tone control position

[David Deen]

Hello all,
Is there a good article someone could suggest to me that discusses the effects of where tone controls are positioned in a circuit? I've wondered what the (sound) difference was between putting tone controls at the end of an overdrive device (such as the Matchless Hotbox, blues breaker, and others) compared to inbetween gain stages.

Thanks.
-David

[Mark Hammer]

RG Keen and I have been batting that one around since the winter of 1991.

There are several impacts of tone control location.

1) Tone controls can be active (e.g., built around an op-amp), or passive tonestacks like one sees in Fender and Marshall amps and some of the pedal designs here.  Passive tonestacks work by selectively bleeding signal.  If you stick them *before* something that is supposed to distort, it will not be pushed as hard, unless you take steps to compensate for that passive loss.  Tonestacks placed AFTER a clipping stage will not take away from whatever clipping has taken place, but they stillbleed signal.  If the signal that hits them is plenty hot, then there may not be enough passive loss to truly weaken that signal, and it may still be strong enough to push subsequent portions of the signal path (e.g., amp) into clipping.  Alternatively, the tonestack can be followed by a gain recovery stage to make up for the passive loss.

2) Distortion stages add harmonics to whatever it gets, in proportion to the amplitude of what it gets.  Boosts to specific bands result in more harmonic content (i.e., multiples) being added to those bands.  Notches/scoops reduce the amount of clipping possible to generate.  Tone controls placed AFTER a clipping stage selectively accentuate or attenuate harmonic content that has already been added in addition to the fundamentals in those bands.