Variable capacitors?

[inverseroom]

Craig Anderton said they existed, but that they were no good for audio use because you couldn't get much capacitance into them.  That was a long time ago, though, before film caps and resistors were everywhere.  Are these out there?  Does anyone use them for anything?  I mean, I could think of a few uses...

[Peter Snowberg]

Yes, they still exist.

The range is very limited so don't think about tuning an oscillator with one, but tuning the value of a treble cap in a tone stack is easily within reach of the right one.

Check http://www.circuitspecialists.com for useable units. See type 42 & type 46 units in the trimmer cap section.

One word of caution.... When adding one to a tube amp, BE SURE to use a blocking cap (something like 0.1uF) in front of the stack so that the stack sits at ground potential. If you don't the adjustment screw will be sitting at B+ voltage.

Using a trimmer cap in a tone stack is just wonderful. If you make the slope resistor a trim-pot and use the right trim-cap for the treble cap, you'll be surprised how much you can tune the output.

[Paul Perry (Frostwave)]

Low value caps are more useful with high impedance circuitry.
There was a wien bridge audio signal generator that used a twin gang radio tuner cap as the variable element (this was a tube unit about 40 years ago from memory). The advantage was, no resistor track to wear out!!
Hmmm...... do I remember a tube wah using a variable cap from a member of this board??

[zachary vex]

variable caps for small radios used to be 0-365 pF, as i recall.  not such a bad range, really.

[sean k]

A condensor mic element is an air pressure variable capacitor...

[zachary vex]

if you can keep up the pressure...

[stm]

Some months ago we had a thread with a couple of circuits that allowed changing a cap form 0 to 100% its value with a pot. Puretube was also involved.

[puretube]

http://diystompboxes.com/sboxforum/viewtopic.php?t=25504&highlight=varicap

[inverseroom]

http://diystompboxes.com/sboxforum/viewtopic.php?t=25504&highlight=varicap

Wow, neat!  I've got to try that...

How does a variable cap work, by the way?

[Joe Kramer]

Thanks Puretube and STM for these circuits!

Question: I see the "input," which is presumably one pole of the cap.  Where's the other pole, the "output?"  I'm thinking in terms of using this circuit to replace something that requires several caps and a rotary switch.  That's possible with this, right?

BTW, here's STMs op amp version:

http://tinypic.com/ab0x1

[stm]

This circuit is intended for a Cap with one "free" end (Cin) and the other end grounded. If you need a true floating cap the number of OpAmps will double. I think puretube or someone else posted a circuit with a floating simulated inductor, upon it may be possible to derive the floating cap.

[Joe Kramer]

Ah!  Perfect for my app then, because it's a cap-to-ground situation.  Thanks STM!

Regards,
Joe

[puretube]

I do prefer my tube-solution, though - it`s got more mojo...

[zachary vex]

i like this varicap.

http://home.earthlink.net/~lenyr/varelec.htm

[stm]

Puretube, could you please post your circuit again? Apparently it's gone from the previous thread.

There is still a chance to retain some of the mojo using MOSFETs or JFETs instead of the tubes as voltage followers, before going into the antimojo OpAmps (well, excluding the NOS JRC4558D devices!).

[puretube]

sorry, stm:
it`s not gone from the previous thread,
but I`ve taken down all my sites (one of which contained a.o. that schem...),
because of the attitude of >some< members* of this and other
forums, and their actions/behaviour here and elsewhere in a public
which they seem to think of as "privacy", but nevertheless spread it around the web...

I don`t like to share my ideas anymore with people that are writing such disgusting things - and try to gather information about me and my stuff,
in order to hinder my future just for fun and out of envy.
(like they try to do with other designers, who always lend a helpful hand here).



I`ll gladly personally e-mail you that little schem, once I received your address at: pure... [EDITED] .

You`ve done great contributions to some far-out topics, that I really dig!
And I wish it were possible to keep on "collaborating" here,
but due to a handful of guys: for me the thrill is gone.

[EDIT:] ooops, I forgot to add an easter-goody:



*: ("non-veterans")

[Joe Kramer]

Hey, Mr.Vex: Thanks for the cool website!  I like the "magnet kicker"-- is there "e-bow" potential there?

Hey STM:  1458 has mojo, and 741 too!

Regards,
Joe

[puretube]

Puretube, could you please post your circuit again? Apparently it's gone from the previous thread.

stm: check your PM...

Mr.Vex: already put it in the MoJo-thread:
http://diystompboxes.com/sboxforum/viewtopic.php?t=25039&start=270
(after you hinted that in a recent thread)


BTW: currently I`m developing my latest device, to achieve variable pos & neg capacitance:

the THRU-ZERO-CAPACITOR,
(aka: "Capductor", aka: "Indacitor")

still needs a little debugging, to go all the way from 10µF to 10Hy...

[zachary vex]

oh, you maniac you.  8^)

[Paul Perry (Frostwave)]

Maniac or not, the latest Silicon Chip mag has a circuit for a synthetic negative capacitance.
Tragically, there was no application given...
Admitttedly, this IS the April issue, but I don't think it is a joke :roll: