Problem with diy mxr distortion pedal

[Pacchio]

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.

 

[indianajo]

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.

 

[bob91343]

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.

 

[Pacchio]

Thank you both. I will try to make your changes and i'll post the response.

 

[Pacchio]

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.

 

[Nigel Goodwin]

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.

 

[Pacchio]

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.

 

[Nigel Goodwin]

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.

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."

I never use breadboards, they are too unreliable and cause too many ill effects (although audio is usually OK).

 

[Pacchio]

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?

 

[Nigel Goodwin]

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.

 

[Pacchio]

Yes! when I play with the clean (via a bypass created by me), the signal is perfect. As soon as I turn the distortion, the gain rises considerably (three times in my opinion) and a noise is added to the distortion of the guitar.

How can I reduce the gain? What IC replace them in case? Or what components?
Later I will try to add the capacitors. : D

Thanks again.Ascolta
Trascrizione fonetica

 

[wakibaki]

Diode clipping circuits do provide some 'fuzz' or distortion, but most people find the effect a bit crude and unmusical.

A better-sounding circuit is the fuzzface, Technology of the Fuzz Face - Frame Definition preferably built with 2 germanium PNP transistors. I recently built one with some germanium transistors (I found a stash of a dozen or so), and I can attest that the sound is good, with individual notes in a chord still being heard, and a certain sensitivity to how hard you play that you can exploit.

This is just a personal preference, you might like the diode circuit.

An externally hosted image should be here but it was not working when we last tested it.

Yes! when I play with the clean (via a bypass created by me), the signal is perfect. As soon as I turn the distortion, the gain rises considerably (three times in my opinion) and a noise is added to the distortion of the guitar.

How can I reduce the gain? What IC replace them in case? Or what components?
Later I will try to add the capacitors. : D

Thanks again.Ascolta
Trascrizione fonetica

There's always a possibility of balance problems with the bypass. You can reduce the output by replacing R10, the 100k log pot with a, say, 500k preset pot above a 1k log pot.

An externally hosted image should be here but it was not working when we last tested it.

I'm just guessing, you might need 10k log. It's best to keep the total of the two ~ 100k as in the original design. You just use the 500k preset to set the max volume.

w

 

[Pacchio]

Yeah, the fuzz is very cool and interesting. I'll build it as soon as i will resolve the noise problem.

Thank you.