Keeley Ultra Mod

[BAM]

Ok, I'm kinda new still to the DIY community, so bear with me if i make an ass out of myself . I've decided to perform the ultra mod on my DS-1. Correct me if I'm wrong, but this allows you to swithc between the Ultra Mod and SEM with a switch. My question is, how do I wire the switch so I can switch between the two mods? And if someone oculd explain what it does exactly (more in-depth, not just syaing it switches mods) that would be great. thanks guys

-James

[BAM]

bump

[Marcos - Munky]

The Seeiyng Eye mod is some changes in caps value and a LED in the clipping stage. The Ultra mod is the Seeying Eye mod plus another LED in the clipping section. The switch add and remove the extra LED in the clipping section.

[BAM]

cool thanks. anyone have a schem for this? thatd be great thanks again

[Torchy]

"Seeing Eye" and "Ultra" mods are for the DS-1 IIRC, as below.

http://fuzzcentral.tripod.com/DS1.html

The SD-1 mods are here ...

http://fuzzcentral.tripod.com/SD1.html

Check both  :wink:

[Mark Hammer]

The whole mod "package" does a number of things but the most obvious are to reduce how much bass is trimmed off the low end by increasing many of the capacitors in the signal path, and to change the clipping parameters.  

As you will note in the excellent color-coded diagram that Phil Bryant generously provided, many of the caps in red are caps the signal MUST pass through to get where its going.  Normally, these are selected to eliminate stray DC leaking through, but if they are a shade too conservative, their cumulative effect can shave off low end.  For instance,  if a single series cap rolls off low end by 6db/octave at 50hz, then certainly power supply hum and stray DC will most certainly be blocked (which is a good thing), but added up over half a dozen such "conservative" cap values, content at 200hz will also start to be affected.  Unless it was extremely poorly or strategically chosen in the first place, it is unlikely changing a single cap will restore lost bass, but the cumulative effect of moving the low-end rolloff point down an octave for all relevant caps will be noticeable.

I should point out that more bass is not necessarily "better".  If your instrument and amp are already kindy of sludgey sounding, you may even find that reducing the cap values to shave even *more* bass off suits your tastes.  However, since the highest amplitude part of the signal is usually the low end, and since the low end contributes harmonics higher up the spectrum when distorted, retaining low end at least up to the point where the clipping is done may be useful, even if you find that shaving the bass off *post-clipping* nets you more of the tone you want.  For instance, you may find that reducing what Phil has highlighted as C13 (.047uf stock, .1uf modded) to maybe .033 produces a less bottom-=heavy sound while retaining the snarl.  Because the tone control only lets you pan between a thick bass-heavy tone and a thin reedy one, you can't really get a warm, vocal tone without either sluggish lows or screechy highs.  Of course, you may well own one of these pedals simply because you don't WANT a mellow warm tone, so ignore my recommendations.

The two back-to-back 1N4148 diodes to ground provide symmetrical clipping whenever the signal reaches around 500-600mv (depending on the individual diode properties).  Replacing one of them with an LED raises the clipping threshold for one half of the waveform (which half doesn't really matter).  So, the clipping threshold might be 500mv on the side with the silicon diode, but 1.6v on the side with the LED.

This does three things.  First, it produces asymmetrical clipping which some people find to be a more pleasing tone, though not everyone loves it equally.  The second thing it does is to increase the apparent volume since lifting the clipping threshold means that signal passing past the diodes CAN go as high as a volt and a half for at least one half cycle, instead of being clamped at a ceiling of 500mv or so.  The third thing it does is create a somewhat more dynamically responsive sound *because* it is not clamped in at such a low clipping threshold.  You will be able to dig into notes a little more with your pick and hear a difference in volume as well as tone....or at least moreso than before.

The various mods do not really depend on each other in any manner.  That is, you could simply replace a diode with an LED and be done with it, OR you could leave the diodes as is and swap out the relevant caps for improved bass response.  There is also no requirement to enhance bass response unless you want it.  As noted above, though, more bass generally equals bigger signal, which in turn equals more clipping since the signal will more often be closer the the clipping threshold.

[brrt]

Thanks Mark! That's a really good & clear description of what's going on inside a (modded) DS-1... Guys like you make this forum worth reading!
Thanks again!

grt, Brrt