# New mini-breadboard



## Chuck D. Bones (Aug 22, 2021)

Jubal81 & I have been messing with a servo-biased Germanium Fuzz-Face.  Here's my rendition.  Controls are FUZZ, BIAS, TONE & LEVEL.  The trimmer sets the servo's operating point.  The collector currents don't drift; the servo keeps the DC component of Q2's emitter current constant.

Thanks for the mini-breadboard, Fig!


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## jubal81 (Aug 22, 2021)

Yass!
I've been having an absolute blast with this one this afternoon. Chuck's cooked up an epic one, guys. Buckle up.


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## Chuck D. Bones (Aug 22, 2021)

Wow! Yours is so neat.  Did you use film for C6?  You should.

This started out as an experiment to see if it could be done.  It's a basic Fuzz Face, I added an opamp and a half a dozen R's and C's to stabilize the operating point.


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## jubal81 (Aug 22, 2021)

Chuck D. Bones said:


> Wow! Yours is so neat.  Did you use film for C6?  You should.
> 
> This started out as an experiment to see if it could be done.  It's a basic Fuzz Face with an opamp and a half a dozen R's and C's added on to stabilize the operating point.


Nah, the film is C1. I used electrolytic for C6, but I could try an MLCC for that. Only had one 2u2 film cap.


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## Chuck D. Bones (Aug 22, 2021)

The servo won't work right with electrolytic for C6.  Too much leakage & dielectric absorption.  Use electro for C1 if you have to, that's what Dallas Arbiter used.


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## fig (Aug 22, 2021)

C6 tantalum 

Nite guys!


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## Chuck D. Bones (Aug 22, 2021)

Even tantalum is too leaky for this circuit.  For now, I have a 6.8K resistor in place of VR1.


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## benny_profane (Aug 23, 2021)

Oh, very interesting. Are you testing it with real world temperature fluctuations?


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## Chuck D. Bones (Aug 23, 2021)

I just put a heat gun on the transistors.  Q2's collector voltage went from 5.499V to 5.788V (about a 5% change) and then settled back to 5.483V when the servo caught up.  Not a very scientific test, but the servo is doing its job.

Without the servo, Q2's collector voltage would have run away.


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## cooder (Aug 23, 2021)

Killin' it! You guys are on a roll!


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## cooder (Aug 23, 2021)

Does the op amp need to be a CA3130 or could it be a TL071 (or TL072 with the second half unused)?


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## fig (Aug 23, 2021)

The 3130 has built-in output voltage regulators. Is that the feature being implemented?


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## Chuck D. Bones (Aug 23, 2021)

The CA3130 is a simple CMOS opamp.  I'm not sure what you mean by "built-in output voltage regulators."

In this circuit, the opamp's inputs and outputs are operated near ground.  Not all opamps have that capability.  The CA3130 does.  The TL072 does not.  The requirements for the servo opamp in this circuit is that it must have very low input bias current (FET input) and the inputs and output must be able to operate at the negative rail (ground).  The CA3130 is not the only opamp that meets these requirements, but it is readily available and relatively inexpensive ($2 ea at Tayda).  The CA3130 shows up in other pedal designs; some by me, some by others, like Bjorn Juhl.  This is the simplest implementation of the servo control.  There are ways to make the TL071 work, but it takes more parts.  There are other rail-to-rail opamps out there.


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## jubal81 (Aug 23, 2021)

Chuck D. Bones said:


> I just put a heat gun on the transistors.  Q2's collector voltage went from 5.499V to 5.788V (about a 5% change) and then settled back to 5.483V when the servo caught up.  Not a very scientific test, but the servo is doing its job.
> 
> Without the servo, Q2's collector voltage would have run away.


I held Q1 up against a hot tube and put it back in and got pretty much the same result.


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## jubal81 (Aug 23, 2021)

jubal81 said:


> I held Q1 up against a hot tube and put it back in and got pretty much the same result.


Sorry, I should have expanded a bit more. I didn't try to get the transistor hot. It felt just a bit warmer than the air. Rushed it back to the breadboard, popped it in, and the fuzz sounded the same. Yeah, not all scientific, lol.

However, in the past, I've noticed even a 5-10 degree change in room temperature can make a dramatic difference with Ge transistor fuzzes.


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## Chuck D. Bones (Aug 23, 2021)

Tayda & Mouser carry the MCP602, a dual CMOS opamp that would work.  Only catch is they have to be powered by 5V or less.  Not a big deal.


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## uranium_jones (Aug 23, 2021)

Wait, a motorized fuzz? I'm confused.


