Rangemaster variable capacitor ?

[timmyo]

I'm reading around stompbox building plucking up courage to get started and read this on the Beavis audio site:

Here's a trick to simulate a variable capacitor, especially useful for tone control applications. Attach two different capacitor values to a potentiometer--moving the wiper then sends more or less of the signal to one of the caps thereby changing the frequency response.

Now my idea for a build  is for a Rangemaster with cap options for hi/full range boost based on one of the caps in the circuit - would something like this allow for a sweepable rather than switch-based cap selection ? If so, can anyone throw a bit more practical light on what is being suggested here ?

thanks in advance,

Tim

[GibsonGM]

The input to the blending circuit (output of something else) is fed to the wiper of a pot (maybe 100k).   You put 2 different caps at each outer leg of the pot.  Use very different values, like 4.7uF and 400pF.  The 2 caps come back together in parallel, and that is the new output.  Place a resistor to ground here to complete the filter - the output is at the junction of the 2 caps and resistor (hope that makes sense!).   You'll have to play with the component values, start with maybe a 1K to ground and mess around.   

In conjunction with the resistor, you're making 2 high pass filters, and the amount of signal that each gets is a function of the pot's resistance.   As you rotate the pot, more of the signal goes to 1 of the caps.  As you approach the larger cap, more bass can go thru; the opposite is true for the smaller cap.

You can figure out the cutoff for each of the cap/resistor combinations using Freq=1/(2pi)RC.   By making the caps very different from each other, you get quite a large usable range...you'll have to tweek to see what it sounds like.

Good luck!

[John Lyons]

I think Timmyo is looking for an input blending cap which is two caps with a 100K linear pot between the input side of the caps.
You can experiment with the cap values. Putting a resistor to ground will cut treble. What the one below does is cut bass and low mids variably.

John     
                   1uf
               I---I I-------I
               I               I
               I---- I        I
   100KB   /      I        I
    POT     / <--I        I
               /               I
               /               I
               I               I
     IN       I-----I I-----I  OUT
                  .0047
     

[earthtonesaudio]

The other route is to get a genuine variable capacitor, and scale the input impedance of the circuit to get a usable range of cutoff frequencies.
Variable caps are scarce though, but if you find a very old, junked radio...

[timmyo]

so basically just parallel 2 capacitors, and have the 100k pot inline with one of them?

If so, I presume it matters which one you put the pot inline with? Or not ?

[John Lyons]

You would be lucky to find a variable cap that would even fit in an empty stompbox.

Timmyo
The pot is in between the IN side of the two caps and the output is taken from the opposite side with the two caps tied together.
On one side of the pots rotation you have two caps paralleled.
On the other side of the pots rotation you have 100K resistance between the In side of the two caps which is basically only having the small cap in the circuit as shown in my (crude) diagram.
You can between a treble and full range sound this way.

Email me if you want a schematic of this.

john

[mnordbye]

Haven't seen the rangemaster schematic yet, but for your information, Futurlec have som variable capacitors. The one with the largest value only has 10 to 90 pF though.

Magnus Nordbye

[earthtonesaudio]

An air capacitor will certainly be too big, but Sprague makes small, panel mount variable caps that will easily fit in a stompbox.
Look for GME11001, GME10301, etc.  The key is the letter "E" which specifies panel mount, 1/4" bushing.

...But using a variable resistor is easier and more practical.

Also you might want to check out this:
http://runoffgroove.com/omega.html
...which might be exactly what you're looking for.