE reasonable-precision stepped attenuators. Either will do. You'll be able to buy 5 to 10 great sealed pots for the price of 1 decent precision attenuator. Is 0.17 dB worth that?

I'd rather have the nice pots.

Never thought about that DC thing. Thankfully, the majority of switched attenuators are hooked to front-ends that are zero-DC ... due to input impedance bias-grounding resistor (which might be the whole cascade-of-resistors in the attenuator itself!)
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