Converting bipolar fuzz to Single supply power

[John Lyons]

How can I convert this to a standard single power supply?



This is an odd one as the ground is floating.

thanks

[slacker]

Looks like it is already single supply to me. The only weirdness is that on the schematic the negative end of the battery and pin 4 of the opamp aren't connected to ground. I'm not sure if that's correct or not. Sorry that's not much help 

[GibsonGM]

Yeah, it just looks like it's floating.  Weird.

[R.G.]

The ground is not floating - it's tied to the output pin through a DC path in the output volume control. The + input is held at the middle of the power supply, so the opamp itself is positioning the power supply to be balanced on whatever the "ground" voltage coming in and out is. The 10K/10K divider tells the opamp which way to pull the power supply. Notice that Rf in this circuit is the 1M resistor and Rin = the source impedance of whatever drives it. I'd expect it to be very sensitive to whatever impedance drives it, and it's probably intended to be a bare guitar output, not the output of a buffer. This is, I think, something like what happens in a Fuzz Face input where the gain is sensitive to what drives it.

[Mac Walker]

BUMP

This schematic is from another site:



It differs from the above diagram in the following ways:

1)  Polarity at the inverting/non-inverting terminals is the same, but pins 3 and 2 are swapped in position only, BUT pin 3 is tied to the center of the voltage divider (4.5 V) and chassis ground.
2)  There is a 10K resistor missing on the output which presumably lowers the output voltage a bit before it hits the 10K pot.

Does the circuit still work without the center of the voltage divider tied to ground?  If it were required you would have to run this from a battery only, or an isolated 9V source, as it would not play nice with other pedals.

Also wonder which one represents the actual production circuit, if anyone knows....

[John Lyons]

Thanks Mac
The Photo schematic is correct. I forgot the Ground connection at pin 3 on my schematic. 
I also forgot the 10k at the output...which may explain some oddities that I've been having and recently
pulling my hair out over. I had this schematic but glossed over that 10K so thanks for pointing that out! 

[EATyourGuitar]

thanks for sharing. I would have never known this thing existed without you john! check it for mistakes. I'm just going on what I see here in the thread. gonna etch me a board and give it a try.

[PRR]

> convert this to a standard single power supply?

Clearly it MUST have input and output caps.

That drawing is so convoluted I'm not sure, but I think this is the common-battery equivalent:



It looks nasty. Input impedance is very low. It will play different from a guitar than buffer or other box. The poor chip can't drive 220r cleanly (which I guess is the point), not to mention the zero ohms if Level is turned way down (but then you won't hear much of anything).

[Mac Walker]

^

According to gutshots of the original floating round the web, it did not have coupling caps, which, I know is.....weird.....

So which is correct, the jumper from 3 to ground, as shown in hand drawn sketch, or from 4 to ground (as shown in your modification)?  Or should the 4.5 V reference float, as shown in the original that John (presumably?) drew up?

I haven't had a chance to bread board it yet....

[earthtonesaudio]

Either is correct.  As PRR drew it, it will work better (i.e. it will work at all) with daisy-chained power supplies like people tend to use.  But then you have to add the i/o coupling caps.
If you build it as original, you have to direct couple either the output or the input, since that's the only place it can get its (non-floating) DC bias.

I would suggest PRR's modified version, since it looks like it will be a little more forgiving.

[PRR]

> gutshots of the original

John's question is "How can I convert this to a standard single power supply?

I'm referring to such a *conversion*, not the original.

[EATyourGuitar]

has anyone else noticed in the youtube videos that the whistling sound comes and goes as you adjust the filter knob? can we fix this without getting too far away from the shit design of the original shit circuit?

[wavley]

I had an Earth Sound Research G-2000 for years, it was one of the best built, great sounding, and reliable tube amps I've ever owned and I'm sad that my wife convinced me to get rid of an amp and it's the one I chose.

I guess my point is that I was really surprised how not very well designed this seems to be, of course that doesn't mean it won't sound great.  I'm gonna have to check some videos out.

[EATyourGuitar]

if anyone would like to use my layout I can share it as a PDF or a panelized PDF ready for etching. I'm going to do some more layouts with the PRR mod for negative supply connected to audio ground. if there are any more changes or suggestions happening in the thread I can volunteer to do more layouts and post as PDF.

[Mac Walker]

^ Cool....

Stumbled across this:

http://www.jimmybehanfx.net/custom/earth-sound-research-graphic-fuzz

Talks about switching a cap in and out to go from stock to "soften the peak and cuts out the oscillating"

Looks like coupling caps added and negative ground based on the DC power jack.....

[John Lyons]

The hand drawn schematic is 100% correct.
The schematic I made in the original post is missing the 10k at the output at pin 6
and also the ground point at pin 3. (sorry about that internets).

The original does not have in/out caps.
The original works as drawn in the schematic (hand drawn one).
It oscillates, yes. Part of why I like it actually. Color me strange...  

Thanks Paul. I'll try that modification.

[EATyourGuitar]

^ Cool....

Stumbled across this:

http://www.jimmybehanfx.net/custom/earth-sound-research-graphic-fuzz

Talks about switching a cap in and out to go from stock to "soften the peak and cuts out the oscillating"

Looks like coupling caps added and negative ground based on the DC power jack.....

if I had to guess what that feedback cap is I would say anything in the range of 680pf to 47pf. I will test it out. I can't understand how jimmy behan has it working on a 9v with other pedals if the only added cap is a power filter? that little light blue cap is the feedback cap obviously. so no in and out caps, change the ground, any ideas? whats his secret?

[John Lyons]

He's also got 3 additional resistors.

[EATyourGuitar]

He's also got 3 additional resistors.

thanks again. I did not think to count resistors. I may have figured it out now. only 0.95 microamp DC going back into the guitar pickups. transimpedance amplifier has been left mostly as is. I stayed up late reading TI application notes about single supply opamps. the only problem with referencing the bottom rail is that your input signal will swing past it and therefor past the operating range of the opamp. the only reason for the caps is to block DC at the input. I think it is more like a safety feature than an actual requirement. yet to be determined is how this transimpedance amplifier responds to something that is theoretically ok if it was not setup for transimpedance. I want to remind the qualified people that I am not a trained EE. I do my best so please correct me where I'm wrong. also interesting is that if you did have a battery in your guitar, it has a a cap already.

[Mac Walker]

Whoops, I mispoke about the coupling caps on the link I provided.....

EATyourguitar, Do you have C3 and C5 backwards in the above drawing (values swapped)?

Also I am still wondering how the switch ties in, there appears to be a jumper on the switch, which I can't wrap my head around...