Sunn O))) inspired distortion

[JohnH]

In my random pedal-googling I came across this distortion circuit, and thought I'd see if I could build it:

http://www.taydakits.com/instructions/sunn-o-distortion

I'm a bit of a noob, and of course this circuit isn't very well supported (no real instructions, slightly confusing circuit diagram, no schematic, no PCB to purchase). I put the diagram and PCB layout in photoshop so I could see where the connections actually were, and came up with this:



Anyway, I fiddled around for awhile (I'm embarrassed to say for how long...) and finally got it to work on a breadboard



It actually sounds pretty good. Better than the other circuit I breadboarded last week. I didn't have electrolytic caps in most of the values it required, so used ceramic caps instead. I don't know if this makes a difference?

From what I understand a pot between pins 1 and 8 of the first IC gives you control over the gain, right? I might stick one in tomorrow arvo and see what happens, and then I'll try to solder it to some veroboard and box it up.

[Marcel]

Half the battle is making it work in the first place... so regardless of the time you spent you are doing well...

Using other caps shouldn't make much difference so long as the uF value is the same or very close. What will change is the tonal curve. You probably won't notice any difference when it is by itself, but when put up back to back with a correctly made one you will be able to say it has more of "this" or less of "that"... and if you like it the way it is then ... well you know....

Try a 10k pot in series with the 10uF cap on pins 1 & 8 of the first IC to vary the amount of distortion (gain of the 1st IC). A pot and cap on the 2nd IC will vary the overall volume of the pedal. A look at the data sheet via Google ( www.ti.com/lit/ds/symlink/lm386.pdf ) gives some examples of how to set the IC up. You could 'play' with (vary the circuit) that gain control to give a bass boost amongst other things.

As the IC is designed to drive a small loud speaker with about 200mW or 0.2W and given your comment on how it sounds this project could be morphed into a sweet little 9V battery powered pocket Amp...

[JohnH]

Thanks for this Marcel, that link is great. I'll play around with it this weekend, see what happens and will keep you posted.

[JohnH]

Played around with this circuit today. The gain control between pins 1 and 8 on IC1 works well, but I just couldn't get the bass boost to work at all. I tried a 10k pot and a capacitor between pin 1 and 5 on both the first and second IC's, and all I got in both cases was a drastic volume reduction, and a kind of staccato stammering in the output signal.

To be fair, I don't actually have a 0.033uf, so tried using both a 0.01uf and a 0.047uf. Capacitor values confuse me, and my understanding of what's happening is limited, so it's possible it was never going to work with those values, but the volume reduction and stammering signal surprised me. Is it likely to be the capacitors causing it? Or is it more likely to be operator error, and I'm just doing something wrong somewhere?

For what it's worth, I thought it may have been a dodgy potentiometer, but tried two and got the same thing both times.

[Andy40]

Two LM386s ...interesting

[DrNomis_44]

Played around with this circuit today. The gain control between pins 1 and 8 on IC1 works well, but I just couldn't get the bass boost to work at all. I tried a 10k pot and a capacitor between pin 1 and 5 on both the first and second IC's, and all I got in both cases was a drastic volume reduction, and a kind of staccato stammering in the output signal.

To be fair, I don't actually have a 0.033uf, so tried using both a 0.01uf and a 0.047uf. Capacitor values confuse me, and my understanding of what's happening is limited, so it's possible it was never going to work with those values, but the volume reduction and stammering signal surprised me. Is it likely to be the capacitors causing it? Or is it more likely to be operator error, and I'm just doing something wrong somewhere?

For what it's worth, I thought it may have been a dodgy potentiometer, but tried two and got the same thing both times.

A 0.047uF should work fine as a substitute for a 0.033uF cap, you might find that it'll let a bit more bass frequencies through though, but it should work with no other issues.


You could also try a 0.022uF in parallel with a 0.010uF, which will get you closer to the specified value of 0.033uF, but the thing is, the value of the cap isn't as critical to the operation of the circuit as you might think, caps tend to have a tolerance of something like +/- 10 to 20% of the marked value, which means that the actual value of the cap can measure anywhere within +/- 10 to 20% of the value marked on it, so you have quite a range of leeway.

[JohnH]

Thanks Doc. I hadn't realised the cap tolerances were so wild! Good to know. I wonder what I'm doing wrong then? Will have to have a play around with it again next weekend and see if I can figure it out.

[DrNomis_44]

Thanks Doc. I hadn't realised the cap tolerances were so wild! Good to know. I wonder what I'm doing wrong then? Will have to have a play around with it again next weekend and see if I can figure it out.

To me, the stammering signal sounds a lot like what's called motor-boating, motor-boating is when the circuit goes into self-oscillation at a low frequency, and sounds a bit like a motor-boat engine idling (it's heard as a regular putt-putt-putting kind of sound), motor-boating is usually caused by poor power supply filtering, basically, the input of an amplifier picks up a signal superimposed on the +V supply, this signal gets amplified and is then fed back to the input again, etc, etc, it's a form of positive feedback, to fix that, you can try putting a large value electrolytic cap from the +V supply to circuit ground, you can use any value of Electrolytic cap from say 100uF to 470uF, just make sure that the working/rated voltage of the cap is greater than the supply voltage, for example, if the circuit is only being powered by +9V you can use an electrolytic cap with a voltage rating of 16V or 25V, or even 35V, as long as it's higher rated than the supply voltage.

[JohnH]

That definitely sounds like what I was hearing. Good to know; I'll try that this weekend!

[DrNomis_44]

That definitely sounds like what I was hearing. Good to know; I'll try that this weekend!

Let me know how it all goes.