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## Chuck D. Bones (Aug 23, 2021)

Clearly.


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## fig (Aug 23, 2021)

Chuck D. Bones said:


> The CA3130 is a simple CMOS opamp.  I'm not sure what you mean by "built-in output voltage regulators."
> 
> In this circuit, the opamp's inputs and outputs are operated near ground.  Not all opamps have that capability.  The CA3130 does.  The TL072 does not.  The requirements for the servo opamp in this circuit is that it must have very low input bias current (FET input) and the inputs and output must be able to operate at the negative rail (ground).  The CA3130 is not the only opamp that meets these requirements, but it is readily available and relatively inexpensive ($2 ea at Tayda).  The CA3130 shows up in other pedal designs; some by me, some by others, like Bjorn Juhl.  This is the simplest implementation of the servo control.  There are ways to make the TL071 work, but it takes more parts.  There are other rail-to-rail opamps out there.


Sorry, I was unclear (at best). I had read the in the datasheet that one of the suggested applications for this opamp was as a voltage regulator and I somehow that translated to built-in VRs...dunno..anyway...unless I'm hallucinating it's being used as such in this case.


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## fig (Aug 23, 2021)

quality_jones said:


> Wait, a motorized fuzz? I'm confused.


If I'm not mistaken, that would be a mechanical servo.


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## uranium_jones (Aug 23, 2021)

fig said:


> If I'm not mistaken, that would be a mechanical servo.


Is there an "electrical" servo? Because google is apparently not my friend today.


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## Chuck D. Bones (Aug 23, 2021)

fig said:


> Sorry, I was unclear (at best). I had read the in the datasheet that one of the suggested applications for this opamp was as a voltage regulator and I somehow that translated to built-in VRs...dunno..anyway...unless I'm hallucinating it's being used as such in this case.


Any opamp can be used to build a regulator.  One of the components in a regulator is a voltage reference.  The CA3130 does not contain one, you have to supply one externally.  Intersil's CA3130 datasheet shows some application circuits, two of which are voltage regulators.  In the example below, IC2 serves as the voltage reference.


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## fig (Aug 23, 2021)

IC what you mean. Thanks! 

Follow-up if I may, if the correct TO package voltage regulator existed, could that be used, and if not why?


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## Chuck D. Bones (Aug 23, 2021)

Are you asking about 78xx and 79xx series regulators?  LM723?  I used to design power supplies, so I could talk all day about that stuff.


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## Chuck D. Bones (Aug 23, 2021)

quality_jones said:


> google is apparently not my friend today.



You know who else says that?  Rick Santorum.


There are all kinds of servos.  In the most general sense, servo control means using a negative feedback loop to control or regulate some physical quantity, be it velocity, position, temperature, pressure, voltage, etc.  Seems that the common parlance is to use the term "servo" to refer to closed-loop position or velocity control of a motor.  You could call the cruise control in your car a servo.

In this case, we're regulating the average current in Q2.  The control loop is very slow, so it doesn't react to wave shape or dynamics.  It does react to slow drift caused by changes in temperature.


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## cooder (Aug 23, 2021)

Chuck D. Bones said:


> The CA3130 is a simple CMOS opamp.  I'm not sure what you mean by "built-in output voltage regulators."
> 
> In this circuit, the opamp's inputs and outputs are operated near ground.  Not all opamps have that capability.  The CA3130 does.  The TL072 does not.  The requirements for the servo opamp in this circuit is that it must have very low input bias current (FET input) and the inputs and output must be able to operate at the negative rail (ground).  The CA3130 is not the only opamp that meets these requirements, but it is readily available and relatively inexpensive ($2 ea at Tayda).  The CA3130 shows up in other pedal designs; some by me, some by others, like Bjorn Juhl.  This is the simplest implementation of the servo control.  There are ways to make the TL071 work, but it takes more parts.  There are other rail-to-rail opamps out there.


What he said....

Thanks for the insights and this looks amazing. Seems that it makes the idea of the Benson Germanium Fuzz with the 'inbuilt heaters' of the carbon comp resistors redundant in a very elegant and better way.
https://www.bensonamps.com/guitarpedals/germaniumfuzz


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## fig (Aug 23, 2021)

Like this;


			https://www.mouser.com/datasheet/2/389/cd00000444-1795274.pdf


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## Chuck D. Bones (Aug 23, 2021)

I like regulators, but I like the temp controlled fuzz _moar_. A clever, if not extravagant solution. Could double as a foot-warmer.

