New project: DOD FX-25 Autowah

[Zero the hero]

Looks like Robert Penfold Autowah.
It features a very powerful sweep. It slightly distorces the guitar sound.
Click on the oranges and follow the project menu.

[moosapotamus]

Nice one, Z. Your web site is looking good, too.

~ Charlie

[petemoore]

  Very cool.
  It slightly distorces the guitar sound. Is this a 'bad' concern, or overclipped sound, mild overall distortion can be cool, but phasers and EF's sometimes get a 'gakk' tone of OA distortion on peaks that is 'bad' or undesirable IMO.
  Two knobs. [?]
  I see what looks like two seriesed opposing diode strings [one on each Signal path oa]...I don't recall seeing that before, the diodes connected at the anodes and the cathodes proceeding to a + and - input...[?]
  Then there are the intersecting loops at the output of the IC's, which are standard duals, what do those loops signify?
  Whoops, I'm getting [what I don't really 'get'], that the IC1 is an LM13600, which I know very little about...

[Paul Marossy]

the IC1 is an LM13600, which I know very little about...

That's a dual OTA (Operational Transconductance Amplifier)

[Mark Hammer]

Pete,

Remember all that stuff about transforming the Ross Phaser and Small Stone, using a "phase-filter" mod?  Well, transconductance-amp allpass (phase-shift) filter stages, whether they use a 3080 (as in the Tonepad Piedrita), a 3094 (as in the commercial Small Stone), or the LM13600 (as used in the Ross as well as the DOD FX20), can also be configured to be other filter types.  In the FX25, the LM13600 is configured to be a 2-pole lowpass filter.

The envelope follower should look pretty familiar to everyone, and is subject to all the usual sorts of mods found in other envelope controlled filters.

Could someone answer me this, however.  Does this pedal sweep up (wah) or down (oww)?  From the looks of it, especially the resistor tied to V+ at the tail end of the envelope follower, I'm guessing it is the latter.

Note that a later issue of the FX25 (is it FX25C?) includes a blend control for mixing in clean and filter sounds.  Better for bass.

Thanks for the work Zero.  You're MY hero.

[gaussmarkov]

thanks zero!  nice project and gorgeous site.   

[StephenGiles]

I wonder if someone could explore the possibility of a "super autowah using my Spacedrum envelope generator here:
http://www.diystompboxes.com/smfforum/index.php?topic=36222.0
and the filter from this great project - as I'm involved in scanning bags full of old family photos at the moment. There would be half of a 13600 spare so I'm sure a use could be found for that!
Stephen

[StephenGiles]

Like this for instance:
http://img.photobucket.com/albums/v517/uncle_boko/superdodFX25.gif
Stephen

[Nikolay]

Any ideas how to convert this envelope for bass??

[Jack]

Any ideas how to convert this envelope for bass??

You don't have to. It already sounds very nice, to my ears at least.
Or did you mean more bass?

[Mark Hammer]

Normally, when adapting a bandpass filter for bass, one likes to drop the filter range down an octave or so by doubling the filter cap values (in this case increasing C9 and C10 to .02 or .022uf).  That tends to be a strategy employed in order to hang onto more low end when using bass, due to the properties of bandpass filters.  In this case, it's a lowpass filter, which means bass is preserved so the downward shift in filter range is really only useful to alter the tone to taste, rather than any sort of tonal triage to keep enough bass in the sound.  You can try .022uf caps though, and see if you like them.

[Zero the hero]

THANKS TO EVERYONE!!!
I like all the repsonses you gave to this circuit, I'll try to reply to everyone.

The distortion I get sounds like a very mild overdrive, it somehow warms the overall effect.

Mark, I haven't notice if the sweep goes up-to-down or down-to-up, I was in a great hurry when I tested the unit! I'll plug it in the amplifier and hear more cerefully. Bye the way, I was thinking on a mod to choose the sweep in both direction with the flip of a switch, like in the Ibanez AFL (just another inverting op-amp after the envelope follower).
And thank for the compliment!

