BMP OPAMP oscillating (answered)

[Anonymous]

I’ve just finnished building a BMP OPAMP version but it is motorboating (self oscillating in low frequency). I double checked the values of all components and everything is OK.

Any clues? Did anybody eles have the same problem?

Thanks…

[Barry]

Most anyone who has experienced oscillation in pedal builds should look for a bad solder joint somewhere. Short of using an audio probe simply re-flow all solder joints both on and off board including pots and jacks. They should be shiny and look like a tiny Hershey kiss. Beware of over heating.

Further help requires photos and voltages however I would certainly bet on a cold solder joint.

[Anonymous]

Barry,

Will re-check all the solder joints, although I have been soldin for more than 45 year, you never know if something went wrong.

I will post the results.

Thank you for the prompt reply.

M

 

 

[Anonymous]

Here is what I found:

• R3 was 33K instead of 330K (I found it yesterday but it did not affect the motorboating)
• Today :
  • there was a short circuit between C9 and R8 (I only saw it inspecting with a magnifying lens)
  • I was using a swith connected to A B and two sets of diodes. I removed it an put Si diodes on the board

My guess is that the motorboating came from the short circuit.

Another issue: I expected the pedal to be very loud (like the transistor BMP) but it is not. With Vol and Sus to the maximum it just a little louder that when bypassed. Is it like this or I made another mistake?

Best,
M

 

 

 

[Barry]

Absolutely not normal.
Some pictures would help to see what mods you made since you brought A/B Pads.
I also suggest an audio probe. See our Guides page for more tips.

[Anonymous]

I will re-check everything with the oscilloscope and so.

BTW… I did not use any A/B pad, just the A-B option in the PCB for the diodes. I abandoned it and put the diodes in teh PCB. Thera are no mods in the circuit now.

Thanks,

M

 

 

[Anonymous]

I double checked all the parts and the weldings. Both were OK.

Then I used a signal generator and an oscilloscope to investigate the gain stages. Here is what I found:

• input: 1 x 500 mV sine
• IC1A pin 1: 7 x 500 mV sine
• IC1B pin 5: 5 x 500 mV sine
• IC1B pin 7: 5.5 V square (BIG gain)
• IC2B pin 7: 7 x 500 mV square
• Tone pot, center tap : 2.5 x 500 mV (here is the big output drop)
• Out (max vol.): 2.5 x 500 mV (a little lower that the input)

No matter how high is the input signal the aoutput does no change much.

My conclusion is that the tone control drops the signal a lot and to get a loud output is necessary another gain stage like the transistor BMP or the Rat.

Any comments or suggestions?

Best,
M

 

 

[Anonymous]

Am I correct that you mean to say that your “1 X 500mV” is 0ne DIVISION at 500mV or 500mV?  Or “7 X 500mV” is 3500mV or 3.5 V?

If so, then IC1a pin 1 is 3.5V.  Compared to the input of 500mV (0.5V) this is a gain of 7.  The gain of IC1a is determined by R3/R2 or 330K/47K which is 7.

Likewise the output level is 2.5 x 500mV (1.25V) compared to the input level 1 x 500mV (0.5 V).  So you have measured an overall gain of 1.25/0.5 or 2.5.

The drop in level through the tone section is normal since this is a passive tone section.

Unfortunately, these measurements are only valid for the specific frequency of your sine wave.  This circuit famously has a mid frequency “dip” as well as roll-off at the bass and treble frequencies.

Based on your results, I believe that the circuit is performing correctly.

[Barry]

As Wilkie points out & based on your readings with your Volume up full you have a Gain of 2.5 times the input signal which is typically enough however if you desire an added Gain Stage we have a circuit which our International Distributors also carry called an After Blaster which would add a clean gain stage at the end of any circuit and conveniently fits over the ON/OFF status foot switch.

[Playsforfun]

2 thumbs up for the After Blaster. I absolutely love using these in my dirt pedal builds.

[Anonymous]

@Wilkie: thank you for your comments. The measures matched the gain calculation (more or less).

I just feel that the overall gain is not enough… the circuit compresses the sound a lot som the gain has to be big for my taste.

@Barry and Playsforfun: I will try the same output stage as the transistor BMP. Will build it by hand, it is too simple to use a pcb and I need it very compact to fit in the enclosure.

I will post the results when I finish the project.

Thank you again for the help and comments.

M

 

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