Help with identifying circuit

[tonedawg]

I recently de-gooped a circuit from an old 2 n 1 booster (Morgan shadow boost), I've attached a pic. Its interesting, one side sort of looks like an electra but with led soft clipping, the other is very odd, one 4148 diode to ground the other I'm not sure where it goes yet. I havent been able to fully trace this yet, was wondering if anyone could make sense of it and maybe point me to a schematic that exists already. One side is 2N3904, the other is BS170, both have volume and gain controls. Both gain controls crackle when turning.

[tonedawg]

Ok, I just traced one side, this one looks like a highly modified electra. Does this make any sense at all? I don't understand what's going on with the series resistors off the emitter.

[Rob Strand]

Good going.  I couldn't see the pic this morning as the image server was down.

I'm a little suspicious that the diodes (actually RED LEDs?) should connect between the caps (as they are now) but the other end goes to ground.

For the other side perhaps check out some zvex SHO schematics on line.   It has similar structure but no diodes.

Emitter resistor looks normal to me.  It being where it is might fend off some of the oscillations people get on the SHO.

[tonedawg]

The other side is definitely an sho, just finished tracing that one.

This side, it's super weird, the led diodes 100% do not go to ground, I tested with a meter to be sure.

[Rob Strand]

The other side is definitely an sho, just finished tracing that one.

Cool.  Good chance it was going to be that.

This side, it's super weird, the led diodes 100% do not go to ground, I tested with a meter to be sure.

Yeah, pretty weird.   These things are what the are.   Sometimes the reasons aren't clear why.

[iainpunk]

The other side is definitely an sho, just finished tracing that one.

This side, it's super weird, the led diodes 100% do not go to ground, I tested with a meter to be sure.

if thats so, they should be gating the signal quite a bit, does it gate?

cheers

[ElectricDruid]

if thats so, they should be gating the signal quite a bit, does it gate?

Iain's right, that's the "budget noise gate" diode connection that you mostly see in metalhead pedals - Boss HM2 is an example, I think, and there are plenty of others.

[Vivek]

Coring diodes ?

[iainpunk]

Coring diodes ?

crossover diodes, they don't exactly core a wave if there is a capacitor in series with these diodes, but it does gate. Andy's Minisagverket is actually designed around such diodes.

edit: also, my Embryo Pink pedal has crossover clippers, which help it get its gruesome sound

cheers

[Rob Strand]

The 47n across the diodes is going change things a lot.   It's not going to work well as a noise gate since the 47n will pass the noise straight through.   The cap minimizes the effect of the diodes somewhat but we don't know the value of the pot it's feeding. 

[assuming silicon diodes - which I doubt is the case]
The 100n + 47n in series forms a high-pass filter cap of 32n.    The collector swings say +/- 4.5V, to the diodes to conductor we need +/- 0.6V across the 47n cap, which translates to +/-1.5*0.6=+/-0.9V across the series 32n.  The voltage across the cap is like a low-pass filter formed by the series caps and the pot.  The clip point is -14dB down (ie. 4.5V -> 0.9V), fclip = 4.9/(2*pi*Rpot*C).   The lowest frequency will produce the most voltage across the cap, so fclip = 80Hz, C=32n so Rpot needs to be less than 300k in order for the diodes to have an effect.    300k is the barely doing anything point.   A 50k pot will make it more pronounced.

So I guess it looks like it could do something with a reasonable pot value.


[Assuming LEDs]

Actually weren't the diodes actually LEDs?.   I'm pretty sure the diodes on the board are for the SHO stage MOSFET protection.
The LEDs are going to have a clip point of 1.7V ie. harder to clip.
+/- 1.7V across the 47n cap  =>  +/-2.55V across the series caps.
4.5V -> 2.55 V => -4.9dB
fclip = 1.44 / (2*pi*Rpot*C)
fclip >= 80Hz => Rpot <=  90k

So a 100k pot isn't going to do much.
A 50k pot is going to be suble.
Could the pot value be lower?

[tonedawg]

Sorry for the late reply, yes the diodes are red LED's. The gain pot value is 1K.

[Rob Strand]

Sorry for the late reply, yes the diodes are red LED's. The gain pot value is 1K.

1K for gain makes sense (ie. the pot on the emitter).

My calcs refer to the volume/level pot.  The pot on the output.

[tonedawg]

Just looked, 100K volume pots.

[Rob Strand]

Just looked, 100K volume pots.

Cool, thanks.   

Based on the calculations, that's verging on the LEDs having no effect.

[tonedawg]

Interesting, perhaps I'll go back and double-check the tracing, though I don't think it's a mistake, there's not much in here!

Can you also help me understand why they ran the resistors the way they did off the emitter? Like, why is there a 47 ohm there?

[Rob Strand]

Can you also help me understand why they ran the resistors the way they did off the emitter? Like, why is there a 47 ohm there?

An unbypassed emitter resistor sets the maximum gain.

There could be more to it.  It might help prevent oscillations.  The fact it limits gain can help prevent oscillations in itself.  A second effect is the resistor provides damping.  The wires to the gain pot have inductance and this can form an oscillator.  in the past, people have had problems with the SHO oscillating and it's pretty well established this is caused by wiring inductance to the gain pot.  Adding the emitter resistor helps prevent this.  Also placing the resistor right at the emitter gives the best chances to prevent oscillations.

The bypassed emitter resistor is there to help set the DC bias reliably without affecting the gain.

[antonis]

It might help prevent oscillations.  The fact it limits gain can help prevent oscillations in itself.  A second effect is the resistor provides damping.  The wires to the gain pot have inductance and this can form an oscillator.  in the past, people have had problems with the SHO oscillating and it's pretty well established this is caused by wiring inductance to the gain pot.  Adding the emitter resistor helps prevent this.  Also placing the resistor right at the emitter gives the best chances to prevent oscillations.

7 sentences for the sake of simplest form of Negative Feedback argument.. 

(you have to admit you tend to fold out into a gabby guy, from time to time, Rob..)

[Rob Strand]

7 sentences for the sake of simplest form of Negative Feedback argument.. 

(you have to admit you tend to fold out into a gabby guy, from time to time, Rob..)

The circuit (potentially) has bigger problems than just feedback.   The feedback part is the first paragraph.