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

[Michael -nil-x]

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).

So far the mods I have done sucessfully on the synth are:

On OSC 1:
Manual PW
PWM via the LFO
X-mod freq via osc 2
Independant LFO to freq
Osc Out

(rather than the LFO affecting both OSC's equally I seperated them to 
independantly affect each)

On OSC 2:
EG to freq (though I need to buffer the eg)
X-mod freq via osc 1
Independant LFO to freq
Osc Out

On VCF:
External audio to filter
External CV to filter freq

EG:
V-trig in

The ca3080 is fried so I haven't messed with the VCA yet, but I'm 
getting some in the mail. I pulled the old one out and added a socket.

I decided not to patch it out entirely but create much more advanced mod 
routings and also build a new box/panel - the keys will be gone (they 
are destroyed)
Many more mods are planned, I'll document all the changes and make them 
available to the community once I have them completed and the project up 
and running.

- Michael nil-x
- Pressure : Penetration
Music: http://www.pressurepenetration.com/
Photography: http://nil.pressurepenetration.com/

WeAreAs1 at aol.com wrote:
> In a message dated 7/1/05 10:50:20 AM, mmanard at columbus.rr.com writes:
> 
> << I'm not sure what you mean. A diode off pin 7? Will that remove 
> the .5v? Will it affect the output of the EG in other ways? >>
> 
> Hello Michael,
> 
> Phillip's diode thing is quite simple, and would work and even help a bit, 
> but it's not the optimum way to deal with the problem, in my opinion.  
> 
> FYI, here's how the diode idea works:  Standard silicon diodes have a 0.6 
> volt "voltage drop".  That is, they reduce whatever voltage is running though 
> them by 0.6 volts.  If you put 5 volts into the diode, you will get 4.4 volts out 
> of it.  The voltage drop also works in such a way so that the diode will not 
> conduct at all until the input voltage rises above the 0.6 volt point.  
> Therefore, if you run your EG output through a diode, the output of the diode will 
> stay at 0 volts until the EG output rises past 0.6 volts.  Sounds good, no?  
> 
> Yes, it would take care of your .5 volt offset problem, but here's what's not 
> so good about this solution:  When your EG starts to rise, it will start at 
> 0.5 volts, but it still has 0.1 volts to go until the diode will allow the EG 
> signal through.  This means you will get a rather strange attack phase -- it 
> will have a short step or bump in the beginning of the attack.  Depending on 
> your attack time setting, it may also feel like a kind of delayed attack.  You 
> will also get a decay/release phase that falls nice and smoothly, then abruptly 
> drops in stepwise fashion at the end.  These attack/release bumps will be 
> *especially* audible when using the EG as a pitch modulation source.
> 
> Another problem the series diode will cause is that it will reduce the 
> overall EG maximum output level by 0.6 volts.  This may give a noticeable kind of 
> lackluster or weak EG performance, especially if this EG's normal maximum level 
> is already relatively low -- say around 5 volts or so.
> 
> In my opinion, the more elegant solution to this problem is to add an output 
> buffering stage with an offset trimmer.  The offset trimmer would be used to se
> t the "at rest" EG output to exactly 0 volts.  If you wanted to be really 
> clever, you could also set up your output buffer to have both positive-going and 
> negative-going (inverted) outputs.  This would allow you to make a whole bunch 
> of new types of sounds that would not be possible with just the standard 
> positive-going EG output.  
> 
> Keep in mind that your MG-1 circuits may have been designed to expect that .5 
> volt offset from the EG, and trimming it out may give you a slightly "weaker" 
> EG sound.  This wouldn't be much of a problem when using it to drive VCO 
> pitches, but might be apparent when using it to drive filters and VCA's, because 
> you will lose .5 volts at your peak, and they may have been expecting that 
> addtional .5 volts.  Hey, there are tradeoffs everywhere in life.  It would, 
> however, be possible to add a small amount of gain to your buffer amp, to make up 
> for the peak voltage lost in the trimming process, giving you the best 
> solution.  The whole buffer amp/trimmer thing could be done with just one opamp stage, 
> or maybe with two stages if you need some extra gain and also want an 
> inverted output.
> 
> Peter Grenader has designed a very nice little circuit he calls the 
> Gizmotron, which would make a very nice output buffer for your EG.  It has the added 
> benefit of having an attentuator pot that gives both positive and negative 
> ouput, with zero output at the center of the pot's travel.  This would be very cool 
> as an EG attenuator.  The Gizmotron has its own offset adjustment trimmer for 
> calibrating its own zero point, but I'm not exactly sure if the trimmer would 
> also take care of your MG1 EG offset problem.  You might need to add a second 
> trimmer for that, but it certainly would not be at all complicated.  You can 
> find the Gizmotron at Peter's site:  http://www.buzzclick-music.com
> 
> Michael Bacich
>