Differential Fuzz Design

[mdcmdcmdc]

Hi folks,

I've been poking around on this circuit for the past few weeks and it's getting to a point where I'm happy with how it's sounding, so I thought I'd share. This is my first dip into assembling something out of a few disparate fragments that weren't cribbed directly from other pedals (which is to say, they were cribbed from old radio magazines instead).

I built Dan Coggins' Dinosaural Tube Bender circuit last year and really, really liked it. It was the first time I'd used a pedal based around a differential amplifier, so that was the jumping off point for this project. This uses a different style of differential amp than Coggins'—a wide-band single-ended design was how it was described from wherever I grabbed it—and most of the distortion comes from stacking two of them together into some good ol' LED clipping. The differential amp has a particular quality to it, you get a lot of articulation and growl rather than smooth sustain.

The input section is a needlessly complicated unity buffer that gives high impedance and adds a kind of pleasant EQ curve to the signal. I found it in an issue of wireless world from 1968 and tweaked it slightly to work with the rest of the circuit. The output is obviously a big muff gain stage minus the clipping. I had started with a germanium output stage, but honestly it didn't really add anything besides more parts. 

Saturation comes from changing the resistance to the emitters of the first differential pair; the sound goes from a medium distortion/crunch to a pretty wild fuzz. I did most of the tweaking to the circuit with a goldfoil-equipped guitar and a vintage jazzmaster. I haven't tried it with humbuckers yet.

Apologies in advance for any errors in the schematic, and if something jumps out as being extremely-not-best-practice I can't state strongly enough the degree to which I have no clue what I'm doing. I'm sure you folks will have some incredibly helpful thoughts on how to improve this, as I think I've taken it about as far as I can. If anyone feels like breadboarding it, I'd love to hear what you think!

Thanks for all the help debugging my wonky builds in the past (and future) and for this great archive of problem solving and pedal nerdery! Here's the schem:

[iainpunk]

thats honesty a sexy schematic. i cant wait to try this!

cheers

[mdcmdcmdc]

Thanks!

[antonis]

thats honesty a sexy schematic.

You definately are a boot(strap) lover... 

[duck_arse]

you have a 200k to ground before the led clippers, and a 100k to ground after the led clippers. I don't know, but I think that is only a legal combo if you have a cap between the clipper leds and the output transistor base. otherwise, one resistor has to go.


ahh, also - there is a cap resistor cap in series between the PNP and first diff transistor. one of those will have to go as well, my money is on the electro.

[mdcmdcmdc]

Thank you! Good catch on the cap between the LEDs and the output stage.
I imagined there would end up being some vestigial/redundant decoupling capacitors in there from stacking the circuit fragments together and not actually knowing what any particular part is doing...

I'll pull out the 1uF see what happens.

[mdcmdcmdc]

Per duck_arse pulled the 1uF and added a 100nF cap before the output stage (thanks again!). Dropped the emitter resistor in the 2nd differential stage to 22K to compensate for what I think was an overall loss of gain.

[mdcmdcmdc]

Spent a few hours on this circuit yesterday, just generally futzing around and seeing if I could make it work a bit better. There was a lot of ultra-high frequency noise coming through which seemed to manifest itself as pretty wild amounts of string wiping sounds that I was trying to chase down. I ended up finding a slightly simpler version of the bootstrapped sziklai buffer circuit to try, which seemed to help a little but in the end I just added a passive bandpass filter after the input to clamp down on everything. There's no lack of output volume in the circuit so the insertion loss is a non-issue.

Similarly, I tried a bunch of different resistor values in the wide band pairs and wound up finding a combination that seemed to give about the same gain but sounded a little better to my ears. Again, these amplifier stages tend to add a lot of high end, so I put a LPF after the second differential stage before the final stolen-from-a-big-muff clipping stage.

I'll probably go back and try to A/B the original buffer stage and see if one actually does work/sound better than the other, but I'm out of breadboards so that'll have to wait a bit. It feels like this one is a tiny bit more scooped that the first version, which felt fairly flat with maybe a bit of a high-mids lift.

Here's the schem:

[edit: a couple of typos in the schematic]

Subjectively, it sort of lives in the space between where a Rat leaves off and a Big Muff picks up.

[duck_arse]

me again. I see 100R to ground following the 22uF out of the pnp. that can't be right. can it?

[mdcmdcmdc]

Again, I'll preface this by saying I have literally no clue what I'm doing but based on the online HPF calculator I checked that should give a cutoff around 72hz?

[PRR]

The 100r will make the first stage, with 3.3k load, distort REAL easy.

That may be the point?

[mdcmdcmdc]

Saying it's 'the point' is definitely giving me too much credit, but may be a happy accident?
Essentially, the buffer circuit as presented had a 22uF at the output and it sounded ok; not understanding that stage well enough to want to start changing things too much but wanting to add the BPF after the buffer stage led me to just look up "what resistor works with this cap to give me the cutoff I need?" et voila: 100R.

I'll swap the 22uF for a smaller value and raise the resistor accordingly and see what happens.

[antonis]

Essentially, the buffer circuit as presented had a 22uF at the output and it sounded ok;

Let me guess..
Maybe WITHOUT 100R shunt resistor..??
(just 22μF coupling cap..)

Again, I'll preface this by saying I have literally no clue what I'm doing but based on the online HPF calculator I checked that should give a cutoff around 72hz?

As Paul told you above, 100R is effectively set in parallel with 3k3 resistor, severely loading n-p-n/p-n-p stage..
If you like to have a 72Hz HPF at this very point, interchange 47k/100pF and 22μF/100Ω positions..

