Gar Gillies' infamous STINGER adapted for stompbox use!

[whoisalhedges]

Well, ooookay, I thought it was working....



I've tried both of these. Yes, the resistors to ground are grounded, and the drain resistors are going to V+... I just neglected to finish the drawing.

Problem is the input signal, not the summing amplifier... whichever way the fuzz-side volume pot is wired, I can't turn it all the way down without the clean side going silent. Additionally, there's a significant attenuation in the clean volume when the fuzz circuit is switched on anyway (they are out of phase - as they are in the original tube circuit - which could be a concern, but their amplitudes differ wildly).

When the fuzz is switched out of the circuit entirely, I've got a glorious dirty boost/low-gain OD from the "clean." When clean is switched out, the fuzz is nearly unlistenable (which is in keeping with the inspiration from the Stinger ). When they're combined, it is an excellent sound - I just want them to combine... well, better.

If I insert resistance between the fuzz volume and ground, there is always some fuzz in the circuit; and if there's no or too little resistance, I lose all volume when turning fuzz down. Yes, I can get pure cleans just by switching out fuzz entirely, but that's not what I want. I want both circuits to be independently controllable at their outputs.

So... I'm stumped.

[duck_arse]

put a blocking cap between each pot and the summing point, and lose the cap that's shown.

[whoisalhedges]

put a blocking cap between each pot and the summing point, and lose the cap that's shown.

Thank you!

That is to say, you solved the problem as stated - when I turn down the fuzz volume to null, I no longer have my clean output going to ground.

However, something new: turned down all the way, I still have a low level of fuzz. I disconnect after the volume pot, I still have fuzz (what?)... I need to disconnect the fuzz circuit AT ITS INPUT (immediately after the first gain stage) to get it out of the circuit entirely. WTF? Is there some parasitic fuzz "leakage" coming back from the GATE of Q2 into Q1 via the drain???


Turn the fuzz down all the way or BREAK the connection between R14 and Q3, there's a low level of fuzz. Break the connection anywhere between Q1's drain and Q2's gate, pure clean signal.

Now just what in the hell is going on here...?

[whoisalhedges]

Oh holy crap, it's getting into the clean circuit via the freaking power supply... I've got fuzz leaving Q2's drain and getting into the circuit via R4 and C2...

:head smashing into brick wall icon:

So, something's gotta go between R4 and R6... something that passes DC and not AC... a capacitor to ground. A big one that passes ALL AC.

Once I install the power filtering hoo-ha (I don't currently have the 100u C15 on my breadboard, because I figured I wouldn't need it on the breadboard) will that be sufficient? I suppose I can find out right now....

edit: the answer is no. That is to say, with a big cap from + to ground, there ain't a lick of difference. Poop.

[whoisalhedges]

That's because it's not via the power rail, it's via the feedback resistor. :brick wall thing again:

So - what's the dead-simple fix I'm missing?

[amz-fx]

Try something like this and see how it sounds:



regards, Jack

[whoisalhedges]

Didn't actually do much...

The LEDs are there to limit the signal going into Q1. Not sure I need to worry about that with my Strat, but if I have another dirt pedal in front, I don't want to overdrive this pedal at the input. The ratio of R6 to R7 is the same (basically) as the ratio of plate to cathode resistor in the original circuit. I don't think there's any reason this needs to be so, but it doesn't sound right if I go far off. 33k & 120 ohms sounds indistinguishable from 330k and 1k2; but I put a 10k in as the drain resistor (my 4k7s were all the way on the other side of the room) and it didn't sound right with either 1k2 or 120....

Series resistance between the pots and the summing junction, I tried that again with the same results as before: it doesn't really have any effect on the fuzz side, and it attenuates the clean side unacceptably - even just 10k! I suppose it might be interacting with the tone stack. Not sure, I just don't much care for it.

I'm pretty sure it's coming via the feedback resistor. I mean, how else could that fuzzed-up signal be getting back to a separate path coming off the drain of Q1? Again, if I break the connection before Q2, there's a really nice boost/OD.

There is no feedback in the original - but a MOSFET isn't a tube, and as I said before: if I play, and it's all gloriously fuzzy, and I remove Rf, the fuzz slowly goes away. Maybe I can ditch the feedback with different drain & source resistors...?

[duck_arse]

as a great man once said:

So, something's gotta go between R4 and R6... something that passes DC and not AC... a capacitor to ground. A big one that passes ALL AC.

you want to de-couple the Q1 supply from the Q2 supply. so put a resistor (?10k?) between R6 and R4, THEN hang a cap (10uF? 22uF? 47uF?) from top of R6 and new resistor to ground. report back.

it might even help if you take top of R17 to the new Q1 supply, but don't " me on that.

[amz-fx]

Q1 will clip long before the LEDs start to conduct. Let's say the Q1 gain stage has a gain of 10x... since the output can't swing more than 7v pk-pk, that means the input must be less than 0.7v pk-pk to avoid clipping. I just made those numbers up, but they are in the ball park.

Here is the filtering that duck_arse suggested. It's a good idea even if it doesn't fix the fuzzy problem.



regards, Jack

[whoisalhedges]

Thanks, guys.

I implemented the suggested changes, but it didn't fix the fuzz bleed - at this point I'm 99% sure it's fuzz off Q2's drain being dumped through C2 via the feedback resistor.

