From mj20279@janus.swipnet.se Sat Sep 28 12:14:30 1996
    28 Sep 96 15:14:16 +0500
    28 Sep 96 15:13:47 +0500
Comments: Authenticated sender is <roberts@mail.pharm.sunysb.edu>
    28 Sep 96 04:03:35 +0500
          by gaia.swip.net (8.7.6swip/8.7.3) with SMTP 
          id JAA02204 for <analogue@hyperreal.com>; 
          Sat, 28 Sep 1996 09:50:34 +0200 (MET DST)
Date: Sat, 28 Sep 1996 09:48:35 +0200 (MET DST)
From: Ricard Wolf <mj20279@janus.swipnet.se>
Cc: Analogue Heaven <analogue@hyperreal.com>
Subject: MG-1 multitrig mod

Hi all!

Here's the multitrig mod for the MG-1 - (just love these ASCII diagrams :-).


Moog/Realistic MG-1 retrigger (and continous glide) mod
-------------------------------------------------------

The MG-1, as all other monophonic Moogs (at least the fully analog ones)
lack multitrig, meaning that if you hold a low note and play a higher one
the envelope won't retrig. (Note: most Moogs are low-note priority - the MG-1
is an exception to this rule.) This can be awkward when playing fast, and
especially on the MG-1, with its, er, limited envelope generator, where
you don't have a separate release phase to smooth things out when you're
using the envelope generator in AD mode.

So, on my unit, I've added a retrigger circuit. The circuit was originally
snitched from the E&MM Spectrum synthesiser (I think) and modified for the
MG-1. I've used it successfully in another monophonic I built a long time
ago as well.

As an added bonus, the multitrig mod adds 'continous glide', i.e. if you play
a note (with glide on) and release it before the glide has completed, the
glide will continue after the note has been released. This is contrary to
the original MG-1 function, where the glide stops when you release the note.
Although this might be useful in some odd cases, often it's just irritating
when you take your finger of the key to quickly and the note remains at some
odd point along the way.

When installing in the synth you can either put it all on a small piece
of veroboard, or do like I did, which was piggyback the opamp chips to other
opamp chips in the vicinity, connecting only the V+ and V- pins, spreading
the rest straight out, and mounting the passive components 'in the air'
between the opamps, or, betwen the opamps and the PCB. All references are
to the main PCB in the 'lid'.

  
                                                            glide pot   |\
                                                                      --|-\
             cut track!                              ------  ------     |  >--
KBD CV -----+----X------------------------+----------| 1k |--| 1M |-*---|+/
(pin #6)    |           +--------+        |          ------  ------ |  3|/
            |           | 6|\    |        |              R61   ^ R63|    U7A
            |           +--|-\ 7 |        |                    |    |    LF353
            |   ------     |  >--*--------*                    +----+ 
            +---| 1K |--*--|+/            |                         | 
                ------  | 5|/ 1/2 TL072   |                        --- C13
                        |                 |                        --- 1uF
                       ---                |                         |
                       --- 1uF            |                        _|_ GND
                        |                 |                           
                       _|_ GND            |                           
                                          |       |___ existing components ___|
                                          |                           
                                          |                                    

                                          |     ___ +15V
                                          |      |     
                                          |      |    
                                          |     ---  
                                          |     | | 
  +---------------------------------------+     | |
  |                                             | |100k 
  |                                             ---    
  |                                 1N4148       |    
  |                                  | /|        |   
  |                               +--|< |--*-----*---+     
  |                               |  | \|  |         |    
  |                               |       ---        |   
  |                               |       | |        |  
  |                 ------        |       | |        |             auto contour
  |               +-| 1M |-+      |       | |3.3k    |                trigger
  |               | ------ |      |       ---        |                            
  |  ||  -------  | 2|\    |      |        |         | 2|\                      
  +--||--| 10k |--*--|-\ 1 |  A   |        |         +--|-\ 6          S7 
     ||  -------     |  >--*------+  +-----*            |  >-----+------*|out
    68nF          +--|+/          |  |     |         +--|+/      |       |
                  | 3|/ 1/2 TL072 | _|_GND |         | 3|/ 741   |   +--*|
                  |               |       ---        |           |   | 
                 _|_ GND          |       | |        |           |   |  * in
                                  |       | |        |           |   |
                                  |       | |3.3k    |           |   |    ----
                                  |       ---        |           |   +----|91k
                                  |  |\ |  |         |           |        ----
                                  +--| >|--*-----*---+           |        R131
                                     |/ |        |               |    
                                    1N4148      ---              |    
                                                | |              | | existing |
                                                | |              | |_ comps. _|
                                                | |47k           |
                                                ---              |
                                                 |               |
KBD GATE ----------------------------------------+--------X------+    
(pin #9)                                              cut track!     
                                                                      

Notes: - The KBD CV and GATE signals are available at the rear edge
         connectors on the board (pins 6 and 9 respectively).
       - TL072 needs +15V on pin 8, -15V on pin 4 (easy if piggybacked to 
         an LF353 or LM393).
       - 741 needs +15V on pin 7, -15V on pin 4 (easy if piggybacked to 
         another 741).
       - Two PCB tracks must be cut, marked with X in the diagram.
       - The 1uF cap in the voltage follower should be a low leakage type,
         e.g. polycarbonate.
       - Even though some signals are specified at their end points (for
         instance, the gate signal to S7), they are often available at
         other places as well - follow the PCB tracks!
       - Some components already present on the board have been included
         for clarity. They are numbered according to the MG-1 schematics/PCB.
         
Circuit description:

The top half of the circuit implements the continous glide function, but
more importantly provides fast voltage changes to the differentiator/
rectifier in the bottom half even when the glide control is advanced.
This part of the circuit is simply a voltage follower in connection with a 
large capacitor to hold the CV, just like the original circuit shown.

The bottom half can be subdivided into two parts. The left part (around
the TL072) is a differentiator which converts the changes in CV to large
voltage spikes. The rest of the circuit is basically a rectifier, which
rectifies the spikes (which occur when the CV changes), so that both
ascending and descending CV's will cause a retrigger, and performs a
logical AND function with the gate signal, in that if the gate is active
and the CV changes, the result will be a break in the gate signal, in effect
causing a retrigger. If the gate signal is inactive, it is not affected.

The output is fed on to switch S7, the auto contour switch, which selects
between the keyboard gate signal and LFO square wave (for repeat).

If there's any interest in switching out the retrigger circuit, I'd
suggest point a switch at point A (after the differentiator) in the diagram 
(open => no retrigger), but I haven't tried this myself.

Happy retrigging!


/Ricard
--
Ricard Wolf (ricard.wolf@swipnet.se)

Analog synth, railroad and Citroen GS fanatic
        -- "Ich bin der Musikant mit Taschenrechner in der Hand" --




