Simple pot question

[squidsquad]

Let's say I'm dyin to finish a box tonight (Fetzer Valve)....the output calls for a 100k pot...but all I have is a 50k...and a 250k.  Is one preferred over the other? Would one be louder?  Which?

[MartyMart]

Hi Squid,
It depends on how the circuit is put together, but if its just an "end of circuit" volume pot, then yes the 50k will be a bit "underpowered" and the 250k should give you a bit of a boost. ( someone chime in if i'm wrong here!! )
For instance, the TS9's gain pot can be increased for more distortion, because of the way that is wired in to that circuit.
I have had to use odd values and swop lin for log sometimes, with mixed results.
Whenever possible, use the correct value. or wire it "for now" and get the correct value pot later.

Marty  

[bioroids]

Hi!

If the output volume pot is wired as a voltage divider as usual, you can use the 250k pot, I don't think it would change the sound at all.

Depending on the output impedance of the circuit, maybe even the 50K pot works too, but I think it's safest to choose the bigger one.

In any case, it's very likely that the differences between the 3 values are tiny

Luck!

Miguel

[Chico]

Squidsquad:

I think that you can use either potentiometer.  Which will work best for you will depend upon the context that you plan to use the circuit.

IMHO, for the Fetzer, you have two primary considerations to look at when choosing a value for the output pot.  First, how does the high pass filter that is formed with the 22nf output cap and volume pot affect your instrument frequency response, and second, how does the output impedance of the Fetzer affect your signal path.

First, the volume pot forms a high pass filter with that 22nf capacitor that precedes it.  Using your basic formula for the cutoff, 1/( 2pi RC ) using the standard values for the Fetzer (22nf cap and 100k pot), you get a cutoff frequency of 72hz.

To preserve this cutoff, simply use the 50k pot, but replace  the 22nf cap with a 44nf cap.  Otherwise, you can use the above formula to compute a new value of the output cap to get the low end response that works best in your particular context.

If you choose to use the 250k pot, then you do not need to replace that 22nf cap.  In this instance, your cutoff freq is lowered to 29hz.  Again, using the above formula, you can compute the cutoff that works best with your rig.

The other issue that you may want to to consider is whether you will have effects that follow the fetzer, and how they will be affected by the output impedance of the Fetzer, i.e., due to the value of the pot.  
If all your effects have really high imput impedances, the value of the pot may not matter.  If you have effects with lower input impedances, you may be able to hear a difference.

As a general rule, I try to keep my output pots to 100k or less so that I do not load down other effects.  

If it were me, I would use the 50k and tinker with that 22nf cap value until I found a value that works for my rig.  The larger the cap value, the lower the cutoff freq.

Best of luck,

Tom



But keep in mind

[squidsquad]

Marty, Miguel & Tom,

Thanks for your help.  Tom, you really helped me start to comprehend what's going on, although I'm not to the point where I do the math on these things.