Fuzz Pedal Oscillation issues

[RandyBeast]

Hello Everyone,

I am new to this site and just got my associate's degree in electrical engineering. It's the start of what I want to start doing. anyways. to get my fingers wet, I've been designing pedals!

I have successfully developed 3 pedals as of right now,

Drive,
Distortion,
and Tremelo.

all very easy to do. But I am at a loss with a Transistor base Fuzz I am working on.

on the breadboard, the circuit works perfectly and sounds fantastic!

however, once I create a two-layer board (PCB). it oscillates like a mad dog. I have to mess with the feedback resistance and gain knob to stop it, but this also affects the gain I receive from the pedal making it not as grungy as I would like.. I've tried all kinds of different layouts for the board and still can't figure this one out. if I could pick anyone's brains, this would be great. the main circuit is based on the FaceFace circuit.

Thanks

[FSFX]

Sorry but you really haven't provided any real details for people here to help with your problem.
You mention things like 'Transistor base Fuzz' and 'FaceFace circuit'. What exactly do you mean by those? I know of the Bazz Fuzz and Fuzz Face which are common circuits but quite different.
You don't tell us whether your circuit is silicon or germanium or plain BJT or Darlington. You don't tell us of the power supply polarity and your powering method, i.e battery, wall wart or pedal power supply.
With no schematic or information about your build, we don't know if you have incorporated any power filtering or decoupling.
As you say you have an electronic education, then you should be aware of the conditions such as phase and loop gain conditions that create positive feedback and oscillation.
The feedback to generate instability and oscillation can come from a variety of sources such as capacitive or inductive coupling of a high level output to a high gain, high impedance input or can be fed back via poorly decoupled power supplies or can be induced poorly designed ground connectivity.

Please provide us with more details of your circuit.   

[blackieNYC]

And... Welcome!

[antonis]

Welcome also..

Without a detailed schematic and PCB pics, the only suggestion should be to place Miller (feedback) capacitors between Collector & Base of each BJT..
And isolate IN & OUT wiring..

[RandyBeast]

Thanks for the feedback, I wasn't sure on what to start with. but here is the PCB design and Schematic. My design is perfect on a breadboard, the only problems are with the PCB. so I really need help with what I'm doing wrong with the PCB layout. as I only had one class on PCB design and they only showed me how to Trace boards.... haha

This is a Germanium PNP transistor circuit. using Positive Voltage instead of Negative as the old Fuzzfaces did. (I didn't want to have a special power supply for just this pedal.)

I used a two-layer board with Gnd planes on top and Bottom using Gnd Vias to connect the two planes together.

In the first design, I had the transistors very close to everything and I thought that was my problem, so I changed the layout putting the Transistors away from everything else. but no change in results.

I've never done this before. so sorry for the lack of info. I'm just new to this world.





I will answer any questions you have on the circuit. the last halve of the circuit after the gain section is a Gate section to clean up the circuit buzz that high gain pedals sometimes create.

Thanks

[idy]

You know more than I about electronics, but admit you are new here... I wouldn't be surprised by this problem. There is a reason why people go for voltage invertors and etc for PNP fuzz. People much deeper than us have knocked up against this and come to the conclusion that it doesn't always work.
I can't find the Geofex page about this...
"in theory you should be able to reverse the polarized components and etc but in reality....

You have a stash of Ge PNP? You can't use Ge NPN?

[PRR]

I don't see any power supply bypassing in this HIGH gain circuit.

IMHO you do not need all those trimmers, or the op-amps, as a first "FuzzFace" build. Steal some popular values and get that working before you get fancy. You may find that minor layout changes or bypassing is the difference in squeal.

[RandyBeast]

Thanks, PRR,

Do you mean on the PCB specifically for the supply bypassing? the 20uF is the only one on the gain stage. where else would you think is a good place to add by passing caps?

The op-amps are crucial for my gate circuit to work. Like I said above, this circuit sounds amazing on the Breadboard. it only squeals on the PCB. if I have to have a Breadboard version of this for the rest of my life I will haha. but I want to make this legit and solve this. if I can get the oscillating under control. this is a great pedal. even when it's squealing, playing it sounds really good.

Thanks

[RandyBeast]

You know more than I about electronics, but admit you are new here... I wouldn't be surprised by this problem. There is a reason why people go for voltage invertors and etc for PNP fuzz. People much deeper than us have knocked up against this and come to the conclusion that it doesn't always work.
I can't find the Geofex page about this...
"in theory you should be able to reverse the polarized components and etc but in reality....

