[sdiy] [slightly-OT] Passive tone controls - coupling/biasing on input signal

[Donald Tillman]

> On Mar 9, 2015, at 5:37 AM, Justin Owen <juzowen at gmail.com> wrote:
> 
> I'm looking to add some sort of passive tone control to a circuit - either the classic 3x pot 'FVM' Bass/Middle/Treble control or the 2x pot Bass/Treble Baxandall type.
> 
> Do either of those circuits have a requirement/preference that the input signal is AC-coupled - or will it receive a positively biased signal just as happily?

You really want it AC coupled.  Otherwise you'll get DC thumps turning the bass control, and any pot scratchiness will be amplified.

It's also important to know that Leo Fender's tone control (also known as "FVM" or "tone stack") is very different from the traditional hi-fi tone control:

The hifi tone control is intended for flat response at 12 o'clock, for subtle boosting and cutting of the highs and lows to compensate for conditions, and for the controls to operate independently.  

Leo's tone control has no flat response setting, the curves of the control are tweaked to make an electric guitar sound great, with wild variations in response settings for creating timbres, and a lot of crazy interaction between the controls.

---

Because the history is interesting:

Leo Fender invented the bass/treble and bass/middle/treble Tone Stack, and the circuit was copied by others with variations.  FVM is "Fender, Vox, Marshall", thus encompassing the variations.

Peter Baxandall introduced active bass and treble controls in a feedback loop.  Passive bass and treble controls were in use previously.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com