Any Bad Side-Effects to Adding Clipping Diodes to Something Like This?

[Ben Lyman]

I'm trying to learn how to properly set up an IC OD/Dist and I have breadboarded something very similar to a Voodoo Lab OD like the one found in this thread:
http://www.diystompboxes.com/smfforum/index.php?topic=64238.0
My OD is almost exactly like that one and I have some diodes in the same position as that schematic shows going to VR. I also have a pair of diodes going to ground after the output cap to give it more distortion/compression/whatever. It sounds better to me this way but I am wondering if there is anything to be concerned about. Any negative side-effects, impedance issues or something?
Thanks

[antonis]

Maybe you'll need some extra output boost because your final maximun signal is restricted to about 650mV..
(in practice, you made a 2-stage distortion (BMPi style) with no final recovery to your max allowed level by the supply rails..)

As for impedance issues, you'll have to calculate diode's internal resistance in parallel with Level pot..

[Kipper4]

"Maybe you'll need some extra output boost because your final maximun signal is restricted to about 650mV.."

As Antonis says Ben you may need an extra make up/booster after the second set of diodes
Is this on the breadboard?

Try something like this maybe after the second set of clipping diodes. LPB. Small parts count.


http://www.diystompboxes.com/smfforum/index.php?topic=109810.msg1005156#msg1005156

see reply no8 by Antonis for schematic.

leave out your currant output cap before the second set of clipping diodes as the lpbs input cap will serve as a dc decoupling cap.

[Transmogrifox]

It looks fine.  TL072 datasheet states unlimited output short, so loading into a 150 ohm R and diode will not cause any damage.

As for stability or TL072 misbehaving in some way under these conditions, let your ears be the guide.  If you were to have these mass produced and stamped off by hundreds from a manufacturer I would analyze it a lot more closely. 

The only "bad" thing I could imagine is if you were swapping op amps and used an op amp that was not able to drive the ~150 ohms when the diode turns on.  Again, I think your ears would tell you something bad was happening.

Most modern op amps are designed to handle unlimited output short circuit without damage...even the 741 was pretty robust against output loading.

[antonis]

TL072 datasheet states unlimited output short, so loading into a 150 ohm R and diode will not cause any damage.

I'm not sure if we deal with "short"..

Taking in mind that TL072 doesn't hit supply rails and diode voltage drop we come in a output current of slightly over 20mA, which is very close to oscillation but definately not a shorting situation..

[Ben Lyman]

...you made a 2-stage distortion (BMPi style)...

Sweet! Maybe that's why I like it 

...As for impedance issues, you'll have to calculate diode's internal resistance in parallel with Level pot..

Thanks Antonis. Do I really have to? I don't like the sound of that "calculation" stuff  Can I just plug it into a few pedals and amps, see if it sounds good and assume it's not doing any damage? Please?

Is this on the breadboard?

Yes, it is.

"Maybe you'll need some extra output boost because your final maximun signal is restricted to about 650mV.."

As Antonis says Ben you may need an extra make up/booster after the second set of diodes...

Try something like... LPB. Small parts count.

leave out your currant output cap before the second set of clipping diodes as the lpbs input cap will serve as a dc decoupling cap.

Thanks, sounds like a plan. What if I want to add a tone stack? Before or after LPB?

...The only "bad" thing I could imagine is if you were swapping op amps ...

Hmm... it's not exactly like the Voodoo Lab OD... I used a 4558 but that's probably okay... right...

...If you were to have these mass produced and stamped off by hundreds...

No, this one is for me but I have a friend who wants something like this too. He has a 10 year old Proco Rat that he said he got just because he "didn't know any better" at the time. He doesn't like it and he wants me to make him something that is better suited to "fast 3 minute, 3 chord songs like The Ramones" and, after I added the extra diodes, I suddenly thought this might be a good candidate for his custom pedal.

[Kipper4]

Not strictly a tone stack but heres an idea for you Ben
Variable capacitor

see reply 4 here

http://www.diystompboxes.com/smfforum/index.php?topic=52421.0

[Ben Lyman]

looks like a possibility kipper. I assume that would be for the input cap?
Truth is, I'm just loving my fuzz tone stack that Gus recommended to me and I want to use it again. It's like it retains this really fat midrange when it cuts the highs. I can't really explain it but it has a sort of "Rat-like" quality or flavor.

[Kipper4]

Your fine mate. Just wanted to introduce you to something differant.
Are you going to post the finished schematic ?
I'd like to see this tone stack.

