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Troubleshooting unwanted distortion in a DIY guitar effect pedal?

I built myself one of Moosapotamus' Bass Paraloopers following his design (the updated one, that is) to the letter.  The only modification I did was to remove the low pass filter, since I'm using it for guitar, not bass.  Unfortunately, it has several problems.

The one I am most concerned about is some unwanted distortion.  It is fairly mild, but noticeable enough to be a problem when playing clean.  It affects both the dry and wet signal (i.e. it is present when the blend knob is set to both 100% wet and 100% dry).  I do not think the signal from the effects pedal in the loop is overdriving the circuit.  How can I narrow down where/what component this distortion is coming from?  Unfortunately, I do not have an oscilloscope to use for troubleshooting.  Any help is appreciated.

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iceng5 years ago
  • You could have a low battery
  • You could be over driving the input
  • I don't like the resistive 4.5 VDC bias generator,  add an op-amp to guarantee the bias holds during play.
A
RelaxedSoup (author)  iceng5 years ago
D'oh. Low battery it is. This thing sucks down batteries like mad, much faster than I thought it would. However, this re-introduces an old problem that, when hooked up to a power supply, this thing has a terrible, terrible hum. Almost louder than the main signal in fact. It's hideous. This guy seems to have gotten rid of most of it, but I'd sure like to get rid of all of it. Any ideas?
Here are some answers for your HUMM and the improved bias.........A
9VDC_REG.png9vREG.jpgparalooperSCH.png
RelaxedSoup (author)  iceng5 years ago
All right, thanks. I will try that.
Definitely +1 on the bias generator.

As I always say, beg, borrow, or steal an oscilloscope to SEE what's going on - one of the units that uses your PC audio card will probably help nail it.

Steve