Simulate a variable capacitor

[JonBass]

Hi guys,

Afraid I'm a bit of an electronics newbie, so be prepared for some foolishness in my question

Hoping for a clever solution to a seemingly insurmountable problem...

I'm building a guitar at the moment and as I'm sure you know, the tone pot usually uses a fixed value capacitor to act as a filter.
I would really like to be able to use a variable capacitor so that I can adjust the threshold of that filter.
Obviously variable capacitors exist, but as they are all in the pF range (at least the ones that would fit into a guitar), I would like to simulate the effect of one.

Can this be done in any sensible way?

Obviously I could use several capacitors with different values and some sort of switching arrangement, but was hoping for something a bit neater.
Perhaps something involving a couple of different capacitors with a potentiometer to vary the amount of signal going to each?

If you haven't pulled your hair out at the silliness of this question, congratulations!

Thanks very much,
JonBass

 

[CDRIVE]

You didn't state the desired range of this cap but I don't think variable caps are practical at audio frequencies. It would demand a lot of real estate.

I'm assuming the filter circuit you refer to is RC. Wouldn't it be more practical to make R variable?

Chris

 

[Martaine2005]

Hi John,
There are such things as 'trimmer capacitors' in the nf and pf ranges.
But they are more of a 'set and forget' configuration. Probably not durable enough to mount a knob for constant adjustment.
I think you would be better off with the switched arrangement. A rotary switch with how ever many caps you want to mount.
Are you playing with caps now? .022uf and .047uf seem to be the norm.

Martin

 

[Martaine2005]

Here is a site that sells custom split pots with different caps on. You may be able to do something like this.

Martin

 

[JonBass]

Thanks for the replies guys.

It seems to me that the best thing to do will be to go for the switched arrangement like you suggest Martin.

Appreciate your time

 

[CDRIVE]

Hi John,
There are such things as 'trimmer capacitors' in the nf and pf ranges.
<snip> Are you playing with caps now? .022uf and .047uf seem to be the norm.

Martin

Martin, I have to admit that I seem to miss a lot of memos at my age. This is definitely one of them. Do you have a link to some of those .022uF, .047uF variable caps?

Chris

 

[CDRIVE]

Here is a site that sells custom split pots with different caps on. You may be able to do something like this.

Martin

I see by this link that my assumption of an RC filter was in error since these appear to be bandpass filters that will include "L". I thought we were talking about LPF/HPF RC networks.

Chris

 

[Martaine2005]

Martin, I have to admit that I seem to miss a lot of memos at my age. This is definitely one of them. Do you have a link to some of those .022uF, .047uF variable caps?

Chris

Hi Chris, those caps are just the normal range values for guitar tone pots.

I see by this link that my assumption of an RC filter was in error since these appear to be bandpass filters that will include "L". I thought we were talking about LPF/HPF RC networks.

Chris

No, not at all. That was just a site for guitar modding enthusiasts.
Very unusual to see inductors on the pots. Usually just single caps on the tone pot.

As for links....All electronics stores sell them. RS, Mouser, Digikey, Farnell etc. They are just 'trimmer capacitors'.
But as I noted above, not really any good for using as an adjustable capacitor. Just a set and forget/calibrate type.

http://uk.rs-online.com/web/c/passive-components/capacitors/variable-trimmer-capacitors/
http://uk.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g94

Martin

 

[CDRIVE]

I read that as variable caps in the .022uF / .047uF range were available.

Chris

 

[JonBass]

After a bit of a 'blue sky thinking' moment, I've decided I'm going to use an 8-position binary coded switch and a few different valued capacitors (probably 0.010, 0.022, 0.047, 0.092uF). Although this isn't truly variable, it'll provide enough granularity for most guitar players.

The switches will be moved very rarely, so with luck this will work for a reasonable amount of time.

Thanks again for the input!
Jon

 

[Martaine2005]

You may be better off with a larger rotary switch designed to be used.
Mount it in the guitat like a 2nd tone pot.

 

[AnalogKid]

OR, use an analog circuit to grow a single-stage voltage-variable filter and control it with a simple pot. Was that the intent of the original question, or did I mis-interpret it?

What kind of filter do you want? High-pass, low-pass, bandpass?

ak

 

[Ratch]

Hi guys,

Afraid I'm a bit of an electronics newbie, so be prepared for some foolishness in my question

Hoping for a clever solution to a seemingly insurmountable problem...

I'm building a guitar at the moment and as I'm sure you know, the tone pot usually uses a fixed value capacitor to act as a filter.
I would really like to be able to use a variable capacitor so that I can adjust the threshold of that filter.
Obviously variable capacitors exist, but as they are all in the pF range (at least the ones that would fit into a guitar), I would like to simulate the effect of one.

Can this be done in any sensible way?

Obviously I could use several capacitors with different values and some sort of switching arrangement, but was hoping for something a bit neater.
Perhaps something involving a couple of different capacitors with a potentiometer to vary the amount of signal going to each?

If you haven't pulled your hair out at the silliness of this question, congratulations!

Thanks very much,
JonBass

Google for "variable frequency low pass filter". Lots of entries come up. Or maybe you want high pass.

Ratch

 

[GPG]

http://www.analog.com/media/en/training-seminars/tutorials/MT-205.pdf
Google"biquad filter" and "state variable filter" for more.
And of course:
http://www.electrosmash.com/vox-v847-analysis