AMZ "Tubetype Distortion" - how does it work?

[thelonious]

Hi all,

I've been reading the forum for a while but this is my first post. I've learned a lot from you folks over the past year or so, and I'm quite grateful!

I just tried incorporating the "Tubetype Distortion" clipping section that Jack Orman posted at AMZ/muzique---just the part that's in the first op amp feedback loop---into a pedal I'm experimenting with. The host pedal is basically a GGG bluesbr...ker modded into a KoT-ish thing. Switching in the tubetype distortion circuit in the feedback loop seems to cause a level boost with lots of growly bass. Sounds great on solo lines, but farty on rhythm stuff, so right now I switch it in when I want a solo boost. I'm running it at 18V as Jack suggested.

I've tried searching to find an explanation of how the tubetype distortion circuit works, but so far I'm not having much luck. I understand that the back-to-back zeners provide softer clipping than one alone, and that it is possible to set the threshold of clipping by choosing the right zener voltage. I'm guessing that the zeners in the AMZ circuit are different voltages so that the clipping is asymmetrical. I also read that the circuit causes bias shifting... of the op amp? Other than that, I'm lost...

Can anyone enlighten me on (and/or point me to a thread that helps explain):
1) What the caps to ground do, and what changing those values would do?
2) What the 1N4148 (or 1N914) is for?
3) How bias shifting occurs in the circuit?
4) How to lower the bass within that feedback loop circuit section to make it less farty - would adjusting cap values help with this?
5) How in the world it creates a boost effect when it's in a feedback loop? Is it shunting some of the NFB through those caps?

Thanks in advance...

[PRR]

> the "Tubetype Distortion" clipping section that Jack Orman posted at AMZ/muzique

Link?

[thelonious]

>Link?

Here 'tis: http://www.muzique.com/schem/louis.gif - and the description is 2/3 of the way down this page: http://www.muzique.com/schem/projects.htm

I'm experimenting with the section that is in the feedback loop of IC1a.

[PRR]

"found the US Patent No. 5,032,796"

Let me Google that patent for you....

http://www.google.com/patents/US5032796

Lots of nice pictures. 3 pages of good (though muddy) text. 21 Claims over 2 pages.

[PRR]

> 1) What the caps to ground do....
> 2) What the 1N4148 (or 1N914) is for?
> 3) How bias shifting occurs in the circuit?

All are related. Diode charges cap and shifts bias.

> 4) How to lower the bass within that feedback loop circuit section to make it less farty - would adjusting cap values help with this?

Without the bias-shift, you would reduce the 1uFd cap.

However the bias-shift scheme foils this, by putting 10K+22uFd in parallel with the leg where you would normally try to shave some bass.

> 5) How in the world it creates a boost effect when it's in a feedback loop?

IC1a is basically a non-inverting amplifier connection. Such schemes can certainly provide voltage gain; "boost".

[thelonious]

Brilliant. I definitely have some reading to do this weekend. I didn't realize that much detail was publicly available on patent apps---that's exactly what I needed. Thanks!

[PRR]

> I didn't realize that much detail was publicly available on patent apps

That's what a patent is.

Work hard to invent and perfect something. As soon as you sell a few, someone copies it, sells cheaper because no development cost, you never recover your time/work/money of inventing.

So why invent?

Well, for the good of society.

So you go to the King and say "I want a monopoly on my new iron plow. Anybody else sells them, I want you to stop them."

But then maybe nobody ever learns all the details of your invention, can't work on improvements to your invention, not a lot of good to society.

Thomas Jefferson struggled with this. The deal he came to was: you do a *full* disclosure of your invention, and you get a 17 year monopoly.

The full disclosure is in part so everybody knows *exactly* what is- and is-not covered. If you are the first to bring the iron to cherry-red heat to work the lip, then you give full details so that nobody else can sell cherry-heat lips (for 17 years).

Naturally, such "full" disclosure is distressing to some inventors. Patent language is a lot about what you don't say, or hide under tedious verbosity. OTOH some patent authors love to recite all of the prior art (previous fuzz circuits) partly for context but also in hope that some of those ideas stick to the new invention.

So yeah, you need to hold your nose and take grains of salt, and question.

But patents ARE intended to be a record of society's inventions; and part of the social contract is that others can study patents for further inventions. (Or non-invention: the engine-valve I invented in the 1980s was invented in dozens of forms from before 1900, none successful--- therefore even though mine is a bit different I dropped the idea.)

[stm]

I can readily see two reasons as to why this circuit doesn't sound as good as expected with chords.

First, there is an issue with the schematic...

These stages clip/limit/squish the positive and negative peaks asymmetrically, however they do not invert the signal like a triode.  As there is no signal inversion between the first and second stage, the asymmetric clipping treatment is applied twice to the same peaks, where in a dual triode preamp the asymmetric clipping is applied differently to each polarity.  A simple way to achieve the same effect is to invert each of the three diodes in the second stage.

[teemuk]

SLM circuit did the inversion. ...and of course one can just reverse the order of the Zeners in the second stage.