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mu-tron III

11/29/1999 12:54 AM
mu-tron III
What does the "peak" knob control, the "Q"?  
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11/29/1999 5:00 PM
Resonance control
I have noticed that other envelope filters have a resonance control. How could I add one of these to this circuit? Also, one of the caps in the follower circuit is responsible for the decay time is it possible to make it adjustable with a pots?  
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!  
11/30/1999 4:35 PM
Mark Hammer

Nice reply Joe. Clear and helpful.  
The state variable filter configuration used in the Mutron can be implemented in a few ways. The two LDR's are run in parallel with fixed resistors. As you note, the change in filter turnover/center frequency depends on the combined value of the LDR and resistor.  
Although the Mutron III has many features, one it doesn't have is the equivalent of an "Initial frequency" control. This can be simulated by sticking a dual-ganged pot in series with the fixed resistors. So, the signal would travel through each section of the dual pot, and then through the parallel combination of fixed resistor and LDR.  
Readers of Craig Anderton's Electronic Projects for Musicians will recognize the Super Tone Control as identical to the Mutron, but without the envelope control. One of the things that Craig pointed out was that the basic configuration of fixed resistors between stages did not permit a wide sweep. I forget the exact specs, but I think the gist was that simply varying the value of those fixed resistors could only get you a 10:1 change in centre frequency (e.g., a 300-3000hz sweep). If the fixed resistors were replaced with pots in voltage-divider mode, however, the sweep could be upped to either 100:1 or 1000:1 or something MUCH larger than the original.  
To do this on the standard state-variable filter, the fixed resistor would be replaced with a 1k resistor, and the output of the op-amp leading to that fixed resistor would be connected to the outside lug of a pot. The wiper would go to the free end of the 1k resistor, and the other outside lug would go to ground. Values of 10k-100k would work fine.  
Given the latitude in pot values, why not stick a dual ganged pot in place of the input-to-wiper segment of that configuration, and put the LDR's in place of the wiper to ground segment? The pots would essentially allow you to tailor range (you might need fixed resistors in parallel with the LDR's anyways). The response of the unit would certainly be different, so it might take some getting used to, but the possibility for wickedly wide sweeps is seductive. This might also mean no need for range-changing caps.

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