Btw my go to for Ramones and a punk pop feel would be a Rat.

[Transmogrifox]

...As for impedance issues, you'll have to calculate diode's internal resistance in parallel with Level pot..

Thanks Antonis. Do I really have to? I don't like the sound of that "calculation" stuff  Can I just plug it into a few pedals and amps, see if it sounds good and assume it's not doing any damage? Please?

No you don't have to calculate anything.  The difference between reality and approximation of ~650mV is too minuscule to be of any material value.

TL072 datasheet states unlimited output short, so loading into a 150 ohm R and diode will not cause any damage.

I'm not sure if we deal with "short"..

Taking in mind that TL072 doesn't hit supply rails and diode voltage drop we come in a output current of slightly over 20mA, which is very close to oscillation but definately not a shorting situation..

Yes, that is understood.  I'm saying if a TL072 can handle a short, then there is no series resistance too small, and hence 150 ohm ok.  In other words you could drive the diodes through a capacitor directly without permanently damaging the op amp.

As for oscillation and other bad behavior...I think Ben has already proved to himself this isn't a problem. 

The 4558 output characteristic is similar to TL072 so it will be fine.

[Ben Lyman]

...I'd like to see this tone stack.
Btw my go to for Ramones and a punk pop feel would be a Rat.

It's the same tone stack as this fuzz pedal I made: http://www.diystompboxes.com/smfforum/index.php?topic=112558.0 . So simple, just a 100K linear pot with 2 .01uF caps and 2 47k resistors. You can also hear some samples in my fuzz pedal demovids.
And, ya.. Rat's are great, I love them. My friend doesn't like his but I don't like it either, hardly a Rat in my opinion. Small box, Chinese import, fizzy and thin. I have a 1980 big box, completely different sound. I have tried a few others, didn't like any of them, the worst was a "boo-teek" clone of the old late 70's "Tone Knob" Rat... ugh... terrible 

[antonis]

What if I want to add a tone stack? Before or after LPB?

It depends on your specific tonestack..

If loss is small (assuming a passive one) you can place it after the LPB, taking in mind the (new) output impedance..

If loss is quite big (more than, say, 4 - 5 dB) I think it's better to place the tonestack before the LPB and trim it's gain (Rc/Re)..

[Ben Lyman]

Thanks guys. Good call on the LPB stage, it helped it a lot. I couldn't really get the diodes to work properly after I added the booster stage though 
I had to move them all the way to the end right after the LPB output cap. Now this thing is a monster with tons of distortion and boost. I have not added the tonestack yet, hopefully find time later.
Another thought: I wonder what would happen if I took out the extra diodes and moved the LPB to input? Maybe it would boost it a lot and make the "light OD" become heavy distortion... 

[Kipper4]

Still on the BB?
Jumper it and try it Ben.

[Kipper4]

You may need to up the output impedance of the LPB. Or the distortion may load it down.
Haven't got the schematic just now. I'll go find em.

[Kipper4]

Maybe try removing c2 and vr1 on the LPB in this thread

http://www.diystompboxes.com/smfforum/index.php?topic=109810.msg1005156#msg1005156

And remove R1 in the VLO in this here..

http://www.diystompboxes.com/smfforum/index.php?topic=64238.0

And experiment with the value of R2 (VLO).
The reason I'm thinking leave the input cap on the distortion circuit is it is likely a part of the pre distortion tone shaping.
You might need to adjust this too according to your rig.
While it's on the BB.
Maybe some other fellas might do otherwise.

[Ben Lyman]

Thanks Kipper, tried all of the above, sounds great, very heavy sound, low noise too. Also put a 100k series in place of LPB volume pot to cut some gain and noise. No need for diodes at the end with all the distortion that it already has. Added the tone stack and the output dropped a little but not enough to warrant another make-up booster. I think it might be a keeper.

[Gus]

Have you seen this? http://www.gmarts.org/index.php?go=217

[Ben Lyman]

Thanks Gus! Just read it and I really learned a lot. great info, written in a way I could understand

[Ben Lyman]

Here's where it's at now on the BB. Sounds great but I don't know if everything is necessary or if I'm missing some important features. After I pulled the diodes out of the middle it sounded much better and louder. I also pulled the 1n cap that was parallel to those diodes, sounds the same to me but I realized it only left one single component on the VR, and the two 10k resistors flanking the diodes and cap were now just a 20k resistor in between the two halves of the op amp.
Any ideas? Anyone?