To drive or to recover?

[raulgrell]

Experiencing a extremely low output in a pedal I am designing, after putting a big muff based tone control has led me to wonder: Is it better to drive the signal before the tone control or after? If 'better' is too subjective, what differences can it make?

I'd try it myself, but I'm all out of transistors/opamps

[R.G.]

To a first approximation, it is better to raise the signal level before the tone control losses because this preserves more signal to noise ratio than amplifying afterwards.

But no one does it that way, either amateur or professional. That is because the tone controls are  usually in a place in the circuit where noise is not a problem anyway, and because there is often not a big enough power supply to amplify the signal up enough without distortion.

Tone control stages usually cause approximately a 20db loss (a loss of 10 to 1) so you need a gain of 10 (20db) to bring it back up. Often there is not enough power supply voltage to amplify a signal by 20db cleanly before a tone control. So the only option is to let the tone control bring it down, then re-amplify it back to make up for the losses.

The third way is to use an active tone control stage, which is quite popular in the pros.

[Mark Hammer]

One of the things that a passive tone control like the BMP control or a 3-band tonestack does is to "tame" the signal, so that it doesn't overdrive the gain recovery stage.  In other words, that 20db passive loss is constructive in some ways rather than pure loss.

Let's think this through a bit.  Imagine I have some sort of gain stage (or maybe even two) intended to achieve clipping.  By definition, that clipping will result in an output signal that is at some sort of maximum.  If I follow it immediately with a gain stage intended to compensate for the passive loss of the tone control after it, then I am feeding that gain stage with an already hot signal which is likely to demand the sort of output signal amplitude that is unattainable in clean fashion from a mere 9v battery.  If I am content with what that gain recovery stage contributes to the already distorted tone, then that's fine.  However, if I like what is hitting my tone control, then I don't want to jeopardize the tone quality.  All I want is the same thing, only louder than it would otherwise be after a 20db level loss.  As such, the place to compensate is after the tone control, not before.  Brother Keen is absolutely right in asserting that, purely in terms of S/N ratio, sticking the gain recovery stage before the control is the right thing to do.  I think it is a pretty fair guess, however, that what you are dreaming up is some sort of signal coloration device, and that if the tonal quality is right where you want it at the moment, then sticking an additional gain stage between the existing circuit and the tone control is the wrong way to go.  Besides, if it is a higher-gain circuit to begin with, then there WILL be some hiss and the overall circuit might benefit from having treble/hiss cutting as part of the characteristics of the post-tone-control gain-recovery stage.

From a strategic point of view, I vote for post-tone control.  Doesn't seem to have hurt the BMP much.  Also nicely isolates the tone and output volume controls so that they don't interact quite as much.