Diode Compression with Op Amps

[WGTP]

Has anyone tried this on the input of op amps, like Joe Davidson's Vulcan?

[MartyMart]

I've only used them on either op-amp feedback loops or to ground like an MXR Dist+, how would that sound ?

Marty.

[jrc4558]

You will have to use them in a cirquit like muff fuzz or black cat overdrive. WHere op-amp overdrives an opamp. I guess...

[Paul Perry (Frostwave)]

Is there some confusion here between distortion (as in flattening a waveform) and compression (as in flattening the envelope of the audio signal)?
Because putting diodes to earth or in feedback loops will do the former, but not the latter. (Well, I suppose enough waveform flattening WILL give a flat envelope as a side effect, but that isn't the primary point of a didttortion/fuzz..)
As usual, if I have the wrong end of the stick please ignore..

[WGTP]

Your correct, there is some confusion.

I'm not talking about the standard diodes in loop and diodes to ground.  

I'm referring to the diodes compression in Joe Davidson's Vulcan, which is not an op amp distortion, but uses 3 - BJT 2N5089 stages with the compression caused by a diode in the middle of the input bias.  I can't really explain it, nor do I understand it, I just know it produces the squishiest distortion I have found.  

Joe said it would work with op amps and I was wondering if anyone had tried it.  Where is he anyway?  

[aron]

I haven't tried it either. Maybe for the new year

[RedHouse]

I havent looked at the Vulcan schem, but diodes on an opamp input are generally referred to as "Clamping Diodes" and are usually used to ensure the input signal doesn't exceed a pre-determined level, often used in measurement and filter circuits.
(the clamping is, to our ears, distortion and not compression)

[Joe Davisson]

I don't think it would work very well. Maybe if you can configure the opamp as a current-amplifier, with a bit of a lower input impedance. That's where I would start, but you might end up with a much more complicated circuit than just using the transistors.

-Joe

[bigjonny]

Interesting discussion.  The schematic available anywhere?

[bigjonny]

O, I see where the schematics reside has been picked up in another thread... Thanks anyway!

[Paul Perry (Frostwave)]

Well, I think I have redrawn the first transistor stage from a PCB layout. A standard single transistor stage, with a cap bypassed emitter resistor, and the usual bias resistors to + and ground, but with a diode added, thus:

signal comes in, 1M to ground, signal contues thru 0.1 cap to another 1M to ground, then to the cathode of a diode, then the diode anode goes to a 4.7M to + and to the transistor base.

Now what I wonder, is whether the diode has the effect of varying the bias of the transistor as the audio level changes?? nyone going to lash it up & throw a CRO on it?
And here is something even more mindbending: remember that the effective impedance of the diode varies as the current thru it varies.. as always, a 'simple' circuit with a pretty complex behaviour. :?:

[bigjonny]

Why two 1MΩ resistors to ground?  I assume the first is a pull-down to drain the capacitor and the second is a volatge divider for the DC, but couldn't the 2nd 1MΩ do both duties?