YJM308 pedal mod

[bass_econo]

OK so I just bought a YJM308 and changed out some stuff. I have been reading the Liquid, 250 and distortion + stuff and considering making more changes based on those. I'm looking to get some suggestions.  I've true-bypassed, changed the input cap to .01uf and added another diode for asymmetrical clipping.  It is 100% better to my ears.  It no longer sounds like fingers on a chalk board to me.   

Will changing the .047uf off pin 2 to .068uf add a lot of bass? 

Also since I want more distortion from it I plan on trying mosfets and fets, I don't currently have any Ge.

How can I overdrive the op amp? 

Thanks

[Ben N]

How can I overdrive the op amp? 

Are you sure you want to?  The conventional wisdom is that overdriven opamps sound pretty bad, probably because of all the feedback that is built in.  There is a reason that most opamp distortions use clippers, as opposed to discretes and CMOS designs, which overdrive the device.  Or so I'm told, but IIRC, there are a few clipper-less opamp distortions out there--Tech21 XXL?

Ben

[bass_econo]

Yeah I'm sure.  I can always undo anything that is done.  I want to take the opportunity to learn this circuit and try different things on it.  I have a hotcake which overdrives the opamp and I love it, so it is possible for it to sound nice.  I also like the black cat which does it and I heard a clip (forget who at the moment) where someone got opamp distortion from a Boss MT-2 and I thought is sounded excellent.

[Mark Hammer]

The MXR Blue Box also achieves its distortion tone from overdriving an op-amp as does the E-H Bass Balls.  Some overdriven op-amps can sound quite nice.

The YJM308, DOD250 and MXR Dist+ all use changes in the resistance from the inverting pin to ground to vary the gain.  Increasing gain involves making the resistance lower.  Fortunately or unfortunately, as gain is increased the bass rolloff moves up so that the tone becomes thiner as you crank the distortion.  The solution to this is to increase the value of that .047 cap connected to the gain pot.  Doubling its value drops the bass rolloff by another octave.  Stock, the low end starts to get rolled off around 7hz at min distortion, but around 720hz at max distortion, which is kind of thin.  Increasing that .047 cap to .22uf will drop that down to 153hz, which is more acceptable for guitar.  A change such as that proposed to .068uf *will* conserve a bit of bass (rolloff at around 500hz), but not enough to prevent things from getting thin.

[Eirik]

You may also want to read Analogmans notes on the OD-250/YJM circuit.
http://www.analogman.com/dodmod.htm

[WGTP]

Try lifting the diodes and see if the op amp isn't distorting already.  I think we get more op amp distortion in our stuff than we think.  In the Rat for example, it is really a 2 stage distortion with the op amp first and then the clipping diodes.  It might be 3 stage if the buffer is distorting as well.  Same may be true of the Boss DS-1.   

[bass_econo]

First off, thanks a lot for the input.  I know I could just build or buy another pedal that could achieve some of the things I'm looking to get out of this pedal but I'm trying to educate myself on somethings.  I do find it interesting that most people say that opamp distortion is not good but when you start naming off the pedals that do it, you get a line of pedals that people generally like.  BTW I found the link to the MT-2 opamp distortion.  You can find a discussion and clip here - http://indyguitarist.com/forum/viewtopic.php?t=63.
 

Mark - thanks for the response I have learned a lot from your responses, but they always bring up more questions, which is a good thing.  They get me thinking or set me down a path to thinking.  I seen some discussion that it is adding bass is better after distortion.  Would that be appropriate here or is changing the cap just getting back to some middle ground?

Eirik - I've been looking at Analogman's stuff too.  I'm looking to get a 1458 chip (I already have tons of 4558 variations- received from Analogman incidentally.).

WGTP - That was my thinking since to me the Rat(my fav pedals in the whole) and the Distortion + seem to be really similar.  I was thinking of getting a blending of the two - opamp distortion and diode distortion. 

[Mark Hammer]

Mark - thanks for the response I have learned a lot from your responses, but they always bring up more questions, which is a good thing.  They get me thinking or set me down a path to thinking.  I seen some discussion that it is adding bass is better after distortion.  Would that be appropriate here or is changing the cap just getting back to some middle ground?

You're welcome.

