Gain control from unity to ... more than unity?

[KarenColumbo]

I wanted to patch together a little booster to colour the guitar sound. I build a "deluxe" fetzer with an amz tone control after a volume pot - which sounds great. The only problem now is that it seems I have just a tiny bit more than unity gain when fully cranked - seems that the tone stack sucks quite a bit of the signal.

Now I wonder if it's possible at all to follow the tone stack with a simple "output gain stage" and wire the volume pot here, so that the output is approx. unity gain when the pot is completeley ccw and ... well, more than unity gain when the pot is fully cw.

I guess I have to wire the pot in series with a unity gain resistance somewhere where it is responsible for just that. I have seen this in the AMZ booster (I think). But I haven't found a suitable NPN gain stage schematic, something simple without much colouring, just a, dunno, BJT contruction (emitter follower?). I guess I'm searching by the wrong keywords. Can someone in the know provide me with such a circuitry?

[Kipper4]

Can you do a drawing of what you have now please.

[Cozybuilder]

Try searching for Gain Recovery Stage

[R.G.]

A noninverting opamp gain stage can be wired with the feedback resistor being a pot set up as a variable resistor. The feedback pot/resistor can go from essentially zero resistance to full pot resistance. The gain of a noninverting opamp stage is 1+Rf/Ri, where Rf is the feedback resistor and Ri is the resistor from inverting input to the reference voltage (or simply blocked with a capacitor).

So for a 50K feedback pot and a 4.7K Ri, the maximum gain is 1+ 50K/4.7K = 11.6. For the other extreme of the feedback pot, the gain is just 1 + 0/4.7K = 1.

[KarenColumbo]

This is what I got so far:



Thanks guys - thanks again, R.G., I will have a good look at what you suggested!

[Kipper4]

I'm pretty certain you can move the 100nf to after the vol/level pot.

Measure the trimmers resistances and replace with the nearest value set resistors (less noise)
Breadboard?

I like that bmp stack with body control.

R.G suggestion is great solution. With your built in unity setting.

[KarenColumbo]

I'm pretty certain you can move the 100nf to after the vol/level pot.

Measure the trimmers resistances and replace with the nearest value set resistors (less noise)
Breadboard?

I like that bmp stack with body control.

R.G suggestion is great solution. With your built in unity setting.

Yea, still breadboard. will move the coupling cap, thx for pointing that out rich! tone stack is mighty fine, lots of good variations there

[R.G.]

Forgot to mention. If you can live with lowish input impedance provided by the inverting input only, the same circuit will go from zero gain - full muting - to the Rf/Ri gain of the pot and input resistor. May take some tinkering with the values or a buffer before or after, etc.

[PRR]

> move the 100nf to after the vol/level pot.

That gives a DC path R5 R7 R6 R8 to ground. While this is little change to the FET, the pots will make scratchy noises when turned.

[KarenColumbo]

I see. Will COPY it there, then I'll just try on different sizes.

[anotherjim]

That BMP network catches a lot of people out - there is a DC path to ground from your R5 all the way through via R8. So the 100nF blocking cap must stay and I think should be big enough at 100nF.

If the tone of the circuit satisfies you, and you don't want any more colour adding, then RG's suggestion of an op-amp is the way to go for the most transparent make-up gain.
As it is, I assume you are auditioning the circuit plugged into a 1M guitar amp, then that 1M is the load your make-up stage should offer. Anything less than 1M will change the tone stack character and drop the level even more.

Rather than chose between an inverting or non-inverting op-amp, I suggest both! Since an 8 pin op-amp might as well be a dual. I'd pick TL072.
add...
cap (10n) > 1M input non-inverting op-amp unity gain > inverting op-amp variable gain x1 to x4 at most (x3 should be plenty) and I think I'd try for a trimmer pot at the feedback R.