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I'm building a circuit that allows me to blend a guitar effect in and out, based on >this< (page 2, bottom circuit). I have, however, replaced the 10k pot and the 15k and 51k resistors with 100k, 150k and 510k.

The blending works fine, however the volume is too low, and the blend pot is prone to noise when in the middle position, which makes me think of an impedance problem. Do I have to replace the 1M resistors at FX in and FX out with 10M ones?

Also, the circuit clips quite early. I checked the voltages at the opamp's pins, and, while the second half is OK, the first half has 3.5 at the non-inverting input and 4.5 at the inverting one and at the output. Those should all be within 10mV from each other, right? Try as I might, I couldn't make it right.

I'm in kind of a hurry, and this pedal has already taken me more time than I could possibly give. I'll be immensely thankful if somebody pointed me in the right direction.

Here's my schematic. Right click and choose "view image" to see it in full. schematic

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  • \$\begingroup\$ "Prone to noise"? Do you mean prone to interference or hum? When a pot is in the center position, the wiper is at its highest impedance relative to other things, so it's easy for other things to capacitively couple with that node. The input and output resistors are just bleeders for setting the DC bias to 0 without loading down the guitar. Leave them as they are. \$\endgroup\$ – endolith Mar 22 '11 at 16:14
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This may seem like a simple answer, but use the 10k, 15k and 51k resistors!

I would expect that to fix your problem...is there a reason for using the much larger resistors?

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  • \$\begingroup\$ Yeah, so I can use the circuit in conjunction with a 100k expression pedal. Isn't there a way to use arbitrary values? \$\endgroup\$ – avramov Mar 22 '11 at 12:19
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    \$\begingroup\$ Unfortunately using arbitrary values will change the way the rest of the circuit behaves. You could recalculate all the values to ensure voltage/current are still in the right ranges. Or you could be better off altering the resistance of the expression pedal - bring it down to 10k, that way the circuit remains as planned. \$\endgroup\$ – Rory Alsop Mar 22 '11 at 12:41
  • \$\begingroup\$ A 11k resistor in parallel to the expression pedal wouldn't work, would it? Otherwise, any hints on modifying the circuit? \$\endgroup\$ – avramov Mar 22 '11 at 14:11
  • \$\begingroup\$ So wait, you replaced the 10k mix pot with a 100k expression pedal? \$\endgroup\$ – endolith Mar 22 '11 at 16:11
  • \$\begingroup\$ Well, a 100k linear slider pot for the time being. AND the neigbouring 15k resistors with 150k. AND the opamp's feedback resistor with 510k, just as described in the document I linked to. As I said, I was in kind of a hurry, and hoped it would work like this. Now that it doesn't, I'll try with the original values and see what happens. \$\endgroup\$ – avramov Mar 22 '11 at 17:43
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This morning, the device works, as far as I can tell, just as expected. I think it has to do with extensively bathing the PCB in alcohol. I use this flux which is conductive and has proven quite hard to remove, and it's also called "FU". No, really. The 3.5V at the + input of the opamp is probably due to my DMM's input impedance. Clipping is because of my amp, and the volume is OK. Made a lot of silly mistakes, haven't I? There's still a lot to learn before I can call myself an electronics engineer.

Thanks everyone for the advice, you've given me some useful insights. Now, onto the bottom half of the schematic (relays), which I built beforehand, which previously worked, and which is probably going to give me some more hell until I get it to do what it has to...

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