My Overdrive: Samples and a "Buzz/Fart" Question (Again)

[railhead]

Okay, I'm getting closer and closer to capturing the sound I'm looking for with my overdrive circuit -- but there's a slight issue with the low notes. First, though, here's a sample of the overdrive, played with my American Deluxe Tele through a Stulce SA-10C set flat.

Little Monster Clip 001

I totally LOVE the tone I got, but that same great tone doesn't flow to the lower strings. In this next clip, I play a few chords, and then slam the low E and do some single note fretting. If you listen, you'll hear a buzz in the tone -- but I don't want it there.

Little Monster Buzz

I've tried using caps to limit the low end at the input as well as right before the diodes, but it's still there -- or things just turn too nasal. Is this buzz simply coming form the 2N2222A? I'm clipping with 1N4004s and 1N5817s. The circuit started out as the Trotsky drive, but I've since turned it into what I have here.

Thoughts? What do I need to experiment with? What else do you guys need to know to assist?

THANKS!

[Zben3129]

WOW!!!! That first sound clip is exactly the sound I've been trying to get! Great work. About the buzz, I haven't built enough to figure out what it is. Maybe I could breadboard it and mess around with some parts (not the ones that make that tone though, thats gotttta stay)


Cheers


Zach

[railhead]

Glad you like the tone -- I'm really happy with it. A .01 input cap, coupled with a .047 pre-diode cap almost totally kills the buzz, but then everything's too thin. In the clips, I'm using a .022uF input and .047uF pre-diode. Pretty much anything under .047uF sounds too thin.

[Isaiah]

Have you tried using a cap between the diodes and ground? (Assuming the clipping is from signal to ground)

Excellent clips by the way!
Great tone, good playing and nicely recorded for a little demo

[railhead]

Have you tried using a cap between the diodes and ground? (Assuming the clipping is from signal to ground)

Excellent clips by the way!
Great tone, good playing and nicely recorded for a little demo

The cap "trick" is the next thing I'll try. I guess just put it in series, right?

Thanks for the nice words re: the clips. That's the progression I usually play when I'm running effects through my "Chord Test" to make sure they chime good.

[nordine]

you could also do a simple wet/mix control, a la Scrambler, so filter the signal then compensate with some clean (for lows loss)

[jakenold]

Hey Maury,

Great potential. I love simple circuits like this, and have built a bunch of them myself. Try to post the schematic, and I'll build it and see what I come up with.

[disto]

as i said in the pictures post great tone!
hmm if its bass notes that are causing the buzz could you try filtering, maybe filtering high frequencies through the distortion and low frequencies straight to the output ie bypassing the distortion?! you might need to play around with the mix of the two signals though to fid the best combination

[John Lyons]

Sounds great, now to get that last little buzz out.
I think Isaiah is on to something. By putting the cap between ground and the diodes (in series) you will be limiting the frequency of the clipping. Hopefully that's all you need to do.
It would seem that the bass notes are pushing the clipping over the edge into blocking distortion.


John

[railhead]

I'll be trying this tonight, if I can get my printed and populated PCB to let me (without having to make a new one).

Any thoughts on the cap value I should start with?

[Isaiah]

Try anything from 0.022uF to 0.1uF.
This seems to be a pretty usable range for electric guitar.
Let us know how it works for you, please

[PerroGrande]

I agree with John's assessment.  The tonal change in the bass notes was noticeable, and I thought that they exhibited more of the sound of a squared-off wave.

The low-e string on your guitar is going to be around 82Hz (unless it is drop-tuned, of course).  The A string above it will be around 110 Hz. 

I might be inclined to start with a 0.22uF or a 0.47uF capacitor, but be prepared to go larger if it doesn't have enough effect.  If you go too small, it will probably negatively effect the parts of the tone you're happy with.  This may prove to be at least somewhat unavoidable, as the cap is going to be there at all frequencies anyway.

[railhead]

Will do. I guess where I'm confused is in regard to what's actually happening by placing the cap in series. I understand using caps at input, pre-diode, and output to limit range coming in and going out, but what does a cap after the diodes do? Should I doing the additional cap as well as removing the cap I have going before the diodes? Here's the schematic:



C3 is the one you guys are suggesting -- but should I leave C2 where it is?

[Isaiah]

Yeah, leave C2 where it is.
C3 is just where it needs to be as well.
Looks like it should work really well!