How does feedback work in FETs vs. tubes? That is to say, can I keep the ratios the same?  The 275:1 ratio of Rd:Rs did give me the gain I was after for fuzz; but a transistor's not a tube, no matter what the transfer characteristics look like on a graph. Ideally, there would be a Rf/cap to ground combo (there is a cap on the grid, after all) that will allow the necessary frequencies into the gate, but filter out the fuzz that "leaks" back to the junction at Q1's drain....

"Best" part is, the fuzz leakage just sounds like crap. The output sounds great when both fuzz and clean are turned up; but the signals being out of phase hurts when you have that leakage - I guess the amplitude of the feedback is just right to cancel out part of the clean signal when they're both traveling the same course; whereas at the desired output, I have two radically different waveforms that complement rather than compete with each other.

[Gus]

Is this built on a breadboard?
How long are the wire leads?
l

[whoisalhedges]

Yeah, it's breadboarded - the leads are the smallest ones that came with the board, 4".

I have another piece of "evidence" that may point me in the direction of a solution: a 100n cap from the gate of Q2 to ground stops the fuzz from leaking into the clean side. However, it also blocks too much of that first gain stage from getting into the fuzz side. Meanwhile, a 10n cap doesn't really accomplish much of anything vis-a-vis getting that leaky fuzz to ground.

I need to try 22n and 47n caps to see if any joy can be found in the middle.

There are always compromises to be made. I'm trying to capture the sound of a relatively primitive circuit. Without making thing much more complicated (which in itself would be a compromise), it may be that the only way for this to function as a semi-clean boost/OD is to switch the fuzz out - I am going to have two footswitches after all; it's not mandatory that I be able to remove the fuzz by turning down its volume control alone. If I can't have the best of both worlds, I want the fuzz to sound GREAT. I can always switch it out for the clean side.

I suppose that compromised position does raise a question for Eb7+9, as he (she?) has a B.T.O. - Does the Stinger ever clean up entirely by turning the volume all the way down, or is the only way to remove it from the circuit to flip the switch? I probably spent 2-3 hours over many weeks playing the one at the store, but never turned it all the way down - because really, why would I? 

[whoisalhedges]

A friend reminded me of a should-have-been-obvious solution: put a buffer in-between Q1 and Q2. Yes, that would allow the output from Q1 into Q2's gate without any attenuation; and also provide a barrier to prevent any AC from getting from Rf into the clean side. But that's not in the original; and though I did not start this project intending to copy Gar's topology, just the sound - I have thus far found that both the best and the most faithful sound has thus far come from following his topology.

That, and a buffer would mean more components, and I don't know how to do PCB. Most of us vero builders like to at least try to keep the parts count low. Is there a readily-available diode robust enough to do the trick? :lol:

[Gus]

New breadboard or older one?

You could have resistance in the ground connections of the circuit if the wire to ground connections are not tight.
Have you checked all the ground connections with a meter
The fuzz is on and running at full gain all the time.  If the fuzz control is at minimum you have the fuzz output driving the ground line.

Have you tried removing the (color)tone section control or cap as well as the volume control at the same time.  The tone is always in circuit connected to ground.

[whoisalhedges]

Hm... I have not checked the ground connection.

The board is only about 2 months old... it is a Radio Shack board, though.

[Gus]

Try disconnecting both the fuzz tone and fuzz volume controls leaving the input to the fuzz connected.

[whoisalhedges]

Try disconnecting both the fuzz tone and fuzz volume controls leaving the input to the fuzz connected.

Ah - to prove whether it's the feedback or ground? a cap at the gate did help....

OK, thanks. I'll try after work.

[whoisalhedges]

I lifted the fuzz output connection before it goes to ground - same thing.

Removed R5, and the leaking fuzz slowly died out after about 20-30 seconds (C3 discharging? When switched in, the fuzz also dies out in 20-30 seconds when I remove Rf - and if I start playing with no feedback resistor, it never gets fuzzy to begin with - which tells me I need some feedback there; I've tried 1M, 4M7, and 10M which all act pretty much the same). Unfortunately, that makes it harder to point a finger and say "j'accuse!" as the fuzz I want dies without Rf, just as does the fuzz I don't... however, the fact that a cap to ground where R5 meets the gate shunts off the leakage does go a long way toward proving my case (while also unacceptably coloring the tone going into Q2 - 22n is as high as I can get it without compromising the fuzz, while 47n isn't quite enough to stop the leak).

[whoisalhedges]

One other thing I should note - while I know more about MOSFETs than I did when I started, they're still quite mysterious to me; I even understand tubes better. Q2's gate is idling at 3.312v while the drain is at 3.344v. That's pretty close, yeah? I mean, it works - when the fuzz is doing what I want (i.e. leaving via the output rather than the input), it sounds pretty much exactly how I want it to. Switching the DMM over to microamps (and logic) tells me this stage is a current rather than a voltage amplifier. A BJT it ain't. 

[duck_arse]

monitor the drain voltage when you remove your "Rf" (which, incidently, is called something else on your circuit). does the voltage drift up to 9V?

you need R5, whatever value, to provide DC bias to the mosfet gate. otherwise, your input signal will need to be VERY BIG to turn the mossfet on. see also SHO, orman boost.