You have a stash of Ge PNP? You can't use Ge NPN?

Hello idy,

I can use NPN as well, but using my PNP in this method sounds great on the breadboard. I don't think it's a component issue as I have tried other Transistors other than my germanium to see if it was a transistor issue. but I got the same results. something else with the PCB Layout is causing the issue. as I don't have any issues on the Breadboard version.

Thanks

P.S.
a 12ax7 tube has 8 pins but the sight is telling me I am wrong. anyone know the answer?

[Dormammu]

P.S.
a 12ax7 tube has 8 pins but the sight is telling me I am wrong. anyone know the answer?

Even aunt Wiki knows - 9 pin.   

[antonis]

I will answer any questions you have on the circuit.

R2/R3 values could be an order of magnitude lower..
C2/R12 could be omitted..
(actually, the whole IC1B circuitry could be omitted or implemented as VB stabilizer..)

[anotherjim]

There is a cap on the raw supply VA. It would be better I think if R5 came first then it forms a filter with C7 (with could be a larger value than 47u).

But, if you reverse the order of R6 and the gain pot, then C10 goes in opposite polarity to ground 0v. That way, the bypassing is to ground instead of VA.

[antonis]

There is a cap on the raw supply VA. It would be better I think if R5 came first then it forms a filter with C7 (with could be a larger value than 47u)

I think OP wants to separately filter "heavy" current stages (Q1, Q2) but it should be better to reverse the configuration..

[amptramp]

Welcome to the forum!  Electronics, like music, has a steep learning curve but once you get better at it, it becomes enjoyable.

I'm not happy with the lines going to the base of T1 crossing near the output jack and the back-to-back location of the input and output jacks almost guarantees oscillation.

Typically, the Fuzz Face uses a 100 K resistor where you have the feedback pot - the pot leads are connected to the input and they can pick up output signal from almost everywhere.  A fixed resistor may help with this.

You have an op amp gain stage with a gain of about 23 at the output of a Fuzz Face stage that was not necessary in the original Fuzz Face.  This would definitely boost the signal to the point where oscillation would be probable with the output so close to the input.

The output and input signals are near each other at the H1 connector which I presume is the cable to the bypass switch.  Although the leads are not adjacent, the input and output run in close proximity for the length of the cable.

The lead for the base of the first transistor winds all the way up to the top of the board to pick up C6 and the 250K pot then comes back down to pick up C3 and C4 and they go to the big cap switch in the middle.  That's a long lead covering a lot of area.

I'm not surprised your breadboard worked but the PCB oscillated.  You may need to revisit the layout to get the input signal and output signal a bit more separated from each other.

[RandyBeast]

I will answer any questions you have on the circuit.

R2/R3 values could be an order of magnitude lower..
C2/R12 could be omitted..
(actually, the whole IC1B circuitry could be omitted or implemented as VB stabilizer..)

Hello

R2/R3 is High Values to keep any Current from entering my Bias signal. C2 helps stabilize the VB and I don't have an R12 I don't believe unless you are talking about my Trimmer in my gate circuit. I want that trimmer so people can adjust the Gate to their liking.

There is a cap on the raw supply VA. It would be better I think if R5 came first then it forms a filter with C7 (with could be a larger value than 47u).

But, if you reverse the order of R6 and the gain pot, then C10 goes in opposite polarity to ground 0v. That way, the bypassing is to ground instead of VA.

I don't want to reverse the polarity of that cap because it would explode with the positive voltage hitting its negative end. however, I am curious to change the VA end on just the cap to GND to see what happens.

Welcome to the forum!  Electronics, like music, has a steep learning curve but once you get better at it, it becomes enjoyable.

I'm not happy with the lines going to the base of T1 crossing near the output jack and the back-to-back location of the input and output jacks almost guarantees oscillation.

Typically, the Fuzz Face uses a 100 K resistor where you have the feedback pot - the pot leads are connected to the input and they can pick up output signal from almost everywhere.  A fixed resistor may help with this.

You have an op amp gain stage with a gain of about 23 at the output of a Fuzz Face stage that was not necessary in the original Fuzz Face.  This would definitely boost the signal to the point where oscillation would be probable with the output so close to the input.

The output and input signals are near each other at the H1 connector which I presume is the cable to the bypass switch.  Although the leads are not adjacent, the input and output run in close proximity for the length of the cable.

The lead for the base of the first transistor winds all the way up to the top of the board to pick up C6 and the 250K pot then comes back down to pick up C3 and C4 and they go to the big cap switch in the middle.  That's a long lead covering a lot of area.

