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R.G.: Neutron Questions

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5/16/1999 3:41 PM

R.G.: Neutron Questions

R.G.: I've been thinking about purchasing the Neutron board. I've got a couple of questions.

1) First off, this is the auto-wah we're talking about (vis-a-vis Greatful Dead/New Bohemians)?

2) Secondly, how hard will it be setting up the LED-to-2-LDR module? I'm talking about setup time and time to select parts. Do you outline a test jig in your instruction booklet?

Thanks!

5/16/1999 8:35 PM

1) it's a clone of the old Mutron 3 auto-wah. Same schematic as the one on C.J. Landry's site (thanks, C.J.!)

2) I document four ways to do that, because the original is unavailable. You can
- use an LED to dual LDR module, which C.J. has throughtfully procured. This is a direct sub for the original, and solders right in. You might want to contact him by email if this interests you.

- use two CLM-6000's, the new CLM600 reissue, or the Vactec VT54C3; these are all single LED to LDR modules, but there is enough headroom to run two LED's, and the PCB is set up to allow these to solder right in

- Use two H11F3 LED to photoFET modules; this has been tried in one of the prototypes, and works well

- assemble one LED and two LDR's on a 14 Pin component carrier, with the LED facing up and the LDR's facing up as well, and covering them all with a reflective cover to act as a light mixing chamber.

The Neutron does not seem to be horribly sensistive to matching, but matching LDR's is not tough. You get a batch and a dmm, and arrange some way to hold successive LDR's in exactly the same position, and use the ambient room light as a reference light. You'll probably get one pair out of four random same-part-number LDR's.

It's possible to get closer matches with test jigs, reference illumination, and so on, but it doesn't seem to matter all that much in this one.

All four options are illustrated by sketches in the book.

5/18/1999 1:52 PM

Re: Hours of Mindless Fun! :-)

Based on your recommendations, I went out to C.J. Landry's site and had a closer look at the schematic. I also found a hand-drawn schematic at the site listed below with what I guess is a clone of the Mutron III that someone released a while ago:

http://www.lynx.bc.ca/~jc/pedals.html

The clone was constructed with NE5532s. Since I had a couple of LDRs and a few NE5532s, I decided to breadboard the Mutron III. Didn't take long -- and you were right: obtaining perfectly matched LDRs doesn't appear to be critical to the circuit. In fact, I found that the LDRs were not overly sensitive to ambient light. You may not even need a reflective hood if you bend the LDR legs so that the LDR faces the LED.

The "hours of mindless fun" I alluded to in the title of this post was when I pulled out a flashlight and began shining it on the LDRs. Hysterical. Someone could come up with a fun-box just for this. If I had to guess, I'd bet that the Morley Wah probably bears a strong resemblance to this circuit.

I found the effect to be a bit difficult to tame. I had to practice with it a few minutes to get just the right picking style and pickup combination. I would guess that a post-Mutron compressor would do wonders.

I made a couple of minor alterations to the circuit which I'll be happy to go into...if anyone's interested, just post a reply.

5/18/1999 3:28 PM

Great! I love tinkering!

As to the ambient light - the relative sensitivity depends on the LDR's; some are high delta-R per photon, some aren't. The ambient makes a kind of general adder, so the effect of the changing light from the envelope extractor gets lessened.

I set the thing up so I could dupe the one-LED/two LDR's from the original; obviously, two LED's facing two LDR's is workable, too. I just guessed that if everyone had the option, they'd buy one less LED.

On the flashlight - I think that what JC was going on about with the "random driver" for the Mutron clone was exactly for that, a random change in response. Should be easy to hack together.

I'd like to see what you came up with.

5/18/1999 5:41 PM

I set the thing up so I could dupe the one-LED/two LDR's from the original; obviously, two LED's facing two LDR's is workable, too. I just guessed that if everyone had the option, they'd buy one less LED.

I noticed that he did that too; he used 2 Clarostat modules if I remember right. I used one LED like the original. First, I bent the leads on the LDRs so that their plates were at a 90 degree angle to their legs. I then placed the LDRs so that they faced each other with the LED between them.

On the flashlight - I think that what JC was going on about with the "random driver" for the Mutron clone was exactly for that, a random change in response. Should be easy to hack together.

Yeah, that GIF didn't open on my computer so I couldn't see his diagram. Could the sample-and-hold he talked about be as simple as a cap in parallel with the resistor beneath the LED?

