Which output buffer to use after a Fender/Marshall tonestack?

[gigimarga]

Hello,

I'm trying to build a Krank-like pedal using a boosted Smash Drive with a Fender-like tonestack after it.
As i'd read in a post, the Marshall/Fender tonestacks, with a 1M pot for volume, have a big output impedance.

So, my questions is, what output buffer can i use after: one like LPB (or Big Muff's last stage) or another one?

Thx a lot!

[petemoore]

  I'm pretty sure the BMP last stage is a 'recovery' stage, basically an amplitude booster.
  I would suggest the buffer goes in to boost impedance before the passive losses of the tone control.
  That said, the LM386 can drive current, I dont know if a buffer would offer an advantage between the LM386's output and the TC.
  If the output is too low *after the passive losses, perhaps a gain recovery stage like BMP...

[R.G.]

Both the LPB and BMP output stage are going to be too low an impedance for a 1M treble pot.

- if you want only impedance matching, a JFET or darlington follower will do it.
- if you want impedance matching plus a bit of gain, a single self-biased JFET will do it.
- there's always a JFET input opamp like the TL07x seriess

But there is another option - impedance scaling. If you take any R-C network and change all the resistors by some factor, say 10 to one; then increase all the cap values by the same factor; the resultant network has the same frequency response as the original, but its overall impedance changed by the factor.

Example: You have a tone stack with a 1M treble pot. You change all pots and all resistors to be smaller by a factor of ten. You must also make the capacitors larger by a factor of ten. The resulting network has the same frequency response, but now can drive a lower impedance load. Works for a factor of 100 too - or any other factor. So you could scale the whole network down to use a 10K treble pot.

Notice that the impedance of a resistor is just its resistance; the impedance of a capacitor is proportional to one divided by the capacitance, so bigger capacitor is smaller impedance.

[gigimarga]

Thx a lot both for your answers!

I know about the scaling, but which method is better: scaling or a buffer?
For R.G.: where i can find more information about the JFET follower and/or self-bised JFET buffer?

Thx again!

[R.G.]

I would use an opamp buffer unless there was a specific reason not to. It's cleaner, simpler, and easier to use.

[gigimarga]

Can you tell me a suitable opamp buffer? I want to boost the signal a little, too

Thx again R.G.!

[frank_p]

But there is another option - impedance scaling. If you take any R-C network and change all the resistors by some factor, say 10 to one; then increase all the cap values by the same factor; the resultant network has the same frequency response as the original, but its overall impedance changed by the factor.

Oh ! Now I understand something about what you said in an other post and it got me confused.  You have to increase the caps values, not decrease it...   I should have asked this question before.  That problem was stuck in my mind.  AHHH Thanks R.G.