MIMEO - variable capacitor

para · Joined: Oct 16, 2006 Posts: 276

i'm pretty busy right now but i wanted to share this with you guys since you've all been so cool.

i know that everyone hates having to use a spdt toggle and a bunch of caps to get more range out of your LFO's and stuff so this will get rid of that for good, and replace it with a pot to smoothly adjust the range.

i haven't had the time to really refine it and experiment that much with it but it does it's job. if anyone gets the time to work with it i'd love to hear your finding's.

UPDATE: NOT WORKING

steven

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MIMEO - variable capacitor

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bugbrand · Posted: Sun Sep 02, 2007 12:31 pm Post subject:

Steven, this is freaking exciting! Anyone else ever heard of a pot variable capacitor 'cos I certainly haven't.?!.

I will definitely have a play sometime.

I'm sure others will agree ---> thanks for sharing this!

para · Joined: Oct 16, 2006 Posts: 276

its so simple that i'm shocked i haven't stumbled on someone else even trying it. at least not like this and certainly not variable

steven

bugbrand · Posted: Sun Sep 02, 2007 12:35 pm Post subject:

Replace the pot with vactrol or transistor for voltage control!
Amazing!

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

WOW!

So max capacitance is 100pf, variable to zero?
Does it matter what the size of the cap is?

Veerry interesting!

C

para · Joined: Oct 16, 2006 Posts: 276

no the minimum cap size is the 100pf or whatever you use in there, it can be anything and the amp multiplies it so it gets huge and off the charts. like over 100uf. i don’t have any way to measure it so i just substituted a 100uf for the mimeo in the same circuit and got similar results. just guessing. then if you want to define the range you choose the appropriate pot value to limit the amount it amps the capacitance

steven

revtor · Joined: Oct 05, 2005 Posts: 79 Location: NewJersey, USA

Steve, is this one of your "parasite" boards??

Funky40 · Posted: Tue Sep 04, 2007 2:06 am Post subject:

thanks para
It's very interesting

para · Joined: Oct 16, 2006 Posts: 276

yes the design is used in a parasite but there is a bit more to that design regarding power and stuff.

also i think i should say that this sort of thing is generally called a "capacitance multiplier" when used at a fixed setting. my use of the term “variable capacitor” may be throwing people off.

steven

germaniac · Posted: Tue Sep 04, 2007 10:54 am Post subject:

Hey Steven, thanks for posting this schemo--very cool!

I've seen a couple "variations" on this, but yours looks the simplest (a good thing IMHO). Here are two more for reference, neither of which I have tried. They may work as well, better, or worse for all I know. With all due thanks and credit to the originators of these designs/schemos.

Regards,
Joe

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para · Joined: Oct 16, 2006 Posts: 276

my last response was lost to board malfunction… lame.. it was something like

---

interesting thanks. but those seem to be the opposite of what mine does. they take a cap and make it smaller and mine takes it and makes it larger.

so with theirs if you use a 100n you get about 0n to 100n ( or close)

and with mine using a 100n you get 100n to over 100uf ( or more i don’t have anyway to measure it )

both are usefully for different situations though

steven

blue hell · Posted: Tue Sep 04, 2007 2:30 pm Post subject:

para · Joined: Oct 16, 2006 Posts: 276

no no… i wasn’t complaining. i completely understand and sympathize. this board is huge and has enough mods and stuff to make upkeep and server issues a serious pain. i had just forgotten most of what i had written

you guys can do no wrong in my eyes!

steven

blue hell · Posted: Tue Sep 04, 2007 3:16 pm Post subject:

para · Joined: Oct 16, 2006 Posts: 276

i keep notepad open all day for exactly that reason but i had just closed it to clean up my desktop. the one time i need it...

shawn · Joined: Dec 13, 2005 Posts: 209 Location: savannah

Any luck with any of you guys trying this? I haven't had much luck yet. I recall you saying that there was some trick into powering it? Would it be bipolar -/+ or gnd/+?

para · Joined: Oct 16, 2006 Posts: 276

? it works for sure and no there is no trick to it. you can power it however the chip you choose wants to be powered.
the + and - are the polarity of the cap similar to an electrolytic

steven

synthmonger · Posted: Sun Nov 04, 2007 11:41 pm Post subject:

Not having much luck with it. I tried a few different ways of powering it and I'm not able to vary the capacitance. You sure this schematic is all good? I need something like this to vary the pitch in my MT240 ;X

para · Joined: Oct 16, 2006 Posts: 276

its been months since i messed with it on the breadboard and then i copied the scheme - haven't revisited it at all. some day soon though.

i was using a 258 at the time i think, with just a 9v for power, and i played with it for days so i was not hallucinating. it did work

steven

synthmonger · Posted: Mon Nov 05, 2007 1:23 am Post subject:

Were you using it on a keyboard or anything? I'm still having such a minimal amount of pitch change with my mt-240 It goes about down about 1 semi-tone. I can swap the onboard caps to a max of 300pf before it gets too low and cuts out, so I know the pitch can go down several octaves.

I tried it out with a basic 40106 oscillator and was getting the same effect.

I tried powering it with a 9v battery, two 9v batteries, and a maxim +/-9V chip with a 9VDC jack.

Would it be possible to test it out on a toy or something? The thing barely takes a minute to breadboard. I would reallllllllllllllllllllllllllllllllllly appreciate it Smile

Pehr · Posted: Mon Nov 05, 2007 1:23 am Post subject:

This is very good! very nice! Very Happy

I actually have a variable cap at home. It looks like a pot. It only turns 180 degrees and the range is very tight. I think it is 20-70 pF... I think it is desoldered from an old TV or radio.

para · Joined: Oct 16, 2006 Posts: 276

just to be clear this is a multiplier, it takes the given value and increases it. it will not make it smaller.

i only tested it on a 40106 osc i think.

s

para · Joined: Oct 16, 2006 Posts: 276

i don't know what to say. i just spent 45 minutes trying everything i could think of and it does not work. i really wish i could go back in time right now.

i went threw all of my notes and research and shit and boarded it up with a tested 40106 osc using a 358 with a single supply and a 072 with a bipolar and grounding the cap to Vref and the rail and it does not work as it did.

i feel like a real asshole, but it really did work. like i said i spent days playing with it fine tuning it and shit and i'm pretty sure i tested it in a couple other things like my shitty vco as a timing cap and stuff and it worked in everything, if not i wouldn’t have made the pcb’s which i guess will be garbage now unless it’s an easy fix. i have to have missed something going from the board to scheme. i was working with like 20 different designs on the table at once so i guess it was bound to happen. sadly i don't have the time to mess with it anymore for a long while. i'll leave it boarded as long as i can before i need that for something else so i can play with it when i get ideas. as soon as i have it working again i'll post the correction.

you can start the name calling now.

steven

sneakthief · Joined: Jul 24, 2006 Posts: 569 Location: Berlin

charlatan! hahah

shawn · Joined: Dec 13, 2005 Posts: 209 Location: savannah

Good to know that I wasn't the only one to get this working but also sad to hear that you made pcbs for this already! My only advice is possibly finding someone who can replicate your findings in the future to prevent that from happening.