Output Volume Control Treble Loss?

[chromesphere]

Hey guys, I had a question today, do you lose treble with the (standard output volume control) and if so does it happen when the volume is up or down?  "Standard volume control" as in the one after the output cap that so many designs use.
I'll have to check this on my oscilloscope but in the meantime, what are your thoughts?
Cheers,
Paul

[R O Tiree]

Hi Paul,

That standard layout you describe acts as a high pass filter so, if anything, treble response will be better than bass response.



I have always liked this explanation, when trying to remember which is which... Think of the CR combo as a voltage divider. For this one (which you're looking at in your circuit), at DC the cap is an open circuit, so its reactance is infinite compared with the resistor value. Therefore, low frequencies will be attenuated. At high frequencies the cap's reactance is very small compared with the resistor value, so a greater proportion of your treble signal will pass to the output.

Altering the volume control will not make any difference to this. Say you feed this with 2 frequencies - a low one that gets attenuated by 60% and a high-ish one that only gets attenuated by 10%. So, what you get at the output, with the Vol pot dimed, is 40% of the bass and 90% of the treble. Wind the Vol pot down to 10%... you'll now get 9% of the original treble and 4% of the original bass signal.  Still the same proportions, though - 9:4. It seems counter-intuitive, but I just ran up a simple Excel spreadsheet and it's true, it seems.

Cheers,

Mike

[Mark Hammer]

There can be significant loading effects with standard guitar volume controls.  But one should not confuse the operating context of guitar volume pots with those of pedal volume pots.

First, the output stages of pedals are generally intended, and designed, to have a much lower output impedance than the typical input impedance of an amplifier or pedal.  Second, the cables from guitar to pedals, or guitar to amp, are much longer, and in the absence of any "outside assistance" (buffers, etc.) will interact with the guitar volume pot much more.

Note that it is also the case that guitar volume pots will most often be up full, in which instance the higher the pot value, the less loading of the pickup signal.  Pedal volume pots will often be at less than max value, in which case the pot resistance between input lug and wiper could pose considerable current-limiting of the output signal.  This is one reason (among many) why pedal volume pots are often low-ish values (although they can be as high as guitar pots in some instances).

Finally, note that pedal volume pots are not always the last component in the pedal's signal path.  Many pedals will have buffers or gain-recovery stages after the volume pot.

[chromesphere]

Thanks for the info guys, i get some of it... 

Using the muzique RC filter calculator, i plugged in a couple of values: 100nf and 10k then 100k.  I noticed with the 100k (pot) the corner frequency dropped from 159hz to 15.9hz.  Im probably neglecting some important factor here, but it would appear increase the value of the volume pot would have a lesser effect on low freq attentuation?  Assuming there is a low freq attentuation...we might be heading into in / out impedenace zone where my eyes glaze over and i reach for the paracedemol.
I might need to oscilloscope this.  Probably highly dependant on the circuit driving it like Mark said, i guess...
Paul

[PRR]

The output *cable* and other stray capacitance *loads* a part-way pot.

Standard-connection pot has highest wiper impedance when set for half voltage ("5" on a linear, about "7" on audio taper). The wiper impedance is then roughly 1/4 of the pot impedance.

10K pot: 2.5K
100K pot: 25K
1Meg pot: 250K

Assume a 30 foot cable. Capacitance is about 1,000pFd. Less for shorter cables. Amp inputs may be 100pFd, or less, sometimes more. Pedals sometimes have 1,000pFd tacked on the input.

But take 1,000pFd as a talking-point.

10K pot with 1,000pFd: 63KHz
100K pot with 1,000pFd: 6.3KHz
1Meg pot with 1,000pFd: 630Hz(!)

But don't arbitratily slap 10K pots on everything. The stage or network that drives the pot also enters into it. If a tube or transistor with 100K plate/collector load, the output impedance of the stage can be up near 100K. Sometimes there's tone-control between last stage and the output pot. A 10K pot will load the heck out of everything.

We see why 100K pots are fairly common. A 1Meg pot's setting interacts with typical cable capacitance to give varying top-cut. A 10K pot may limit other aspects of design by being hard to drive. 100K is a safe compromise.

The in-guitar pot is a special case. It has capacitance on its output but also a strongly inductive source at its input. The math gets messy. (Interestingly a 250K guitar volume pot tends to swamp-out in both directions and gives pretty smooth interaction both ways.)