Son of Screamer with switchable diodes

[ghostsauce]

I'm starting to understand these things a little better, I think.  Been doing some research in my spare time and thought it'd be a good time to try Jack Orman's SOS.. http://www.aronnelson.com/gallery/main.php/v/jdub/SOS+perf.PNG.html?g2_imageViewsIndex=1

I want to increase the input/output caps to give it more bass in and out of the circuit, can you guys help me identify them please?  Looking at it, I think the in is C3 and the out is C6. Also, I'd like to try putting the diodes on a switch so I can swap between germaniums if I want.  I don't understand how this can work though, how would you guys do it? If it was just D3 and D4 maybe I could use a DPDT but I dunno, and what sort of germaniums should I look for?  I thought I remembered them having 3 leads..

I must suck at searching cause I can never find the answer I'm looking for. XD

[ghostsauce]

From what I can gather, this is what I think I want.  It looks like this will give me the options of "On with silicone", "Off for clean boost", and "On with Germanium".



Can anyone confirm?  Also, what type of germaniums would you guys suggest I try?

[fatfoohy]

i dont know much about germaniums, but i would think using a 1n34a or similar would be best. this is the way that i would switch the diodes, super simple and only requires a spdt switch

[ghostsauce]

Oh, I like that. Very sexy!   So, in your diagram the diodes you've got there are D2, D3, and D4 from the SOS circuit, right?  I wasn't sure if D2 was part of the clipping section or what use it has.

[bacanador]

@ghostsauce: Should't the diodes be in parallel and in opposite direction?

[ghostsauce]

I've no idea, that's why I'm posting here to pick the brains of the guys who've done this a million times. I've gotten most of what I need from searching the old threads, but I'm stuck on this part

[bacanador]

@ghostsauce: You idea is correct, but back to back diodes in series will be like an open circuit, at least in theory (I've never tried), one of them will block current flowing from left to right and the other from right to left, so it is like an open circuit

You would want them in parallel and opposite directions for tube screamer type symmetrical clipping
       

[fatfoohy]

oh, D6 in my schematic should be reversed, and yes this would replace the original D2 D3 and D4

[bacanador]

@ fathoofy: I figured you meant to reverse d6, yours is for asymmetrical clipping, I wanted to do something similar sometime ago but with more options, basically make a rotary switch select different clipping arrangements, but after some tests, I figured it was better just to select the one I liked and go with that one.

[ghostsauce]

Thanks for the help guys

Any suggestions on what type of germanium might work for this circuit?

[bacanador]

Germanium diodes don't seem easy to find these days, I got mine from Aron's Store

http://diystompboxes.com/zencart/index.php?main_page=product_info&cPath=13&products_id=6

[ghostsauce]

Man, they're more expensive than I thought..  I'd probably need to order a boatload to find a matched pair too. Eh, screw it. I'll fool with diodes & LEDs, thank you though. 

I've a question about the IC needed for the SOS though.. It calls for a LM353, which from searching here I'm told is interchangeable with a TL072.  Thank goodness because LM353 doesn't exist on mouser >_>    So I found a bunch of TL072s, I think. I ordered a TL072BCP and a TL072IP. Any idea if I'd run into problems with these?  I can't for the life of me figure out how to read the datasheet. There is just way too much information there for a noob like me to tell if either of these chips will work. And whether or not it's even useful to try both, cause maybe they're the exact same thing. /shrug 

[ghostsauce]

Cool, I was able to confirm that those 2 ICs will work for this circuit.  So, I think I'm all set now.. thanks everyone for your help!  I'm going to put this and a modded Boss OS-2 into a box together, should make for a really fun pedal. Gonna try to etch it too *crosses fingers*

Should look something like this when it's done: http://ghostsauce.net/random/_Masamune.swf
SOS on the right, OS2 on the left, with some mods on switches, and the OS2's blend control on the south side, so that it's symmetrical. Attack = Volume, Magic = Gain, Critical = Tone. Kudos to anyone who recognizes any of this.

