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Variable caps - fake them?

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GCA

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Here's something I found on a website while looking for easy ways of not putting an air-variable cap in a tone circuit:

Here's a trick to simulate a variable capacitor, especially useful for tone control applications. Attach two different capacitor values to a potentiometer--moving the wiper then sends more or less of the signal to one of the caps thereby changing the frequency response.

There was no example provided and no math. I'm curious - has anyone ever done this to fake a varicap? Say, use one as the bottom C and one as the top C, with the spread between them being the Q?

Ok, the real question: how would these be best hooked up, and how to find the f center? It seems a tad funky to me actually since I've never done this. Would it be similar to a wah-wah circuit used in old guitar effects?

If it works as advertised, I wanna try it in a tone stack and see if I can't get what I'm after with two caps and not have to use a variable anything cept a pot. 'T would be beautifully simple and stupid for me :)
 
It probably WAS a tone stack. And it's just RC filters, you change the resistance with a pot to vary the response, no variable caps needed.
 
It was. I've just never seen a variable RC filter done that way, but apparently it's way more common than I thought. It's a lack of experience on my part... I've only done a cap in parallel with a pot on a guitar to make the usual low-pass filter.

Any idea how I'd hook that up? Or better, you have some sort of reference I could dig up to see the specifics with a couple of examples?
 
What, a tone stack? Just Google it.
Adam's Amplifiers: Tone Stacks

Look at the schematic for the Marshal at the bottom, or the Blondie halfway up the page - it's got to be one of those two.
 
mm... close. Good diagrams of some good stacks.

Semi-para is what I'm really aiming for, without a Q control. I'm looking for ways of sweeping frequency between two centers or a reference discussion of this kind of thing (if I'm not totally off my rocker):
Code:
      |---------| |-------|
      |           C1        |
      \                       |
<---/ R1 (pot)           |----->
      \                       |
      |                       |
      |---------| |-------|
                  C2
Sorry for the crappy ASCII art, especially on R1.. best I can do while on lunch at work. Very wrong also (no signal path in here); I'm just hoping to get the C1/C2 idea for a clue as to how they'd combine.

Very few tone stacks in guitar or bass amps provide a real variable frequency adjustment. The most common way of doing it is to use a switch to bring different caps into the circuit, a la Ampeg SVT and the newer (yet still discontinued grrr) SVP-CL. It's not pot-sweepable though and is essentially a modified Baxandall if i understand it right. The ones on that page all seems to have fixed midrange frequencies... not necessarily the best arrangement for bass.

Am I barking up the wrong tree entirely? I'm looking to work it into a bandpass filter and haven't seen it used in an example without an inductor or an op amp like in a wah circuit. Boost/cut I got. Variable frequency I don't :(

May just have to build one and shove it through a real spectrum analyzer and fiddle with the knobs... a real pain when I can't get it simulated because I'm not sure how to even hook it up!

Let me know if I'm up a creek here!
 
Oh, I see what you're after (crappy ascii art is better than a page of text sometimes) - you want a 100% passive bandpass RC network that gives a continuously variable attenuation and a continuously variable center frequency.

Uh, um, er, well...

I can think of how to do it with opamps, I can think of how to do it with a variable inductor LC filter... but just with caps, pots and pads... that's going to be tough.
 
haha... pretty close. I'm thinking it would be better to go with a tube-based gyrator. I get to make up my gain loss that way, too. This method looks like a total dead-end... there's gotta be an op amp in there somewhere that they didn't mention. After all, they were talking about stompboxes. If this method were actually workable there would probably be 1000 schems on the web using it, but there aren't.

Maybe I can find a Boss pedal or something else that does it as an example? (nah). Oh well, thought it was a blind alley, looks like that's the case ;P
 
Yeah, I think you're going to have to add a tube or an inductor with an adjustable core or something. I serously doubt an effects pedal would have anything like this.
 
Tube is already on the list.. i intend to use 1/2 a 12ax7 or 12au7 for each channel. I'm just fighting with the variable frequency. Seems I can't find a variable simulated inductor without look at patents and I'm not gonna check those out. IEEE has locked up all the good documents.

Inductors are big, heavy, and won't fit in a 1U rackmount at the values required, will they? Gah. Why does varying f cause so much trouble. I'm trying to keep transistors and opamps out of the signal path because i tend to drive gear and they clip horribly.
 
They don't have to be THAT big, it's just signal-level stuff, right? That's the saving grace. If there was any power involved, it would be have to be pretty big - but I've seen 1H coils the size of a marble. If you had a coil that was adjustable from 100mH to 1mH, and had a 100µF NPO cap... uh, that component might actually be bigger than the coil... anyway, you could tune the tank circuit from 50 to 500hz.

You need one with an adjustable core, though. The usual adjustable core units are for RF work and have uselessly small inductances. You may wind up having to make your own.
 
Might as well go with an air-variable cap in that case, at least they're available and easier to make than a hand-wound variable inductor...

Back to square one. How do other people do it? Anyone do it without opamps?

*edit: not afraid of active components. Would simply prefer tube to transistors or opamps since I will already have the power supply to back it up.
 
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You can do it with diodes, preferably a varactor, but if you put a few diodes in parallel the junction capacitance will add. Then you can use a variable resistor to control the amount of reverse voltage across the diode/diodes to change the capacitance.
 
Not usable at AF. You'd need a ton of varactors. I looked into it and the capacitance isn't high enough. Need uf, not pf. Adding them in parallel would make for a giant monster. Unless you got a source for some that could do it.. my mind isn't set in stone on it :)

Gawd I wish I could just do what the guy on the website said!
 
What wesite? Got a link? Maybe we can figure out what he's doing, or if it's BS.
 
Beavis Audio Research

^^ here's where that quote originally came from.
If the "variable low pass filter" at the bottom of the page is any indication, I would take everything there with a grain of salt. That circuit does nothing without a load, and the performance with a load depends on what type of load you put on it. In any case, it's not a low pass filter.
 
Heh! That's a HIGH pass filter!

The cap will only "attenuate higher frequencies" if it's across signal and ground, in series like that it will do the OPPOSITE.
 
Heh! That's a HIGH pass filter!

The cap will only "attenuate higher frequencies" if it's across signal and ground, in series like that it will do the OPPOSITE.
And like I said, without a load, it will do nothing.
 
Heh. Like I said; lost cause XD

Ok. Who's got the variable 1-transistor version I can then tubify?
 
So now you want a one-tube variable frequency bandpass filter with a Q adjustment?
This time you're in luck -

**broken link removed**
 
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