Fuzz in MXR DynaComp Common?

[Willthebold]

Hey all, I recently finished putting together a DynaComp and everything seems to be working right.  However, with the sensitivity pot up about half-way and beyond, there is a kind of fuzzy sound when two or more strings are played at the same time; especially noticeable on bass strings.  I'm playing an SG and wondered if that might have something to do with it.  I haven't had a chance to try it with single-coils yet so I'm left to ponder.  Anyone know how much sensitivity you can dial in and remain clean?  Thanks for your help.

Will

[Willthebold]

After fiddling with the pedal some more, I found that the fuzzy sound is there even with the sensitivity all the way down if you play the bass strings.  I know that probably isn't right.  Also, does anyone know of any other IC's to use in this circuit?

Will

[Mark Hammer]

The CA3080 is well-known for having a low tolerance for input signal.  Give it more than 100mv and it starts to distort.  Enough that Anderton uses it in a voltage-controlled distortion module (DEVICE,Vol 1:4 at hammer.ampage.org).

Is it at all possible the signal level arriving at its input is in that critical zone?  Is it possible that there is either a mistaken component or something that wouldresult in it not being attenuated appropriatrely for the 3080 (you will note the presence of a resistor network at the chip input that seriously chops the level).

[Willthebold]

How do I measure the signal at the moment it enters the IC?  That's probably what's happening.  I'll check all the resistor values at the beginning of the circuit.  It's gotta be something like that.  Everything is working perfectly with nice compression, but there's this kind of faint fuzz that gets more noticeable with the sensitivity turned up.  Thanks for your help, and if you could let me know how to check the input signal to the IC I'll post it here.  Thanks again.


Will

[Mark Hammer]

If you have a DMM, set it to a range that will permit you to identify AC signals in the 2-200mv range.  With your guitar plugged in and one of the leads securely connected to ground, measure the signal level at pin 3 of the CA3080.  Ideally, even with the guitar cranked, and slamming power chords, you probably don't want to see a whole lotta signal at that pin.  Check at the base and emitter of Q1 as well, just to see where any excessive signal (should that be the problem) is coming from.

If that's not it, then you need help from someone with more savvy than me.

[Willthebold]

I checked the voltages and here's what I got:

Pin 3 of IC: 3.3v

Q1 C: 8.4
    B: 1.82
     E: 1.3

I don't know if that IC voltage is right, but that's what I kept getting.  I tied the ground of the MM to ground and touched the positive to pin 3.  That sounds to me like it's way too high though.  Any ideas?

Will

[Willthebold]

I used the audio-probe to determine that the distortion appears first at pin 6 of the IC.  The signal is clean going in, and dirty going out of the CA3080.  I also got all the IC voltages:

Pin
1: n/a
2: 3.07-3.23 (depends on where the sensitivity pot is)
3: 3.07-3.21
4: 0
5: .61-.69
6: 2.51-3.64
7: 8.36
8: .01

Hope someone can help decipher all this.  Thanks for the input.

Will

[Mark Hammer]

If those voltages readings are accurate (i.e., they are AC not DC), no wonder you can't get rid of the distortion.    Distortion increases noticeably in the CA3080 with more than 100mv input, which is about 1/30th of what you're feeding it.  I'm not sure where all that extra signal is coming from, but clearly you have to do something to tame it.

Just out of curiosity, what sort of readings do you get if you turn down the guitar halfway?

[Willthebold]

I checked the voltages again while turning the guitar volume pot, but there was no change in voltage.  I think I'm measuring it right.  I tied the ground of the MM to ground and poked around with the positive side on all the pins.  It almost sounds like there is too much bass being fed to the chip, but I don't know if that's right.  I'm not really very experienced with this stuff.  I guess I could record a sound clip and see what you think.  Thanks for being patient with me.  

Will

[Willthebold]

I was wondering if anyone knew what capacitor determines how much bass is fed to the circuit, and if that could be the cause of the fuzz?  What would be a good value to replace that capacitor with to cut the bass?

Will