DOD FX-25 filter-type mod confirmed

[Kipper4]

Eli
C7 is not in the audio path.
Its part of the envelope detector. I very much doubt its responsible for the extra bass.

The OTA lm13600/ lm13700 is used to make the filter through which the audio signal passes,
the envelope detector detects a signal and sends a voltage/currant to the filter which alters freqauncy of said audio.

I hope this helps
Rich

[Elijah-Baley]

Thanks you. I also was starting to doubt about C7.

I'm finishing the ideas. I didn't touch the output cap, but seems to me the problem is somewhere else.

Has someone verified this layout: http://4.bp.blogspot.com/-m3EQYiHWXKA/UlbObQx7G-I/AAAAAAAAA18/6OEJwqZwHA8/s1600/DODFX25_rev8a.png

Or the layout is right so (and I want to correct it) or I made a mistake something.

[Kipper4]

What happens when you use the range pot? Still too much bass?

Cant help you with the vero layout. Its doing my swede in just glancing at it sorry.......
I dont do vero

[Elijah-Baley]

Try to explain. When I set low the range pot I get a "less wide frequency", a bit less treble, and so less low.

I have to verifiy the decay pot or some near to it, because it seems so subtle, almost useless.

[Mark Hammer]

I should probably add that using a Schottky diode for the series diode in the rectifier (not the one that goes to ground, but its partner) will get you a little more sensitivity and responsiveness.  Keep in mind that the higher the forward voltage of that diode, the stronger the amplified signal from the op-amp has to be to "break through" and force a filter sweep.  I can't say with any certainty, but the difference between how long it takes for an amplified string to reach the forward voltage of the 1N418, vs a Schottky type, probably adds a couple of milliseconds to when the filter starts to sweep.

[Elijah-Baley]

Thanks for joining Mark Hammer.
About the sensitivity the sound is alright, I'm using 1N4148. By the way, the ICs are LM13700N and NE5532P.
I hear no effect when I turn the decay pot, it could be meaning, but it is wired exactly like the layout show, and the multimeter said me it is well connected.

Just a detail, I don't know what it could be considerable. The Sensitivity pot (it works) has the lug 3 connected on the board. I started to build the circuit months ago, then I have to stop, so I don't know why but I connect lug 1 and 2 together, and connected it to the ground. Do I misread the schematic?
http://3.bp.blogspot.com/-zfoc_H5RK04/UiGU3o2QKiI/AAAAAAAAApk/FoqIFM8sBQs/s1600/DODfx25.sch.gif.

[Mark Hammer]

The op-amp should probably be a 1458 or 358 to get a proper sweep.  The Decay control mod needs to have an appropriate ratio of change.  So, it there is a 330k fixed resistor and a 500k pot in parallel with that 22uf cap, you likely won't hear much effect of the added control because (500k+330k)/330k is only a little more than a 2:1 ratio of change.  100k and a 1M pot will give you 11:1 and 47k plus a 1M pot will give you almost 22:1.  The bigger the ratio, the easier it will be to notice differences in decay time.

You can also skip the pot and just go for presets with a 3-way toggle.  In the middle position, it doesn't connect anything in parallel with the cap.  On one side position it connects 330k, and on the other side position it connects 47k, giving you slow, medium, fast....which may be enough for many users.

[Elijah-Baley]

I used NE5532P because it was suggest on topopiccione web site. But I tried a JRC4558D with no changing. I don't have other ICs.

I follow the layout, so I have a Decay pot 1M and a 220k resistor. I'll do further tests in different combination.

And about the wiring of the Sensitivity? Somebody had take a look?
I have lug 3 to the board and lug 1 and 2 to the ground, but now I'm not sure if it is correct.

[Mark Hammer]

As long as the total resistance of the pot to ground gets smaller as you rotate clockwise, you're in good shape.

[Elijah-Baley]

I'm sorry, but I need really help. I didn't find anybody who used this layout, so I wish this thread could be useful to someone else.

I didn't get my built working fine. I tell again: I got the wah effect, but sometimes I have a sort of bump of bass on the first note when the circuit is just powered, and I have this bump of bass frequency, while I play, in the decay of the wah just before it fade.
It's almost impossible play it at max range. I can take off this bass lowing the range.

I noticed the pot decay do nothing. And I have some doubt about the wiring of the Sensitivity pot.
I want to attach the scematic and the vero layout (it includes filter mode switch and the decay pot).





Could be useful this PCB with not mods:


The layout show just a wire on the lug 3 of the Sensitivity pot, but not the other lugs.
How you would connect the other lugs?

About the bass bump, thought I tried low values for C4, C5, C9, and C10, I lost bass, but never the bass bump in the decay. Could be the problem around the no working decay pot?

Thanks!

