Capacitors in relation to a varitone type application

Use this:
http://www.muzique.com/schem/filter.htm
Caps by themselves don't have a frequency value, but in combination with a resistor they form a filter that does. So it depends on the resistor as much as the cap. Yes, you can combine caps. Solder them in parallel (not series) to increase the total value.

davelowell2 said:

Resistor? There's only a switch and caps and jacks. I don't have a resistor?

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Your tone pot is the resistor.

In this case the volume pot is the resistor. So to calculate the frequencies, use the ohm value of your volume pot.

Yes

davelowell2 said:

So, I put in 250000 for the volume pot, then .01 for the cap, I got 6.4. What is 6.4?

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That's Hz, Hertz, the corner frequency. The larger the cap value, the lower the frequency, and vice-versa.

However, as I'm looking at it, the very high value of the pot is throwing off what I would expect from the filter. So it may be that I'm totally mistaken about the volume pot acting as R in the varitone circuit.

Anyone else here able to correct this misunderstanding?

davelowell2 said:

I assume this is in response to the question about the order of things?

I like that I can use two caps together for more possibilities. That will be cool.

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You do understand that two caps in parallel gives a result exactly that same as if you'd used a single cap with a value equal to the SUM of the values of both caps, right? It doesn't exactly give a different effect except for the value being different. There are ways to switch a few caps to give lots of values. Some decade boxes do this.

bongomania said:

That's Hz, Hertz, the corner frequency. The larger the cap value, the lower the frequency, and vice-versa.

However, as I'm looking at it, the very high value of the pot is throwing off what I would expect from the filter. So it may be that I'm totally mistaken about the volume pot acting as R in the varitone circuit.

Anyone else here able to correct this misunderstanding?

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Yes. Let me clear this up a bit. First off understand that a pickup is a coil of fine wire. That creates an inductance (as seen in the pickup specs) and also a resistance (also seen in pickup specs). And in addition there is a capacitance of each turn of wire to it's neighboring turns.

This means that every pickup is an LCR circuit. It is a RESONANT circuit and typically without any additional cap the frequency of that resonance is usually at the higher audio frequencies or beyond. By changing these various values (by changing the amount and kind of wire used in the pickup etc.) you can change the amount and center frequency of the resonant peak of the pup. That is what give various models and brands of pickups their characteristic tone.

Now the C value above can be changed by simply wiring an additional capacitor across the pickup. That will move the resonance DOWN in frequency making the peak deeper in tone. Since the output rolls off rapidly above resonance, the more C you add the more deep any less bright the tone.

But I don't want to over-simplify this because it's NOT simple! When you have an LRC circuit not only is there a resonance frequency, but the presence of R changes what is known as the "damping" of the circuit. That determines the sharpness of the resonant peak. And obviously the R seen by the pickup circuit is MORE than the R of the coil of wire. There is the R of the volume control plus the added R of the amp input hooked to that. And worse than that the value of the R may change with the setting of the volume control. We've all seen instruments where the tone changed drastically as we adjusted the volume.

The bottom line is that yes, it is possible to figure out how the given pickup will work with various caps across it but you'll probably need a simulation program like SPICE to do it. But, really who cares? You really have no need of a total theoretical analysis of this. You've got a bunch of caps and a switch. So hook them up and see what they do. Use your ears. If you want more fun sub a 250k volume control for a 500k or vice versa and see if it makes any difference.

If you go look at some of the curves for commercial tone adjusters you can see what I'm talking about as for the higher cap values the roll off moves lower in frequency and the amount of peaking that occurs can vary from none to fairly sharp hump before roll-off just depending on the equations that determine that sort of thing.

You've got the right idea, just try things and see how they sound. You can always put it back the way it was if it sounds like crap.

Thanks for that! Makes more sense than the simplistic volume knob "theory".

A slightly simpler explanation is that the tone circuit is not an RC low pass filter - the entire circuit including the pickups have to be taken into consideration. And it gets even worse with more than a single pickup.

SPICE, as bassbenj alluded to, is the only way to know the response ahead of time, and there's an additional problem with that - while some pickup manufacturers publish the resistance (R) and the inductance (L), I haven't seen too many publish the capacitance (C) of their pickups.

Go for it. Basically when you combine caps in parallel, you add their values together. .0047 + .022 = .0267, in other words not a huge difference from .022, but maybe worth trying.

bongomania said:

Go for it. Basically when you combine caps in parallel, you add their values together. .0047 + .022 = .0267, in other words not a huge difference from .022, but maybe worth trying.

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If it were me, I'd try to make gradually increasing steps of capacitance from zero (no capacitors) to your max value.

I would try to space them evenly like
none, .000680, .001, .0022, .0047, .0068, .01, .022, .047, .068. .1, .22 which gives 12 values. If really large or really small values give tone that is not usable, then remove those. If the jump between two values seems to give a large difference in tone, then split the difference and add a value in between those two. You can get any oddball values by putting caps in parallel and adding up the values of all of them.

This should hone you in on a nice selection of tone capacitors.

I've been scouring google for a while now and haven't been able to find some similar info, though this thread keeps coming up.

Ive decided Im building a box with a varitone for creating a low-pass filter and having another for creating a high-pass filter. I found this video on youtube and the guy shows different caps with different freqs that they cut off at. How did he calculate that? I want to have the high pass one do like 20, 30, 50 and maybe 80 hz or something similar...

here's the vid.