I'd like to see what you came up with.

A couple of things. The first was from JC's site: using a transistor to run the LED. I used a 2N2222A just because it was in easy reach. You can see it in his diagram: the transistor is outfitted as an emitter-follower. Collector straight to +Vcc, op-amp output feeds the Base, Emitter is tied directly to the LED. Feedback resistor to the op-amp upped to 220k (although this shouldn't matter since I ~ 0). There's also a series resistor beneath the LED. More on that later.

With this setup, I found that I didn't need the "Drive" switch setup. There was more than enough drive leading me to:

The second thing was the series resistor beneath the LED. The LED I used needs 30 mA for full brilliance so I started with a 220 ohm. I then added a 10k pot in series with the 220 ohm. (10k may be overkill -- a 5k would probably do. Again, it was handy.)

With this pot in place, I found I could adjust the amount of "wah" to exactly where I wanted it. In other words, I had complete control over how wide the filter opened (i.e. where the Q swung to).

My next quest -- and R.G., maybe you have some ideas on this -- is to find a way to control how fast the filter opens. I.O.W., slowing down the LED's response time. A cap across the LED might do it -- the filter would open more slowly as the cap charges, then the cap could discharge through the LED for a rapid shutoff...

Off to try it out!

5/18/1999 8:47 PM

You know, I get two kinds of people who ask about effects - those who want to get in and mess with it until it works for them, and those who think anything not "original" is at best a sly trick, and probably won't sound as good. This leaves me in a clear dilemma over what to put into repro boards - original or better?

What I've come to is making the boards so you can build them with the original parts/circuit/etc. but also modify them. This seems to keep the purists happy, but it also looks a little backwards to the real tinkerers.

In any case,

quote:
" using a transistor to run the LED"

Not a bad idea, easier on the opamp at the cost of another part. I think the original used the opamp only to keep cost down. You can mess with the value of the current sense resistor to get the same effect with the stock setup. The transistor driver is especially useful with old-style low sensitivity LED's. With modern superbrights, the opamp by itself will do fine.

quote:
"a way to control how fast the filter opens"

If you think about it, the LED driver follows the envelope from the signal rectifier very quickly. It's easier to slow down the envelope by using another R-C delay at the input of the current driver stage rather than a bigger cap working at a higher power level across the power section driving the LED.

5/18/1999 11:45 PM

Since you're getting philosophical :-)

:-)

You know, I get two kinds of people who ask about effects - those who want to get in and mess with it until it works for them, and those who think anything not "original" is at best a sly trick, and probably won't sound as good. This leaves me in a clear dilemma over what to put into repro boards - original or better?

Ah! It's that key word: "better." The purists would say "says who? What type of better? Better on paper or a better sound?"

To the purists, I would say that from a practical standpoint, an original circuit -- truly original, without substitutions -- may simply be impossible. For example, a Fuzz Face Ge transistor or the Tube Screamer op amp, parts that may simply not be manufactured anymore. That's just scratching the surface: there's a whole line of Ge parts that aren't manufactured anymore and, of course, tubes...

Even if you can find all vintage original parts: one guitar player may be looking for a completely different sound than another guitar player. And not just because of different styles of music; I'm talking about different guitars: different woods, different pickups, different wiring schemes. And different amps. This is the reason I got into tinkering with the FX circuits (besides the cost of tracking down an original box). A minor modification here or there -- like, say, change a cap value to raise/lower frequency response at a key point in the circuit.

FWIW, I appreciated your Fender Blender kit just like it is. It's as close as reasonably possible to the vintage circuit with modern parts, yet you showed where a couple of minor modifications could be made to improve performance and even provided the substitute part for free. Tinkerer though I am, I would've been disappointed had the kit contained a lot of modified sound circuitry.

It's easier to slow down the envelope by using another R-C delay at the input of the current driver stage rather than a bigger cap working at a higher power level across the power section driving the LED.

Let's see if I've got your thinking right: op-amp output to resistor. Resistor connects to two things: the base of the transistor and a capacitor connected to ground. Charge time set by the resistor/cap combo. Discharge path through the Xistor/LED/Re. In order to make the discharge path faster than the charge path (to keep the sensitivity reasonable), the new resistor at the op-amp output should be significantly larger than Re. Could also make this new resistor a pot so that the charge time could be set. Interesting...

Is that what you're suggesting?

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