DOD FX55 schematic?

[LucifersTrip]

On a few quick searches, I'm finding every else except for the FX55, for instance:

http://www.schematicsunlimited.com/d/dod

Does anyone have a schematic (not layout, not the Supra FX55-B) laying around?

thanx

[LucifersTrip]

got it

[holio cornolio]

Hauling this one back from the dead.... Firstly hi everyone. I've lurked here under one username or another for years, but just recently got back into pedal building. Long story short(ish) I've always loved the old FX series DOD pedals and recently spotted one of these distortion units on EBay for a good price. I know that they're not exactly a classic, but I took a punt and it arrived today. I am something of a Johnny Know Nothing when it comes to electronics, I just solder by numbers, but looking at this schem made me wonder if the circuit is a little different from the norm. Anyone have any background on the circuit and what the big 14 pin chip is all about?
I have to say, this is a pretty nice distortion to use. it is not touch sensitive or transparent or any of those modern buzzwords, but it's got a nice chimey crunch at low gain settings, and dimed it will conjure up feedback at will. I'm a little surprised that I've never ever ever seen one of these on another pedal board ever. It's pretty cool.

[CheapPedalCollector]

It's a continuation of the 250 preamp/overdrive. Stage 1 is a soft clipping circuit like any TS or OD-1/SD-1, it is symmetrical. This drives a hard clipping circuit which is identical to the old 250.

The CD4007 is setup as an edge triggered flip/flop and drives the gates on the 2 FETs. It goes through the distortion circuit on one polarity and through the buffer on the other polarity. It has a transistor setup as a current limiter to drive the LED and also keep it from making a pop when switching. You can find an article here https://hammer.ampage.org/files/dodswitch.gif it's by David Di Francesco, one of the designers at DOD.

It's a nice pedal, I like it. I think it sounds better without the FET based switching though.

[Fancy Lime]

... I think it sounds better without the FET based switching though.

Really? How so? I have a hard time imagining that the switching circuit and JFETs would affect the sound in any way when the pedal is on. In bypass, sure, buffered or non-buffered makes a difference depending on the setup before and after the pedal.

And also: Welcome to the forum!

Andy

[Rob Strand]

It's a nice pedal, I like it. I think it sounds better without the FET based switching though.

The J113's aren't the best choice.  J112 would be better.

[Fancy Lime]

It's a nice pedal, I like it. I think it sounds better without the FET based switching though.

The J113's aren't the best choice.  J112 would be better.

Why is that?

Andy

[Rob Strand]

Why is that?

It comes down to the JFET VP.

Without any signal the JFET's D and S sit at 4.5V (Vref).    No problems here.


However when large signal swings are present the D and S swing above and below 4.5V.  When the JFET is switched on the gate sits at around 6V.  For large positive swing the D and S can rise above 6V the lower VP devices and start to turn off on the positive signal peaks.  The lower VP devices will turn off at lower signal levels than the higher VP devices.

The problem with the high VP devices is they don't turn off properly and can leak or turn-off and on in a non-linear manner with the signal.

So you have this small zone where there is a right VP and the JFET can turn on and off without corrupting the signal with a signal of reasonable level.

In addition to that you want the RC networks on the gate to overlap the switching between the on and the off paths in a manner that doesn't have any "gaps".

That's why it's good to stick with JFETs that have VP's in the same region as the original Boss/Ibanez parts.

[Vivek]

Last month, I sold my DOD FX54 attacker compressor and DOD FX55BB Supra Distortion. I had them for maybe 25 years or so.

A reborn guitarist came to buy them, he said COVID made him look at guitar again, and he wanted these pedals since they were the first ones that he had when he was a teen, and he wanted to recreate that moment in time.

[Rob Strand]

I could have trimmed the VP for a better example but this should show the basic issue:

JFET-ON


JFET-OFF

[Fancy Lime]

Why is that?

It comes down to the JFET VP.

Without any signal the JFET's D and S sit at 4.5V (Vref).    No problems here.


However when large signal swings are present the D and S swing above and below 4.5V.  When the JFET is switched on the gate sits at around 6V.  For large positive swing the D and S can rise above 6V the lower VP devices and start to turn off on the positive signal peaks.  The lower VP devices will turn off at lower signal levels than the higher VP devices.

The problem with the high VP devices is they don't turn off properly and can leak or turn-off and on in a non-linear manner with the signal.

So you have this small zone where there is a right VP and the JFET can turn on and off without corrupting the signal with a signal of reasonable level.

In addition to that you want the RC networks on the gate to overlap the switching between the on and the off paths in a manner that doesn't have any "gaps".

That's why it's good to stick with JFETs that have VP's in the same region as the original Boss/Ibanez parts.

Hi Rob. OK, thanks, I understand the reasoning. However, why are the gates only at +6V? Should the 4007 switch not provide close to positive rail voltage?

Andy

[Rob Strand]

Hi Rob. OK, thanks, I understand the reasoning. However, why are the gates only at +6V? Should the 4007 switch not provide close to positive rail voltage?

Yes the 4007 switching circuit will swing higher and have less problems with the positive peaks - and it follows it will have less problems with a J113 than a Boss/Ibanez circuit.  (I'm just so used to giving examples for the Boss/Ibanez circuits, that's where the 6V comes in.)

EDIT:
forgot to say the J112 will probably overlap more smoothly when switching than the J113.

[CheapPedalCollector]

Sometimes they don't turn off all the way and you can hear effect signal bleed through too in bypass mode. It's really common in DOD pedals and I usually add another FET and diode to squash the signal at any gain stages. I've posted a mod elsewhere for that with the FX50B/FX55B/FX52/FX53 as they all use the same circuit.