P.S.
Even after the above suggested mod, input buffer is still loaded a bit due to 10k//120k//3.3k//(47k+100R) = 2.3k effective AC Emitter load..

[mdcmdcmdc]

Ah, ok that makes sense. so, bear with me here... if I swap the 22uF for say a 220nF (which still seems like a reasonably large output cap?), raise the 3.3K to 7.5K and use a second 7.5K to complete the HPF I should have about the same emitter load (7.5/2=3.75) and a cutoff that's also in the same ballpark (64hz)?

[antonis]

 

I presume you aren't very familiar with transistor bias techniques..

By doubling 3k3 value (7k5) you halve Emitter & Collector quiescent current hence halving buffer's current driving ability..

If you don't like to do what people tell you here (pas mal..), make 22μF -> 10nF and 100R -> 220k (for 72Hz) and earn peace of mind..
(both for you and us all.. )

[mdcmdcmdc]

It's not that I don't like what you're saying, I'm just trying to understand what's happening as this is all far beyond my pay grade! I'm trying to inch my way to understanding more...

I interpreted what you had said about the two resistors being placed in parallel to mean that they'd end up effectively acting as a single ~97R resistor. So going from a 3.3K//100R to 7.5//7.5 would get the emitter resistor effectively back to where it had been previously (3.75K instead of 3.3K), and then adjust the coupling cap of that stage to get the HPF cutoff in place. But again, this is just a guess and I'm trying to figure it out as I go.

Maybe a better way to ask the question is: I'd like to drop a passive BPF between the buffer stage and the first differential amplifier stage. What's the best way to do this without messing up either what comes before it (the buffer) or after it (the differential amp)?

The buffer circuit as I found it includes that 22uF final capacitor—I have no problem swapping it for a 10nF or 100nF or etc, I just want to try to understand WHY that solves the issue.

And (of course!) thanks for taking the time to talk me through this. It's greatly appreciated.

[iainpunk]

It's not that I don't like what you're saying, I'm just trying to understand what's happening as this is all far beyond my pay grade! I'm trying to inch my way to understanding more...

I interpreted what you had said about the two resistors being placed in parallel to mean that they'd end up effectively acting as a single ~97R resistor. So going from a 3.3K//100R to 7.5//7.5 would get the emitter resistor effectively back to where it had been previously (3.75K instead of 3.3K), and then adjust the coupling cap of that stage to get the HPF cutoff in place. But again, this is just a guess and I'm trying to figure it out as I go.

Maybe a better way to ask the question is: I'd like to drop a passive BPF between the buffer stage and the first differential amplifier stage. What's the best way to do this without messing up either what comes before it (the buffer) or after it (the differential amp)?

The buffer circuit as I found it includes that 22uF final capacitor—I have no problem swapping it for a 10nF or 100nF or etc, I just want to try to understand WHY that solves the issue.

And (of course!) thanks for taking the time to talk me through this. It's greatly appreciated.

we like to see bigger resistors and smaller caps in builds and designs, since these need less current to fulfill the same function. the buffer doesn't really like to drive the ''high current'' filter in your design, and the transistor might live way longer if you ask it to conduct less current.

cheers

[antonis]

I interpreted what you had said about the two resistors being placed in parallel to mean that they'd end up effectively acting as a single ~97R resistor.

For AC only (signal).
For DC, 22μF cap is considered "open circuit" (blocks DC) so 100R is "isolated" from CFP Emitter resistor (3k3)..

So going from a 3.3K//100R to 7.5//7.5 would get the emitter resistor effectively back to where it had been previously (3.75K instead of 3.3K), and then adjust the coupling cap of that stage to get the HPF cutoff in place.

As said above, 7k5 resistor in place of 100R one is considered in parallel with whatever value Emitter resistor for AC ONLY..!!
For DC, new Emitter resistor value (7k5) represents itself..

Maybe a better way to ask the question is: I'd like to drop a passive BPF between the buffer stage and the first differential amplifier stage. What's the best way to do this without messing up either what comes before it (the buffer) or after it (the differential amp)?

I suggested you two altenative solutions above..
(either interchange positioning between 22μF/100R and 47k/100pF or lower 22μF down to 10nF and raise 100R up to 220k..)

The buffer circuit as I found it includes that 22uF final capacitor—I have no problem swapping it for a 10nF or 100nF or etc, I just want to try to understand WHY that solves the issue.

The buffer circuit, as you found it, DOESN'T include 100Ω resistor shunting to GND..!!
(you don't read thoroughly what is already written..)

[antonis]

As for that "notorious" CFP" (Sziklai) buffer, make 10k upper voltage divider bias resistor a 82k one (or 100k by also making lower 120k resistor a 150k one) and make 150k bootstraped resistor a 47k  one..

The former for a more healthily biased Emitter follower (symmetrical undistorted swinging ability) and the later for more stiff bias configuration (more current through bootstraped resistor + voltage divider Thevenin equivalent one)

P.S.
Bootstrap factor is about 180 so 47k resistor is "seen" as 8M5 one..
(higher apparent value makes no difference 'cause total input impedance is dominated by h_FE x (R_Emitter//R_Load)

[mdcmdcmdc]

The buffer circuit, as you found it, DOESN'T include 100Ω resistor shunting to GND..!!
(you don't read thoroughly what is already written..)

Thanks again for your help with this—I'd just like to add that there's a difference between "not reading thoroughly" and "asking for clarification when you don't understand something." There may be a slight language hurdle (your english is definitely better than my Greek!), so I just wanted to make sure I understood WHY you were suggesting the changes rather than just asking for a solution. Yours and Ianpunk's responses above cleared it up—the general preference for smaller caps and bigger resistors; the AC working in parallel vs the DC being blocked by the cap.