[acresm22]

I recall reading somewhere that a good rule is to use a multiple of 10 to decide which volume pot to use in a passive pre. In other words, if your amp has an input impedance of 100-150 k ohms and your CD player's output impedance is sub-1k ohms, a 10k ohm volume pot is ideal.
Is that a good rule? I'm curious because I see passive volume controllers that use 50k or even 100k pots, and I'm wondering what effect (positive or negative) these would have over a 10k pot in my system.
BTW...I use a SET amp that has an input impedance of 150k ohms.
Thanks.

[Steve]

I recall reading somewhere that a good rule is to use a multiple of 10 to decide which volume pot to use in a passive pre. In other words, if your amp has an input impedance of 100-150 k ohms and your CD player's output impedance is sub-1k ohms, a 10k ohm volume pot is ideal.
Is that a good rule? I'm curious because I see passive volume controllers that use 50k or even 100k pots, and I'm wondering what effect (positive or negative) these would have over a 10k pot in my system.
BTW...I use a SET amp that has an input impedance of 150k ohms.
Thanks.

Good question. At least two considerations.

1) Does not limit bass response by Passive being too low in input impedance (Z).

2) Interconnect cable (IC) and amplifier input capacitance does not load the output of the Passive, thus causing high frequency response problems.

Even 10k ohms output Z will be loaded by the IC capacitance, unless very very low capacitance.
Anything above 4k ohms can and will cause high frequency response problems. (That is why passive volume controls should be mounted in the amplifier chassis, to avoid IC capacitance.)

Yet 4k ohms and less will cause bass response problems unless the source is DC coupled out.

Hope this helps.

[Uptown Audio]

I'd go with 100k if I was inserting a passive level control in there. Steve is correct in that the cable to the amp should be as short as possible. Using a higher value allows you a little bit more adjustment range and it more closely matches the input impedance of the amp.
-Bill

[acresm22]

Thanks guys, though it looks like your suggestions are at opposite ends of the spectrum.
If I'm understanding you correctly, Steve, a 5k pot using the shortest and lowest capacitance ICs will yield the best results, correct?
Bill, what advantage is there in matching the input impedance of the amp as closely as possible? If Steve says anything above 4k will cause high frequency loss, it seems that a 100k pot would round off the top end considerably.

[Steve]

Thanks guys, though it looks like your suggestions are at opposite ends of the spectrum.
If I'm understanding you correctly, Steve, a 5k pot using the shortest and lowest capacitance ICs will yield the best results, correct?

I was not as clear or complete as I should have been, and thus misled you. My bad 22. (Will keep math to very simple and general, as complex equations are normally need when dealing with reactance.)

Short answer: 5K will be the best if your source is DC coupled. High frequency response will be excellent.
If output is capacitively coupled then you will have bass response problems.

Further explanation:

Midpoint resistance of a volume control is where the highest output Z and therefore lowest high frequency response will be.

With a 10k ohm control, midpoint output Z will be approximately 2.5k ohms, while the input Z will be 10k.
Above or below mid point resistance setting the output Z will be lower (assuming the source output Z is quite low).

High frequency response problems will be minimal so long as the IC capacitance is low, generally 100pf or less and the amplifier input capacitance is 100pf or less. Approximately 200pf total, give or take a little.

With a 20k ohm control, midpoint output Z will be approximately 5k ohms, while the input Z will be 20k ohms. The high frequency response has already started to suffer compared to 2k output Z component (unless mounted in the amplifier or extremely short IC used).

For example, 2k ohms output Z with Fr of -1db at 100khz, now becomes -1db at 40khz (-.4db at 20khz) with 5k output Z. Perceived difference will be noticeable.

20k ohm control, high frequency roll off is of concern.

Now lets check bass response with different input Z of passive control.

If DC coupled, then little if any problems. If capacitively coupled, then the output capacitor will need to be huge, or bass output will be shy.

For instance, 5k ohm passive control, 5k input Z. With an output capacitor of 10uf, the capacitive reactance at 20hz will be 800 ohms. You will be losing some 13% of the bass signal at 20hz, some 6% at 40hz.
If the output capacitor is 5uf, we are talking 24% and 12% loss of bass signal respectively.

If one uses a 10k passive input Z, output capacitor 10uf, one loses
8% of the bass signal at 20hz, 4% at 40hz.

20k ohm passive input Z, 10uf, one loses 4% at 20hz and 2% at 40hz.

As one can see, we are dealing with tradeoffs. One has to deal with high frequency losses vs low frequency losses.

Hope this helps 22.
Cheers. 

[Uptown Audio]

Thanks guys, though it looks like your suggestions are at opposite ends of the spectrum.
If I'm understanding you correctly, Steve, a 5k pot using the shortest and lowest capacitance ICs will yield the best results, correct?
Bill, what advantage is there in matching the input impedance of the amp as closely as possible? If Steve says anything above 4k will cause high frequency loss, it seems that a 100k pot would round off the top end considerably.

Steve has already replied to clarify his post earlier. What I suggest is to not worry about HF roll-off with the amp you have. You have a relatively high input impedance and those types of amps generally need a pretty good boost in the front end to get any volume going, so you don't want to lower that. Most old tube amps had input attenuators of 100k up to 1m. It was not uncommon to see 250k and 500k values. Not that you have an antique, but I doubt the principle has changed much for the newer product using the old tube designs. You are not going to lose much HF unless you have a long run of cable, especially a high capacitance cable from the volume control to the power amp. We have already covered keeping that short, and that doesn't change when using a higher impedance pot. You definitely don't want audible bass roll-off, which could very well occur with too low of a value. Most older tube amp designs have lower output at frequencies greater than 15k, despite some having response to 100k. You'd need a great output transformer for that to be near flat.

The ideal solution would be to install input pots on the amplifiers or directly in front of them to avoid the losses. Without seeing the schematic, it's hard to say but you might even be best served by reducing or replacing a resistor in the amp with the level pot, maybe adding a capacitor as well. Rather than re-invent the wheel, I suggest just going with a higher value pot and retaining bass response. You could choose 50k or 100k and be fine. That way, when you do get an active preamp, you will have the most direct power amp configuration (stock). If you are considering the passive for sound quality reasons rather than cost, I'd rethink that and just get a good, active preamp.
-Bill
 

[acresm22]

Thanks, guys. Very helpful.