Variable Capacitor

[duffrey]

I have been working on this variable sweet cap thing for my Vox Wah rebuild for a while now, and I have just about all but given up!  For the time being I have just stuck the 0.01uf cap in it so that I can use the pedal, and it sounds sweet.  I went with the fulltone inductor and pot.  The pot made a huge difference even with the stock board.  

Anyway.  
I have now gone down to the basics of my problem.  
I have a 0.0033uf cap (metalized poly film) hooked up to pin 1 of a 100K cermet pot, then the positive side of a 4.7uf electro cap hooked up to pins 2 and three.  Then the other leg of the of the small cap and the negative side of the electro hooked together.  When I put it on the meter I get values starting at 4.9uf and then up as I turn the pot, not 0.0033 and up like I want.  I have tried hooking it up every other way possible, making sure to test all the possible combination of leads, but still can't get the results I would like.  

The cap I'm trying to replace is 0.01uf by default.  I would like to be able to sweep between 0.0033uf and 0.1uf, just for some variety, and the ability to use it with a bass sometimes.  
How can I achieve this?

[niftydog]

many have tried, so far - all have failed.

Getting a variable cap that's bigger than a few hundred puffs (picofarads) is not an easy task.

Check this effort out!

[puretube]

We had this topic shortly ago, look at this thread:
http://diystompboxes.com/sboxforum/viewtopic.php?t=24307&postdays=0&postorder=asc&start=0

practical volunteers welcome, and urgently looked for!

[duffrey]

How then is Joe Gagan seemingly able to use just about any two caps he wants in most of his pedals?  
He uses the exact same circuit for an input buffer or other function in just about every pedal he has designed.
How does it work for him?

Jeff

[niftydog]

can you point us to such a circuit?

I doubt he's varying the capacitance. More likely it's a filter with variable resistance.

[puretube]

duffrey: maybe a 6pos. switch with 2n2, 4n7, 10n, 22n, 47n, 100n caps
would help you?

[duffrey]

I wanted to try and get away from the switch idea in my wah.  I wanted to have everything adjustable via trimmers on the board.  A set and forget type of thing, but with infinite possiblilties.  

Here's one of Joe's schematics.  
http://www.ampage.org/sounds/files/newdinofuzzschem2.gif
You can check out http://sounds.ampage.org/ for a bunch more examples.  

It also says in the DIY faq that it is possible to do this.  Its under the input cap switch section.  

Well I went out and bought an 8 switch dip switch today with a bunch of caps just in case I can't get this working.  But I still have faith.  

Thanks for your help

[puretube]

the trimpot affects the effectiveness of the electrolytic,
but does not alter the capacity of the parallel combination.
An ideal capacitor doesn`t have a series-resistor.

[niftydog]

The FAQ does mention Joe's circuit, but it is not a variable capacitor at all. It is in fact just a simple mixer circuit that varies the amount of signal that passes through the larger of the two caps.

What you have built will work in the same way, but it will not reflect a varying capacitance on a meter.

To get an effective "range" of between 0.0033uf and 0.1uf I would suggest you need something more like a 100nF (0.1µF) cap instead of the 4.7µF cap.

Even so, if all you want is to have a "bass guitar" mode on the pedal, a simple SPDT (or even SPST?!) switch will allow you to do this quite effectively.

Trust me, if Joe had come up with a way of making a variable capacitor useful in the audio frequency range using two caps and a potentiometer, then he would be a God!!!

[duffrey]

I originally had a 0.0033uf and a 0.1uf cap in parallel with the 100K pot inbetween them, and absolutely nothing happened.  So I took the whole thing out, put it on the meter, and just assumed that because there was no change in capacitance that I had found my problem.  Guess not.  
So in theory it should work.
I guess I'll keep plugging away.  

Jeff

[niftydog]

perhaps that capacitance range is unlikely to make any effect. Have you tried substituting the original cap with either a 0.0033uf or a 0.1uf to see if it actually makes a difference?

I guess that's why Joe uses a huge cap like a 4.7µF.

[cd]

You're thinking of it the wrong way.  The circuit you're describing is (as niftydog pointed out) more of a mixer than a variable capacitor.  The cap affects frequency response by cutting highs or lows.  If you start with a .01u cap, it has a certain frequency reponse - if you graph it out, it looks like a hill.  If you change capacitors, you change the Q so the hill moves up and down in frequency (the center frequency, or Q, changes).  This has the effect of changing the sweep of the wah.  

When you bypass this cap with a resistor and another cap, the RC combo does just that - it allows certain other frequencies to bypass the cap unaffected.  With the RC combo (with variable R) you change the frequencies which are bypassed.

In terms of a wah, adding this RC combo simply adds "dirt" to the bottom or middle of the hill - the center frequency (Q) is widened (or flattened) on the bottom, not changed.

In other words, you cannot change the frequency reponse (cap value), only bypass it (allow certain frequencies to not be affected by the cap).  Since your goal is to change the frequency response, the RC combo will not work.  Get a big switch or a DIP switch bank and go nuts with 6-10 different caps.  Caps in parallel add together (so two .01u caps in parallel are effectively a .02u cap).

[tclixx]

So that's why on the wahs I have seen with ajustable sweep or Q actually have a rotating switch that is conected to several different size caps and each different setting is just swicthing to a different size cap. What would be the name of such a rotating variable switch. I would want one that only switched between 2 - 4 different cap sizes.

[niftydog]

a rotary switch is the beast you seek. First you need to know how many postitions you want to be able to select, then get yeself a suitable rotary switch! There's more info around here if you want to have a search around.