modding a DOD Fx66 (flashback fuzz)

[fuzzo]

Hi guys,

I've just received that pedal today. And I'm very despointed abouth it, It's very gainy and has a lot of high frequencies. Really ugly sound. Absolutelly not a fuzz like the manuel says.

So, here's the schematic :

http://www.schematicheaven.com/effects/dodfx66_flashback_fuzz.pdf

I think the gyrator circuits are a key of the sound produced by the distortion box. How know the frequencies boosted by them ? How transform it ?

See you another things to do  ?

Actually, reducing the gain and high frequencies could be a good start.

Thanks

[fuzzo]

No one has idea to change that pedal ?

I plugged it today again, and that sounds really bad 

[Mark Hammer]

You can reduce the graininess and high frequencies by increasing the value of some of the o-amp feedback capacitors.  In most instances, they seem to be chosen simply to eliminate oscialltion, and not really for tone shaping.

So...C10 can probably stand to be bumped up as high as 470pf, and C18 can probably help if increased to 330pf or so.

[fuzzo]

Thanks Mark.

I'll try that if I've the good values cap .

I was thinking about reducing R17 to 4K7 or maybe less to have a gainless sound. Remplacing the 1N4148 clipping diodes for Red Led can help too ?

So, I'm wrong, the gyrator haven't big importance ?

[Mark Hammer]

The gyrators are critical to the FX66.  Not critical to whether it will work or not, but critical to it being the FX66 and not some other pedal/sound.  DOD has many different pedals that use combinations of pre-eq and post-eq (in the form of gyrators) to produce different sound personalities. 

When you say "too gain-y", I'm not sure what you mean.  Do you mean you want less distortion?  less sustain?  less volume?  Your sonic goals are not clear to me.

[fuzzo]

I mean too much gain. I'd like something with less gain.

Actually I thougth that was a "real" fuzz (imating fuzz face, tonebender ect.... sounds with AOP )  but that sounds more like a distortion with a lot of high frequencies (Imagine a big muff with only high frequencies, when you increase the gain you increasing more the highs). Any bass even if I  put the "low" at max whereas the "high" works well, it adds a lot of highs.

I tried to see what kind of gain I'd like to have, and after 12H00 I can't hear any difference with the "drive" pot.

Plus, I supect that PCB is using for other DOD products, there are missing components on the PCB (empty holes)

[fuzzo]

I tired to calculate the frequencies boost for both gyrators with Jack Orman's page : http://www.muzique.com/lab/gyrator.htm but on Flashback fuzz the bias resistors are missing (I think the bais is taken from the feedback loop, where the gyrators are connected) so how calculate that ?

I put the feedback loop resitor (220K) in the equation but that gives a boost at 135Hz, I don't think it's the good value.

[zombiwoof]

I've got one that I got for cheap on Ebay, it's definitely a fuzz, probably just not the fuzz you want.  It reminded me of the Maestro/Bosstone sound.  I don't think it was intended to be a Fuzz Face, there are many flavors of fuzz, it is just another one!.

Al

[fuzzo]

Maestro/bosstone ? really ? I made a bosstone and it doesn't sound like the dod does.

everytime I plugged it,  it sounds like a bee partouze

Anyway I'm sure I can mod it to have a real fuzz sound I like (fat and dirty) . the only thing, I've to find the frequency boost created by both gyrator and tweak it (and maybe put a trimpot to control the post and pre-EQ frequency).

[fuzzo]

Well, sorry to bump that old thread but I need some help . I  just started modding it, I placed a 20K trim instead of R17 to reduce gain (works pretty good)

But the kind of "fixed wah" stay present , I pretty sure that be caused by the two gyrators made by Q5 and Q7. I think I can tweak it to have a "moveable" frequency range, I simulated some responses changing the resistor which link the base of transistor to the bias (here : http://www.muzique.com/lab/gyrator.htm) but the DOD gyrator haven't the same "architecture" . There are not resistor linking the transistor to the bias so how change the frequencies response without chaning cap combo ? .

So how I could change those gyrators ?  With the resistor in the feedback loop of AOP attached to these gyrator ? That will change the gain too ?

Otherwise, here's what I wanna do (but in my opinion the big change will come from the gyrator modification)

RC/C9 : cut at 318Hz (maybe changing RC (10K stock) with a 10/20K trim to change the response and allow more basses ? )

Changing the gyrator  resistance (God there're a lot of that stuff in DOD pedal) of the one controlled by the "low" control to select the bass frequencies

increase the tinny cap (50pF) in feedback loop to tame highs.

What do you think about it ?

[fuzzo]

I opened up my DOD and found out the missing resistor , the ones with the transistors from gyrator. So R9 is 47K attached to Q5 and gives a boost at 310HZ , R36 is attached to Q7 and gives a boost at 5KHz (actually, now I understand why that pedal is so ugly )

I'm gonna make those gyrator moveable in adding a trimpot (or maybe pots) instead of R9 et R36. R9 gonna be 470K pot with 470R.

The module created by Q7 will change, I'll put another values for caps around it and create a identical module to the one formed by Q5.

With that I'll have a "pré-" and "post-" EQ.

I don't know if that will be really useful but , that sounds so awful that I don't care   

(if that can be useful for someone else)