Stephen, this envelope extractor is great! I have to dig around it an see what happens to the DOD FX-25 with your mods. But I think I have no breadboards free now... I'm working on a fully transistorised ring modulator.
May I post your schematic (super DOD FX-25) on my site with credits to you?
I was very interested in making a PCB for the SpaceDrum and for another tunable drum I planned to do (did you see that Sonic Boomer clone in my site?) but the CA3094 stopped me from doing it.... This will be the starting point for further job on it, I'm sure.

Jack, did you build this project? Or do you have an original FX-25?

Thanks again, people!
Days ago I was thinking to stop with this kind of hobby, since I really have few spare time for stompboxing.
Now I'll try to keep these electrons running very fast!

[StephenGiles]

Please feel free to use my circuit on your site, but of course it is untested in that format. It probably needs a little messing about with the envelope levels going to the filter control pins.
Stephen

[moosapotamus]

Well... while we're just talking, here... how about also throwing in the blend feature from the FX-25B? Zero has the scheme posted on his cool site.

~ Charlie

[Jack]

Jack, did you build this project? Or do you have an original FX-25?

I had an original, but ended up selling it before the switch died on me.

[moosapotamus]

I have an FX-25B, and it does sound good on bass. 

Attack and decay are both fixed... fast attack (almost instantaneous) and slow decay. The sweep goes down, but the start of the sweep (the high point) is determined by how hard you play. Play harder and the filter starts at a higher freq point. The "range" control sets how far down the sweep goes. The "blend" control mixes in the clean uneffected signal to keep things sounding phat, especially when hitting hard with low "sweep" settings. With my passive bass, the "sensitivity" control only gives action in about the last 1/4 of the pot's rotation.

Having controls to set the start and end of the sweep, as in the mod that Stephen suggested, would be totally way cool.
Attack, decay and a reverse sweep switch would all be nice to have, too.

~ Charlie

[StephenGiles]

Time permitting, I will post the circuit explanation of the envelope generator in the Super Dod FX25 - originally appearing in the EH Space Drum using a CA3094, so I need to convert the pin numbers to tie in with the 13600. It works in the same way as the EG in the Microsynth, so it would benefit from using sliders for the Start and Stop Frequency pots. Then you can see if the Stop control is set higher than the Start Control in order to get a reverse sweep. Alternatively, a dual pot could be used so that there is normal sweep at one end, nothing in the middle and reverse sweep at the other end.

As is, two aspects (or more - who knows!!) of the circuit need attention. Firstly the input preamp, taken from the Bi Filter follower may provide an output too high for the filter. I needed this level in order to get a high enough signal to trigger the envelope generator. Secondly, the Envelope Generator output voltage may need further processing to convert it to a current suitable for driving the filter through a useable range.

Another thought is that the filter may benefit from an output buffer, in which case there is a use for the spare half of the 13600 used in the generator. My original thoughts when experimenting with this Envelope Generator, which at the time was linked up to the filter from the Bi Filter Follower, was to keep it simple, preferably using just one LM 324 and one LM13600, but I now think - what the hell, make it decent. The ideas keep coming - why not have 2 filters tuned to different ranges and the facility to send different levels of Envelope to each?

Must eat my dinner!
Stephen

[bioroids]

why not have 2 filters tuned to different ranges and the facility to send different levels of Envelope to each?

This is good, with the right amount of envelope on each filter you're into vowels zone 

Luck

Miguel

[KarbonHed]

The fX-25B schem is up on http://experimentalistsanonymous.com

[Zero the hero]

why not have 2 filters tuned to different ranges and the facility to send different levels of Envelope to each?

This is good, with the right amount of envelope on each filter you're into vowels zone 

Luck

Miguel

Sounds good to me. Do you have any infos about vowels frequency or how to tune a wah to get vowels tone?