[PerroGrande]

Just think of that cap as a frequency-dependent resistor.  In the case of a cap, the higher the frequency, the lower the resistance.  The value of capacitance and frequency determines the resistance (Xc = 1 / (2 * pi * Freq * C).  As the frequency rises, the cap looks more and more like the original wire to ground.  But -- as the frequency drops, the cap looks more like a resistor between the diodes and ground.

You can get a feel for this checking out the "Warp Controls" article at AMZ.  In that article, Mr. Orman is specifically using a variable resistor to act essentially as a volume control for overtones (produced by the clipping process).  You're producing the variable component by using a capacitor -- which makes it frequency-dependent.

[railhead]

So doing this won't change the overall tone -- it will just cut low frequencies before they start to block clip? I guess my limited knowledge understands that the input cap is only allowing certain frequencies through to begin with, so that's one place to cut low end.

As for the pre-diode cap (is there a better words for this?), is it more for limiting the frequencies passed to the diodes, or is it holding the charge from the transistor before pushing the diodes? I can see how either would change the sound, either because less low frequencies are pushed through the diodes to clip, or because less juice is flowing to the diodes, thus they clip less. So, basically, C3 will act more like "frequency bleed" of sorts?

Am I getting close to understanding?

[bipedal]

Had to look up Stulce amps -- hadn't heard of them before, but they look pretty cool.

That first sound clip sounds great; a nice match with the Tele.  Gritty bite and clarity without sounding thin.  Is the grit coming entirely from the clipping in your overdrive circuit, or was it pushing your amp into breaking up a bit too?  Just curious.

Hope you'll post a final schematic of this one when you get it running the way you want --

Cheers,

- Jay

[railhead]

Yeah, I meant to record my amp with the pedal bypassed, but didn't. But to answer your question, that's just 100% effect pedal -- no amp pushing at all.

[John Lyons]

Maury
Caps block AC (guitar signal in this case) and when used between the diodes and ground they block low frequencies (for the values that were given above).
Check out that AMZ article as well that PerroGrande mentions. It similar but all the frequencies are held back by adding a pot in series with the diodes to ground. With the cap you are just trying to block the low end of the spectrum.

What you want to do ideally is stop the low freqs from clipping as much as they are now. You want to find the point where the lows do not affect the signal that is clipped, which is causing the fart/crackle/blocking distortion. The tone should not change much at all.
In the first clip the low end is not very strong so the diodes are clipping nicely.
When the low end is increased with the loud/full chords the diodes are overloaded and forced to hard clip which causes that gross splatty clipping. This happens with amps all the time in DIY although not with diodes. You just need to find the point where the diodes are clipping nicely and the bass is reduced just enough via that series diode cap to ground.

I'd like to see what value you are using when you are done. Keep us posted on how this turns out.

John

[PerroGrande]

You *could* cut low-end at the input via using a smaller coupling capacitor.  The coupling capacitor forms a high-pass filter with the parallel resistance of the biasing network and the impedance of the amplifier stage.  A smaller cap on this input stage would cut low frequencies *before* they reach the transistor.

This is something to keep in mind.  If you have a pickup that is particularly hot (and I've seen them), it is possible that when you really dig into the low notes Q1 could be hard clipping (which has that nasty sound).  If this is happening, the diodes aren't the ones squaring off (or maybe they both are!) -- they're clipping/limiting an already clipped wave.

I actually experienced something similar to this possibility.  I was building a circuit that needed a high-impedance buffer.  I went with a simple JFET source follower.  The only problem was that when I really hammered on the low e-string, the follower farted... The reason?  The negative swing of the input was sufficient enough to pinch off the jfet and hard clip the wave.  It had that hard, farty sound -- much worse than the example you posted.  But -- there was nothing clipping in the following stages, so it came through in its full "glory".  The solution was, of course, to not self-bias the gate but instead to lift it a few volts, which solved the problem. 

The theory on your sound is that the diodes are overloading and "splatting" the sound -- only on the lows.  This can be solved, theoretically, by making them clip a little less aggressively.  This is what the cap does.  At lower frequencies, to AC it "looks" like a resistor to ground (instead of just a wire to ground).  This effectively raises (lifts) the point (voltage) at which the diode starts to do its thing.  For higher frequencies, the signal sees much less resistance -- more like the wire from the original design, so the sound for these frequencies is unchanged. 

So the real question will come down to -- where is the fart occurring?  If it is happening in Q1, doing this may not clean up the issue as much as desired.  On the other hand, if the diodes are, in fact, overloading (and it does have that sound to it), then this should take care of it -- once you dial in the right value.