Quantum QM-168 EQ Mods

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Well-known member
Aug 8, 2009
Provo, UT
Hi All,
Seems that I'm not the only one with a quantum 168 board kicking around these parts so I decided the upload some info on the EQ circuit I'm modding.
Its a very simple circuit, and the idea is to add a potentiometer to sweep the frequency of the boost or cut. These mods are very simple and can all be done without take the board totally apart. You just have to lift the top and drill some holes for the pots and solder a few quick connections. there is plenty of room to mount the potentiometers.

Sorry, my laptop computer crashed so I had to use paint and the image quality is not very good. Also for the same reason I can't post a modified schematic, but the modifications are pretty straight forward os you probably won't need one.

here is the un moddified link to the schematic

Idea: we are taking out the resistor that goes to ground for each gyrator and replacing it this a series resistor and potentiometer. thereby changing the value of the inductor thereby changing the value of the frequency of the circuit.

I came to each of the values of the resistor and potentiometer by testing different values and watching the EQ graph realtime via Q-Clone in pro tools. They are picked for maximum frequency variation without  a big change of the Q or maximum boost/cut

Take note: I am not a qualified technical expert at this stuff and though the Eq modifications worked just fine in my console you might get varied results! But hey thats half the fun of doing things yourself.

Maybe someone could chime in and say what kind of taper would work best - log? anti-log? i'm not sure - log seemed to be working just fine

ok for the low band we are taking out the 180k resistor from the gyrator to ground and replacing it with a 120k resistor and a 1meg pot to ground. Also you want to lift the 360?K resistor when the eq is switched into low shelf mode. This will give you more variation at the corner frequency.

Replace 68k resistor from gyrator to ground with 8k in series with 250k potentiometer. this will give you Aprox 500hz to 2k in the 800hz switch latched and 1.5k to 4k with the 1.8k switch latched.


this resistor is  a little deeper in the pcb but still not hard to get to when you locate it. Its a 120k resistor and you want to replace it with about 30k in series with a 500k resistor. and there is no variation of the high shelf, because it is just a capacitor. 


Good luck! I still am ordering all the parts but will post how it goes when I finish it.