Hard Clipping Diodes

[swinginguitar]

So I was contemplating hard clipping diodes to ground vs soft clipping in a feedback loop.

And I thought, why not try adding hard clippers in my current breadboard project (a run of the mill dual opamp overdrive with soft clippers and treble cut control).

So, between the output of the first opamp stage and the tone pot, I placed 2 diodes in opposite directions to ground. The circuit immediately quit passing signal at all.

Why?

[krister]

Sounds like you are dumping all of your signal to ground.

[swinginguitar]

that's what i was thinking, but wondering what i missed...i know i just blindly threw them in there...

looking at the Rat for instance, that's how they do it...perhaps the voltage coming out of IC1A is too high and the diode is conducting too soon....?

[ashcat_lt]

It could be any number of things, but it's futile for us to speculate without answers to quite a number of questions.  Luckily, a schematic of what you've got would answer most of those questions.

[wilrecar77]

I think you may have dc on your diodes. Try using coupling caps before and after the diodes.

[Mike Burgundy]

Do you have a dual supply for your opamp or are you using a special single-supply opamp?If not, you're probably using "virtual ground" at 1/2 supply voltage - connect the diodes to that instead of ground, or isolate them for DC with a cap as said (that cap is a great tool for voicing - smaller cap = less bass clipped so more (tight, non-flabby)bass in the end result.

[swinginguitar]

Do you have a dual supply for your opamp or are you using a special single-supply opamp?If not, you're probably using "virtual ground" at 1/2 supply voltage - connect the diodes to that instead of ground, or isolate them for DC with a cap as said (that cap is a great tool for voicing - smaller cap = less bass clipped so more (tight, non-flabby)bass in the end result.

DOH!!! Yes, I have Vref...thanks!

[Processaurus]

You want coupling caps, as was mentioned, before the diodes, but you also want a resistor.  Opamps have a low output impedance, so they can drive fairly heavy loads, but not infinitely heavy loads.  Without a resistor between the opamp and the clipping diodes, you are essentially connecting/short circuiting the opamp output to ground, once the diodes start conducting.  This will either destroy the part or cause it to shutdown, if it has built in circuit protection.  That's why you see a 470 to 1K resistor in series between opamp outputs and an output jack on pedals, so that the opamp isn't damaged by guitarists who abuse the opamp by accidentally connecting the output to ground.

Check out the proco Rat, it has a resistor (and cap) before the clipping diodes.

That series resistor on the output makes an appearance in this Geofex article, "What Are All Those Parts For?"

[swinginguitar]

OK...revisiting this.

I breadboarded this:



Before adding the resistor as mentioned, I got no signal.

Using various combinations of cap and resistor (ranging from 100R to 10K), i get signal, but it remains unchanged...still no hard clipping. i'm running them from the output of opamp stage 1 to either ground or Vref...

Any thoughts?

[Jdansti]

What you are trying to do is similar to the clipping stage of the Tube Screamer. You might want to read this, especially the section on the clipping circuit.:

http://www.geofex.com/article_folders/tstech/tsxtech.htm

[swinginguitar]

What you are trying to do is similar to the clipping stage of the Tube Screamer. You might want to read this, especially the section on the clipping circuit.:

http://www.geofex.com/article_folders/tstech/tsxtech.htm

ummm...not really. "t'ain't no hard clipping to ground/vref on that there".....that's diodes in a feedback loop

[Jdansti]

What you are trying to do is similar to the clipping stage of the Tube Screamer. You might want to read this, especially the section on the clipping circuit.:

http://www.geofex.com/article_folders/tstech/tsxtech.htm

ummm...not really. "t'ain't no hard clipping to ground/vref on that there".....that's diodes in a feedback loop

Sorry-I missed what you're trying to do. The only methods that I've seen for diode clipping is diodes in the feedback loop (Tube Screamer) or on the output to ground (Proco Rat).  There may be other methods that I'm not aware of, though.

[Earthscum]

What you are doing here is attempting to clip a signal that is already cllipped. The signal from the op amp is going to just barely get your diodes to ground to conduct.

Two solutions are boost the signal before clipping, or use diodes with higher forward voltage in feedback than in clip to ground. A suggestion that will give you some sonic goodness is red LED and schottky silicon or germ in feedback, and a pair of silicon or schottky clippers to ground.

[Quackzed]

what you could do is insert a 10k resistor in series after the last cap of the schem, but before the 10k vol pot, then put your diodes to vref there.
Then you can raise that 3k3 resistor in the fbloop of the 2nd opamp to say 10k, so the second opamp will amplify enough to hit your hard clipping diodes enough to let em clip.