Variable capacitor

[JTEX]

R2 can go as low as U1 can comfortably drive without distorting, so probably not much less than 1k if you're using a TL07x. At the other end of the range, I probably wouldn't go over 5k to keep the noise low.

When choosing a P2 value, keep headroom in mind. 1+ P2/R2 is the U2 gain, and too much gain will cause clipping. So, basically, I don't think a P2/R2 ratio of much more than 10 is a good idea. P2=50k and R2 around 5k should work just fine.

I haven't built the physical circuit yet, but I see no reason that it shouldn't work exactly as it does in the simulator.

 

[Cybercow]

R2 can go as low as U1 can comfortably drive without distorting, so probably not much less than 1k if you're using a TL07x. At the other end of the range, I probably wouldn't go over 5k to keep the noise low.

When choosing a P2 value, keep headroom in mind. 1+ P2/R2 is the U2 gain, and too much gain will cause clipping. So, basically, I don't think a P2/R2 ratio of much more than 10 is a good idea. P2=50k and R2 around 5k should work just fine.

I haven't built the physical circuit yet, but I see no reason that it shouldn't work exactly as it does in the simulator.

Not being a true engineer, I was surmising pretty much the same thing, but wanted to ask. Thanks!

 

[JTEX]

I'm not a real engineer either. Best to try this on a bench or in a simulator, check for stability and so on.

 

[YourGuitarist]

I read somewhere that there's a floating version, but haven't investigated further.

There are a couple but they are all complex and component heavy to my knowledge.

 

[almondcity]

Man I'm actually tempted to do another ssbs F*** overdrive with one of these. Theoretically you could dial in the strength of the sag effect

 

[DDad]

R2 can go as low as U1 can comfortably drive without distorting, so probably not much less than 1k if you're using a TL07x. At the other end of the range, I probably wouldn't go over 5k to keep the noise low.

When choosing a P2 value, keep headroom in mind. 1+ P2/R2 is the U2 gain, and too much gain will cause clipping. So, basically, I don't think a P2/R2 ratio of much more than 10 is a good idea. P2=50k and R2 around 5k should work just fine.

I haven't built the physical circuit yet, but I see no reason that it shouldn't work exactly as it does in the simulator.

This is interesting. But a gain of 10 doesn't yield a wide range of capacitance. Are there any op-amps that have a gain of 100? Or at least significantly higher than 10?

 

[Cybercow]

This is interesting. But a gain of 10 doesn't yield a wide range of capacitance. Are there any op-amps that have a gain of 100? Or at least significantly higher than 10?

Opamps, by themselves, for the most part, have a rather large gain range which is primarily determined by the feedback resistance. In this case that would be P2. And note what JTex said in reply #21 about higher gains and headroom (clipping potential):

"R2 can go as low as U1 can comfortably drive without distorting, so probably not much less than 1k if you're using a TL07x. At the other end of the range, I probably wouldn't go over 5k to keep the noise low.

When choosing a P2 value, keep headroom in mind. 1+ P2/R2 is the U2 gain, and too much gain will cause clipping. So, basically, I don't think a P2/R2 ratio of much more than 10 is a good idea. P2=50k and R2 around 5k should work just fine."

So, there is a fine balance to consider when keeping the signal clean.

 

[DDad]

Opamps, by themselves, for the most part, have a rather large gain range which is primarily determined by the feedback resistance. In this case that would be P2. And note what JTex said in reply #21 about higher gains and headroom (clipping potential):

"R2 can go as low as U1 can comfortably drive without distorting, so probably not much less than 1k if you're using a TL07x. At the other end of the range, I probably wouldn't go over 5k to keep the noise low.

When choosing a P2 value, keep headroom in mind. 1+ P2/R2 is the U2 gain, and too much gain will cause clipping. So, basically, I don't think a P2/R2 ratio of much more than 10 is a good idea. P2=50k and R2 around 5k should work just fine."

So, there is a fine balance to consider when keeping the signal clean.

I meant about this part - 'to keep the noise low'. What I should have asked is - are there op-amps that are less noisy than the TL0x's, that could be run at higher gain?

 

[Cybercow]

I meant about this part - 'to keep the noise low'. What I should have asked is - are there op-amps that are less noisy than the TL0x's, that could be run at higher gain?

That's quite an interesting question and deeper than I've ever asked myself. I can only imagine that there's no single factor (parameter) that would determine that. In as such, I went to AI and asked . . . .

"An op amp begins distorting when the required output violates any of these:

Vout > Vswing(max) [Does the opamp run at rail voltage or less?]
or
f⋅Av >GBP
or
2πfVpeak >SR (The slew rate is too low.)
[Where:
f = signal frequency
Av = closed loop voltage gain
GBP = Gain-Bandwidth Product]

or the input/output stages exceed their linear region."

All of which seems to make sense and suggests that a 9V single rail supply for the circuit is a very limiting factor,. And the greater likelihood of using a ±15V dual rail supply might more easily withstand using a gain of 100. Still, for a clean gain at 100, the input should probably be below roughly: 100mVpeak or about 70mV RMS. So things are not as simple as just using a lower noise opamp.

It would seem that one cannot turn a bicycle into a helicopter simply by pedaling harder.