Pots in OP-Amp feedback loops and Gain Law

[rossbalch]

In Doug Small's book "Small Signal Audio Design" he suggests we get better control over our gain control by having the pot straddle to two feedback networks. However, I barely ever see pedal designs utilise this. Is there a downside? One I can foresee is that we might change the value of the high pass filter, which would affect frequency response. Maybe this is a good thing though if the highpass filter becomes higher in frequency as gain increases?

 

[Fama]

I am very dumb when it comes to stuff like this, but I wonder if that is more about improving the "law behavior" when using a linear taper pot, and the issue is sidestepped by using audio taper pots instead, which could work in a similar manner without the extra resistor? At least the TS9 I think uses an audio pot as the gain control. I'm not sure if it is the same thing or something completely different.

 

[rossbalch]

I am very dumb when it comes to stuff like this, but I wonder if that is more about improving the "law behavior" when using a linear taper pot, and the issue is sidestepped by using audio taper pots instead, which could work in a similar manner without the extra resistor? At least the TS9 I think uses an audio pot as the gain control. I'm not sure if it is the same thing or something completely different.

Earlier in the chapter he actually explains how the log law of audio pots is a crude approximation and this way gives better results.

 

[Fama]

Earlier in the chapter he actually explains how the log law of audio pots is a crude approximation and this way gives better results.

So it's similar in effect, but not as good as doing it this way? To be honest, it could just be that people don't really feel the need to "go the extra mile" in this case. Gain is usually set by ear, and as long as it's "good enough" where it won't jump a huge amount from a small bump on the pot, it doesn't really matter that much.

That would be my guess, but hopefully some real experts will also comment. I do have the feeling that if you were to improve pedal designs by doing things better from an electrical engineering standpoint, there would be a bunch of higher priority things to do first before something like this. Plus there's the whole "art" vs. "science" aspect, half of the market seems to be driven by "mojo parts" and "Same Circuit From Back In The Day When They Didn't Know What They Were Doing Because The Old Stuff Is Better Because It's Old". And since pedals are mostly used to make art (it's fine to just collect them too, I don't want to attack anyone), it is also a valid approach, in a way.