DOD - FX10 BiFET Preamp  [schematic]

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Maxwell
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Post by Maxwell »

I don't know how well-known this pedal is, but it's a pretty nice clean booster which can boost bass or treble and it can be found used (it's discontinued) rather cheaply. When I bought mine, it included a schematic which I have scanned and attached.

I thought this might be a fun one to mod. I thought of changing out the TL062 op amp. Would it also be possible to increase the gain to get a larger boost?
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Maxwell
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Post by Maxwell »

Well as an experiment I changed out the op amp for a BB OPA2134. It has a fuller tone now I'd say which I like. More bass and clearer highs. I'm going to leave it in.

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Post by JiM »

It's funny how it's name has little to do with the actual circuit.
You would expect something with two JFET gain stages, or something like the MOSFET Booster from AMZ ...
In fact the J113 are switches and the 4007 CMOS acts as a latch for the stompswitch. The onlt FETs actually amplifying the signal are the first stage of each opamp, just like any other opamp-based pedal !

What is the actual sound difference when the pedal is activated, as it's not bypassed ? Do the controls still operate somewhat when disengaged ?
I only give negative feedback.

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Post by Maxwell »

Yeah I was wondering a bit about that...I expected to see a discrete FET buffer or something like that.

I don't notice the controls having any effect when it's off. The bypass seems pretty good but I keep mine in a loop anyway.

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Post by ironman28 »

JiM wrote:It's funny how it's name has little to do with the actual circuit.
You would expect something with two JFET gain stages, or something like the MOSFET Booster from AMZ ...
In fact the J113 are switches and the 4007 CMOS acts as a latch for the stompswitch. The onlt FETs actually amplifying the signal are the first stage of each opamp, just like any other opamp-based pedal !

What is the actual sound difference when the pedal is activated, as it's not bypassed ? Do the controls still operate somewhat when disengaged ?
BiFET refers to the opamp design that uses a BiPolar output stage and a FET input stage, nothing new there as there are whole families of devices that can be called "BiFET".
The idea was to optimize the input and output stages as FETs are high impedance/low current devices whearas the bipolars are better driving a low impedance load. Not all opamps have FET inputs however, the 4558 types as an example.
The FX10 has a very high input impedance as it was originaly designed to match HiZ piezo type pickups to a guitar amp input. I find that it can be almost too sparkly when used with standard guitar pups. The DOD 410 BiFET studio preamp was the predecessor to the FX 10. It had a single opamp design with a similar tone control, no footswitching and a XLR connector for out put to a mixer all in the smaller diecast DOD box.

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Post by Fuzzer »

Thanks for the explanation.

However, I have to agree that it's a little misleadiong, I had just assumed the same as Jim when I first read about that box ( it was about Soundgarden's guitarist, Kim Thayil) Rig.
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Post by ironman28 »

yeah I can see how someone could infer that there are 2(bi)-FET's in the box. The term is technically accurate so I don't think DOD was trying to fool anybody.
There were DOD FET preamps before they made the 410 or the FX10 so maybe it was an attempt to differentiate them from the earlier FET preamps.
At any rate they are a nice pedal as a booster or a buffer first in the chain as the buffering is always active. Thayil used after his wah to provide a constant load and maintain the full sweep of the pedal kinda like the FoxRox retrofit wah buffer. I have the FX10 and the 410 versions I like the 410 tone control sweep better it is a little different than the FX10. It even sounds good with a passive bass, it helps retain the upper harmonics and transients. I did a stripboard layout and schematic over at the other forum a while back for the 410.http://www.aronnelson.com/gallery/main. ... 0.JPG.html

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Post by rune-hammer »

Awesome! Been looking for that!

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Post by CableBling »

Hi, greatings from a newbie!

I have an FX10 which I love... I'm looking to self build another for a friend who wants to use it following a Big Muff clone to tame/tone it. The new FX10 will be in a bypass loop with the Muff so it has no need for a footswitch/LED etc.. it will always be on whenever the Muff loop is selected.
I've modded the above FX10 schematic to remove the components (I believe) will not be required, just leaving the buffer/gain and tone circuits in place. Whilst waiting for my ordered parts to arrive I wonder if a more experienced builder would mind given my circuit a quick look over to see if I've made any glaring mistakes?

Many Thanks, a great board by the way.
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FX10 bypass removed.jpg

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Post by JiM »

CableBling wrote:Hi, greatings from a newbie!
Welcome !
CableBling wrote:I've modded the above FX10 schematic to remove the components (I believe) will not be required, just leaving the buffer/gain and tone circuits in place.
Looks fine to me.
I only give negative feedback.

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Post by CableBling »

Hi, thanks for the reply.
Here's a couple of gut shots of mine in case anyone's interested.
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FX10-pcb2.JPG
FX10-pcb1.JPG

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Post by Psychotronic »

Trying to fix mine, and I need some help re-wiring the old 9v jack back in (there is a battery clip there now). Not sure which lug on the jack is which, and what wire goes where. Can anyone take a picture of theirs so I can match it to mine? Thanks, in advance, from an electronics n00b.

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Post by CableBling »

This should do it.
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wiring.jpg

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Post by Psychotronic »

That ought to do it, thanks so much!

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Post by adam79 »

Do the three different versions of the FX10 all have the same circuitry? Was it just cosmetic changes?

Thanks,
-Adam

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Post by adam79 »

ironman28 wrote:Thayil used after his wah to provide a constant load and maintain the full sweep of the pedal kinda like the FoxRox retrofit wah buffer.
I saw a picture of Thayil's pedal board and noticed that he had two FX10s back to back.

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Post by Ben N »

Obviously, if you want it always on, or to be able to remove it entirely from your signal chain, it makes sense to remove the switching. But the "bypass" setting on the FX10 is a very good buffer, so one could think of it as a quality high impedance buffer for your board with a very clean boost/tone control added as a bonus. BTW, I believe that the FX10 is similar in topology to the FX50B overdrive.

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Post by mictester »

Maxwell wrote:I don't know how well-known this pedal is, but it's a pretty nice clean booster which can boost bass or treble and it can be found used (it's discontinued) rather cheaply. When I bought mine, it included a schematic which I have scanned and attached.

I thought this might be a fun one to mod. I thought of changing out the TL062 op amp. Would it also be possible to increase the gain to get a larger boost?
The gain of this is governed by the 330k feedback resistor. Increase this value, and you'll increase the gain. Try 470k. Be careful - if you go too far, you'll start to get the outputs of the op-amps "hitting the rails" which gives rise to really nasty distortion.
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Post by tchewtch »

I have a Dod FX-10, which I bought used. The output from the unit is equal to the input at about halfway on the volume control, below that the volume reduces. I took a look inside and the only thing I could see was that the volume control had been replaced at some time. I suspected that maybe it was the wrong value (500K according to the schematics I've seen), but it is correct and tests OK. Does anybody have any suggestions as too where I should look for the problem?

Thanks, Dave

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Post by mictester »

The circuit is working as it should. If you turn the gain pot down, below a certain point it will become an attenuator. This is actually deliberate - the manufacturer calls it a "buffer and preamp". You an use it to reduce level a bit too. I know a player who has one set up for slight level reduction. but treble emphasis, so that he can still be heard in the band mix.
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