Can I patch in diode compression in this pedal?

[aab0mb]

http://www.hobby-hour.com/electronics/s/od2-turbo-overdrive.php

discrete op amps.

I'm looking at the joe davidson discrete op amp in the beginners section and i like the diode compression idea.  I've never tried it out but the discrete op amps in the od-2 seem like a good place to try to insert one for testing.

The problem is the od2 uses a different transistor configuration for the discrete op amp so i'm feeling a little turned around as far as where to patch the diode in line.  I usually work on the turbo section so something like in line with the input of q15?

[Eddododo]

ok, im quite under-qualified here, so let someone else chime in before you trust me...

I *think* the diode is specific to the needs of this circuit, rather than a useful add-on to another discrete op-amp topology, as it were (  )

from what i could guess by my own lack of useful knowledge, I think the diode might be there to 'even out' the transistors to keep them from being overdriven?
I think a lot of people are biting on the 'compression' tag for this project, but I do not get the impression that it is a musical compression as much as a functional compression for the circuits' behavior

[Eddododo]

Maybe it [the diode] keeps the transistors from not only overdriving, but even perhaps from saturating? Is an 'unsaturated transistor' suitable for musical applications?
maybe i am being silly, but thinking of the terms 'transparent op-amps'  and 'tube-like transistors' made me think that the diode is there to keep the voltage low enough to not impart that 'musical breakup' sound of transistors?

does anyone know how long it will be before I know what i am doing?   I guess its time to invest in literature..

[mistahead]

Its not quite a true OP-AMP but it can replace true OP-AMPs, to my knowlege "diode compression" can be achieved in a couple of ways but trying to do anything significant with "discreet diode compression" is pricey and low-qual. when put next to IC's.

Diode (one way junction, two ends) Compression (consider its place RE the transistors, their feedback networks, what are the "ideal opamp" qualities...), pull your opamps and try this in their place - watch the difference between using it in the place of an opamp used to create a gain stage and those for LFO (sch. triggers / true operations / etc).

[aab0mb]

OK this could really be interesting actually.

The od-2 is set up to specifically overdrive the discrete op amps.  Might be a good test to see what the diodes can do. 

I'm willing to hack at this pedal so we can all learn if some gents are willing to help me out along the way.

[Ice-9]

The turbo switch allows the signal from the Q18,19,20 section to pass to the tone control and output section, D9,10,11 form the negative feedback clipping diodes that you would normally find fitted to an opamp loop. (same job here)

The normal mode allows the 2 upper discrete op amps to create the overdrive.

It depends on what it is you want to modify, ie normal or turbo. You could try 2 back to back diodes across C28 in the first discrete opamp section based around Q13,14,15 or 2 back to back diodes to ground from the junctions of R43,C26,27  and of course other combinations of diode clipping on either stage opamp. (this could cause a volume drop which you would have to work out)

You could also add some hard clipping onto the turbo mode to try out, again diodes to ground on the output of the turbo opamp section.

Just a few suggestions to look at.

[aab0mb]

We're talking about the "diode compression" idea being implemented here. Not the diode clippers. 

Seems like you gave a good grasp on how the pedal is working.  Take a look at diode compression and let us know what you think.

[mistahead]

Yes this is intended as a "pin for pin" replacement for a single op amp (ummm TL061 I think is a single..) to operate compression/dirt circuits I believe. O'Squeezer and some old distortion boxes, and modded TS clones have reported good results if I recall.

But it could get interesting if (mis)used in other areas.

[aab0mb]

For the record:  I'm familiar with the replacement of a regular op amp with the discrete diode compression. 

My goal here is to patch in a diode within one of the discrete op amps in this boss circuit in question.  NOT a clipping diode.  An inline compression style diode. I think i'll be able to figure out where it goes with some further comparison of the circuits.  I'll probably get in the pedal and try it at some point soon.

I got out the breadboard tonight.  I'd been looking at The Vulcan and a couple other diode compression designs. I had an lpb1 on the board already so i just made another one cascaded.  lowered the gain of the second lpb1 circuit and put a diode in series between the two  building blocks.  with a little tweaking on the second transistors gain I was able to get a nice smooth overdrive happening.  The diode has a definitely cool effect once it's tuned in with the circuit.

This whole thing is kind of an exploration just for the sake of different dirt tones.  Endless journey...

[slacker]

I think the simple answer is no you can't. The diode comp opamp uses BJTs as the input transistors, the Boss uses JFets, I don't think the series diode will do the same job with fets.
You could of course replace the Boss discrete opamps with diode comp opamps.

[aab0mb]

I think the simple answer is no you can't. The diode comp opamp uses BJTs as the input transistors, the Boss uses JFets, I don't think the series diode will do the same job with fets.
You could of course replace the Boss discrete opamps with diode comp opamps.

As far as i know the diode is patched in before Input of the Output transistor.  If i'm correct we're trying to soften the distortion character of the output tranny.  In that case, I can't see how the transistor really matters.  Like i said, i breadboarded a simple 2 transistor circuit to see how a diode in between stages worked.  Kind of like a Vulcan overdrive.  It requires a bias tweak but that's not a horrible sacrifice.

I'll get around to trying it on the od-2 in the next few days. After letting it rattle around in my head for a few days I think I know what i'm going after.

[slacker]

oops sorry I think you're right I was thinking the diode did something to the input trannies.

[Davelectro]

It shouldn't matter whether it's a bjt or jfet. I mean...there's a jfet Vulcan which also uses the diode trick, right?

[aab0mb]

It shouldn't matter whether it's a bjt or jfet. I mean...there's a jfet Vulcan which also uses the diode trick, right?

rrrright!  I just need some desoldering braid and a new tip so i'm not toasting pads and this experiment is a go.  Should be a couple days.