Problem with diy mxr distortion pedal - diyAudio
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Old 14th September 2010, 01:21 PM   #1
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Wink Problem with diy mxr distortion pedal

Hi everybody.
I'm Luca and i want to ask you a question:
some days ago i've built a simple distortion pedal, following this project:

tonepad -- FX projects

and this schematic:

http://www.tonepad.com/getFile.asp?id=115

The pedal is a mxr distortion plus and i can realize it, but i encountered a problem.

When my guitar's jack is not linked to pedal (and when it is linked too), i heard annoying hum and whistle (like a very bad feedback) when i turn up the volume of my amplifier. Why?!

The problem persists whether i use a battery or whether i use a 9V power supply.

I'm coming crazy.

Thanks to everybody.
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Old 14th September 2010, 02:36 PM   #2
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I don't seem to be authorized to see the schematic, but your unplugged input may be acting as an antenna. Solder a resistor between 1k and 5kohm across the input and see if the problem stops. Thanks for the link, I've been looking for bare PCB's like tonepad's microamp.
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Old 14th September 2010, 04:16 PM   #3
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You apparently are getting a signal into the unit from elsewhere. It's possible your grounding scheme may be faulty. The unit has quite a lot of gain and that may be the problem. Try reducing the value of R8, maybe by a factor of ten or so.
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Old 14th September 2010, 04:44 PM   #4
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Thank you both. I will try to make your changes and i'll post the response.
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Old 14th September 2010, 10:29 PM   #5
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Sorry guys, but the problem persists.
The input jack works effectively as an antenna, because i can heard a radio station, lol.
Moreover, when my hand approached the jack, the hum decreases...

Tomorrow i'll try to put an RC high-pass filter just before the output's jack for remove the
strident whistle, but i do not think it will work.

Thanks.
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Old 15th September 2010, 08:58 AM   #6
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The circuit seems rather lacking, for a start no PSU decoupling capacitor.

Have you built it on the supplied PCB layout?, and are you using a 741 (or something less ancient), I would also suggest building it in a metal box so it's screened.
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Old 15th September 2010, 11:22 AM   #7
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So I built the circuit on a breadboard spacers, and I still put everything into a metal box. The IC is a 741.
Where should I place the decoupling capacitor?

I also read, browsing the web:
"Use shielded wire at the input (jack>switch and switch>PCB). Ground the braid at one end of each wire. This helps reject the output signal that is radiating back into the input."

Thanks.
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Old 16th September 2010, 10:07 AM   #8
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Quote:
Originally Posted by Pacchio View Post
So I built the circuit on a breadboard spacers, and I still put everything into a metal box. The IC is a 741.
Where should I place the decoupling capacitor?
Directly across the supply pins of the 741.

Quote:

I also read, browsing the web:
"[I]Use shielded wire at the input (jack>switch and switch>PCB). Ground the braid at one end of each wire. This helps reject the output signal that is radiating back into the input."
I never use breadboards, they are too unreliable and cause too many ill effects (although audio is usually OK).
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Old 16th September 2010, 10:47 AM   #9
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Ok, but what value i've to choose for this capacitor?

However, I added a low pass filter before the output and I solved the problem of the whistle.
Nevertheless remained the problem of background noise. I read on the web that could be a problem of stabilizing the voltage .. is it true?
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Old 16th September 2010, 03:30 PM   #10
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Quote:
Originally Posted by Pacchio View Post
Ok, but what value i've to choose for this capacitor?

However, I added a low pass filter before the output and I solved the problem of the whistle.
Nevertheless remained the problem of background noise. I read on the web that could be a problem of stabilizing the voltage .. is it true?
Stick a 100uF directly across the battery, and a 0.1uF directly across the chip supply pins - neither value is in any way critical.

The amp is running at very high gain, instabilty is certainly likely to be an issue, so layout could be your problem. But without any signal input you're probably going to get noise, it's not a quiet opamp, and it's very high gain.
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