[sdiy] MG-1 Mods (patching it out)

[WeAreAs1 at aol.com]

In a message dated 7/2/05 11:20:45 AM, nil at pressurepenetration.com writes:

<< Ok, so I tried the diode and it kinda worked. The OSC doesn't go sharp 

when the EG is at rest, but it pulls down the eg amount to the VCF.

I will have to use an opamp and have a buffered output. Not a big deal 

as I was going to have a board on the side for analog noise source 

anyways and another EG (ADSR). >>

Hello Michael,

I looked at the schematic, and it right next to U13B, it says that the EG 
output is supposed to go from 0 volts to +8 volts.  They don't mention anything 
about it starting at 0.5 volts.  This makes me think that there might be 
something wrong with your envelope generator circuit.  I think you should try to 
sort that out before you go any further in your attempt to modify it to work the 
way you want.  It's a fairly simple circuit with only a couple of active 
components (U14 - the 4007 CMOS IC, and U13 - the 4558 dual opamp).  For now, 
remove that new diode and try replacing the opamp, then the 4007, and see if things 
start working better.

If it turns out that EG is in fact working properly, and that the diode is 
still needed, then you can quickly fix your other new problem (the low VCF EG 
amount) by simply changing one resistor value on the existing circuit.  R114 
(39K) determines the VCF EG sensitivity.  You can find R114 near the bottom 
center of the schematic page, just below the filter circuit, next to the Filter EG 
amount pot (labeled "Contoured Cutoff").  If you change it to a smaller value, 
you will get higher sensitivity, and the VCF will have a higher peak response 
to the EG.  Start with a value of 33K, and if that's not enough, then go 
smaller a bit at a time (27K, 22K, etc.).  Those are the standard resistor values 
that are available in that range, so you may need to put two resistors in 
series in order to get the exact response you are after (or choose from some more 
exacting 1% resistors, if you have some).  Changing this resistor will get you 
the response you want.

Michael B.