ground question

[bortlett]

I was messing around with my breadboard this weekend and ran into an issue that I can't seem to figure out, mostly because I haven't been able to figure out the words to use in my search.

I was trying to run a couple of fuzzes in parallel, but I could never get this comparator fuzz circuit to produce any sound when I hooked it up with other circuits. Both fuzzes and the splitter-blend all worked fine on their own, but once they were sharing the same power supply, the comparator fuzz stopped working all together. Everything else still worked.

Here's the schematic of the problematic circuit:


I'm pretty sure the issue was they way this cirucit is grounded. The 100nF cap and 100k resistor connect to ground. Anything else that is going to ground (except for the IC) has to go through those first. The other circuits that I was working with had a more typical ground setup where I could connect directly to the ground pin on the DC jack.

I tried a few things to make this thing work with other circuits, but short of adding a second power supply, I was at a loss. Of my various attempts, I thought adding a virtual ground would be the most promising. I tried a couple of different options from this page: http://tangentsoft.net/elec/vgrounds.html, but with no luck.

Anyway, I have tried to figure out some solutions on my own, but I'm running low on ideas for words to use in my googling so I can dig farther. Is there a term for what's going on with the ground on the comparator fuzz? The closest thing I've been able to find is that it looks kina like a ground lift.

I was really just trying to learn some stuff with my breadboard, so this isn't necessarily something that I'm trying to turn into a pedal. At this point, I'm just curious if there is a simple way around a problem like this or if anyone has some suggestions of places to look for more info, because I'm kind of guessing on my search terms.

[multi_s]

The 100nF cap and 100k resistor connect to ground. Anything else that is going to ground (except for the IC) has to go through those first.

ya it will not turn out because the way he has it setup, the ground symbol in the schematic atpin 3 of the opamp has ~4.5 volts on it, you can see this because the voltage from the battery is split half and half over R3 and R2, leavnig 4.5 at the junction. so if you connect that to chasis ground, the chasis of this fuzz is at 4.5 volts and the chasis of your other fuzz is likely actually 0 v. so there is a short when they are connected via a patch cable or try to share the same supply maybe and most likely this fuzz wont work properly.

a better look that might work is to add a capacitor inbetween R5 and the bypass switch. take pin 3 of the opamp off the ground network that is also going to your a jacks etc and instead connect pin 7 (v-) of the opamp to that ground network. This way the 'ground' of this circuit is actually 0v dc and it will play nicer with your other pedals.

it might be a mistake on the schematic to begin with since im not sure why you would ever seet it up this way.

[bortlett]

thanks for the info. i still have the thing on my breadboard, so maybe i'll play around with it some more just to see how it works if with that suggestion. i was really just curious what the guy was doing here, but it sounds like it's not typical at all.

[multi_s]

ya tbh im not too sure either but he is a famous guy so maybe he knows more than us

A lof of his circuits neded +/- 9 V so maybe you are meant to have 2 9v batteries in series and then put the switch to the circuit ground to thw middle of the 2 batteries. then the ground will actually be ~0V. not too sure thw switch would actulally turn it off in that case though

If you are trying to run it off a 9v DC power suppy, i would go with my first suggestion.


edit:
also here is a useful application note on single supply opamp implementation if you want to look through it.

http://www.google.ca/url?sa=t&rct=j&q=& ... WM&cad=rja

The main issue with the way the power is in the schem above is that the symbols imply the 'virtual ground' used to bias the opamp is also used as the ground reference for the input/output signals. i can t think of a good reason to do that with pedals unless virtual ground = 0v.