troubleshooting this guitar effect circuit

[egasimus]

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.

 

[FvM]

I have, however, replaced the 10k pot and the 15k and 51k resistors with 100k, 150k and 510k.

Must be expected ro increase the noise level by some amount. Another noise generator is however the 1M series resistor in the circuit input.

Observing around 4.5V (mid-supply) at the OP outputs would indicate correct biasing. Input terminal measurements may be wrong due to instrument's resistance loading the circuit. Low volume isn't quite understandable, because the circuit should have a gain > 1.

 

[egasimus]

here's my schematic. a bit crammed, sorry for that.

 

[FvM]

Yes, that's what I understood from the link and your additional despription.

 

[egasimus]

Are r5 and r6 ok at 1Meg? I still fail to understand the concept of impedance in its full, but there's a difference between a 15:1000 and a 150:1000 voltage divider... Maybe that's where the signal gets attenuated, and I get a volume drop? This is just guessing, of course. But when audio-probing the circuit, the volume starts to drop exactly around the junctions of the 150k resistors.

 

[FvM]

The 1 M parallel resistors to ground have only small effect on the signal transmission, their purpose is to charge the coupling capacitors to avoid loud plops when plugging or unplugging the effects.

I must confess, that I don't understand the relay circuit you added to the original design and can't follow the signal path.

 

[egasimus]

Okay, so I'm going to leave them alone for now. You can ignore the relay stuff, just imagine that there's a pot connected between A and B with its wiper to ground

 

[FvM]

You can ignore the relay stuff.

Then the circuit is supposed to work and to allow an overall gain of about 0.1..10. Some noise is however involved by the resistor dimensioning and the operation principle of the input stage. If the gain is considerably lower, wiring errors or defects should be assumed.

 

[egasimus]

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 for the advice, you've given me some useful insights. Now, I'm going back to the relay schematic, 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... Cheers