Dual cap tone control

[roadtonever]

I once read a thread on this subject which I now can't locate for the life of me. IIRC the original poster hoped to create a vari-cap and JohnH did some simulations and generated some graphs and the consensus was that it didn't really work.

What I'd want to do is slightly different, though. I want one cap to be added across a pickup "always on" with an additional cap across the same pickup that's variable via a pot. So more like a blend than a vari-cap in effect I suppose.

The objective is to vary a bridge pickups tone from warm to brassy while keeping a clear and sweet tone on the neck pickup via cable capacitance alone. I've already figured out the capacitance values required.

As some of you might know I don't always manage to figure out these things myself so I'm asking that some kind soul bail me out. Please fill in the blanks in the image below in MS Paint. Thank you!

[newey]

RTN-

I want one cap to be added across a pickup "always on"

Well, we had this debate as to whether this was worthwhile or not, but certainly easy enough to do if you're of the mind.

with an additional cap across the same pickup that's variable via a pot

Isn't this a description of a tone control?

[D2o]

I don't know why, but the first thing that came to mind for me was ChrisK's free woman tone, and - REALLY don't ask me why - I then thought of ChrisK's free neck on ... possibly because it's also free, and certainly very cool.

While neither of those are what you were thinking of, they are free and always come to mind as being worth it for someone to try if that person wanted to get some different tones without losing any bones.

D2o

[roadtonever]

newey said:

Isn't this a description of a tone control?

Why, yes! I erroneously though a typical tone control is in series with the pickup. EDIT: Sunstroke hit me while I had my alligator clips out earlier today as well.

Will this work?

[roadtonever]

Or like this?

[roadtonever]

I'm wanting the caps to be in added parallel in when the pot is providing the least resistance.
Basically if "always on" cap is 100pF and pot cap is 820pF I want to be able to acess a "range" of 920pF to 100pF. In other words an 820pF cap extablishes a soft falloff point variable via the pot while the 100pF "always on" is a hard cut and the capacitance adds in parallel.

[newey]

Neither will work as intended. On the first one, you show both caps attached to the pot shell. If that pot shell is also grounded, which is the usual reason for soldering stuff to the back of a pot, then the "always On" cap is attached to ground at both ends and does nothing.

OTOH, if the pot shell isn't grounded, then there's no reason to solder there, you might as well twist both caps together in series. What you have then is a tone control with the combined value of the 2 caps- not what you're after.

On your second version, you've got the wiper and the CW lug of the pot both wired to ground through caps and the tone control won't work.

[roadtonever]

Any hints? Please see my latest edits in reply#5.

[newey]

I thought what you meant was this:



The "always on" cap will act as a permanent bass cut (I think) and the rest is just a tone pot circuit. But that's more of a guess on my part so let's get a second opinion.

[newey]

The other alternative would be this. where the "always on" cap is "across" the pickup, like a tone control permanently turned down.



But, again, let's get a second opinion before surgery!

Of course, we haven't shown the rest of the circuit. The hot wire also (presumably) comes from, or goes to, a volume pot, which is in turn connected to the outjack tip.

[roadtonever]

I pulled out my alligator clips and followed your second diagram. It works!
The spare no-load pot I used also worked as I'd hope by letting the "always on" cap through alone. You're awesome, Newey!
Initial impressions are a very bad-butt tone and a subtle but useful range of adjustment. If want to know about the method by which I determined the required cap values for the tone desired I used this handy online LC resonance calculator: www.calculatoredge.com/new/resonance.htm The "always on" cap established a 4kHz resonance, brassy/twangy/harsh tone resulting, while the other cap set the variable falloff at 2kHz which makes for a rather aggressive growl.
That said a radio tuning cap is a nice tool to determine ones favored tone as you can go by ear.

Thanks for helping me realize this idea!

[JohnH]

Its good that you like the results. What cap values did you settle on?
J

[roadtonever]

100pF "always on" and 820pF "variable". I got more range with the pot when tried 82pf and 1000pF, with the values I have right now I mostly leave it at 10. The pickup is 5.0 Henry and the cables capacitance is 230pF.
With the resonance I set, using a no-load the resulting tone from activating the bypass is really a special FX tone, maybe something to terrorize your bandmates with or becoming the envy of country musicians ;P Maybe with a 500k+1M pot one could get a decent bright yet not over the top sound, maybe not I'm just speculating on that.

I could have used just one 1000pf cap and this is the way I tried it at first. The benefit I hear with the "always on cap" is a nice slightly darker yet "bolder" characteristic with the pot on 10 and also a clearer tone when combined with the neck pickup in series(mini half-parallel).

If you're curious to know the 3.2 Henry neck pickup right now gets 230pf capacitance from the cable alone, no tone pot, resonance end up at 5.9kHz. I'm liking the way it sounds.

[roadtonever]

It dawned on me a large part of what the circuit achieves is simulating the electronic properties of a hotter pickup. Perhaps something worth trying if for example you have a Strat of LP with a set of traditional wound pickups and you're considering a hotter bridge position.

[sbgodofmetal]

Jun 11, 2011 18:28:26 GMT -5 roadtonever said:

It dawned on me a large part of what the circuit achieves is simulating the electronic properties of a hotter pickup. Perhaps something worth trying if for example you have a Strat or LP with a set of traditional wound pickups and you're considering a hotter bridge position.

This sounds very interesting, and may be a way to get some usefullness out of el' cheapo pickups without breaking the bank to replace them.

[ashcatlt]

Exactly what is the difference between replies #4 and #9?

[newey]

Good point, there isn't any difference, so long as the pot shell isn't grounded on #4.

[Glass Snuff]

roadtonever, I'm glad you're happy with what you've got. You've "discovered" using a small value cap to adjust the fundamental resonance of a pickup, a trick that doesn't get enough press, IMHO.

To others attracted by the thread title, newey got it right in post #6. Basic EE tells us that any combination of capacitances and resistances will sum, and you can recreate the action of any complex tone network with a single cap and pot of the right values.

I wish it were otherwise, I've been stubbornly revisiting this idea over the years, but you can't change a cap's value with a resistor - you need a variable cap.

[sumgai]

Jun 12, 2011 7:31:43 GMT -5 Glass Snuff said:

..... you can't change a cap's value with a resistor - you need a variable cap.

Either than, or one of these little goodies.....

Wikipedia entry on Switched Capacitors