Digitech PDS 1550

[dan5150]

Digitech/DOD PDS 1550 "Programmable Distortion"

I just scored one of these off of eBay for $16. It makes me think that the Digitech/DOD guys were WAY ahead of Vai and Ibanez with the whole "Jemini" thing. :-)

It doesn't sound bad at all, and I know if I spend some more time with it, I can get some great sounds just with tweaking all of the settings.

Digitech was cool enough to send me the schematic. I have it shared out here: http://gg2lpg.bay.livefilestore.com/y1pmXBQj4AJnCZgukohsxwSkjkgKU5kvO502NYuWdwNioqZc537Rb-rcCKI-syFgnqYbDPCaWUKonci8RW-LKlMoRgj_YiSAZ_Y/PDS1550%20schematic%5B1%5D.pdf?download

In reading the schematic, the overdrive looks to be of the TS808/SD-1 variety using symetrical soft clipping through a 1458 model Op Amp. However, there is also a pair of hard clipping diodes right after the op amp as well.

The distortion side reminds me more like the HM-2 with asymetrical soft clippers through a TL062, then a pair of hard clippers and a pair of GE diodes (which I think are used more as a noise filter).

I still don't have my head wrapped around the Parametric EQ and Delay parts of the circut, but I am not sure I want to mess with that stuff anyway.

The great news is that all of the IC's are socketed, so I can really experiement with different Op amps for both distortion sides.

The main thing I need to do is replace the DC jack (it uses the old DOD barrel kind with a standard BOSS style so I can power it with my pedalboard.

If I could make it happen, I would like to get the overdrive side to replicate my TS/SD-1 clone, and the Distortion side to replicate the sound of my keely DS-1.

I think I can get close with some clipping diode removal/replacement and some Op Amp switching.

Anyone have any other suggestions? :-)

[Paul Marossy]

Sounds like you're on your way to modding it. Don't expect to hear much of a change with just swapping out the opamps, though. Changing the diodes will be far more effective.

[dan5150]

Yup, I cleaned it up and played with it (stock) last night just to get a better a "feel" for the tones it produces now.

The "A Channel" (The "Metal" channel) has WAY more gain. Plus it is really fuzzy. I am pretty sure this has mostly to do with all of the clipping diodes.

The "B Channel" is quieter and has a "lo-fi" kinda thing. (Meaning I like my TS clone way better). I am going to try and replicate the clipping diode section in my TS clone and see if that helps.

Someone please remind me, is it more diodes = more gain, or vise-versa?

[dan5150]

Ok, in the process of modding it, and I have a few questions on the schematic.

The Op amp values listed in the schematic were NOT what was actually in the pedal.

First off, Channel "B". - This is supposed to be the "Overdrive" channel. I replaced the Op amp with a JRC4558DD (removed from an SD-1). I think that helped to smooth things out a bit, since what was in there was a 1445 which according to Analog Man, is a Dual version of the LM741. At a minimum, the replacement has higher fedelity. :-)

Here is the schem from that section:



It appears to have the standard symetrical clippers in the feedback loop.

I also thought that D4 & D5 were hard clippers. However, when I replaced them with jumpers, I no longer got any sound from that channel. Therefore it is now back to stock.

Can anyone help explain what those diodes are used for?


On to Channel "A" - This is supposed to be the "hi gain" channel. Very fuzzy/buzzy when the gain is cranked. This channel is also noticably louder than the other.

I replaced the stock Op amp (TL022CP) with a TL071.

Here is the schem from that section:



This channel looks ALOT like the BOSS HM-2. It has asymetrical clippers in the feedback loop, a pair of GE diodes (I think that are used as a noise filter) and a pair of hard clippers to ground.

Since I want to get this side closer to a Keeley DS-1, if I simply remove all of the soft clipping diodes (D15, D16, D17), do I need to alter the values of (or also remove) the resitor and cap in the feeback loop as well?

Thanks for any help!

[diggum12]

Hi, can you place this schem on another hosting site?  For one thing, it doesn't work when I click in it now.  But it's also blocked at my work too.

Thank you!!!

[dan5150]

The full schematic is a small (342k) 3 page pdf file.

I can try to get the link working again, or I can e-mail it to you?

-Dan-

[Paul Marossy]

I also thought that D4 & D5 were hard clippers. However, when I replaced them with jumpers, I no longer got any sound from that channel. Therefore it is now back to stock.

Can anyone help explain what those diodes are used for?


This channel looks ALOT like the BOSS HM-2. It has asymetrical clippers in the feedback loop, a pair of GE diodes (I think that are used as a noise filter) and a pair of hard clippers to ground.

Since I want to get this side closer to a Keeley DS-1, if I simply remove all of the soft clipping diodes (D15, D16, D17), do I need to alter the values of (or also remove) the resitor and cap in the feedback loop as well?

D4 & D5 are clipping diodes. Don't jumper them, just remove them. If you remove the soft clipping diodes, you can probably leave the resistor and cap as is. You can experiment with some different values of those just to see what happens. I would think if anything, maybe you might want to lower the value of the 220K resistor a little bit to make up for the removal of those soft clipping diodes.

[dan5150]

The part that had me stumped is that even though I replaced them with jumpers, I still should have had signal (as oposed to removing them outright), right?

Since I want to remove the soft clippers from the feedback loop, is there a "best practice" on what values to keep in the feedback loop? What does that resistor/cap combo control?

[Paul Marossy]

The part that had me stumped is that even though I replaced them with jumpers, I still should have had signal (as oposed to removing them outright), right?

Since I want to remove the soft clippers from the feedback loop, is there a "best practice" on what values to keep in the feedback loop? What does that resistor/cap combo control?

Not sure why you wouldn't have any signal with the place where the diodes were jumpered. Doing that essentially makes it a voltage follower, if you were to pretend that the gain control wasn't there.

The resistor sets the amount of gain and the cap affects the high frequencies. Generally speaking, making the feedback cap smaller makes it sound harsher/brighter and making it bigger mellows it out/makes it smoother.

[CZ101]

Maybe I should start a new thread about this, but has anybody experienced the clicky/ticking sound from this pedal while in bypass mode? The ticking seems to be synchronized with the LEDs flashing, fwiw.. Any ideas on how to fix this?