Help debug my schematic, maybe biasing issue?

[winslowj]

Yo! I've been designing a distortion pedal, and got it sounding perfect on my breadboard, but then sketched & ordered some pcbs and it's acting up. I'm only testing the first 2 sections right now, but basically when I plug it into my amp, turn everything up all the way, and play my bottom E string, it produces a super low rumble. That's pretty much the only sound I can get out of it at all. I once had a similar problem that I fixed with editing the bias, but I'm not sure exactly what to do in this scenario. Here's the schematic:



The 9v wires are reading correctly, and the powered side of R15 is reading ~4.5v, but the other side is reading ~3v. The positive input on the op amp is reading the same, the negative input is reading ~1.2v, and the output is reading a whopping ~8.2v. Does anyone know what could be going on? The only thing I've really tried so far was taking out R5, cuz I was afraid it was making a voltage divider with R15, R6, and R7. Didn't really change anything.

Edit: I updated the schematic to the most recent iteration. Sorry about that.

[iainpunk]

welcome to the forum

what are the JFET voltages? JFET's are notorious for not being consistent, one JFET can bias perfectly with a certain set of resistors while another JFET will totally not work at all, only passing signal through the parasitic capacitance.

do you have an audio probe??? if not, i recommend making one


cheers, Iain

[antonis]

If Q1 is intended for buffer use, get signal from Source..
If it's used as it is, interchange R20 & R3 values (as a starting pont..)

For both the above cases, make R10 value 1M, C11 value 10nF and delete C10 & R2..
(if you wish to keep the R2/C1 LPF, place a 2.7pF across R8..)

Also, delete R5, R6, R7 & C3..

P.S.
Too many consecutive LPFs/HPFs result into loading interaction..

[winslowj]

Oo, I should totally build a probe!

One thing I forgot to mention, I switched R3 (my source resistor) to a 330 ohm resistor. My drain is at 5.62v, my source is at 0.72v, and my gate is at 0v.

[winslowj]

Wait! I just realized that this is a slightly old schematic! There's now a 50k trim pot going into the Drain, should really be 10k or maybe lower. Sorry about that.

[winslowj]

Antonis I'll try that out, I might try just bypassing most of the filters just to see if that helps out

[antonis]

DELETE R5...!!!
(or place a cap somewhere between R5 upper leg and R7 right leg..)

As it is, R15 & (R7+R6+R5) form another Vbias voltage divider..

edit: Plz post a schematic of final circuit 'cause we're confused a bit..

[winslowj]

Updated! I'm very sorry about that. I'm gonna mess around with the filtering section right before the op amp for a bit, see if it is just creating a divider or something like that.

[m_charles]

or just bail the jfet, us a dual op amp, and grab the other half as your buffer. less parts too

edit: Use the other half as your booster. less parts too

[winslowj]

So, I took out R5 and bypassed R6, C3, and R7, and nothing happened. No sound came through, and the voltages on the op amp were all still wonky. I also tried bypassing the jfet section completely, no dice.

m_charles, I tried that out while breadboarding and I did dig it, but I was just personally a fan of the jfet boosting system. But I really think that the section after the jfet is the problem, I tried bypassing the op amp section (everything after C2) and the jfet boost worked like a charm. That on top of the op amp having wonky voltages, I think it really is something between C2 and the diodes.

[m_charles]

you have a boost going into a mxr dist + so its pretty straightforward.
R15 should be much higher I think (its hard to read on your schem). Def start with 1M.
R6, I don't see a need for that, I'd just bail it.
C8 I'd raise to 10 or 22uf just for the heck of it.
C6 should be nowhere near .1uf. Thats going to kill the whole signal and make it sound like a pillow is over the speaker. It'd be better to have nothing or a .001uf (470pf maybe). change that first.
start with those things...

[winslowj]

The thing that's confusing me the most is those voltages around the op amp - like 8v on the output?? 3v & 1v on the positive & negatives?? it doesn't make any sense, could it be that I just have a bust TL071?

But thanks for those tips! Kinda embarrassing, but what exactly does R15 do? I feel confident about everything else in the pedal, but why does R15 have to be so high?

[willienillie]

could it be that I just have a bust TL071?

It sure could.  Maybe it was damaged somehow, maybe it's a fake.  If you purchased it on Ebay, Amazon, or Alibaba, probably the latter.

what exactly does R15 do? I feel confident about everything else in the pedal, but why does R15 have to be so high?

I would call it a "bias feed resistor" but there's probably a better name.  In the MXR Dist+ it contributes to the input impedance, and that's why they used a 1M (I think, someone will correct me if I'm wrong).  Here it's not so critical, because you have another stage before it.  I'm not smart enough to tell you what the best value would be, but 470K will function fine, and it's the same value as used in the DOD 250, which is a variation of the Dist+.  If you're happy with the sound, use it.

[m_charles]

The thing that's confusing me the most is those voltages around the op amp - like 8v on the output?? 3v & 1v on the positive & negatives?? it doesn't make any sense, could it be that I just have a bust TL071?

I doubt you fried the op-amp. they are much tougher than you'd think, and in this circuit, even a crappy knock-off will work. 99% chance you just wired it up wrong. something isn't getting ground, something is shorting something it shouldn't be, cold solder joint, etc.

Bottom line, your VB should read 1/2 the battery or power supply, period. Most often 4.5v obviously.  even with no opamp, that should still be the case because if your voltage divider is correct, then it does just that. Divides the voltage. If that point isn't reading 4.5v (approx) then the whole thing isn't going to work right.

But thanks for those tips! Kinda embarrassing, but what exactly does R15 do? I feel confident about everything else in the pedal, but why does R15 have to be so high?

Sorry man, there are pages and pages about that topic, too much to rewrite here. Find a break-down of how an MXR dist+ (DOD 250) circuit works is a good place to start.

First make sure everything else is working, let us know how it goes...

[duck_arse]

are you currently debugging a breadboard circuit or a pcb circuit? if the former, can we see pictures of what you have built, please? if the latter, then, as someone once said, can we see photos of your build, please?