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 28 Sep 96 04:03:35 +0500 by gaia.swip.net (8.7.6swip/8.7.3) with SMTP id JAA02204 for ; Sat, 28 Sep 1996 09:50:34 +0200 (MET DST) Date: Sat, 28 Sep 1996 09:48:35 +0200 (MET DST) From: Ricard Wolf Cc: Analogue Heaven 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" --