Because I keep running into dead links while looking this stuff up, so I'll try to put the info in subsequent posts. (Shower thought: I should probably make a big clearinghouse thread for the FX5X series with as many backed up posts/links/pics as I can...)

Hey y'all, I'm back in the saddle after a longer than expected break. You may have seen my previous post on building a custom DOD FX53 Classic Tube PCB, which I did before I knew MusicPCB and a few other makers had some nice FX5X-based PCBs available. https://www.diystompboxes.com/smfforum/index.php?topic=131556.0

I have two leftover boards, which means I can build any 2 of: FX50 (OD+), FX55B/C (Supra distortion) and FX52 (Classic Fuzz), so I'm looking for schematics and board shots. I've found plenty for the FX50, 52, and 55-B, but I cannot find a confirmed schematic or non-blurry board shot of the elusive FX55-*C*. Does anyone have a schematic/board shot available? All the links I've been able to find (looking at you americaspedal) are dead.

I've seen chatter both ways that it's either just the Chinese version of the FX55B, or it's actually got different component values and a brighter, thinner sound; either way I want pics or it didn't happen. It looks like early runs happened in the US so it's possible either is true depending on which unit you get.

Second, I've also seen some references to people modding the FX55b/Cc in various ways. This user says Wampler used to mod them but I can't find the original thread [1]. Does anyone have more info on this, especially the Wampler mod? I don't know which capacitors they're referring to because I don't have a board shot. I'm *doubtful* it's the 120p LPF in the feedback loop but you never know. The diode mods seem to be on point and I'll give 'em a try since I have parts and too much free time. Other mods I've found [2][3]

Third, I saw one version of a clone for the FX5X boards that had an additional 1M resistor to inject vbias into the buffer (insert link and pic when I find it again) into the inverting input/feedback loop. As far as I can tell, the output buffer is coupled to the tone stack, which is getting its vbias through the volume knob's virtual ground. Does this make a noticeable change? It seems like it should work with or without, no harm done.


TLDR
1. Does anyone have schematic and board pics of a Chinese (V)FX55-C or USA FX55-C?
2. Any info on Wampler's old mods for these?
3. Does adding injecting vbias into the output buffer's input do anything extra?


[1] https://www.harmonycentral.com/forums/topic/1522079-how-to-mod-a-dod-fx55c-supra-distortion-into-a-really-good-sounding-distortion/ TBD put in quote reply
[2] https://www.freestompboxes.org/viewtopic.php?t=31350 More mods
[3] https://www.freestompboxes.org/viewtopic.php?t=16676 More mods
[4] http://nicosonic.com/boss_tr2_df2_od1_dod_fx55b_pedal_modifications More mods
[5] http://abrammorphew.com/notes/2014/09/16/dod-fx55b-supradistortion-bigbuff-mod/ More mods (this link just stopped working today (!) but the image is still visible: https://i0.wp.com/abrammorphew.com/notes/wp-content/uploads/2014/09/dodfx55b_bigbuff_mod.png
[6] PedalPCB PCB for FX5X builds: https://www.pedalpcb.com/product/fx5x/ (possibly includes FX52)

I have a fx55 here but don't know the letter b or c.   I'll look when I get home.  I've bought it used/ cheap and never really bonded with it.  I do remember looking for mods and may have a file folder with some stuff. 

fx55b
https://fuzzcentral.ssguitar.com/schematics/dodfx55b.gif

PCB pic of fx55B
https://mirosol.kapsi.fi/2015/05/dod-fx55-b-supra-distortion/

From the PCB pic and the schematic you might be able to fill in some blanks.
The obvious solution is to trace the pedal or trace the differences.
No need to retrace the footswitch circuit.

---

The fx55c is here,

https://www.freestompboxes.org/viewtopic.php?t=27298

 I guess i did mod mine a bit. Top chip is in a socket and is LF353N, which i know i have a bunch.  2 diodes D1, D2 are lifted. There are a few caps that aren't greenies but look stock so i didn't think i changed them, would have to lift the board and check for resolder to make sure.  4 pieces of thru hole J201A in there, i should steal them for other projects!
Weird, looks like the older b model had a LF353 and the C had 2 4560's, so maybe the mod was to use the old opamp. D1,D2,D3,D4 are named different between B & C . I lifted the D1, D2 which would be the soft clippers on the C model. Probably softened up the whole pedal was my goal.

Quote from: Rob Strand on May 03, 2024, 06:01:34 PMfx55b
https://fuzzcentral.ssguitar.com/schematics/dodfx55b.gif

PCB pic of fx55B
https://mirosol.kapsi.fi/2015/05/dod-fx55-b-supra-distortion/

From the PCB pic and the schematic you might be able to fill in some blanks.
The obvious solution is to trace the pedal or trace the differences.
No need to retrace the footswitch circuit.

---

The fx55c is here,

https://www.freestompboxes.org/viewtopic.php?t=27298

🙏you're the best! I couldn't find that (maybe I missed the pdf link going too fast). TY!

Quote from: mozz on May 03, 2024, 07:42:50 PMI guess i did mod mine a bit. Top chip is in a socket and is LF353N, which i know i have a bunch.  2 diodes D1, D2 are lifted. There are a few caps that aren't greenies but look stock so i didn't think i changed them, would have to lift the board and check for resolder to make sure.  4 pieces of thru hole J201A in there, i should steal them for other projects!
Weird, looks like the older b model had a LF353 and the C had 2 4560's, so maybe the mod was to use the old opamp. D1,D2,D3,D4 are named different between B & C . I lifted the D1, D2 which would be the soft clippers on the C model. Probably softened up the whole pedal was my goal.

I wonder if it's an audible difference? I'll pick up some opamps to experiment. I think the 353 and 4560s are actually the only two common ones I don't have on hand at the moment.

For a comparison data point, my FX53 (Classic Tube, same PCB) has a 353 for the top (drive) opamp and a 1458 for the bottom (buffer) opamp. My clone uses TL072s. I've been having some volume issues with it but I think I might have changed a component and forgotten about it--I doubt it's the opamps since they're similar types (JFET) and slew rate, etc. Time to make another one!

I think it's simultaneously cool and annoying that the DOD pedals from this era are so wildly different between production runs. It's like they have lore and backstory and individual character. It's why I'd like to make a single thread with all my findings because there's so much scattered info and variants in the schematics.

Quote from: tootsMcgee on May 03, 2024, 12:36:34 PM---- Shower thought: ...)

this. they are always the best, even if they generally come too late.

Hang on...looking over that FX55C schematic and board pic...did they remove the 3.9M resistor from the feedback network?

If I'm reading this right (probably not, very tired right now): 100k gain knob max and 200R resistor to ground yields ~500x gain on the FX55C

On the FX55B, the topology is a bit different because one end of the pot is connected instead of left to float. Does this mean that at max gain, it's effectively 97K (parallel 3.9M and 100k) and the gain is 97K/27R = 3600x?! (I think Mark Hammer mentioned this in my thread for the FX53, which has an even bigger gain factor of 5600)

Now to be fair, past a certain point, the pedal just sounds like orange juice on toothpaste tastes like, so the difference is probably not that huge, but it's still something that caught my eye...maybe that's why people noticed a change between the 55B and 55C

This sounds like I need to do some experiments with socketed resistors and a jumper to tie in the other end of the gain pot...maybe with a standardized test audio file from my audio interface...

EDIT: FX50B schematic, has 500K pot and 470R to ground, = ~1000x, more than the FX55C (??)