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previous: nic Resonance control -- 11/29/1999 5:00 PM View Thread

Circuit Analysis (long)

11/30/1999 2:48 AM
Joe Fuzz
Circuit Analysis (long)
Hi, nic. I saw your post earlier but I wanted/needed to get home and look at my notes. I built this and modified the &*%! out of it.  
The Mutron III is an ingenious use of a two-integrator-loop second-order filter circuit. I found the circuit snippet in "Microelectronics" by Sedra & Smith.  
I'll be referring to the following diagram on CJ's site:" target="_blank">"> (It might help to have this schematic printed out and on the desk in front of you as you read this.)  
Your first question: What does the "peak" knob control, the "Q"?  
Simply put, yes.  
The actual equation for Q is:  
(R9 + RPeak Pot)/R5 = 2Q - 1
Again, this is referring the CJ's schematic. I'll leave you to solve the above for Q. But you can see that varying the Peak Pot will change Q.  
The ingenious part comes in when you look at the equation for the resonant frequency wo. The equation for the resonant frequency is:  
wo= 1/CR
where "C" = C5 || C4 = C7 || C6  
and "R" = R10 || RLDR1 = R12 || RLDR2  
where the symbol || is shorthand for "in parallel with."  
In other words, in order to change "C" you have to change the capacitance in both of the feedback loops for A3 and A4, theoretically by the exact same amount. (That's the reason why a dual switch is used here - if you switch in C4 you must also switch in C6 at the same time.) Same thing with "R" - it's not enough to only vary the resistance between A2 and A3, but the resistance between A3 and A4 must also be varied, theoretically by the same amount (although R.G. contends - and I agree - that there's a little play here in the tolerance. Well, okay, a lot of play).  
So what the designers of the Mutron III did was to vary the resistance "R" based on how loud the signal going into the box is. By varying the resistance "R" at this key point in the filter, they change the resonant frequency, wo. The louder the sound is, the brighter the LED lights up which causes the resistance of the two LDRs to go down. So, "R" goes down and wo wahs open. As the signal fades, the LED gets dim, the resistance of the two LDRs goes back up again, "R" goes up, and wo wahs closed. Neat, hunh?  
This should give you some ideas for modding. When there's no signal, the "R" is primarily determined by R10 and R12 so varying these might be something to look at (i.e. a dual pot so you can vary both equally). When there's a loud signal, the "R" is primarily determined by RLDR1 and RLDR2 so you could say that the LDRs (and the LED) determine how "wide" the wah opens.  
This should get you started!  

Mark Hammer Nice reply Joe. Clear and helpful... -- 11/30/1999 4:35 PM