aion QUARK Manuel - Page 5

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BUILD NOTES

EQ switch

The original OD-880 uses an op-amp low cut arrangement similar to the Tube Screamer. The EQ switch

changes the resistor (which sets the gain ratio) and capacitor (which sets the low-cut frequency) for

alternate values. The values in the parts list work well as a starting point, but you can tweak if you want.

• For more available gain in the modded EQ mode, reduce the value of R9 (e.g. 2k2).

• For more bass cut (i.e. less bass), reduce the value of C4 (e.g. 100n or 220n).

• For less bass cut (i.e. more bass), increase the value of C4 (e.g. 470n or 1uF).

Make sure to use an on-on toggle switch for this since there is no center-off mode as with the other two

toggles. The center position of a center-off switch would cut out the signal.

Treble switch

The original OD-880 has a steep treble cut after the clipping diodes and no discrete tone control. The

default treble cut works pretty well, but you may find you need more for certain guitar & amp combos.

Or, you may want more treble on tap so you can turn it up and down directly from the guitar.

The stock capacitor value is 4n7. This has been modified so that it has 1n2 and 3n9 capacitors in parallel

with an always-on 820pF capacitor, which creates 3 different settings: 4n7 (stock), 2n2 (less treble cut)

and 820pF (least treble cut).

By tweaking these three values, you can end up with different modes. For example, by using a 4n7

capacitor instead of 1n2, you can have one mode with more treble cut, meaning there is one setting

above and one setting below the stock mode.

C5 capacitor

The original OD-880 does not include a feedback capacitor in the op-amp gain stage. This capacitor is

found on nearly every similar drive circuit that came afterward and is seen as best practice to limit the

bandwidth of the op-amp for better stability. It's extremely subtle and has nearly no impact on the audio

frequencies except at higher drive settings.

QUARK VINTAGE OVERDRIVE