Opamp motorboating in fuzz pedal

[RLawlor]

I've modified the mxr blue box schematic for just a fuzz without the octave but no matter what I try I can't seem to get rid of a motorboating noise. I've got the circuit on a PCB. I tried a resistor before the second opamp stage but still there was a motorboating sound. Any help would be much appreciated.

[duck_arse]

try this thread ....
https://www.diystompboxes.com/smfforum/index.php?topic=122775.0

[RLawlor]

That was actually a thread I started hahaha. I tried some of what was suggested and none worked and/or changed the sound beyond what I wanted. Thanks anyways

[antonis]

As far as I can see on your current schematic, you didn't try what suggested..

56k is still placed as shunt resistor and Vref cap is set across power supply..

What actually changed is "motorboating" instead of "oscillation".. 

[RLawlor]

Sorry I actually did put a cap from 4V5 to gnd I just forgot it on the schematic. I tried swapping the 56K but it made no difference and resulted in a worse sounding fuzz also. Just thought I wouldn't swap it on that schematic if it didn't work.

[RLawlor]

I tried them all, just didn't work so I excluded the changes from my schematic that's all. Sorry for any confusion

[bettsaj]

I had an horrendous motorboating issue on a power boost clone I built..... I was working on it for 3 or 4 weeks....... Ended up being Sky Q of all things.... not that this is your root cause, but just saying 

[amptramp]

The prime suspect is the 4VS supply.  You have a midband gain of 471 in your first stage so any variation in 4VS could cause oscillation from feedback through the power supply.  The second stage is a pure differentiator and that has its own stability problems.  Try putting the 56 K in series with the 47 nF instead of having it go to 4VS.  Otherwise the rising gain with frequency meets the falling open-loop gain at some frequency and the feedback becomes in phase, causing oscillation.  You also need some capacitance from the output of the second stage to the inverting input to get some feedback lead.

[duck_arse]

hmmmmm ......

I just went and looked at a circuit for "a" bluebox [tonepad] - compairing their circuit and yours shows you have your second stage gain parts in the (-) in feedback loop, whereas their circuit has it in the (+) in f/back loop. perhaps just shift the 22k from the -in to the +in pin. (and the electro across your Vb to ground.)

[RLawlor]

So I was messing around to see where the noise was and I clipped the lead of the 10uF capacitor in the feedback loop of the first opamp stage... and ... no noise. Also a completely different effect, more of a distortion than a fuzz. Any one know why removing the low pass filter would decrease the gain?

[amptramp]

So I was messing around to see where the noise was and I clipped the lead of the 10uF capacitor in the feedback loop of the first opamp stage... and ... no noise. Also a completely different effect, more of a distortion than a fuzz. Any one know why removing the low pass filter would decrease the gain?

Cutting the 10 µF capacitor drops the first stage gain from 471 to 1.  No wonder the noise disappeared.  This may appear to be a lowpass filter but it is actually something that cuts the feedback at midband and above frequencies so that the gain stays at unity.  The non-inverting gain is 1 + (Zfb/Zin) where Zfb is 470Kohms in parallel with 330 pF and Zin was 1 Kohm in series with 10 µF.  By cutting out the 10 µF, Zin goes to infinity and you are left with a gain of 1.

The differentiator in the second stage is still unstable - the 47 nF input capacitor needs some resistance in series.  The 56 Kohm resistor to 4Vs would be a good candidate if you disconnect the 4Vs end and put this resistor in series with the 47 nF capacitor.

[RLawlor]

I'll try swapping the 56K resistor again, but from memory it was as if the gain had decreased. I'll post after I change it.