Author Topic: Second build: still not working  (Read 6961 times)


Second build: still not working
« on: March 05, 2010, 03:19:37 PM »
Hi, I'm building this project for the second time (had the wrong op-amp first time). At first I thought getting the proper op-amp would solve my problem (no distortion).

I got it built the second time, and now I get a noticeable fuzz (a slight warmth), but it's nothing like the full-on distortion I've heard from the original DOD. I don't have RA pots, and I'm wondering if this makes a difference.

Would there be any way to substitute a resistor for the gain pot in such a way that I could get the maximum gain possible? (just to troubleshoot).

Also, one of the capacitors needed is 25pF (C3). I only had a 24pF and I'm wondering if this makes a difference.

Also, I'm not using an LM741 (digikey didn't have any), but the LM741CN which I understand is an acceptable substitute. I've noticed that this project leaves some of the pins floating, might it help to ground the unused pins?

I can make and post all my voltage readings if anyone is willing to help me troubleshoot this. I've read other people with the same problem, and people tell them to check the gain pot, but mine seems to be hooked up properly.

EDIT: Now this thing is getting all intermittent on me. I tried grounding some of the pins on the LM741CN (the unused ones), and I would get intermittent cracks of nice distorted signal, but it seems that this requires extremely hard strumming, and the effect dwindles to nothing when the strings volume goes below some arbitrary threshold.

Also, would having really long capacitor leads effect this circuit in any way? I don't feel like clipping them down till I know this works, but if it would impede the circuits ability to function, I'd consider clipping them.


Re: Second build: still not working
« Reply #1 on: March 05, 2010, 03:36:14 PM »
Measure the DC voltages at the op amp pins. They will indicate what's wrong.


Re: Second build: still not working
« Reply #2 on: March 12, 2010, 08:39:10 PM »
Part I have:

battery (out of circuit): 9.48 V
rail to rail (with battery): 9.48 V

IC (LM741CN)

(all measurements with reference to ground rail, nothing plugged in to mono in/out)

Pin # FunctionVoltage Reading
1offset null7.87 V
2inverting input3.86 V
3non-inverting input  6.40 V
4V-7.88 V
5offset null7.87 V
6output7.63 V
7V+9.48 V
8NC0.00 V

Thanks in advance to anyone who can spot an anomaly. I'm not that familiar with op-amps.
« Last Edit: March 12, 2010, 08:48:50 PM by stkmtd »


Re: Second build: still not working
« Reply #3 on: March 12, 2010, 09:34:58 PM »
Are you sure about this? OK, pin 4 should be ground - so zero volts. Start with that. What layout are you using?


Re: Second build: still not working
« Reply #4 on: March 13, 2010, 02:28:32 AM »
wow, I feel like an idiot. So easy to miss a jumper when the jungle of components slowly start filling up the breadboard. Everyone's sleeping now, will have to give it a spin in the morning :)

Good eye. Surprised I didn't catch it when I made the table. Guess I was frustrated.

EDIT: I followed the breadboard diagram stickied in the forum as closely as I could, but not close enough.
« Last Edit: March 13, 2010, 02:30:16 AM by stkmtd »


Re: Second build: still not working
« Reply #5 on: March 14, 2010, 03:11:14 AM »
Okay, now it works. One thing I notice though, if I run it into my amp's clean channel, I get a nice clear distorted tone.

If I pass it into another layer of distortion (built in to the amp), the signal seems to get compressed, and the noise floor comes way up. Is this circuit less noise prone when you get it on a PCB? I'm guessing that the breadboard is just acting as a giant antenna for noise to creep in, as all my patch cables are shielded. I also notice that playing legato isn't as clear, so maybe it's a signal strength issue.

Any expert advice in this area would be appreciated. This sound is nice and warm, but the noise is a bit of an issue.

And for curiosity's sake, what could I do to this circuit to create a heavier clipping? Would I have to get an op-amp that creates a larger gain? Or simply adjust the feedback loop somehow?
« Last Edit: March 14, 2010, 03:52:39 AM by stkmtd »


Re: Second build: still not working
« Reply #6 on: May 05, 2010, 10:49:33 PM »
most of that is probably just the noise that the breadboard is picking up that your amplifying, breadboarded distortion circuits are usually noisy on a breadboard, i'd build it. it will be quieter once its on a pcb and in a shielded box. if its breadboard noise, then your amps gain channel will also be amplifying that noise and it sounds like thats what your hearing, long smooth notes can sound like thy have interference, due to real high freq oscillation. hard to say how noisy it will be in the end, but it should be much quieter boxed up
nothing says forever like a solid block of liquid nails!!!