Distort-o-matic XI Stompboxology

[soggybag]

***
Edit: a few mistakes were pointed out to me, and as I re-copied the drawing I found another.
R15 10K off the emitter of Q3 was missing.
R3 between gate of Q1 and V+ was miss numbered as R7.
***

Here's a drawing of another effect from stompboxology Vol 11. No. 2.

This is described as a distortion box with an agressive tone control. The tone control does look very interesting. Does anyone have any comments on this? The descriprion says the inductor was taken from a 1989 Dunlop wha, and a similar .5L inductor from mouser did not have a high enough Q to work as the tone control.

There are a few notes in the back of the issue about replacing the coil with a simulated inductance made with a transistors and a few other parts. I could make a drawing and post it if anyone is interested.

The rest looks a little like the Blender with a FET input stage, the afore mentioned abressive tone control and some clipping diodes.

[Mark Hammer]

Thanks for the drawing.  The few schematics I have seen reproduced from Boscorelli's stuff are generally hard to follow because of how they are drawn.

The mirrored pair formed by Q4,5 is clearly used to derive an octave up, as in the Green Ringer, Superfuzz, Blender, and so many others.  I don't know what the inductor will add to the tone beyond perhaps a different passband, but much of what the tone controls do here can be mimicked with a simple modified notch filter.  (Boy I must sound like a broken record these days!).  If you look at the Univox Superfuzz, it includes a tone switch that selects between the notch filter (midscoop) and an equal-volume passive attenuator network made of 2 resistors.  With some simple exerimentation, I found that if one simply inserted a variable resistance between the cap to ground on the scoop network and ground, you could reduce the scooping effect and bring the mids and upper bass back up for a throatier sound.  The disadvantage is, of course, that you have to adjust the volume to compensate, but some may find the variable aspect worth the trouble.

If you look at the Foxx Tone Machine (a cousin), you'll see that the tone control plays an interesting trick with the midscoop design.  It turns the second resistor in the network into a fixed+variable resistance, and takes the output from the wiper.  As you rotate in one direction, the resistance in series with the treble bypass cap (3300pf) gets smaller, letting the balance shift a little more in favour of the high end.  Rotate it the other way and now the entire value of that pot is in series with the bypass cap, pushing the treble in the background and changing the "shape" of the scoop.

An inductorless version of what Boscorelli shows might involve a similar midscoop, but with a variable resistance in series with the treble bypass cap, and a variable resistance in series with the cap to ground.  One lets you adjust midscoop, the other lets you adjust treble, which is a big part of what he shows with the circuit built around the transistor and inductor.  Where his circuit improves upon what I've suggested is that I gather it provides both boost and cut for both controls, where what I've suggested provides only cut.  On the other hand, all Marshall/Fender tonestacks do is provide cut and people seem pretty happy with them.

[soggybag]

I have to agree, the schematics in Stompboxology are very confusing, which is why I redrew this. My drawing is not that great. How do the rest of you do such neat schematics, is there software? I like the style of the schematics on Tim Escobedos site, they are very clean and clear.

The documents say that the Bass and Treble controls provide 30db of cut and boost each. Which makes it sound like a pretty extreme tone control.

[Mark Hammer]

I gather 30db = +15/-15, which is a reasonably common figure, although for me that amount of boost and cut is probably more useful ahead of the clipping circuitry, rather than after it.

[soggybag]

I am far from expert on the subject, but I would have guessed that having the tone control after the clipping stage would give the filter more to work with.

Mr. Boscorelli says  "...Q6, whose nominal gain is 1; however, pots R21 and R22 allow Q6 to generate bass/treble boost/cut in excess of 30dB.

He also mentions: "The adjustable preamp bias [R18] lets the player move the bias closer to V+. This lowers the threshold of unipolar clipping, giving ealier onset of mild distortion."

I notice the level pot, R20, sets the gain of the output buffer Q6, rather than using a using a pot as a volatage divider to ground, as is seen in most effects. Boscorelli uses this in Boost-o-matic IV also. I posted a drawing of this earlier in the week here:
http://www.diystompboxes.com/smfforum/index.php?topic=46867.0

After taking a second look at this one it may be something different.

[R.G.]

The tone control does look very interesting. Does anyone have any comments on this?

It's a version of the old LC based boost/cut graphic equalizer.

