Suggestions Wanted: "Variable Capacitor" For Variable Bass Cut

[railhead]

I'm looking for suggestions as to how you guys would address something I'm wanting to do.

I'm making a lo-fi type effect, and the main thing it does is cut bass to get things nice and tinny. I'm liking cap values between 220pF and 600pF for the cut, but I thought it would be nice to be able to "dial-in" how much cut I may want (in case I want 800pF for one song, and 300pF for the next, etc.).

Variable capacitors don't jibe with a 1290NS, so I was wondering how you all would address the issue. I could quick-n-dirty setup a rotary with set values, but a "regular" pot would be nicer.

Thoughts?

[SonicVI]

Maybe use an active semi-parametric eq band rather than a passive RC circuit.

[Solidhex]

I'd go with a blendable input cap...not exactly what you described but effective I think...

--Brad

[Gus]

It should be easy.  I would not vary the cap.  Without knowing what the input of the circuit is like and other things your question can not be answered correctly.

[railhead]

Like I said, I'm using a cap at input to kill low end ala the LofoMofo. The original schematic uses a 220pF which shaves a bit too much off when used with high-gain signals, so I'd like to be able to "switch" to a different cap on the fly. My desire was to be able to "simply" turn a knob and go from, say, 220pF to 800pF.

This is why I said a quick and dirty solution would be to setup my circuit with 220, 330, 440, etc. caps and use a rotary switch to go between them -- but I'd like to implement this functionality with a smooth pot if at all possible.

Maybe the rotary knob is the best option...

[Papa_lazerous]

I would have thought a rotary switch would be the only feasable option

[caress]

might not be exactly what you're looking for, but peep this...


pretty cool.

[Papa_lazerous]

Care to explain how that works 

[caress]

i just had it recommended to me...i can't explain it!   
the negative/positive squares are used like the poles on a polarized cap.

[Papa_lazerous]

Hmmmm RG....................Can you pretty please explain it as I dont understand it.  Or somebody else who knows

[caress]

Hmmmm RG....................Can you pretty please explain it as I dont understand it.  Or somebody else who knows

you should write steven, the guy who worked this up.  his website is on the image and you can contact him through the site...

[caress]

Hmmmm RG....................Can you pretty please explain it as I dont understand it.  Or somebody else who knows

you should write steven, the guy who worked this up.  his website is on the image and you can contact him through the site...
this might only be the bones of his whole circuit, though...i have yet to try it out.  as stated on his site, "the mimeo is a dream come true for me. its a variable capacitor like no other. it has an enormous range from 100p to well over 100uf. all accessible through a smooth single turn of the pot."  i'm guessing it has some stuff missing...possibly the 100uf cap?  more?

[WGTP]

A simple one that has been used on fuzzes is to start with a small cap at the input and then use a pot, 250K or so between a larger cap, 10uf or so.  Turning the pot reduces the resistance between the 2 caps and gradually brings the large cap in parallel with the small one.   

[Gus]

go to geofex and look for the  low fidelity effect.  Make the high pass sweepable.

Back to the lofomofo (you did not have that circuit in the first post)  Problem is it is a low parts count circuit sometimes that makes it harder to  "add on" to.  One thing that might work is to use a dual opamp both sections as noninverting buffers
between the opamps have a cap and then a resistor and pot to Vref.  f = 1/ 2pi RC.  You can then scale the values to something that would work Maybe R a min of 2.2K with a 10K pot.  2.2K with the C for the highest freq.  470k input R Buffer, simple RC highpass,  Buffer then part of the Lofomofo

However use a bigger cap for the 220 pf.  You might want to put the 1n5817 and 500K pot before the first opamp buffer then cap couple it to the first buffer

[ehofherr]

  Maury, I'm working on something similar right now!  I've taken the input capacitor and replaced it with a rotary switch and 6 caps.  In doing so, there's a "loss" in signal level getting to the gain stage with every stage of bass cut.  The solution I'm working out right now is to take the other side of the rotary and place resistors there. 

  I do not want a noticeable change in output level when I set the rotary to a different capacitor.  Fixed resistor with the "bass tone", and no fixed resistor with the "treble tone".  Of course I'll still have a Gain pot for dialing in.  I have proven this will work and am finalizing my resistor choices.  I should have it completed by the weekend.

  My vote is Rotary Switch for ease, simplicity, and accuracy.  Change on the fly to your desired setting every single time!  +1 for Papa. 

[wampcat1]

I'm not understanding why you wouldn't just use a simple gyrator circuit?
bw

[amz-fx]

might not be exactly what you're looking for, but peep this...

It is a gyrator with the filter response flipped so that it behaves as a capacitor instead of an inductor -  the inductor simulator will have the positions of the pot and capacitor reversed. I have a circuit almost exactly like that drawn in one of my notebooks.  I saw the basic fixed capacitance multiplier in one of the big circuit collection books, and realized that it could be made variable. It just goes to show you that it is easy for people working independently to come up with quite similar circuits.

When I had the idea several years ago, I tested it on a breadboard to see if it worked, but that's about all I did with it, expecting someday later to find a place for it in a circuit, possibly a wah.   Don't know how it would sound.

regards, Jack

[Mark Hammer]

There is a difference between the amount of cut at a given bass frequency, and the frequency at which a given amount of cut begins.  The request for a variable capacitor is essentially the second type of cut. 

The alternate-bypass-cap that was used to excellent effect in Joe Gagan's NVN designs (and I gather is also used in the Reverend guitars bass-cut control) provides a sort of compromise between the two.  That is, one has a default input cap value that provides the greatest or most bass cut one might conceivably desire.  In parallel with that is a much larger cap and a variable resistor.  When the resistance is high, lower frequencies still have a hard time passing.  As the resistance decreases, more and more low-end is able to pass through the larger cap and essentially leapfrog over the small input cap.  Works great, and I imagine you could probably have some interesting possibilities if you had, say, a large variable resistance (e.g., 1M) set up as a bidirectional pot where the wiper goes to one end of the default cap, and the two outside lugs each go to one of two possible bypass caps/paths, one of which could even be an inductor/cap combo or simply an alternate cap.  In the middle position (500k in series with each alternate path), the dominant factor is the default cap value.  As the control gets rotated in one direction or the other, one of the alternate paths starts to have an impact.

Another alternative is to have a series of caps and a rotary switch with the clicking detente feature disabled so that it feels like a pot in its smoothness (even though it is still stepped).  This is wired up such that the common shunts different numbers of series-connected caps.  As an example, if we had six 100nf (.1uf) caps in series, that would yield an effective capacitance of .0167uf.  Shunt one of them so that you now have 5 in series, and the capacitance goes up to .02uf.  Move the switch over to shunt two of them, and that now goes up to .025uf, and so on.  The change from step to step can be arranged to have a certain "taper" if one picks the cap values just right.

Alternatively, what you describe seems to me to be a request for a variable highpass filter that provides the same amount of cut but at a continuously variable cut-off frequency.  That particular objective is easily possible with a "normal" dual-ganged pot, an op-amp and a couple of passive components, or even a single pot if you are content with a shallow slope.

[Freddy205]

How would you calculate the cap value that the "MIMEO" circuit above produces? I've never seen anything like it?!

[brett]

Hi
Why not a tuning capacitor (a "gang") from an old radio? They typically go from 10 to 200 pF, but some go to 500 pF.
If you put a 200 pF one in parallel with a 220pF cap, you'll shift the roll-off by a whole octave.
cheers