Tonepad Phaser DIY, Part 3

Started in on the mods to the phaser. As previously noted, it works well in stock form. Now I want to do the mod to correct the 'volume drop' issue.

Some background on this mod:  the 'color' switch on a Small Stone inserts feedback into the signal path.  The resulting tone is thicker, darker, and "whooshier" sounding.

Basically we're increasing the gain on the input transistors.  But increasing the gain means that the gain on the feedback loop is also increased.  If the gain of the feedback is greater than the original signal, it will, well, feed back.  There will be a screeching tone in the signal when the feedback is switched on.  The trick in the mod is to increase the input gain, but to tame the feedback gain so it remains below the level at which it actually feeds back and squeals.  There is a great explanation of how phasers work in R.G. Keen's article, "The Technology of Phase Shifters and Flangers."

The mod involves changing the value of two resistors - R11 (3.3K) and R42 (4.7K). If you search for 'Small Stone volume drop,' you'll find a lot of information on the interweb on this issue. Some recommend changing both resistors to a fixed value (generally 10K for both, but this can vary).

However, after researching this some, I decided to go with a 10K trimmer for R42 and a 10K linear pot for R11, which will let me fine tune the feedback, and offer (maybe) another tonal option.  The picture here shows R11 removed, and the location of R42.  It's going to be a tight squeeze to get a trimmer in place of R42.  I could put one external to the board, but I'd prefer to have one right on the board.

R42 is the input resistor, and R11 is in the feedback loop.  On the right, you can see the two pots that replaced the resistors.

To do the basic adjustment for the signal, I turned the R11 pot full up, and then adjusted the trimmer until it was just below the feedback level.  Then I can adjust the R11 pot for the actual level of feedback.   My plan is to mount the R11 pot on the top of the pedal.

I tested this for a bit, and I don't hear a huge tone change with the pot.  It seems to be pretty subtle.  I'd like to spend a bit more time with it before I wrap it all up, but I'm close I think.