Also sometimes they have weird design and the VCC is lower than normal (see FX50B) because of how the volume control is hooked to the tone section and causes other problems.

As Rob also pointed out you can get clipping too, so they often lowered the amount of output available to compensate for that, and they barely get above unity gain which is not really desirable for me at least.

[Vivek]

It had been noted before that there is a slight bleed of distorted sound even in bypass mode on some DOD pedals.

[holio cornolio]

It's a continuation of the 250 preamp/overdrive. Stage 1 is a soft clipping circuit like any TS or OD-1/SD-1, it is symmetrical. This drives a hard clipping circuit which is identical to the old 250.

The CD4007 is setup as an edge triggered flip/flop and drives the gates on the 2 FETs. It goes through the distortion circuit on one polarity and through the buffer on the other polarity. It has a transistor setup as a current limiter to drive the LED and also keep it from making a pop when switching. You can find an article here https://hammer.ampage.org/files/dodswitch.gif it's by David Di Francesco, one of the designers at DOD.

It's a nice pedal, I like it. I think it sounds better without the FET based switching though.

thanks. This is super helpful. So the cd4001 simply handles the switching between buffered bypass and distortion circuit? And the distortion circuit is a soft clipping 'input stage' into an od250? So a few simple mods, could turn this into a hybrid of an mxr distortion plus right? Someone must've tried that...

[Fancy Lime]

Ok thanks. Sorry if I am being a bit of a nuisance. Point is, I am trying to design a pedal using a 4007 + JFETs switching system that avoids the worst problems if possible.

So, just for the sake of understanding: the distortion bleed through in bypass mode comes primarily from the high Rds(on) of the J113, not the low Vgs (off), right? Unless we happen to have a J113 with a Vgs(off) of less than input signal + one diode drop, agreed. The datasheet gives ranges of -0.5 to -3V for the J113 and -1 to -5 V for the J112. So essentially, the low end of the J113 is too low but the high end of the J112 is too high, no? Do we need to select these for proper Vgs(off) range? That would be inconvenient...

Andy

[CheapPedalCollector]

thanks. This is super helpful. So the cd4001 simply handles the switching between buffered bypass and distortion circuit? And the distortion circuit is a soft clipping 'input stage' into an od250? So a few simple mods, could turn this into a hybrid of an mxr distortion plus right? Someone must've tried that...

Yeah, it's basically a higher gain tube screamer into the hard clipping section of a 250. The soft clipping probably doesn't make any audible difference, easy enough to hook up a switch to disengage the diodes to see.

I've modded the FX50B with various mods, I can post a list here in a new thread if someone is interested. Most of them can be applied to the aforementioned pedals which use the same circuit/pcb and just have minor tweaks.

Ok thanks. Sorry if I am being a bit of a nuisance. Point is, I am trying to design a pedal using a 4007 + JFETs switching system that avoids the worst problems if possible.

So, just for the sake of understanding: the distortion bleed through in bypass mode comes primarily from the high Rds(on) of the J113, not the low Vgs (off), right? Unless we happen to have a J113 with a Vgs(off) of less than input signal + one diode drop, agreed. The datasheet gives ranges of -0.5 to -3V for the J113 and -1 to -5 V for the J112. So essentially, the low end of the J113 is too low but the high end of the J112 is too high, no? Do we need to select these for proper Vgs(off) range? That would be inconvenient...

Andy

I think you can use either really but you'll get more headroom out of the J112. To stop the bleeding what I did was hooked up a 3rd J113 and a diode in the gain stage to drop it to inaudible levels. I'll post the schematic here.

[holio cornolio]

Would love to see those mods 🙏

thanks. This is super helpful. So the cd4001 simply handles the switching between buffered bypass and distortion circuit? And the distortion circuit is a soft clipping 'input stage' into an od250? So a few simple mods, could turn this into a hybrid of an mxr distortion plus right? Someone must've tried that...

Yeah, it's basically a higher gain tube screamer into the hard clipping section of a 250. The soft clipping probably doesn't make any audible difference, easy enough to hook up a switch to disengage the diodes to see.

I've modded the FX50B with various mods, I can post a list here in a new thread if someone is interested. Most of them can be applied to the aforementioned pedals which use the same circuit/pcb and just have minor tweaks.

Ok thanks. Sorry if I am being a bit of a nuisance. Point is, I am trying to design a pedal using a 4007 + JFETs switching system that avoids the worst problems if possible.

So, just for the sake of understanding: the distortion bleed through in bypass mode comes primarily from the high Rds(on) of the J113, not the low Vgs (off), right? Unless we happen to have a J113 with a Vgs(off) of less than input signal + one diode drop, agreed. The datasheet gives ranges of -0.5 to -3V for the J113 and -1 to -5 V for the J112. So essentially, the low end of the J113 is too low but the high end of the J112 is too high, no? Do we need to select these for proper Vgs(off) range? That would be inconvenient...

Andy

I think you can use either really but you'll get more headroom out of the J112. To stop the bleeding what I did was hooked up a 3rd J113 and a diode in the gain stage to drop it to inaudible levels. I'll post the schematic here.

[Steben]

the soft-before-hard thing is found in notably the Nobels ODR-1 as well. Elektor had a distortion circuit in 1986 with the same idea.
It is very subtle, but it plays with some onset clipping region thingies. And it prevents the first opamp clipping, which results in somewhat less harsh content.

[CheapPedalCollector]

Would love to see those mods 🙏

OK, I'll draw up a schematic and start a new thread on it.