[duck_arse]

e-b - can we see photos of your work? have you posted "the" circuit diagram (I can't connect to anything at atspace.com, including topopicione)? that vero layout is to be frowned upon, as it shows four places with two links or component leads sharing a single hole. did you make those four in a goodly manner?

and the perf layout sugests that "the other end" of the sens pot is grounded, which agrees with the boxed note in the vero dia. (SENS: 1 and 2 to ground {the rest needs me to scroll down, then across, so I can't see it}.)

[Elijah-Baley]

I had resume the schematic, now we can see it.

I checked the contact of the jumpers and of those resistors. Seems alright.
In the boxes...  I missed it! Now I know why I had connected the wires like that. Indeed, that pot works! Ok that is ok.

http://i.imgur.com/KC0IXDa.jpg

http://i.imgur.com/0cOVwhn.jpg

Some caps are on socket, because I made some tests. No changing after I add the sockets. Diodes on socket, too.

I stared and checked it along. No contacts between stripes, wires well connected and in the right place, caps are ok... Also the resistor seems right, I measured it, but some are hard to measure, so I compare the colours with the other. I didn't find no mistake.

I really can't hear any effect with the decay pots, anything it should to do it doesn't. I think there could be the problem, or some mistake, but I really can find it.
This circuit are driving me crazy.

[jez79]

Decay knob is wired normally, in parallel with the envelope cap shunting it to ground.

EDIT:
Instead, connect the decay 1 wire to the =9V spot just above it, and remove R9 (1M). Essentially replace R9 with the pot/R24.
Your decay will become an attack knob, and it will make audible differences, much more than any decay mod i've tried.

[Mark Hammer]

Decay on an envelope-governed circuit is a bit like an alley-cat: if you keep feeding it, it won't go away.  As a result, despite being far more effective in my experience than unbuffered attack mods, it is easy to mistakenly interfere with a good decay mod.  If one lightly mutes the strings with the butt of your picking hand, you'll find that the decay mod results in clearly audible differences in sweep.

None of that is to take away from the value of a good attack-time mod.  The trouble is that, in most unbuffered contexts (which the FX-25 is) it is difficult to achieve audibly different attack times without detracting significantly from the sensitivity of the circuit.  Remember that both attack and decay revolve around the time it takes to charge up, and discharge, that electro cap to ground.  And any resistance placed in series with the output of the rectifier gain stage reduces the current feed to that cap. in a manner akin to having less envelope signal.

[Elijah-Baley]

Anyway, if I really can't hear no effect with this pot. I like have the Sensitivity pot much high, else I have less attack. It is like an attack control.
I have the sensation could be something wrong around this no working pot.

If I right, this decay control includes the pot and the 220k resistor R24. So I think I can take off this components getting the "standard" Alex Petrini's schematic. And, I wish, the circuit should work.

This bump of bass I have on the first note when the circuit is just powered could be some charge caps? (Maybe the 22uF cap?) I had low C4, C5, C9 and C10, but this bump had never go away.
Sorry, I'm trying to guess.

[Elijah-Baley]

I take off the Decay pot and the 220k resistor, but no changing.
I replace C11, the output cap with a 100nF polyester cap, I get a bit less bass, but I have the bump. I have this bump in the decay of the notes but also on the first note when the circuit is just power. I'm using a NE5532 instead of 1458, but I think thet is not the problem. Sorry if I repeat myself.

Should I disconnect even the filter switch? Where this bass bump coming from? It is not normal, it is like an earthquake.

[Kipper4]

I wouldnt expect an earthqauke.
The 1458 is a compensated chip not sure the 5532 is...........
pm me ill send you a 1458.

[Elijah-Baley]

[...]
pm me ill send you a 1458.

pm sended .

I wouldnt expect an earthqauke.
The 1458 is a compensated chip not sure the 5532 is...........

In the topopiccione web site there is a recommendation about this IC, because the FX25 could have a bit of distortion.
This is the desription: «As previously stated, the sweep is very powerful, maybe too much... cause my unit slightly distort the guitar signal. Changing the op-amp is reccomended, and I had a major improvement in the sound using an NE5532P by Texas Instruments: the unit seems more quiet now.
Unlike Dr. Quack, the DOD FX25 seems not very IC sensitive, so it will work with many op-amps: I tried TL072, TL062, NE5532, LM1458, RC4558, LM358 all with good results (they all make both filters sweep, but there are some differences about distortion).»

I'm beginning to think could be some ground problem. There's no more the "Decay section", I change the value of several caps, I have this bump of bass on the any first note. I connected the ground from the board and that from the Sensitivity pot together, adding the negtive of the snap battery. Am I missing something there's no in the layout?

I'm afraid there's no much things to do.
I like this effect, but I can't use it.

[Elijah-Baley]

This could be my first failed project. I don't understand if there's something wrong or the effect is right so.

I was thinking about to Snow White Auto Wah, it should be pretty similar, and I can use recover some components.
I didn't want to abandon this DOD FX25 because I like it, but I really don't know what I have to do.

[jez79]

Finished some mods to an FX25b unit.
Cool synthy sounds possible!