[bipedal]

Just glanced through this thread to see what the project looks like...

This comment caught my attention:

I'd probably need to order a boatload to find a matched pair too.

There's no need to worry about finding matching pairs of germanium diodes.

When people refer to needing matched pairs of components, it's likely in the context of germanium transistors, where varying values can have a dramatic impact on the sound of a circuit.  (See Fuzz Faces.)

Sure, the clipping voltage of germanium (and silicon, for that matter) diodes can vary piece by piece, just as the measured resistance of a 100 ohm resistor may not exactly match 100 ohms, but for the application of clipping diodes, that slight variation probably won't result in an audible difference.

Germanium diodes may be a bit more expensive per unit than silicon diodes, but they do have a little different clipping sound to them compared to silicons, so it's worth trying them out to see how your ears like them.  For germanium diodes to be used as clippers, 1n34a is a common type, as is 1n270.

Best wishes,

Jay

[ghostsauce]

That cleared up so much, thanks!

[ghostsauce]

Hmm, having trouble with this circuit. I've gone over it reflowing the joints and checking for bridges and mistakes but can't find anything wrong. It bypasses properly, and the LED lights up when on, but the circuit is dead. I'll post pics in a bit but can somebody verify this layout please?  The cap off on it's own seems to make it less than reliable... http://www.aronnelson.com/gallery/main.php/v/jdub/SOS+perf.PNG.html

[Ben N]

For your clean boost option, you should probably be switching in some resistance rather than leaving that part of the circuit open. Otherwise, you will have a huge increase in output from the first opamp, and very likely overdrive the second opamp.

[jdub]

Hey Ghostsauce-

Sorry it took so long to chime in, haven't had a chance to get on the forum recently. I can verify the layout; I use the pedal frequently.  Just to be sure, though, as soon as I get a chance I'll double-check the layout with my build just to make sure I didn't mess something up in the DIYLC file.  Sounds like your OBW is fine, and I assume you checked for bent opamp pins, orientations, etc.  Have you measured voltages?

BTW, which "cap off on its own" are you referring to?

[ghostsauce]

Thanks jdub! 

The cap off on it's own.. well there are two round things on the pcb layout that aren't connected to anything. Looking at it now I'm guessing they are standoffs though.. I thought they were caps that weren't connected to anything, which made it seem like an unfinished or rushed layout. I did make sure the opamp pins were slotted properly, and that it was swinging the right direction. I've noticed that some of the solder traces get hot to the touch, which weirds me out. So I triple-checked the polarity of the electrolytics and they were a-ok. I haven't measured voltages, or even taken pics. It's sitting in a box cause i needed to walk away from it for a while.    

[jdub]

Believe me, man, I know the feeling...

OK, so I did some checking, and I really screwed the pooch on this one...the layout I (accidentally) posted was an earlier work-in-progress version that is NOT correct.  It and the final version had very similar file names, and I selected the wrong one when I posted it and evidently didn't check very closely.  My sincere apologies for any anguish this has caused you...

As soon as I'm done writing this I will post the correct version.

Now, for the erroneous areas:  first of all, since this was among the first layouts I did, and since I didn't have much experience at the time, the electrolytic caps have the striped (negative) side facing the wrong way.  For some reason, when I started using DIYLC, I didn't make the connection that the little light-blue stripe on the electro cap image represented the negative lead (like on a real electro cap...duh), and I used them to represent positive(!?).  I know, doesn't make much sense.  I now know better, and the new layout has the negative leads facing the correct way.  In your build, your caps may be facing the wrong way.

Secondly, R7 (which lies between IC pins 1 & 5) should NOT be connected to ground.  Must have overshot with the trace on that one.  Finally, and very importantly, the trace running from C5 to the Drive pot must be run on the component side of the board and NOT make contact with +9v running to IC pin 8.  This is likely the source of your hot leads and hopefully hasn't damaged anything.

Again, my apologies, man.  I've had that layout up for awhile and should have checked more closely to ensure it was the correct one.  If there's anything else I can do to help, give a shout.

John