Help with salvaged two stage npn SE4010 circuit

[walkman]

Hi all,

l salvaged this circuit board many years ago from some sort of audio equipment. It is a two stage NPN SE4010 circuit.

I’m not very familiar with transistors but think have seen this topology before, though I’m not sure how it works or what is called or used for.

At a guess would it be a high gain first stage with feedback to the input of the first transistor?

Perhaps a mic preamp? Or possibly to directly drive a headphone or small speaker?




Can this be used as a preamp or guitar pedal with added hi z input 1m /68k , 9v V+ and a 50k ? volume pot added at the output.

How could I put this circuit to good use for audio ?


Cheers
Walkman

[mozz]

Looks like a fuzz face circuit, no output capacitors though.

[Helmholtz]

It's a textbook 2-stage, high gain small signal amplifier circuit. The NFB works for DC only and provides the bias for the first transistor.
The 1M resistor looks wrong; value should be much lower (maybe 100R).
As Mozz said, a DC blocking output cap is missing. Maybe the 0.01µ cap is conneced wrong?

Circuits like that were used "everywhere" including e.g. tape recorders.
Not suitable for headphone or speaker.

[walkman]

Thanks Helmholtz, I rechecked the board and it’s 560 ohm not 1M.

The 0.01 looks drawn correctly, but I can see part of a component lead in the board at the output side of the second transistor at that must have broken off, likely then an output cap as you say.

[Mark Hammer]

Mozz and Helmholtz are correct. Unless there is a DC blocking cap on the output of that 2nd transistor, or unless there is a DC-blocking cap on the input of whatever it was supposed to go to, that output, as drawn, is a recipe for problems. As noted, though, it was "salvaged from some sort of audio equipment" so it may well have anticipated appropriate DC-blocking in some other part of the equipment it was in.

It could be transformed into a distortion/fuzz circuit, but not without a number of changes.

[Mick Bailey]

A coupling cap and volume pot are perhaps all that's needed, depending on your guitar and personal preferences. I'd certainly try it as a guitar preamp, especially if you're running it into a tube amp.

[35L6]

Hook it up ! If it stays clean it's a boost , if it clips it's a fuzz box . Either way you win .

[walkman]

Thanks all for the encouragement and advice, after looking at a few of the many sites about the fuzz face I’ve altered the existing as found circuit and put it into a temporary home.


Old 3.5” disk cary case, good for a quick test build ;-)


Now that I know it works, I can add a fuzz control. May post a clip and the voltages.

Does it fuzz, yes it does, sounds quite good and very little noise.

(perhaps a little too much compression? Can that be tightened up for a little quicker response? )


p.s noticed that I had left the 330r from the emitter of the first stage. It is still in the circuit what effect will it have to bypass it ?


Cheers
Walkman

[mozz]

Yes you need that cap from Q2 emitter to ground. Also your input cap should have the positive side towards the transistor base. Then you use your guitar volume to clean up.