A bipolar transistor has two inputs: the base and the emitter. The gain from either one is the same, but the input impedance is different, the emitter being quite low impedance. But it is possible to set up a transistor as a "differential amplifier" by using both. The base is the inverting input, the emitter is the noninverting input.

In this tone control, the user can do a treble boost/cut by turning C7's pot to the emitter where it cuts any emitter feedback out and thereby boosts treble, or toward the collector where it cuts treble out of the output. Same for the c6/L1 network, but it boosts or cuts around the LC resonance, which if I did the math right is 82Hz, the frequency of low E.

R19 crossfades between boosted signal at Q2 and the octave at Q4, 5 collectors.

Yeah, very like a blender with a different tone control, perhaps something from that adjustable bias in the JFET/bipolar feedback pair at the input.

[Gilles C]

That Distort-o-matic circuit looks like 2 modules in one to me, an input buffer part, and a distortion part.

I liked seeing this schematic, because the input buffer looks a lot like the kind of preamp I began experimenting with these days.

The disto part is also nice.

I should have bought the Stompboxlogy series when it was available. I like the circuits you draw. Thanks.

Oh, R2 value is 104 ohms, right??? Or is it 10 something?

Gilles

[wampcat1]

Also, there is free software available to draw schems -- Try tinycad, it's fairly easy. Also, do a search for "schematic software" or something like that, there should be a ton of posts about it with links.

bw

[tiges_ tendres]

I use chunk schematic for my stuff.  It's free (to a point) and easy to use.  The library is limited, but you can code your own stuff for points if you are so inclined.

Chunk is an Australian company that make bass pedals. 

[puretube]

where does Q3`s emitter go, DC-wise?
(my guess: ground)

[Phorhas]

That Distort-o-matic circuit looks like 2 modules in one to me, an input buffer part, and a distortion part.

I liked seeing this schematic, because the input buffer looks a lot like the kind of preamp I began experimenting with these days.



Gilles

I also am very interested in that direction. I saw this "gain stage" in an old high-school electronics text book and it kinda popped out and made me wonder...

How does it work for you?

[Mark Hammer]

where does Q3`s emitter go, DC-wise?
(my guess: ground)

Yeah I thought that was a little weird, too.  Q3 is set up in anticipation of being a phase splitter feeding Q4/5 the way it is done on the Green Ringer and the Dynacomp rectifier path.  Usually this involves equal-value resistors on the emitter and collector.

I was able to get hold of a copy of the original graphic from someone, and it appears the redraw from Soggybag accidentally omitted a 10k emitter resistor to ground.  Once that is inserted, I think the circuit will make more sense.

[soggybag]

**** R2 10k ****

[soggybag]

A few mistakes were pointed out to me I fixed them and made a note and updated the original post.

[StephenGiles]

"My drawing is not that great"

It looks very good to me, it's not easy to draw circuits freehand and in any case it's a form of art! Many thanks.

[Gilles C]

I also am very interested in that direction. I saw this "gain stage" in an old high-school electronics text book and it kinda popped out and made me wonder...

How does it work for you?

Very well...

I also saw that kind of gain stage a few times before, and always wanted to try it. Well, I did a few times finally.

I tried both schematics from Jensen (as I posted a while ago) that use this configuration, and with the last one I built, the one with some gain, I added a Mouser 42TM018 XFMR to make it an isolation box.

It works very well on the bench, but i still have to try it with an amp.



The only problem I had was selecting the FET to get the correct DC level at the output, or trying to find the best FET Drain resistance. I forgot I could use a bias trimmer as in the Distort-o-matic schematic. It just hit me in the face when I saw the schematic.

Oh, and I used a J201 for that one, but will use a 2SK30A for the next one, and will have to change the layout for the DGS configuration instead of the DSG one.

I will post the project when it works perfectly.

Added: Phorhas, I finished a layout for the input buffer part of the Disto-0-Matic schematic to be able to experiment with it, and compare the result with the ones I already built. If you're interested in trying it yourself, just tell me so, I can send you the layout.

---------------------
Thanks for the value of the 10K. I thought it could have been 100 ohms, but 10K seemed a bit high to me. Finally, it seems it's not...

--------------------
soggybag, you forgot to identify R18 on the updated schematic.

Thanks again for posting it.

Gilles