Stratobooster mod help???

[jallenfuzz]

I was interested in taking an alembic stratobooster circuit board from GGG and adding a tone filter cap to cut out the low end via a switch. To allow me to toggle between a full stratobooster boost and a treble boost.  I was looking for some advice.. I was thinking to put a .0022 or .0068 cap between the input and the transistor.  I would be switching between no cap and cap.  Does this sound like it would work??

[Mark Hammer]

The pot and 10uf cap to ground provies a path to ground for audio frequencies that bypasses the 12k resistor.  The lower the resistance of that  "alternate" path, the more current is pulledfrom V+, and the louder the output.  Use of a 10uf cap makes that happen for pretty much the entire audio spectrum, producing a full spectrum boost as you reduce the resistance setting of the 10k pot.  If you make the value of the cap smaller, however, that gain is applied to higher frequency content only.  Where that gain starts being applied is a function of the cap value.

So, let's say you had the following:

- the same 10k pot goes from the drain of the FET
- a 1k resistor and 470nf cap, connected from the ground end of that pot to ground
- a 33nf cap from that same end of the pot to ground

So, instead of a large value cap to ground, whose path resistance is varied by the pot, we have an arrangement whereby there is always a slightly higher resistance for a medium-value cap to ground, and a second smaller-value cap to ground.  As before, reducing the resistance setting of the pot gets you more gain, but the most gain is applied to the upper mids and higher, with some additional gain being applied to the lower mids, and little or no gain applied to the bottom end.

I'm just guessing the values here, but they should be in the ballpark.  Feel free to experiment.

[Gus]

A link to the circuit does help I did a search and found the circuit.

Search for Doug H range control
OR
To get a treble boost with a 1.5meg input resistance you will need a small cap. 
Looking at the schematic I think you used, R6 is not only switch pop it is part of the biasing when the gate does not have a DC path to ground
1/(2 pi RC) for the frequency.

say you want 1KHz
 C =1/(2 pi 1.5 ^6ohms 1^3 Hz) = about 1^ -10 = .1nf about 100pf

The way the circuit is DC coupled you might will get a pop when you switch the cap in and out.  I would always have the input cap in circuit and bypass it with the switch for full range

You can recalculate the cap value for R6 1meg gate to ground or 470K etc  and/or select a different - 3dB frequency.

Be aware if this is mounted in the guitar you are missing the cable capacitance that forms an EQ between the input of the booster and the LRCs of the passive guitar.