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previous: Kremator Hi,that's true, but if you ... -- 11/2/2000 2:28 PM View Thread

Re: Mesa graphic EQ

11/2/2000 3:56 PM
Stephen Conner
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Re: Mesa graphic EQ
OK this is the schem for an op-amp gyrator  
 
 
 
 
 
 
 
and here are the design equations  
 
 
 
L=R1*R2*C  
 
Rdc=R1  
 
 
 
Normally R2 will be between 100k and 1M. R1 should not be less than about 50-100R. A good rule of thumb is to make C the same as the tuning capacitor for that band and then calculate R2 to get the right inductance. You can assume that the DC resistance is dominated by the 1K series resistor Mesa put in, so R1 will be between 1k and 1k5.  
 
 
 
Easy as pie ;)  
 
 
 
Regarding split supplies, in your case it's easier to run the op-amps off the same single supply as the rest of the EQ circuit. You don't need extra caps since the cap in the LC circuit already blocks DC. You just need a reference of Vs/2 which you can easily make with an extra op-amp and a potential divider.  
 
 
 
Also, I'd decrease the supply to -30V so you can use standard op-amps. (You see the same EQ circuit with -30V supply in the Mark IV so it's probably fine) TL074 or MC33179 are good quad audio op-amps. LM324 are a bit noisier but very cheap and easy to get.  
 
 
 
If you're determined to go for the full -56 then you should probably use discrete transistor gyrators instead. I can post a schematic for one if you want.  
 
 
 
Steve C.

 
Replies:
Kremator Hi again,If you'... -- 11/2/2000 10:07 PM