Variable Bypass Capacitor Idea

[kgull]

I recently came across puretube's varicap and stm's mod to remove the first buffer gave me an idea. After removing the first buffer, it places the value of the pot across the cap.

If we use a 1k pot, it ends up looking like a modified fuzz face gain control that's turned all the way up (ok a bit of a stretch but that's what I saw).


Tested it in a sim and it behaved identically to changing the cap. Wired it up to my fuzz face test board and while the fuzz face worked fine, neither the sound nor the measured cap value changed.

Have I completely overlooked something?

[Jazznoise]

A much simpler way of doing this I've seen on here is to parallel two caps and set them up on lugs 1 and 3 of a Potentiometer so it acts as a divider. Bam, varicap. Sorta.

[kgull]

Like the bottom one here?

Problem is the cap is not going straight from emitter to ground anymore so it lowers the gain.

[Jazznoise]

It can. Obviously when the pot is on either extreme the Emitter sees an AC short to ground. A 1K pot should make a difference, we're not talking a "true" short anyway. Paralleling resistors between lugs 1 and 2 and 2 and 3 can reduce it further. The interaction might become abit weird - but that might be cool.

If this is sounding undesirable, recreating the original varicap circuit might be the best idea. Then tinker from there. Another point is that the bypass cap may have to be decreased massively to change the low end. Since the emitter cap increases AC gain.

I've seen people insert an Emitter-Base capacitor to simulate the softer clipping of older Ge transistors. That might be worthwhile experimenting with.

[kgull]

The original varicap would work fine and makes for an interesting tone control from classic fuzzface to treble booster (ala rog may queen) to overdrive-ish tones.

I was hoping this approach would work because it seems slightly more, idk...elegant? I'm mostly curious as to why it doesn't work when it seems to in the simulator.

[earthtonesaudio]

What is the DC voltage at the top of the 1k?  Could be too low for the opamp to work.

[kgull]

1.2V across the resistor
2V at the inverting input
9V across the power rails

maybe a larger pot? To try and center the voltage a bit better?

[kgull]

Nope, forgot the fuzz face does it's own weird biasing thing (10k reads the same as 1k). Maybe that's what's throwing it off?

[earthtonesaudio]

Those voltages show that something is not right.  If the opamp is working, the voltages at its + and - inputs will be equal.


Most opamps have an input voltage range which is a couple volts above their negative supply, and a couple below their positive supply.  1.2V is too low for many, such as TL07x. 


Possible solutions:
1. Use an opamp which can operate down to its negative rail, e.g. LM358.
2. Give the opamp a bipolar supply so +1.2V is nearer to the middle of its range.

[kgull]

Giving the opamp it's own power supply works well. I'd assume then something like a LM358 would work too then? I'll have to throw a couple on my next order...

Realized a slightly easier way to do this would be a PNP buffer, works just fine:


Thanks for the help!

[earthtonesaudio]

Even better!  I like that solution.