My Neutron Filter build experience, and a couple of questions for the community

[JudMan]

A large helping of thanks go to RG Keen, Mark Hammer, and the others who have helped make the Neutron Filter a workable project.

I've always wanted that auto-wah/envelope filter sound.  I've tried the envelope filter on my old digitech GNX3, and didn't quite like it.

So, I ordered parts, etched a board, and started building.

At first I used the H11F3 opto-FET's, and a 7066 charge pump.  When I fired it up for the first time, I thought I had miswired something, and spent about three days debugging, and tearing my circuit apart section by section to find out what was wrong.  I can say, I know a lot about the wave forms of this circuit now!  (It is certainly helpful to have an o'scope)

The sound I was getting was a terrible high-pitched constant whine.  Which I knew was caused by the 7066 charge pump running at 10k.  Ok, so I used my bench power supply to remove that while I troubleshot the rest of the board.
The next problem was a horrible distortion.  I could get the auto wah effect, but the gain knob only had a tiny amount of adjustment where I got no signal, and then it jumped to full on distortion.  

I eventually got to the point where, I realized, there wasn't anything wrong with the circuit board,  (granted my etched board was rather bad), or the parts.  

So I tried using a different opto.  VTL53C.  And this basically 'fixed' my project.

Now it sounds like I expect.  

So, I've been playing with it for a few days now, and want to improve it.  

1)  the attack is great, but the decay sounds like maybe the ldr's aren't matched, it kinda has a beat freq. to it.  Sort of a distorted warble.

2) the sweep direction works great in one direction, but I really have to adjust everything to get it to sound good in the other direction.  Even my amp volume has to be adjusted
(I wish I could remember which direction sounds better, I'm pretty sure it's down that is perfect)
I'm thinking i should fool around with the resistors for the bias setting of the led driver opamp when it's in the up(down?) direction to try and even it out.

3) the range switch is almost useless for me, I like the lower freq, the higher freq sounds very tinny and shrill,
I'm thinking maybe to adjust the range caps so I get a lower freq on the low range, and a slightly higher freq on the high range.

4) I can't see using the HP filter much at all, it also sounds shrill to me

In case anyone cares, the guitar i use most has two humbucker pickups, one of which I can tap for a single coil (sort of) sound.

So my real question(s) is(are) which suggestions do other people have for improving/calibrating/tweaking the neutron filter?

Oh another idea I thought might help was to add a unity gain buffer amp at the input, and only use the 'gain' opamp to drive the envelope detection circuitry.
This might help out when I bypass the effect.

[K Zustang]

Hi There,

congratulations on this filter build. It's a tough one....
Have you solved the whining noise by changing the 7660?\
I am starting this project right now and I am really afraid after reading so much about problems with the optocoupling and the voltage charge pump.

Can you elaborate on how you solved the decay by matching the VTL53C?

Thanks
Doron

[bagudan]

Mine does exactly the same thing, on all the points You described! Maybe it's the design ofthe neutron? Haven't had a chance to play an original mutron, so cant say if it compares... But Got tired of messing with it, and settled for one great sounding setting. Bandpass, downward sweep, it,s the best sounding filter I have.....but then there's the volume drop:(
But still..... It does Sound sweet and funky as hell one the one setting...

[digi2t]

OK, I'll try to translate some of my findings from my Foxx Guitar Synth clone build (basically a Mutron III rip off). Although I rolled my own copy of the Mutron dual Vactrol (see http://www.diystompboxes.com/smfforum/index.php?topic=95269.0 ), the quirks and fixes are about the same. I too used the dual VTL5C3's at first, but in the end, I saved my VTL's and used my custom job because I could sort through the LDR's and match them. Besides, it had better whonk to my ears. For the power supply, I used a MAX1044. Never had a problem with whine.

1)  the attack is great, but the decay sounds like maybe the ldr's aren't matched, it kinda has a beat freq. to it.  Sort of a distorted warble

I found that using a trimmer as a current scaling resistor allowed me to find a perfect range for the Vactrol to operate in, and get rid of the warble. Where "Rx" is in the Neutron schematic, I used a 250K trimmer, in series with a 270R resistor as backup not to burn the LED. Once the range is acceptable, dial back your input gain and/or your resonance a bit.

2) the sweep direction works great in one direction, but I really have to adjust everything to get it to sound good in the other direction.  Even my amp volume has to be adjusted
(I wish I could remember which direction sounds better, I'm pretty sure it's down that is perfect)

Once you get point 1) smoothed out, then attack point 2). Sweep up will always sound "better" than sweep down. Sweep up is what we associate with the Mutron sound (think Stevie Wonder's "Superstitious"). As for getting the sweep down to sound better, or more balanced vis-a-vis the sweep up, then I recommend replacing R18 and R19 with a 330K trimmer. Or a 500K trimmer (easier to find) and use two 510K resistors, one across poles 1 and 2, and the other across 2 and 3. The resistors will cheat the trimmer into approximately a 330K. With this trimmer, you'll be able to balance the down against the up. You should be able to get close enough that you should not have to mess with the input gain to get a decent sweep either way.

3) the range switch is almost useless for me, I like the lower freq, the higher freq sounds very tinny and shrill,
I'm thinking maybe to adjust the range caps so I get a lower freq on the low range, and a slightly higher freq on the high range.

I agree. I'm on Lo all the time. But, don't forget, the Mutron was designed to be used with a variety of instruments. I assume that the Hi function probably has it's merits in some applications.

4) I can't see using the HP filter much at all, it also sounds shrill to me

Refer to point one above. Using a trimmer for the LDR current adjustment adjusts the range, so dialing in a slightly lower range should make all three ranges usable.

Ultimately, the Mutron reacts differently to different pickups, so you'll have to make slight adjustments in the input gain to get the right whonk. Nature of the beast.

Here is a schematic I drew for the Foxx. Use it to see how I arranged my trimmers (T1 for the LDR, and T2 for down sweep balance).



Hope that helps.
Dino

[alfafalfa]

Saw your video Dino and I must agree with you it sounds terrific !
Great job.

[digi2t]

I found that in mine the down warbles more if you try to stretch the sustain/ramp (via the T2 trimmer) too much. The down isn't so much a mirror of the up, I consider it just a different envelope effect. Mine is pluckier than the up. Though I tend to use the up mode most of the time, for bass or keys in lower registers, down sounds pretty neat. With a good amp, and speakers capable of pushing a lot of air, you can really rock the rafters.

One of the functions of the Foxx that REALLY helps tune the up versus down balance, is the LFO function. With the LFO on, I could switch between up and down "on the fly", and adjust the trimmer to the best balance. Then the final tweaking is done at the end by plucking the strings. This is where one has to be objective though, since after all, this is an effect that responds directly to playing dynamics. I must admit, once I found the sweet spots, I did find that I had to develop a certain nuance in my playing between the two modes.

Like I mentioned before; nature of the beast.