Volume pot value too high?

[AC439]

I have read discussions in the forums for a few hours but still am not sure if I have chosen the right volume pot value.

I have been using 100kA pots for the volume control of the tripath amps. The input caps have been upgrade to 2.2uF MKT type. I use my ipod touch 4G's line out to drive the amps. I can't find the Ipod's line out impedance from the internet. All people discussing the ipod output impedance are referring to the 3.5mm output for the headphone.

The TA2024 mod page refers the volume pot to 50k. Since I don't have a 50k, I just use 100ka in which I have on hand.

Comments ?

[Einric]

What kind of 100k pot are you using?
I have a 50k Alps Blue when I should have gotten a 100k.

[theAnonymous1]

You usually want to go with a pot value LOWER than the input impedance of the amp. Most Tripaths have around 20k input impedance.

I've always stuck with 10k pots on these amps. You could even go lower if your source can handle it.

[Redshift187]

Given that there is usually an output capacitor on your source, a lower value pot (assuming it's the first thing connected to the input) will increase the high-pass frequency and reduce bass.

A higher value pot also makes it easier for your source to drive. I don't see the problem with a 100k pot.

[DF96]

With a 100k pot fed from a low source impedance the maximum output impedance from the pot will be at -6dB when you get 25k. If this feeds a 20k amplifier input then you will lose some signal but that might not matter too much. The important thing is that the 20k input impedance must be constant, otherwise you will get non-linear distortion. That is why smaller pots would generally be used.

[AC439]

So it seems to me that the optimum value needed to be determined by the listening level ?

Say, if I usually listen at 1/2 dial (50%), then this has to match the input impedance of say 25k. Therefore, my volume pot value should be 50k since I am dialing in at 50% which matches the input impedance.

Therefore, if I usually listen at 1/4 dial (25%), then my 100k pot is fine because the -6db (25% of the pot divider) is 25k which is matching the input impedance of the amp.

Am I correct ?

[DF96]

No. Impedance 'matching' is not the issue. Audio equipment usually works best when there is a significant impedance mismatch.

A volume pot presents its peak output resistance at 50% volume; higher or lower volumes have a lower output resistance, although the curve is broad. The problem with a high resistance is that it could create distortion if the amplifier has an input resistance which is low and non-linear. It needs to be both to create a problem: low or non-linear are each fine by themselves. The snag is that some power amps do have both issues: low and non-linear input impedance. You won't find this in any data sheets, but it may appear in hidden form in good reviews. If the item seems to be fussy about what equipment drives it, and in particular seems not to work too well with a passive preamp, then it might have this problem. Be aware that some reviewers might mistakenly think this 'fussiness' is a good sign; a sign of 'fine discrimination'.

[Redshift187]

Good to know. I've been using a 50k on my LM3886 amp, and a 20k on my TA-2020 just because that's what it came with.

[AndrewT]

There can be a variety of passive filters around the interconnection between two pieces of audio equipment. Some are deliberately placed there to improve audio performance.
Add in a volume control to that interconnection and you have increased the potential for even more passive filters. Some of these filters can interfere with audio performance.

If you want to examine the input arrangement in detail, you need the source equipment details and the receive equipment details and the volume control details and the details of the cable feeding the receiver and the details of the cable feeding the volume control.

You may find that the plugs and sockets can even affect what is happening, but this is more likely to be a very small effect, if any.

The passive filters in the interconnection have a big effect on the final sound.
You need to give details for a well rounded summary of the audible effect.

[AC439]

Thanks for all comments. I read Andrew's similar comment somewhere in this forum and that's mostly where I get stuck. The details as mentioned are probably impossible for me to obtain. Even I tried to find the ipod touch's line output impedance to no avail. The piece of 7' link between the ipod dock to the RCA input has no spec on, say, pF per meter on the spec. Inside the amp, I use solid copper wires pair twisted by hand for connections between the RCA, vol pot and the amp's input. I don't have a capacitance meter to measure this short link. I do aware that every element in this path has L,C and R in it and they will act like filters together (probably a strange filter after everything combined).

What I'd like to know is basically in general if I'm using a pot with too high of a value in this simple application. Maybe I should try several value of pots to see which one gives the bast match ?