Technology of the DOD FX-17?

[Paul Marossy]

I have been looking at the DOD FX-17 schematic and trying to understand how this thing works, but I am kind of at a loss. The kind of stuff I am wondering about is what do those 4007 chips do? What is up with the parallel J113s after the input buffer? How does that 13600 chip create a wah circuit? How the output jack is connected doesn't make sense, either...

Anyone want to give me a hand in understanding this thing a little better?
Schematic: http://www.diystompboxes.com/cgi-bin/webbbs_scripts/webbbs_config.pl?read=201

[Paul Marossy]

OK, this much I know:

4007 - dual complementary pair and inverter with 3 N-channel and 3 P-channel enhancement mode MOS transistors

13600 - dual OTA with two current controlled transconductance amplifiers with differential inputs and push-pull outputs.

LM3080 - an OTA

LM 358 - dual opamp

J113- N-channel JFET


The 13600 is really confusing me now!  :?

[puretube]

the jfets do a boss-like bypass switching, actuated by the right 4007,
which also does part of the routing.
the left 4007 is a HF-oscillator based "proximity control,
together with the MPS and half 358
providing a "distance to voltage" converter,
that feeds a VCA and a VCF;
the 3080 is the VCA;
the 13600 are 2 VC lowpasses in series (however strangely fed and fed back - needs a second look...)
[edit]: hooked up as a statevariable bandpass

[cd]

The way the bypass works, as PT mentions it's electronic - imagine the B FET being very very high resistance while the A is low resistance, and vice versa.  So in "bypass" mode, the signal goes directly from the first opamp stage to the output jack.  In "effect" mode, the "B" signal path is blocked by the very high resistance of the FET, and goes through FET "A".  Here, the signal goes through the 3080, whose gain is controlled by the footpedal, and the wah circuit (built around the 13600).  Then the outputs of each is selected, and is fed to the output jack.

[Mark Hammer]

The classic DOD switching circuit is shown here: http://ampage.org/hammer/files/dodswitch.gif

You will see how the 4007 is used.

As for the LM13600 chip, I'm going to take a closer look later today.  It may well be possible to convert the pedal into a foot-controlled phaser as well by flipping a connection for those .01uf caps from pins 5 and 12.  But I'll need to verify.  At the very least, you can control the range of the wah by changing the value of those caps, smaller for higher up, larger for lower down.  For bass, for instance, you may want to switch them to .022uf.

[Paul Marossy]

Thanks for the replies! That makes it a lot more clear. Seems like a sophisticated design. Hmm... a foot controlled phaser would be very interesting.  :idea:

[Paul Marossy]

As for the LM13600 chip, I'm going to take a closer look later today. It may well be possible to convert the pedal into a foot-controlled phaser as well by flipping a connection for those .01uf caps from pins 5 and 12. But I'll need to verify.

Any more thoughts on this Mark?

[Paul Marossy]

I am tempted to buy another one for some modding experiments...  :lol:

[BigT]

Paul,

Thanks for all your work on this FX-17.  You have a fantastic web site and I really enjoy the fine pix you have put up there.  I have one of these little monsters and it really is a different kind of wah sound - not bad - just quite different.  It's neat how small and sturdy they are too.  I look forward to any mods you come up with.  I will probably tear into mine and try some of your suggestions.  Again, thanks for all the work and please keep us all posted.

Also, a big thanks to all who have/will respond to you about this wah.

BigT

[Paul Marossy]

Paul,

Thanks for all your work on this FX-17.  You have a fantastic web site and I really enjoy the fine pix you have put up there.  I have one of these little monsters and it really is a different kind of wah sound - not bad - just quite different.  It's neat how small and sturdy they are too.  I look forward to any mods you come up with.  I will probably tear into mine and try some of your suggestions.  Again, thanks for all the work and please keep us all posted.

Also, a big thanks to all who have/will respond to you about this wah.

BigT

Glad you like the site and stuff. I'm still waiting on Mark Hammer to see what he comes up with on the foot controlled phaser idea. That stuff is still over my head...  :oops:

BTW, I used to live in Albuquerque, too. That was from 1975-1980 and I lived off of Justin Drive in Paradise Hills on top of the hill the whole time I was there. I loved it there! What's it like there these days?

[BigT]

Paul,

While we are waiting for the incredible Mr. Hammer, I remember that fOXX made a Foot Phaser of some sort back in the 70's.  I have heard that Brian May of Queen used one???  Maybe somebody out there has some info on that fOXX pedal.

About Albuquerque... I moved here when you left!  Dec 1980.  Moved up from San Antonio, TX (great music, great people, great food, BAD humidity).  Love the DRY here, but like any city, it has its good and bad.  The worst thing is the city is groing way faster than the inferstructure(sic) can handle it.  This is one booming city.  Tons of growth esp. since 1980.   We do have Grandma's Music store here though- great store - www.grandmas.com. Come back and visit sometime and we'll jam.

BigT

[Paul Marossy]

Paul,

While we are waiting for the incredible Mr. Hammer, I remember that fOXX made a Foot Phaser of some sort back in the 70's.  I have heard that Brian May of Queen used one???  Maybe somebody out there has some info on that fOXX pedal.

About Albuquerque... I moved here when you left!  Dec 1980.  Moved up from San Antonio, TX (great music, great people, great food, BAD humidity).  Love the DRY here, but like any city, it has its good and bad.  The worst thing is the city is groing way faster than the inferstructure(sic) can handle it.  This is one booming city.  Tons of growth esp. since 1980.   We do have Grandma's Music store here though- great store - www.grandmas.com. Come back and visit sometime and we'll jam.

BigT

Well, here in Las Vegas, it is insane what is going on in terms of new contruction and housing prices.  :shock:

Anyhow, I have never heard of that foxx foot phaser that you speak of.