I'm not surprised your breadboard worked but the PCB oscillated.  You may need to revisit the layout to get the input signal and output signal a bit more separated from each other.

Hello amptramp,

Thanks for your insight, I did on my very first design have the transistors up closer to everything and I thought, if I moved them away it would help. but nothing changed. the issue is space, I don't really have much choice in the input and output location. I could change the PCB-mounted jacks to wired and run the wires on opposite ends of the enclosure.

All the trim pots are for personal adjustment, everything can be tweaked to liking. these values change all the time with Transistors because the HFE is never the same. so I went with trimpots so I didn't have to calculate resistance values every build.

The opamp gain is necessary for the Gate circuit, any circuit buzz that is under 3.3Vs while not playing can't go through the GR LEDS. so they bypass through the 10K and 500K trimpot resistances. 470/510= .9ish gain which means the buzz is basically killed. once you play the GTR, the signal is brought back up to 4.5V and the LEDs open up. but with a voltage drop across the LEDS the signal needs to be brought back up. but 10K/470K adds the gain that was dropped.

I don't have a ton of choices with the layout because of volume and Gain knobs input/output jacks. however, on the breadboard, the input and output are pretty separated where they are not on this board. It's definitely a Layout issue with the PCB. so thanks for your input! I will look into trying another layout. My other issue is cost, spending money on new protoboards is starting to rack up. shipping alone costs $25 an order.

[FSFX]

The opamp gain is necessary for the Gate circuit, any circuit buzz that is under 3.3Vs while not playing can't go through the GR LEDS. so they bypass through the 10K and 500K trimpot resistances. 470/510= .9ish gain which means the buzz is basically killed. once you play the GTR, the signal is brought back up to 4.5V and the LEDs open up. but with a voltage drop across the LEDS the signal needs to be brought back up. but 10K/470K adds the gain that was dropped.

With R8 being 470 ohms, R10 being 1k and the BIAS2 pot having a value of 10k then I can't see how the signal hitting those green LEDs is ever going to get anywhere near high enough for them to start conducting in any meaningful way.

[RandyBeast]

The opamp gain is necessary for the Gate circuit, any circuit buzz that is under 3.3Vs while not playing can't go through the GR LEDS. so they bypass through the 10K and 500K trimpot resistances. 470/510= .9ish gain which means the buzz is basically killed. once you play the GTR, the signal is brought back up to 4.5V and the LEDs open up. but with a voltage drop across the LEDS the signal needs to be brought back up. but 10K/470K adds the gain that was dropped.

With R8 being 470 ohms, R10 being 1k and the BIAS2 pot having a value of 10k then I can't see how the signal hitting those green LEDs is ever going to get anywhere near high enough for them to start conducting in any meaningful way.

Hello,

That's why I have a test point, you plug the meter into that spot and adjust the trimmers to get 4.5V at T2 collector. I have no problems at all getting the LEDS to conduct correctly, and it sounds great. but this is beyond the scope of this conversation.

Thanks

[idy]

On this page, reply #8 there is a diagram of bypass caps. They look almost like the caps attached to fuzz control on FF, but you look carefully and they are going to "the other place." This is the kind of bypassing to get rid of oscillation.

https://www.diystompboxes.com/smfforum/index.php?topic=122458.msg1155570#msg1155570

I guess I would also try to isolate the oscillation. Does it happen if the IC stages are bypassed somehow?

[FSFX]

That's why I have a test point, you plug the meter into that spot and adjust the trimmers to get 4.5V at T2 collector. I have no problems at all getting the LEDS to conduct correctly, and it sounds great. but this is beyond the scope of this conversation.

Thanks

That 4.5v is a DC 'Q-point' or quiescent point voltage. It is nothing to do with the maximum AC signal voltage that you will be feeding to the green LEDs which is determined by the potential divider formed by the resistors in the collector of Q2 and the ratio of them. In your circuit, the maximum AC voltage swing of the output from Q2 feeding the diodes is never going to exceed about 1 volt peak-to-peak and typically when biased correctly will be more like 350mV peak-to-peak. If you want to drive those diodes with a larger signal then you need to increase the value of the 470 ohm resistor considerably and reduce the BIAS2 pot resistance or just take the signal directly off of the collector of Q2 to feed to the diodes.

[johngreene]

The most useful tool I have for tracking down oscillation problems is my finger. Poke around the PCB touching wires, parts, traces, pads, until you find a spot that at least changes it or kills it. That will help you to focus on an area that is providing the necessary positive feedback to oscillate.