Variable Capacitor Wah Wah

[mac]

I was wondering what to do with a dual ganged variable capacitor I had removed from an old radio.
The variable cap seemed to be perfect for a MFB bandpass filter
I estimated the capacitance up to 330pf - 470pf, so the resistor in the feedback loop had to be of some mega ohms.
Before building or breadboarding I ran some simulations.
I modified the MBF filter a bit and I had two modes, one having a Cry Baby sweep.

These are the schematics and plots for mode I and mode II,

Mode I Schem



Mode I Plot



Mode II Schem



Mode II Plot



The working pedal proto,



The cylinder covered in aluminum foil is a variable inductor, another of my projects

Mode I has one of the caps fixed which gives a Cry Baby-like sweep. You can fix the other way to get a mirrored result
Mode II is closer to the MFB filter. It has a wider sweep but possibly less Q.
I added some resonance. You can play with Rres, Cres, and RQ to fine tune to your taste.

The addition of Cmin and Cpop in Mode I schem was necessary because plates touch sometimes giving a nasty pop. Cpop prevents a short, and Cmin is the minimun capacitance.
I did not added those caps in Mode II to keep the schem simple, but they should be there.
I also put some vaseline gel between the plates to avoid external vibrations to get into the circuit, and to increase capacitance a bit.
It would be better if the cap were into a closed container full of vaseline gel. Capacitance should be 2x or 3x higher.

Opamps can be anything you have at hand.

The pinion is from tape mechanism, a bit bigger than a quarter dollar, 2 euros, or one pound.
To build the rack I used epoxi stick, an ice cream stick, and a bit of zinc sheet.
I put epoxi over the wood stick and I passed the pinion over and over again. A few minutes later it was pretty hard.
Remember that for your particular box the relation between the maximun rotation and the rack lenght is s = angle * radius.
The bigger the radius the faster the sweep. The zinc sheet is flexible like a spring as the rack has some horizontal movement.

Sound clip soon.

For educational purposes only.
And if you dont' like it, but you live near places like San Andreas Fault, it makes for a nice seismic monitor

mac

[joegagan]

that is awesome! i love your thinking this way.

i understand the sim as i spend time in LT spice with  inductor wahs. that really is a wide sweep it is showing.

[PRR]

> what to do with a dual ganged variable capacitor I had removed from an old radio.

Sell it. These things are getting hard to find.

> I estimated the capacitance up to 330pf - 470pf

Standard AM radio RF cap is 365uuf(pF) max, ~~35pF min.

The LO side is 180pF max.

The plates should not touch. Careful de-bending is possible but tricky.

I have never heard the can of vaseline idea; yes that would increase C. So would oil. Might also raise the maximum voltage (not a problem here!). In either case (especially oil), shaft leakage would be a major problem. Which might be why it was "never" done.

[GibsonGM]

What paul said....you can get decent money online from radio collectors/repair guys, who do that the way we do pedals here.
I have a few I've been meaning to unload.

I can't use them for much (well, I make a shortwave receiver now and then), as their capacitance is just so low....

I like the idea, though, mac!!  Something creative.

[Kipper4]

Intresting Mac.
Id love to hear it please mate.

[mac]

Intresting Mac.
Id love to hear it please mate.

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=51926&g2_GALLERYSID=bb8f1e9ec4289cec32492fa132a95a97

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=51923&g2_GALLERYSID=bb8f1e9ec4289cec32492fa132a95a97

I made RQ and Rres potentiometers to change the response of the filter. You can notice that in the second clip, I increased RQ a bit for more lows in the 4th sample.

It needs a bigger gear for a faster freq change. The gear's radius is critical to have a Cry Baby feel.

Windows Media Files. Clicking on the links should open player or browser complement.
If the links don't work go to my gallery. Click on the camera icon and in the next page click *Download movie*

mac

[mac]

Mode 2 sound clip, a more noisy than mode 1,

Mode 1 sound clips,

The gear and rack, the whole thing is fragile so I couldn't abuse of the effect. 

Not a Cry Baby, more like a variable phaser, but I proved it can be done. 

mac