Build report - DOD FX25

[Mark Hammer]

I had a fairly productive weekend, getting my Bluebox working - finally, and building a Baseballs and DOD FX25 from the project info available at Alessandro Petrini's site: http://topopiccione.atspace.com/PJ11DODfx25.html

The FX25 layout works fine.  It's a pretty simple OTA-based filter with a sensitvity (well, rectiier gain actually) and resonance (what they call "peak") control.  The response time is fairly fast, and the range of tones about as good as a DR. Q.  Filter range would likely need to be altered by changing C9/C10 for .022uf or maybe even .027uf if you were planning on using it for bass.

As is my habit, I stuck a variable attack time control in mine by inserting a 1k pot (I actually used a 10k with a 1k resistor in parallel) between D3 and the R9/C7 junction.  Because this cuts down sweep amount at the same as it slows down the initial attack, I needed to max the sensitivity control.  Looking over the schem, I realized that at max gain, this has a rolloff around 338hz in the envelope detector section, whic struck me as putting the bass notes at a disadvantage, so I increased C5 to .33uf just to be on the safe side.

The lowest resonance/peak settings were not particularly inspiring, so I upped the value of R15 to 33k to nudge the overall range of resonance settings up a bit (higherresistance values of R15 + P2 = greater resonance/peakiness).

In later versions of the FX25, a blend control was added to mix in some full spectrum clean signal with the filtered signal.  I can say that this is likely a very useful addition in this instance.

Although I haven't tried it out yet, and the ways of OTAs are a mystery to me, I see a parallel between the Dynacomp/Ross compressor and this configuration.  I gather R9 recharges C7 in some manner, and provides a fixed recovery time.  I'm just guessing in the dark here, but my instinct is that a variable decay time could be achieved by having something like a 510k fixed resistor and 500k pot/variable resistor in series to replace R9.  I'll see if I can wire it up later today and verify.  Stock, the FX25 has a nice whip-fast attack, but the pleasingly largish (22uf) smoothing cap might make settling time a little too slow for some tastes or songs.

[Zero the hero]

I've always wanted to revise the DOD FX-25 layout in order to add all the mods taht came up months ago in this forum... But I never had the time!
I would like to add your impressions and mods into the project page, with credit to you, of course. OK?

Thanks for building this project! Your suggestions are always welcome!!!
Have you seen "The Sniper"?

[Mark Hammer]

Thanks, and thanks for the layout.  Extremely easy build with no problems whatsover in fitting components in.  I like Francisco's layout very much, but occasionally I reach into the parts bin and the only caps I have of that value simply won't fit.  In this case I did not have that problem.  The only complaint, I suppose is that it would be an advantage if all pads for any switches or pots were reasonably close together.  P2 is a little "spread out".  As well, there is no ground pad for P1.  Not a big problem since I could just tuck the other wire under the board and solder it to the ground trace on the copper side, but its' absence might confuse some folks.

Generally, very nice project and writeup.  Between your site, Tonepad, and GGG, these are golden times for DIY-ers!  Those folks just starting out within the last 3 years should consider themselves VERY very lucky.

[Bernardduur]

Ha, I was just checking that sniper out. It looks so cool.....

[Zero the hero]

I thing that I love of FP layouts is panel mounting pots. I actually managed to do this in the Gretsch Controfuzz only.
I'll work harder to do this in my future projects. 
PS Today I noticed how the amplifier of the Sniper looks like the MXR Headphone amp on GGG. I simply didn't aware of this design... I wasted hours on designing that thing...

[cab42]

Generally, very nice project and writeup.  Between your site, Tonepad, and GGG, these are golden times for DIY-ers!  Those folks just starting out within the last 3 years should consider themselves VERY very lucky.

Well Mark - I do feel lucky. I started out 6-7 years ago, but gave up until I found this forum two years ago. Not only I can build the most fantastic effects, but this forum  has also become my daily therapy. When stressed I just need a few minutes reading about an input buffer on a TS then I'm cool again 

On the topic: I added the DOD FX25 to my build list. I need a filter succes after failing so miserably with the Phuncgnosis. Everyone praises it, and I have built it twice with different layouts but both distorts like crazy.

Regards

Carsten

[Mark Hammer]

Well, since you mention the "therapeutic" effects of talking about effects, I'll just add that I finally got a Blue Box working this weekend after having it sit useless for the better part of a year.  The same thing happened with a Dr. Q about 4 years ago.  Eventually, the light comes on, you think about something that han't occurred previously, and you find the source of the problem.  Thank goodness that, as repairman, you are your own most patient customer!

Now, if I can get the PAiA Hyperflange that I bought in 1992 working, THAT will be my big success!