Coupling caps when bypassing the EQ

[ted01]

Hey all,

If I was to add a switch that would let me bypass the EQ, would I need to add an additional coupling cap? In my crudely drawn addition you can see that I'd like to install a switch that would let me bypass the EQ. Is the 330pF cap (paralleled with the 220K resistor) sufficient, or are the caps in the EQ part of the DC blocking stuff. If I need to add a cap (the one with the question mark next to it) what should it's value be?

Ted

[Bob-I]

Check out the Dumble files section for ODS101 schematics. You'll see another way of doing this by disconnecting the bass pot. It's very effective, adds a nice boost and mids to cut through for a solo.

[ted01]

I see what you mean. However, I'd prefer completely bypassing the EQ for analysis purposes.

[Bob-I]

I see what you mean. However, I'd prefer completely bypassing the EQ for analysis purposes.

Then yes you need a coupling cap. I'd go with .022uF

[Firestorm]

Yes, you do need another cap to block the DC. The new cap, along with the 1M volume pot forms a high-pass filter with a frequency of 1 divided by 2*pi*R*C. So you need to make your cap big enough to allow all guitar (or bass) frequencies to pass unattenuated (and to swamp out the influence of the source impedance, which is pretty large anyway). A .02 cap looks like it would be enough for guitar, but I'd just go with a .1 and eliminate any phase shift at the relevant frequencies.

[butwhatif]

There are too many variables to consider, along with all the hypothetical consequences. You'd be best to hook it up to a cab, and use a guitar that you like to play that has a possible big bottom sound, like a LP with a big HB pickup in the neck pos. Then you can evaluate what cap will give you a playable sound without a big messy swamped out bottom that you can't deal with. My guess it would then be something in the range of .001 to .05 mf.

[Firestorm]

My guess it would then be something in the range of .001 to .05 mf.

For "anaIysis purposes," I assume you want to eliminate any frequency attenuation. In this circuit, a .001 cap will produce a filter with a cut-off frequency of about 160Hz (roughly an "E" played at the second fret of a guitar's "D" string; with a .02 cap, the cut-off frequency will be more like 8Hz; with a .05 cap, the cut-off frequency will be just over 3Hz. The source impedance will have a miniscule, but measurable, effect so you want to eliminate it, too. I'd use at least .05 to see the effect of eliminating the tone stack.

That said, if you go back to Bob-I's initial response, you can almost completely eliminate the effect of the tone stack by lifting the ground connection at the bottom of the middlle pot. Just put your switch there and make your life easier.

[jaysg]

I did this in my F2-B clone. 0.047uF in series with a 10M which gets the switch across it. Cap attaches to the plate side of your 330pF and the resistor attaches to the other side of the 300pF. I didn't bypass the treble pot, fwiw.