Softer Fuzz Break-over

[Madkatb]

I built a modified version of this circuit for a friend. (Russian MP16B Germanium trannies, PNP, D1 1n4742, D2 white LED)
When he plays softly there is no fuzz but as his picking becomes harder it suddenly "breaks over" into distortion. He has no complaint about any of that as it suits his playing style dynamically. But... is there any way to minimize or soften the break over point, so that it doesn't happen so suddenly? Series resistor between D1 and collector of Q1?
http://www.home-wrecker.com/buzzbox.png

[ghostsauce]

I'm kinda just guessing but I assume a pot before the gain stages would do the trick since it would lower the signal going in, and then you could turn up the post gain volume pot to where it should be.

[Madkatb]

Adjusting the input level by using the guitar volume had no effect on what I described other than to need harder picking at reduced guitar volume. I'm thinking it has more to do with the input signal/ gain. When it reaches the right level the diode starts to conduct and clip the signal. I'm looking for a way to "soften" the onset of clipping. The pedal response goes from clean to distorted quite suddenly.

[~arph]

mmm.. try using diodes with a lower forward voltage... a germanium for D2 for instance (like the original schematic you linked)

[Mark Hammer]

Would a small resistance (e.g., 1k) in series with each of the diodes do it?

[Madkatb]

The series resistance was what I was thinking. Changing the diodes would change the voltage where the clipping starts but I'm trying to adjust "how" the clipping starts, trying to smooth out the abruptness.

[Mark Hammer]

Well, it is well beyond the perimeter of my expertise, but I was under the impression that the knee or break-over point on many diodes was sensitive to the current applied.  And certainly a series diode will tinker with that current.

[~arph]

Yup, and germaniums have a softer knee

[LucifersTrip]

I there is no fuzz but as his picking becomes harder it suddenly "breaks over" into distortion.

what you are describing actually sounds like a misbiased fuzz...which is probably what's happened since you used ge's instead of the silicon

[Jaicen_solo]

Could be blocking distortion. Try a 10-100k resistor in series with the 10uF interstage resistor.

[pinkjimiphoton]

mmm.. try using diodes with a lower forward voltage... a germanium for D2 for instance (like the original schematic you linked)

that's cuz the led doesn't clip until you're really on it. led's make it louder, sorta, but they kinda kill the distortion unless driven really hard.
especially a white one, they take more to make 'em break down and conduct.
try a 3mm green or red one with a germanium diode in series with it, maybe a 1 k pot between the diode clipper and ground. you should find it easier to get a "crossover" point that's useful.
led's need something like 3 volts to conduct... an 1n34 or something is more like .3v.

you COULD try adding another led in parallel there, but not sure if it would make a difference in the conduction point.

but the prob you describe is absolutely the LED imho. i've grown to really hate them, i used 'em a lot in early attempts at building toys...they can sound wonderful, but it takes a lot to make
'em scream.

[Kesh]

have found Ge diodes do this breaking into and out of distortion in other diode feedback clipping circuits.

i think what's going on is the large germanium leakage, being roughly flat for any level of signal, when in negative feedback, effectively expands the signal, but in your circuit this would only happen on half the signal, when the 1n34a is reverse biased. this is the half of the signal the diode won't clip, and the half that overdrives the transistor

expanded dynamic range means transition into distortion is sudden.

[Electron Tornado]

I tried this out on the breadboard this afternoon. I preferred the sound with the 1N34 on Q1 and the 1N4148 on Q2, but it still wasn't that great. I didn't like the sound as the notes were allowed to decay. So, after some fiddling, I came up with this:

The pot on the input would probably be better as 150k or so (250k was too much), but 100k was what I had on hand. It could be just left off entirely as the circuit cleans up nicely with the guitar volume control. Putting a cap in parallel with the diode on Q1 really helped the sound of the decay - much smoother. Using two diodes in series on Q2 really improved the sound to me. The LED is a 3mm red LED.

Edit: This is the revised schematic:

[Kesh]

i'd suggest that if you want a Ge level of clipping, but without the weird decay, use a Schottky diode like the BAT43

[mac]

Try ading a series resistor between stages. A 100k at the collector and a low impedance load does not look good, imho.

mac

[Electron Tornado]

Try ading a series resistor between stages. A 100k at the collector and a low impedance load does not look good, imho.

Not sure if you were addressing my comment about decay, but I tried a resistor between the stages, and then with one on the input as well, and there was no real change. Using a 1N34 diode on Q1 did help, however. Using a 1N4148, like my schematic shows gives a more aggressive tone, though.

Since this bears a certain resemblence to the Big Muff Pi, I took a look again at this discussion:  http://www.diystompboxes.com/smfforum/index.php?topic=78111.0   I'm wondering if adding a resistor to either or both transistors to slightly lower the gain will help smooth the decay.


I hope this is useful and I'm not hijacking this thread. 

[Quackzed]

some other things you could try...
--try a 500 or 1k (even 2k) btwn the emitter and ground of q2 or both?... should lower gain and alter bias a bit for less sputter...
--add a cap in parallel with the  diode/s on q2 as well as q1
--lower q2's collector resistor to @ 50k or as low as 10k even... lower gain and alter bias for less sputter as well. at least i think.
but if a cap worked for q1 it should do the same for q2
... i remember simming the bazz fuss and theres a definite fine line between bias point (diodes) and collector and / or emitter resistors
i know the emitters are to ground in this schem but theres no reason you can't add an emitter resistor....

[Electron Tornado]

some other things you could try...
--try a 500 or 1k (even 2k) btwn the emitter and ground of q2 or both?... should lower gain and alter bias a bit for less sputter...
--add a cap in parallel with the  diode/s on q2 as well as q1
--lower q2's collector resistor to @ 50k or as low as 10k even... lower gain and alter bias for less sputter as well. at least i think.
but if a cap worked for q1 it should do the same for q2
... i remember simming the bazz fuss and theres a definite fine line between bias point (diodes) and collector and / or emitter resistors
i know the emitters are to ground in this schem but theres no reason you can't add an emitter resistor....

Adding a 470 Ohm resistor from the emitter of Q1 to ground seems to have cleared things up. I tried values smaller and larger, but 470 Ohms seems the best. Leaving the 1N4148 on Q1 sounds much better as well. I tried the cap on Q2 earlier, but it didn't help. I also tried reducing the gain on Q2, but it's best left where it is.

The schematic above has been revised.

[Madkatb]

I checked bias on the two transistors and made some changes. Now have -0.7V on Q1 (collector to ground) and -4.2V on Q2. Sounds a lot better. Fuzz from low level all the way up so I managed to get rid of the "clean, abruptly fuzz" issue. Now, if I could just get the clean/ wet blend control to work... (Tried this after making the changes.)

[Jaicen_solo]

Having looked at the schem a bit more, it occurs to me that the abrupt transition into distortion may have something to do with the forward voltage of the diode. When it starts to conduct, it's going to mess with the bias on the base of the transistor. So the harder you drive it, the more it wants to turn on the transistor.

It's probably inherent in the design that it behaves this way, its likely it was designed to be clipping all the time.