Fuzzstain (Bernardi)

[jimbeaux]

I'm in the process of building Bill Bernardi's Fuzzstain; however, I'm a little confused about the wiring of the rotary switch  (S1).


* Are both wiper contacts (a & b) of the rotary switch bridled together and connected to the junctions of R13, pin 1 (IC1D), C7, R14 and the LEDs in the optoisolators ?

* edited

[Mark Hammer]

The rotary switch is not critical to the functioning of the circuit, although I suppose the 4 presets provide a convenience.

If you look at the circuit for the Marshall Bluesbreaker pedal (and King of Tone, JHS Morning Glory, yadda yadda yadda), you'll see a feedback resistor in the clipping stage, and a 6k8 resistor in series with the clipping diodes,  That resistor "softens" the clipping, bu providing some reduction in negative feedback when the diodes start to conduct.  IN the Fuzztain, R18-R21 do the same thing.

At the same time, R13-R15, and R17 provide the input resistance to IC1D that helps set the gain.  The way the switch is set up, there is a default input resistor of 1k (R13), and R17, 15, and 14 place another resistor in parallel with R13 to reduce the effective input resistance to IC1D and increase its gain.  So, to calculate, in the full clockwise switch position, R15 is placed in parallel with R13, yielding a combined parallel resistance of 404 ohms.  Along with the 1k5 feedback resistance of IC1D, that gets us a gain of 3.7x for IC1D.  At the same time, as rotating the switch turns up the gain from 1.5x to 3.7x, increasingly larger resistors are placed in series with the 2+2 set of 1N4148 diodes.

I'm not sure what the logic underlying that approach is, and would probably have to read the original article to grasp it.  BUt there is absolutely NO reason why you could not substitute the left-hand side of the switch with a 390R resistor in series with a 1K pot, and the right hand side of the switch with a 100R resistor in series with a 2k pot.  This would not provide he convenience of the 4 presets, BUT all the sounds of the presets would be attainable with some knob-twiddling, in addition to achieving some other tones that the original lacked.

[idy]

Yes, the two "wiper contacts" are joined.

[jimbeaux]

- Both answers much appreciated !