Update: The simulation failed because I don't have enough memory? I tought that was a small circuit. Anyone knows why the simulation fails just by changing one inputs freq.?

 

I really don't know why, but by changing the capacitor before the opamp from 0.1u to 330n and my circuit now simulates. Everything seemed fine this morning, I'll be ordering the components tonight, just after doing an AC sweep and a Monte Carlo analysis. I will breadboard it and see if I can get any sound from it... I know frequencies over 5kHz won't sound great, but at least I want to test the functionality first.

 

Any suggestion for the 1/4" mono female plug on what part I should get? I was thinking about 3x Switchcraft's SC1121-ND ([URL]http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=SC1121-ND[/URL]). Any other suggestion here? Everything in the end will be soldered to a perf board, so I'm ordering a DIP for the TL072, trough-hole for the J201. I'm I forgetting something?

 

Good news! Everything is up and running, thanks to those who gave me some advices! A small thing though: When I soldered my circuit, I wanted to test if the sound was going through everything. At that moment, only the first input/preamp/mixer/output was done. I played through the circuit with no problem on my electric amp. I then built the missing input/preamp to the mixer. When I tried the circuit again, an almost constant electric noise was coming out of the electric amp. I then tried on a tube lamp with two guitars, no electric sound. Finally, I tried back with one guitar on my electric amp: the problem is back. It seems like a problem interfacing with the electric amp. The thing is, it worked fine before I built the second input/preamp.

Any suggestion on where can that sound come from?

 

The mixing resistors R7 and R8 and the coupling capacitor C6 to the input of the mixer IC must be very close with short wires to the mixer IC's input pin or the wires will pickup all kinds of interference.

The cables from the guitars must be shielded audio cables with the shields connected to a ground point near the inputs of the preamp.

 

But why does the same circuit work with a tube amp?

 

Maybe your circuit causes a ground loop, that would make a mains frequency buzzing sound.

 

I tested the voltage from the gate of one of the JFETs and that amplified the buzzing sound. So I unsoldered and soldered again that gate with the rest of the circuit. Next thing I know, the thing works on one input. I played a while with no buzzing sound. I tried the other input, the buzzing sound came back, and on both input this time...? I tried your suggestion of limit the wires before the mixer's cap, but no luck there. The gates of the other JFET now amplifies the buzzing sound in the amp if I test with my voltmeter.

 

I think the input that buzzes has a poor ground connection on its input cable.
If you touch the input with a lead from your voltmeter then it is normal for the circuit to buzz because the lead picks up mains hum like an antenna. Shielded audio cable has the grounded shield to block hum pickup.

 

I think the input that buzzes has a poor ground connection on its input cable.
If you touch the input with a lead from your voltmeter then it is normal for the circuit to buzz because the lead picks up mains hum like an antenna. Shielded audio cable has the grounded shield to block hum pickup.

I think I got it. The two phone jack inputs are out of the circuit, with wires "far" from the circuit board. When I plug only one guitar, the other one creates an open circuit. Since there is a wire going to the input, this one is un-grounded and must act like an antenna like you said. If I ground that wire, the buzzing sound is gone. If I plug a shielded jack in the second input, the buzzing sound is much lower. When my brother comes with his guitar next time, I'll try it with both guitars in, see if that was the problem. Thanks a lot for the help Audioguru, I'll confirm later if the circuit works fine so that those looking for a simple cheap way to play on a single input amp will have one-a-many solution.

 

You must use shielded audio cable, not just a shielded jack.