I have seen precision crystal oscillators that have their own build-in heater to regulate the temperature of the crystal.

Engineering guy: "We could design a temperature compensated circuit." 
Marketing guy: "Temperature controlled would sell better."


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## cooder (Aug 23, 2021)

The idea of the temperature controlled' as in Benson Germanium Fuzz seems a bit possibly laden with inconsistency wouldn't it? Fancy and somewhat cool, but the servo option seems a lot better engineered me would think?

Over to Chuck....


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## benny_profane (Aug 23, 2021)

cooder said:


> The idea of the temperature controlled' as in Benson Germanium Fuzz seems a bit possibly laden with inconsistency wouldn't it? Fancy and somewhat cool, but the servo option seems a lot better engineered me would think?
> 
> Over to Chuck....


I would think that it would only work until the ambient temperature exceeds the temperature they’ve set the control for.


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## jubal81 (Aug 23, 2021)

Found an AC176 today, threw it in there and sounded/worked great. It's worked with every transistor I've thrown at it. I've got one that noticeably lower gain, but it still works and sounds pretty good.

Unfortunately, one of my AC127s crapped out. Was playing nice for about 10 seconds, then dead. My component tester thinks it's a resistor sometimes and two diodes other times.


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## Chuck D. Bones (Aug 23, 2021)

They just don't make transistors like they used to...

Spent a little more time playing thru it.  Changed VR2 to C1K because the bottom-end of rotation was useless.  Retuned the TONE stack.  Reduced R4 to get a little more volume and expand the BIAS control range.  Added C10 to reduce the high-freq "trash" above 5KHz.  Still need to install and test the CONTOUR control.


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## Chuck D. Bones (Aug 24, 2021)

Attention Cooder:
I found a way to make a TL072 (or TL071, TL081, etc.) work.

1. Install a green LED between Q1-E & GND.
2. Increase R7 to 3.9K
3. Add another green LED in series with D2.
4. Reset the bias trim (TR1).

This moves the opamp's inputs and output far enough from GND for a TL081 to work. The cost of doing this is two more LEDs and some volume. There is also a risk that the opamp will latch-up during start-up because some JFET opamps do weird shit when their inputs get too close to GND.  Me, I'll stick with a rail-to-rail opamp like the CA3130.


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## jubal81 (Aug 24, 2021)

Got mine updated to the latest version this morning. 
I tried to set it up so the bias pot full on was loudest/smoothest and all the way off is 80's toy "8-bit" sound. Fun. As. Hell.
What I did is swap the bias pot the other way around, turn it all the way up, then adjust TR1 until the collector voltage on Q2 is about 4.5V. It takes a good bit of time for the voltage to stabilize, so patience is required.
You also have to be a bit patient when changing the bias control. It takes a little time to catch up. Not too long, but it's just not instantaneous. 
1K for Gain control is great.
The other note I have is I still prefer the original tone control. It has less range, but for me the whole sweep was in the sweet spot.


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## Chuck D. Bones (Aug 24, 2021)

I have mine set up so BIAS at 7:00 makes symmetric or close to symmetric clipping.  That puts Q2-C at 5.66V on mine. Turning BIAS up increases volume, mid presence and even-order harmonics.  Above 3:00, it gets sputtery.  I could have just as easily oriented the BIAS control the other way.  Since it's B-taper, it's easy to switch over.  Changing the BIAS setting settles in under a second.  The bias trim is slow as molasses.  As it should be.

For the readers, the "original tone control" that jubal81 references predates the first schematic above.  It had C4 = 10nF, R4 = 47K and VR3 = B50K.  Like he said, a little less range.  Check it out on TSC or breadboard it.  They are very similar. 

This is the v0.3 tone control. Green is tone at 0; blue is tone at 5, magenta is tone at 5.5 (flat), white is tone at 10.  Note: the TSC and schematic ref des do not match.  Can't be helped.  I made R2 (below) very large so it behaves like it's not in the circuit.  There's always a way to get a simulator to do what you want.




Whether C3 is 470nF or 1uF is _mox nix_.


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## cooder (Aug 24, 2021)

Chuck D. Bones said:


> Attention Cooder:
> I found a way to make a TL072 (or TL071, TL081, etc.) work.
> 
> 1. Install a green LED between Q1-E & GND.
> ...


Following this glued to the screen, and I'm not worried about getting a CA3130 for 2 dineros at Tayda since your reply made as expected total eyeopening sense. Not interested in possible weird shit that might happen with a cheaper opamp.
Thanks guys, riveting stuff.