There are virtues of adding bass both before *and* after distortion.  If it weren't for the fact that guitars have greater signal amplitude at the low end than at the high end (thick strings make bigger output), this would be a pointless conversation.  By eliminating the de-emphasis of bass (which is different than boosting bass) that the 308/250/Dist+ normally have, you will not only have more "body" to the sound at higher gains, but will likely get more distortion as well.  Why?  Because the difference between presence or lack of bass de-emphasis is like the difference between being asked to walk around the attic in sandals vs Gene Simmons platform shoes.  Obviously in the one case you will be far more likely to hit your head.  Making the signal sit on top of a nice solid bottom when facing those clipping diodes will bring much more fo the signal that much closer to the clipping threshold.

My advice is to first change the cap and see what else is added to the quality of the sound when that happens before you start chasing any other mods.  I'm not saying those other mods aren't useful or potentially pleasing to your ears.  Rather, eliminating the bass de-emphasis brings a number of other changes with it that may well accomplish all you wanted to accomplish.

[bass_econo]

OK.  I just replaced the .047 with a .22uf tantalum (the only thing I had close).  The sound has definitely changed, it sounds really good now (to my ears).   It is now very thick and throaty with a hint of of a wolfy sound.  It is more distorted but I'm still liking it best with maxed out distortion.  Almost what I'm looking for. I'm going to try some mosfets after supper. 
Does it matter which way the tantalum positive end goes?  I think I have the positive end facing the drive pot.

[bass_econo]

OK, some dumb questions and then some updates...

I've changed out the diodes for BS170 mosfets (GD->|S).  They sound exceptional to my ears.   I think I'm going to add another in there to make it asymmetrical.  So far the sound is pretty good.  It's making a good heavy 80's metal distortion now.  Not as fuzzy as I would have expected from the mosfets.  They do sound very three dimensional though.  I like them a lot.

I also changed out the 4.7K resistor(gain side) to 3.3K in the attempt to add more gain to overdrive the chip.  The problem is that is that the output is now so high that I can't get more that 3 o'clock before it is out of hand.  Can I gut the output on the back end by add resistance before the output pot?  I think I would want to let the high output hit the diodes, or would that be wrong?

Again thanks to everyone who has helped.

[Mark Hammer]

I also changed out the 4.7K resistor(gain side) to 3.3K in the attempt to add more gain to overdrive the chip.  The problem is that is that the output is now so high that I can't get more that 3 o'clock before it is out of hand.  Can I gut the output on the back end by add resistance before the output pot?  I think I would want to let the high output hit the diodes, or would that be wrong?

Your reasoning is flawless.  If you are using a 50k output level pot, then perhaps a 10k-22k fixed resistor in series with the input lug might work out nicely.  If it's a 100k output pot, then maybe 18k-39k.

One of the things that the added series resistor buys you is the opportunity to insert the SWTC (Stupidly Wonderful Tone Control http://img.photobucket.com/albums/v474/mhammer/SWTC.gif ) instead of the fixed resistor.  I stuck one of these just ahead of my Dist+ volume pot and I like it a lot.  Gives a choice of full sizzle or warmer tones.  Let your ears guide as to cap values.  For a 50k output pot you could use a 10k tone pot or 25k-50k with the right parallel fixed resistor to bring down the overall value.  A 25k pot would likely work fine for a 100k output pot.

[WGTP]

See the attached thread about using Mosfets for clipping.

http://www.elixant.com/~stompbox/smfforum/index.php?topic=38581.0

[bass_econo]

Thanks Mark, I'll try that this weekend.  I think all newbies (like me) should try just taking a circuit and experimenting.  I've build several circuits and some mods to some I've found here and on the net, but it was just removing black box A and replacing with black box B.  Now though I'm actually starting to understand a little and I'm getting a "feel" for some of the things.

WTGP, thanks.  That is actually the post I was reading yesterday for the idea to put the Mosfets in.  I chose the DG - S since it was said to be the most different from regular diodes and LEDs.   

[Mark Hammer]

Personally, I don't think there is anything special about the Dist+, DOD250, or YJM308.  None of them would ever be my "go to" pedal, modded or not.  At the same, they are terrific circuits for learning a number of useful principals that can be very fruitfully applied in many other circumstances.  When newbies are able to get theirs sounding the way they want, then I know they've learned something useful and moved up a notch or two in their DIY chops.

[bass_econo]

Honestly, Mark - Me either.  I'm a Rat and although I hate to say it TS fan myself.  Oh and I'm a new Hotcake convert.  I can't seem to turn that thing off.  That was part of the reasoning for tooling around with the 308.  I bought it pretty cheap and it did not place nice with my hotcake.  Either way I can how some would like it.  It just doesn't fit in to what I like.  It will probably make a nice present for someone soon.