Maestro Envelope Modifier

[flanger]

Hello Maestros!

I need some info that I can't seem to find anywhere.

I bought a Maestro Envelope Modifier a few weeks ago. It is a very decent exemplar, but, as it turned out, not complete. From the service manual I could figure out that the missing part is a four-pin optical isolator but I haven't any clue about its type and model.

Of course, I'd like to restore my Maestro to the original specification. My last hope is that maybe there is someone out there who can tell me which brand/type/model of isolator was originally fitted (or has the same specs as the original).

Here is the schematic:
http://www.generalguitargadgets.com/diagrams/envmod.jpg

(In my despair, I even tried to pick up some clue from the schematics of the Boss SG-1 Slow Gear and Electro-Harmonix Attack Decay but, much to my disappointment, they are of totally different construction.)

Thanks in advance for any help

[R.G.]

LED/LDR modules are not high precision devices. The LDR itself ranges over maybe a thousand to one range of resistance, and there is no particular attempt to match or sort them by the makers. What you are interested in is speed. You want a fast one.

In today's market, the best you're likely to find is the NSL-32.

Like I wrote in my Guitar Effects FAQ at GEO in 1997, put in what you can get and enjoy the effect during the months and years you're looking for the perfect replacement part. Then put the perfect one in if you ever find it.

[Mark Hammer]

And just to add to what RG noted, the role of the LDR is to form (in conjunction with R6) one leg of a virtual pot. R6 and the varying resistance of the LDR form onwe leg, and R7 forms the other leg. The "wiper" is the junction between the LDR and R7. the rest of the circuitry is essentially intended to operate this virtual volume pot in a systematic way so that the "volume gets turned up from zero" very slowly each time a new note or chord is played.

Like ANY voltage divider (volume pot), the overall impact of resistance changes to one side of the wiper will depend on what exists on the other side of the wiper. If R7 were, say 47k, then the change in resistance on the other side of the LDR/R7 junction would need to go from fairly high (probably on the order of 500k or so) to fairly low (we'll say less than 10k) in order for the pedal to produce a level change that went from near silence to near maximum/normal level.

Even IF the required LDR dropped to some ideal of zero ohms, R6 and R7 assure the volume will be cut by at least half. Happily, Q1 likely adds some gain in anticipation of that.

This is all the very long-winded way of saying that you can probably score yourself any LED/LDR optoisolator with suitable response speed characteristics, and just adjust R6 and R7 values to get a decent overall response from the unit. My guess is that you could probably get away with using a 4k7 resistor for R7, in series with a 25k or 50k trimpot to add resistance beyond the 4k7 minimum, so as to achieve a usable amount of envelope-controlled volume change.

[R.G.]

This thing practically screams to be reworked into opamps.

[Mark Hammer]

Maybe. The circuit blocks that can be converted into op-amps don't immediately jump out at me.

BTW, what exactly does the Perc/Bow do? Is that a switch between slow attack and sudden turn-on with quick decay? (a quick "ping"?)

[R.G.]

BTW, what exactly does the Perc/Bow do? Is that a switch between slow attack and sudden turn-on with quick decay? (a quick "ping"?)

I think so.

I've always intended to slog through doing a good envelope detector, then doing a good VCA and the putting the requisite envelope generation stuff on it to do a higher quality ADSR module for guitar, but I always get sidetracked.

I think I'd use Bissell's envelope detector, then generate a gate and a trigger from the envelope with comparators, probably LM339's. After that, there are lots of ADSR setups to use. And finally I think I'd try to compress the variation out of the dry signal to get a more constant signal to impress the ADSR envelope onto. I had some thoughts about doing the ADSR inside a compander to kill off residual noise and transients.

I wonder If I could make the ADSR constants be controlled by the initial peak of the envelope, too.

Always more to play with than time to do it.

[Mark Hammer]

"Always more to play with than time to do it."

In the case of envelope modifiers for guitar, that is true in more ways than one, isn't it? The challenge always seems to be the brief duration of the source signal, and especially the way the harmonic envelope changes with time. As a result, I would think that one of the components of a decent-sounding envelope modifier for guitar would be one that:

a) clipped the living daylights out of the signal to achieve something of reasonable duration at a constant level,

b) filter the dickens out of the clipped signal to restore it back to something sounding more like a "polite" guitar.

THEN, you could have something where there was a real choice of attack-time lengths and decay times. Hell, you could even have something with variable attack AND decay times, or even an ADSR format.

[flanger]

RG, Mark:
many thanks for your kindly contribution. Your advices were very helpful - and encouraging - for me, now I'm just digesting the bulk of new info I found in your posts.

Mark:
Although I didn't intend to modify the unit right at the start, the way you write about the tweaking possibilities makes me eager to do some tuning; it seems to me from your explanation (and from that of RG as well) that it doesn't make too much sense to hunt for the "authentic" factory setting. Well, thus prepared, I feel much more free to experiment.

I hope I soon will be able to report of some result.

[flanger]

I don't know – maybe something like a vox repeat percussion? Here is what Jacques writes on his site:
http://www.geocities.com/stompbox2001/newfinds.htm
This is a difficult box. You can spend HOURS to dial in something interesting. I will save you time: two sounds are worth it:
1- "slow gear" attack killer type. Not bad but not great.
2- percussion sound a la the who - baba o'riley intro. Very nice but difficult to settle
You could have guess that by the MODE switch.