blend control caps

[peterg]

On circuits that that have a blend control there appears to be two caps after the Q1 emitter - one towards the control and one towards the remaining part of the circuit.
Attached is a sample. Why not split the circuit after 1 cap? Or are different value caps needed for the wet vs dry sounds?

[GibsonGM]

Sorry, your pic didn't come thru. You have to upload it to a place like Photobucket, then put a link to it in your message here.
Need a pic in order to know just what we're dealing with )

[peterg]

Dropping the scheme. I notice a note about not posting them without written permission on the site I got it from.

[GibsonGM]

On circuits that that have a blend control there appears to be two caps after the Q1 emitter - one towards the control and one towards the remaining part of the circuit.
Attached is a sample. Why not split the circuit after 1 cap? Or are different value caps needed for the wet vs dry sounds?

Ah, pretty cool circuit, huh?     Looks to me like they are tapping off the E of Q1 and 'calling it good'.  This is a clean signal.  They also split the signal here, and run it thru the rest of the stages to do more shaping on it.   

The additionally modified signal "rejoins" our tapped-off signal at the blend control. 

I see 2 reasons to have two caps at the emitter of Q1:  first, to block the DC bias voltage flowing in Q1, keeping it off the blend pot and out of the rest of the circuit and second, to shape the signal a bit (admittedly, 5uF isn't going to trim much bass, though!!).   

[duck_arse]

the Q1 emitter has "a" voltage level on it, the Q2 base has "another" voltage level on it, and the blend pot is ground referenced (yet another voltage level) at both ends, as it's connected to output. so you need 2 caps to keep the 3 V levels apart.

[peterg]

https://www.dropbox.com/s/4k6wkykz176v2vw/fuzz%20schematic.PDF

I created the attached circuit and it seamed to work on the breadboard. Do you see any issues with it? I'm guessing there could be concerns with there not being a cap between Q3's collector's cap and Q2's base. Is the output at the end of the circuit being fed back into Q2?

[duck_arse]

there will be a dc level, however small, coming from the base of Q2, appearing on VR3 and VR4, which will be bad for your following input. maybe even "crackle ok" pots. how come you used such a big R15? I would guess that would mean your mix knob is always at least 750k/2M+750k away from the Q3 output. my maths might be wrongly expressed.

[peterg]

there will be a dc level, however small, coming from the base of Q2, appearing on VR3 and VR4, which will be bad for your following input. maybe even "crackle ok" pots.

Thanks. I see that now. I'll throw a cap in between Q2 base and VR3

. how come you used such a big R15? I would guess that would mean your mix knob is always at least 750k/2M+750k away from the Q3 output. my maths might be wrongly expressed.

I worked out the R15 value by trial and error. A large value was needed to get as close to a clean sound as possible when the pot is dialed completely CCW. With a smaller resistor I wasn't able to completely filter out the fuzz even with a 2M pot.

[GibsonGM]

the Q1 emitter has "a" voltage level on it, the Q2 base has "another" voltage level on it, and the blend pot is ground referenced (yet another voltage level) at both ends, as it's connected to output. so you need 2 caps to keep the 3 V levels apart.

"I see 2 reasons to have two caps at the emitter of Q1:  first, to block the DC bias voltage flowing in Q1, keeping it off the blend pot and out of the rest of the circuit"

[peterg]

Thanks Mark. I'm getting it slowly but surely.