EPFM Ultra-Fuzz Problem

[thecooldude]

I have recently built the Ultra Fuzz pedal from Craig Anderton's book with partial success.  There seems to be a problem in driving the pedal.  I can get a nice fuzz sound out of it for about a second before the sound cuts-if I pluck lower notes.  It seems to me, as the note decays there is not enough voltage being generated to drive the pedal.  To be sure it wasn't just my guitar not generating enough voltage, I also played the pedal through a sine wave on a synth and even then, the output volume of the synth had to be about halfway up in order for the pedal to sound.

I have checked the circuit multiple times and double and triple checked the polarity of the electrolytics and the diodes.  I also made sure to use the low level preferences of this circuit.  I have also completely rebuilt the circuit on bread board and it exhibits the same symptoms.  This is the first pedal I have built, so I'm not really sure where to troubleshoot from this point.

Is it common to have to build a pre-pedal signal boost?  I wouldn't think this would be the case with a circuit design in this book.

I would appreciate any input or ideas.

[MartyMart]

Did you convert Craigs "Ancient" IC pin setup to a modern 8 pin IC properly ?
Is this a +9/-9 volt circuit ?
MM

[Mark Hammer]

My guess is that you got hold of the schematic but not the book itself, or at least the project description.  The design is intended to do exactly what you describe.

The Ultra-Fuzz is essentially a comparator.  If the input signal is above a certain critical voltage at any point, the second op-amp goes high.  If it is below that voltage, it goes low.  In this way, the unit gives a sort of pedal-to-the-metal square sound (hence "ultra") as long as the input is hot enough, and then "self-squelches" to provide a relatively low noise output at the pioint where the signal might become unstable or glitchy.  It will NOT fade out at the end of a note.  It WILL just flat out die.  That's how its made.

I don't have the schematic handy but you will likely see the sensitivity pot connected to V+ and the wiper feeding one of the input pins on the second op-amp.  The pot and any fixed resistors on each end of the pot, will set the range of valid comparison voltages that will produce an output.  What you want is to have a reference or comparison voltage that is low enough that your input signal will exceed it for a longer time.  To do that, the resistance on the V+ side of the wiper needs to be much larger than the resistance on the ground (or V-, I forget which it uses) side of the wiper.  You can do that by either reducing the resistance on one side or increasing it on the other.

Finally, you can also consider increasing any gain in the first op-amp so as to assure that the signal easily exceeds the threshold, as well as sticking a compressor between the input source and the Ultra-Fuzz to make sure that the signal remains fairly stable for a longer duration.

[sfr]

It's been a long time since I've looked at any of the Anderton projects, but you're description of this thing just made this box sound a heck of a lot more appealing, Mark.  Now I need to go dig out my book.

[Mark Hammer]

http://img.photobucket.com/albums/v474/mhammer/ultra.gif

[thecooldude]

Thanks for the input Mark.  I think its gonna work much better now.  Also, I actually have an understanding of how the thing works now!