Question concerning this schematic

[fldrvr]

So I would like to take a shot at this one, unfortunately I don't have a breadboard or much time for experimenting. I've been told that there is an error in the schematic, but I'm having a hard time spotting it, I was hoping someone could help me out with identifying the error and correcting it. I'm also having trouble figuring out the circuit as compared ot the gut shots I've seen.

I've attached 2 versions the first being the one with the supposed error (not sure about the second one), and gutshots can be seen here: http://www.effectsdatabase.com/model/earthsound/graphicfuzz

Thanks, any help or comments would be appreciated.

[mikebike]

this is just a geuss...

but, in the first circuit the pot labeld filter, doesnt appear like it will do much "filtering".
at best its a "mix" knob.

the second, though, does have a cap, headed to ground, i think that gives you a low pass filter!

hope this helps

[fldrvr]

Thanks for your comments. I did notice that the 2 schematics are a bit different. I think the first may be more accurate but is missing a resistor coming off of pin 6 of the IC. A look at the gut shots of the original pedal do show more resistors than are on the first schematic.

I guess I'll just need to get a breadboard at some point and find some time to test out the circuits and see where it takes me.

The Idiotbox Blasteroid is a clone on this pedal design, as well as the Basic Audio Dirt Halo, which has modified this design heavily.

thanks again

[ve3wwg]

This is a fairly weird circuit. From pin 6, through the 1MEG pot, to pin 2, you have a feedback circuit for the opamp. The 0.1uF cap from pin 2 to ground is a bit worrisome at first, but is likely correct in the schematic.

If you look at the 1MEG pot and the 0.1uF cap together, you do see a low pass filter (in the feedback circuit). With the pot fully open (1MEG), the low pass filter cuts off at about 10 Hz. With the pot turned so that it's resistance is near zero, then it passes the full audio range.

But...

The pot also effects the amount of feedback (and thus gain). So when the pot is at about 1MEG, the opamp is operating at near open loop gain. Thus I suspect in this mode, the signal is clipping against the rails and is perhaps the muddiest lowest pitched signal produced. With the pot near zero ohms, the gain becomes "unity".  At this extreme, I would expect a clean but low level signal coming out (with little if any filtering applied).

Low ohmic settings would produce low level signals with the cutoff frequency increasingly lowered (as the pot increased in resistance). Moving further, the feedback would start causing the opamp to clip as the gain increases and the cutoff frequency is further reduced.

I do believe the output resistor (10k) should be there to minimize capacitive loading on the opamp. It may even be reducible to 1 to 2k, AFAIK.

The one thing I don't like about the circuit is the lack of an input resistor   going to pin 2.  Without that, the gain of the circuit is somewhat at the mercy of what is plugging into it (gain is related to the pot and input resistor).  You may want to experiment, but I'd suggest a 5 to 10k input resistor. This limits the maximum gain to 500 to 100, respectively. This would reduce the likelihood of it not playing well with different inputs.

Since the opamp clips rail to rail (I suspect) for distortion, you may get better tone by trying different opamps. Perhaps those that are designed to operate rail to rail might produce more pleasing sounds.  Also be aware that some opamps "latch up" when they hit the rails.

[fldrvr]

Thanks for your reply, there's som,e helpful information there.

Here are some gutshots of the pedal, which may help someone determine which version fo the schematic is more correct (I have a tough time figuring it out but I'm still new).

Thanks

[mikebike]

wow, thats dirty

[fldrvr]

Yep- I think the only ones that have appeared online, the batteries have leaked at some point.

[ve3wwg]

The best way to be certain is simply to trace it all out. Those are good photos, but that is a tough job to trace it out from an arm chair

But I think your hand drawn schematic will work just fine. I would just add an input resistor (experiment), starting with 10k and 2k and see which works best for you.  If you wire this according to the schematic that you drew, you won't go wrong.

This is a very simple opamp circuit. I may look daunting to a beginner because of the number of parts involved. But most of those are simply there to provide the virtual ground (provide the opamp with a split power supply from a single battery).