Help with EHX Knockout mod

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lykwydchykyn
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Post by lykwydchykyn »

So I acquired an EHX knockout attack EQ with the idea that I could mod it into a multiband distortion. As you can see from the schematic, the thing is a couple of filter networks that can be blended with the dry signal.

My thought was to drop in clipping diodes at the bases of Q1 and Q2, thus keeping the clippers isolated to each band. Problem is, the AC voltage at that point is SUPER low (~100mV on the high and clean channel on my humbuckers if I really spank the strings, more like ~300mV on the low channel). My lowest Vf diode on hand is a 5819 with Vf ~190. So, not much clipping going on.

The levels seem odd to me since my guitar will output ~200mV of just raw output, and the pedal has a voltage converter to run at 18v. You'd expect much higher voltage levels going on inside this thing.

Tried experimenting with some soft clippers in the op-amp feedback loops, but this would be harder to solder in. Apart from locating super-low Vf diodes, any ideas on how I could make this thing do some new tricks?

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stolen
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Post by stolen »

Hi!

IC1a/IC2b add gain on the respective filter before hitting the buffer transistors. We'd leave the diodes where you had them, clipping base of both transistors towards ground, and just increase gain of the OPAs. Gains are defined by (R10/R9+1) and (R20/R19+1), crank the gain of both to about 100 and you should get some OD with most guitars, for example with R19=2.7k and R20=270k. Trimmers/Pots might make sense here.

The low volume behind the filters is normal; they do filter out pretty wide bands, so even with the ~x10 stock OPA gain there's some loss expected.

For the low channel it might make sense to put the diodes at the r11/r12 junction instead of its transistor base so that there's some filtering happening after the fuzz to reduce its overtones, but that's a matter of taste.

All the best,
stolen

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