DOD FX55 Distortion mods?

[ian87]

hi, all. might be kind of a polishing-a-turd situation here, but i got an FX55 for dirt cheap and i'm thinking it might be fun to try and make it suck less.

found a schem for the FX55B on Fuzz Central, but i'm not sure how similar that is to the non-B, which i have. the opamp is different, at the very least.

so is there any point in changing the TL06_ to a TL07_? (i only ask cuz i have a ton of the latter sitting around!) are there other, better opamp recommendations? any other advice or a link to a non-B schem would be greatly appreciated.

tia,
ian

[cd]

Aren't the -B versions the ones in the upgraded case, the ones with the battery door that you can't lose?  I have a -B Grunge pedal which is the same as the non-B schematic that's floating around.  Knowing Digitech/DOD I strongly doubt that there's any difference - even old DOD pedals have been recycled as new Digitech pedals lately.

Anyway - the TL06x series is a low noise variant of the TL07x series.  Not much difference other than that, but you know the drill: swap it with a socket and then you can plug in different opamps to your heart's content.

As for other mods - ask yourself what you don't like about the pedal, then you can change parts accordingly to suit your taste.  Too much gain?  Too little?  Too much highs?  No range in the tone control?  etc. etc.

[ian87]

thanks, CD. i don't have the pedal in front of me, so i can't tell how close the B schem is -- as i mentioned, the opamp appears to be different -- the B has a dual JFET (LF353) and the non-B that i have has a single (TL061). how significant is that?

i'll probably mess around w the tone caps to get it voiced properly, and may experiment w some different clipping diodes to try and remove some of the harshness. anyone have recommendations?

-ian

[Ben N]

Polishing a turd, indeed!  Lemme jump on this bandwagon!

I actually tried selling mine (FX55B or C, I don't recall) a few months ago, but ebay in their wisdom pulled the post because it said "not MXR".  OK.  Since then, I have seen what these go for on ebay, which is nothing.  OK, since that is really what I think it is worth.  So thanks for that schematic link, because I might as well try to mod it, if someone can indeed come up with a way to polish this turd.  

So what is wrong with it?  All thin and fuzzy, no bottom or girth, insufficient control range, no dynamics, no tone.  The tone control in particular sucks.

So what to do?  Now that I see the schematic, I would look at disconnecting one or the other clipping pairs, or socketing them to experiment with other kinds of clippers.  Same for C9 & C13--I'll bet something bigger in there would get rid of some of the harshness.  I'm not sure where the lows are getting rolled off, so I don't know how to deal with that.  Also, what to do with that useless two-pole tone control?  And would there be any benefit to changing the input buffer to a JFET opamp?

Finally, can anyone explain why the FET switch to ground the effect (Q2) is between the LF353 sections, instead of at the end, or why there is a need for an output buffer in this thing at all.

Ben

[toneman]

doesn't the "B" stand for "bass"
like in bass guitar???
just guessing........
i know a hammond 1596BB is *not* a double bass

[ian87]

doesn't the "B" stand for "bass"
like in bass guitar???

i don't think so. it seems the brain trust at DOD added a "B" to distinguish the regular 55 (Distortion) from the SUPRA Distortion...    \m/  :twisted:   \m/

[Ben N]

No, it's not bass--just a variant.

Ben

[j0shua]

HEHE

The Supra distortion is one of the oldest pedal from DOD/Digitech and still on road......

is a variant of the old ( AMERICAN METAL )  but just for hard rock, glam metal, Hair metal " DOD says that"..... ( what a joke! )

anyway the tone/ bass appears never works hehe, but you can change the pot for lower value to optain better results.

also you can change some CAP near of FET i don't remember well and change the diodes


if you can get a inside pic i can edit you on my pc

[ian87]

ok, so i dropped a socket in and tried a whole slew of differ ICs and settled on a TL072. also removed one pair of clipping diodes, and it sounds *better* -- but still not good.

i can't seem to find a non-B schem, so if anyone has a clue which caps on the FX55 are responsible for voicing/tone shaping (the FX55 has no tone knob), i'd love to know. there's like *zero* bass available ...

[cd]

You know, there are at least 4 variants of the FX55 - FX55-B, FX55-C, FX55 (3 knob), FX55 (2 knob).  Have you gone through what you can with the circuit to see how similar it is to the schematic you already have?

If it's at all similar, most of the low end is lost in the clipping section.  FYI I'm referring to the schematic linked above.  Look at the two ends of the DISTORTION control.  Come to think of it there may be an error there - the pin 2 and pin 1 connections should be reversed... if so, then:

Max gain (100k fully in parallel with 3.9M resistor): the 27 ohm resistor and the 1u cap set the low-end rolloff, in this case it's approx. 5.9Khz!!!!  To put it in perspective, the Tubescreamer rolls off low end at 0.72Khz (720Hz).  Stock, the 27/1u combination is a treble boost effect.

Lowest gain (100k fully in series with 27 ohm resistor): zero gain, but low end rolloff at 1.6Hz - no audible bass loss (but no gain either).

So in an nutshell, as you up the gain, the low end gets cut off more and more.  What can you do about it - well, again given the 55B schematic above, I would get rid of R12 (remove it completely), then take the wire that goes from the DISTORTION pot to the 27 ohm resistor, and move the pot end to pin 2 (use the hole that previously held the 3.9M resistor).  Then change the 27 ohm resistor to something more reasonable, like 470ohms or 1k, that will reduce the gain slightly but will hold the low end rolloff steady... change D4 and D5 to LEDs or some other higher threshold diode, and leave D1 and D2 in, but connect them to ground with a 1k resistor to soften the clipping (lift their ground ends in the air).  What else... disconnect the tone control completely, all it's doing is cutting more bass - replace it with a 10k resistor to ground.

[ian87]

hey cd -- i don't have anything resembling a 27 ohm resistor in my FX55. the two resistors coming off the Dist pot are 1k and 22k.

[ian87]

bump.

[ian87]

last bump b4 i give up.

anyone? bueller??

[j0shua]

Did you get the pics inside the DOD?

[ian87]

Did you get the pics inside the DOD?

sorry, joshua -- i don't have a digicam.

[cd]

Have you tried tracing out a schematic?  Or at least how the clipping section is wired in your pedal?

[Elektrojänis]

Anyway - the TL06x series is a low noise variant of the TL07x series.

Um... This is not probably that relevant to this topic... But acording to the TI datasheets the basic version is TL08x. TL07x is a low noise version of TL08x. TL06x is a low power version of TL08x.

The weird thing is that the same datasheets specify the same noise specs for TL08x and TL07x. Equvalent input noise voltage for TL062 is specified to be higher but there is no equivalent input noise curren specified.

fixed a typo

[ian87]

Um... This is not probably that relevant to this topic...

might not be relevant, but it's interesting.

i'm guessing there was a typo in your post here:

The weird thing is that the same datasheets specify the same noise specs for TL08x and TL08x.

what should it have said?

-ian

[Elektrojänis]

i'm guessing there was a typo in your post here:

The weird thing is that the same datasheets specify the same noise specs for TL08x and TL08x.

what should it have said?

Argh... You are absolutely right. It should have said TL08x and TL07x

-Elektrojänis

[ian87]

thanks, janis. that is indeed odd that the noise specs are the same...

i can't remember if i tried a TL08X in the FX55 -- i'll hafta check again.