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## Chuck D. Bones (Sep 1, 2021)

As requested, here is a PNP version of the SFF that does not require a charge pump or isolated power supply.  Same parts (except for the transistors) and same topology, I just reoriented everything that has DC flowing thru it.  At the moment, I'm running a П28 for Q1 and a 1T30B for Q2.  They're both low-leakage and HFEs are in the mid 60's.  Nothing magic about those part numbers, they were in the parts tray so I plugged 'em in.  I'll try some other PNPs and see how they sound.  C11 was added in the previous revision; it turns the CONTOUR control into a TIGHTNESS control.  The C50K could be replaced with an A50K, just swap pins 1 & 3.


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## cooder (Sep 1, 2021)

Duuuuuuude! Celestial indeed! I wonder who the goofball was who requested that...


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## Chuck D. Bones (Sep 1, 2021)

The reason this works is because from an AC point of view, ground and the power supply rail are the same thing.  It's important that the power supply is clean (no hum or noise), that's always a requirement with a FuzzFace and most other discrete designs.


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## jubal81 (Sep 1, 2021)

Chuck D. Bones said:


> The reason this works is because from an AC point of view, ground and the power supply rail are the same thing.  It's important that the power supply is clean (no hum or noise), that's always a requirement with a FuzzFace and most other discrete designs.


Would it be worth it to add a small resistor in series from D1 to C9?


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## Chuck D. Bones (Sep 1, 2021)

Wouldn't hurt.  Current draw is around 2.5mA.  I wouldn't go any higher than 470Ω.  I'll give it a try on the breadboard...


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## Chuck D. Bones (Sep 1, 2021)

Make that 3.2mA.  700mV drop in a 220Ω resistor.  Seems to work ok.  I know you set the bias lower on yours so the current draw will be less.


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## jubal81 (Sep 1, 2021)

Chuck D. Bones said:


> Make that 3.2mA.  700mV drop in a 220Ω resistor.  Seems to work ok.  I know you set the bias lower on yours so the current draw will less.


Dunno about yours, but what I've got on my board is eerily low noise.


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## Chuck D. Bones (Sep 1, 2021)

Mine isn't "noisy" but it does pick up a lot of hum if I crank the gain.  Once it's in a closed box that will all go away.


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## cooder (Sep 2, 2021)

So would the PNP version possibly more susceptible to noise compared to the NPN? A question that nags my brain...
Also, regarding noise suppression, Peter Rutter uses inductors in some of his designs for that, would that be a good one to nudge in?


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## fig (Sep 2, 2021)

Mine is dead silent. Probably due to the fact I haven't built it, but I haven't yet finished diagnosing, so I'm not going to commit to that possibility.


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## Coda (Sep 2, 2021)

fig said:


> Mine is dead silent. Probably due to the fact I haven't built it, but I haven't yet finished diagnosing, so I'm not going to commit to that possibility.


You must have built the MXR noise gate instead…


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## Chuck D. Bones (Sep 2, 2021)

cooder said:


> So would the PNP version possibly more susceptible to noise compared to the NPN? A question that nags my brain...
> Also, regarding noise suppression, Peter Rutter uses inductors in some of his designs for that, would that be a good one to nudge in?


I don't think the PNP version would be more susceptible.  Either polarity requires clean 9V power.

Power filtering is as much empirical as it is analytical.  Filter inductors won't hurt, but they may not help either.  I have built a couple VFE boards and put in the 1mH choke.  Was it necessary?  I have no idea.


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## Chuck D. Bones (Sep 2, 2021)

fig said:


> Mine is dead silent. Probably due to the fact I haven't built it, but I haven't yet finished diagnosing, so I'm not going to commit to that possibility.


Is there a typo in that sentence?


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## fig (Sep 2, 2021)

Beyond several grammatical errors, the sentiment (a failed attempt at humor) seems intact. 

I am not resigned to non-existence being a cause for silence?


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## Chuck D. Bones (Sep 2, 2021)

To quote Henry Lloyd Moon, Jack Nicholson's character in the movie Goin' South: "A joke is when you make other people laugh."


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## fig (Sep 29, 2021)

Just saw this! So true. 

What is a joke if nobody laughs? 

_Useless_


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## Chuck D. Bones (Oct 14, 2021)

The bias trimmer is too sensitive, there needs to be a resistor at each end of the trimpot to limit the range.  I like to set it between 1.0V and 1.8V.


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## Chuck D. Bones (Oct 17, 2021)

See Chucks Boneyard for latest rev of the PNP design.


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