Posted by
induction on
Nov 14, 2016; 6:41pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Tone-Stack-Question-tp34153p34197.html
Hi Travis.
The general rule is that a passive tonestack fed by a stage with a low output impedance will have stronger high frequencies than one fed by a stage with a high output impedance. Whether you need a buffer depends on how much high end you want, and where the tonestack fits into the circuit. If the tonestack comes after an active stage, the difference will be less noticeable than if it comes right after the pickups, which is not to say it won't be noticeable at all.
Phil Abbott over at FSB has published an excellent EQ circuit (google 'Phabbtone', there are too many threads to link them all here). Phil argues that contrary to popular belief, passive pickups will provide plenty of juice to feed a lossy, passive tonestack (the Phabbtone is based on the HiWatt tone stack, which is well within the Fender/Marshall family). I wanted to test his claim, so I built the Phabbtone with a switchable input buffer and put it directly after the guitar. I found that without the input buffer, I lost a significant amount of treble and I could not recover it with post-eq. On the other hand, with the input buffer engaged, I actually had too much treble, and had to set the treble control close to zero, otherwise it was overwhelming. The best result required an inline resistor between the buffer and the tonestack, to increase the effective output Z of the buffer. Now I use the Phabbtone (input buffer switched off) as my slave amp tonestack, in a re-amping setup.
So the answer to the question boils down to setup and sonic goals. I would not want to use the phabbtone with the input buffer as pre-dirt EQ, because the dirt would get shrill. After dirt, the input buffer is generally not necessary impedance-wise, and again can preserve too much treble, but the result is less piercing and brittle than the pre-dirt scenario.
Bottom line, you want the output impedance feeding the tone stack to be in the goldilocks zone for the result you're trying to achieve. If you're going for classic rock guitar tone, your 'just right' will probably different than if you're going for modern metal, etc. Your best EQ for clean tones will probably be different than for pre-dirt EQ, which will also be different than post-dirt EQ, not to mention whether you think of 80's power ballads vs. The Black Keys when you dial in your 'clean' tones.
Even the idea of presenting a consistent output Z to the tone stack can be debated, as you may find the EQ change that you get from rolling off your guitar volume knob (and the resulting increase in output Z that feeds the tone stack) to be welcome or unwelcome.
I guess you've noticed that I've studiously avoided a direct answer to the question, but I hope I've helped give some guidance as far as how to approach the question for yourself. In the end, design questions are mostly dependent on goals and tastes (as long as nothing blows up or electrocutes someone). 'Good design practice' is far more open to debate in analog guitar equipment than in most other fields of electronics.
Edit:
Kinski wrote
Perhaps an SHO, with the stock pot as a trimpot to set top volume. Then add an external volume pot on the output of the SHO?
That would be my suggestion.