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 Forum index » DIY Hardware and Software
Moog MG-1 reset LFO cycle mod
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steenamaroo



Joined: Aug 03, 2012
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Location: NI

PostPosted: Fri Aug 03, 2012 4:52 am    Post subject: Moog MG-1 reset LFO cycle mod Reply with quote  Mark this post and the followings unread

Hi all,
I saw this* thread recently and it got me thinking about something I want to do.
* http://www.electro-music.com/forum/post-264007.html

I'm interested in adding LFO reset to my Moog MG-1, so that pressing a key resets the LFO to cycle start point.
Perhaps later I could have a click track or a pedal do it, but that's getting ahead of myself.


Is there any way to use the gate signal (mentioned in the other thread) to do this?

I spoke to a chap who said all I have to do is convert the gate signal to a pulse (using a resistor and a cap), then route it to the comparator portion of the LFO.

If anyone could elaborate on that I'd really appreciate it.


Schematics are at www.steenaudio.com/mg-1.pdf


Thanks in advance.
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steenamaroo



Joined: Aug 03, 2012
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PostPosted: Sat Aug 04, 2012 3:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

For want of something more sensible to do, I tried two things today.

One was to put a momentary switch across the rate slider, which would force the rate to maximum and then back to the slider's value very quickly.

This sort of worked but didn't always pick up at the same point of the cycle.

Next I made a break before the rate slider and popped a switch in there.
Same thing but in reverse. Rate would temporarily be knocked to zero and back.
This seemed to result in the cycle being paused, then resuming from where it left off.


Anyway, worth a try, I guess.

I've been looking at the way OSC2 can sync to OSC1. Presumably I'd want to emulate that idea?
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bod



Joined: Apr 28, 2009
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PostPosted: Mon Aug 06, 2012 2:04 am    Post subject: Reply with quote  Mark this post and the followings unread

ive used this in LFO and triangle core VCO's with perfect results, but all you need is a gate to trigger convertor to give a short 1-10ms pulse, feeding a transistor that shorts the capacitor in the LFO.
I also used a resistor from the output of the op amp to the transistor (2n3904), if you have a look at the 909 drum schematics and Marc Bareilles LFO schematics there are similar ideas in there. http://m.bareille.free.fr/modular1/cvsynclfo/cv_lfo.htm
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steenamaroo



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PostPosted: Mon Aug 06, 2012 7:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Bod,
Thanks for taking the time to reply.

I've been trawling through schematics the last few days looking for an example to work from, and I think I may have found what you're describing.

Here's a little screen capture from arp odyssey.
www.steenaudio.com/ex2.jpg
That seems to be the same method as your link, right?


The only things are:
I know I can get a trigger from within the MG-1, at the contoured and keyed positions of the 'tone sources' switch in the contoured section. (extreme right of mg1 schematic)

If memory serves, it blips 4.5v at every keystroke.
I don't know how long that pulse is though; I'll have to look into that.
It's the same pulse they use to trigger env attack.

For the actual LFO, I assume I'm dealing with U15B in the mod rate section of the schemo?
It's set up slightly differently from the odyssey, and your suggestion.
I don't see the cap that would need shorting out.

Thanks again for taking the time to help a noob.
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bod



Joined: Apr 28, 2009
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PostPosted: Mon Aug 06, 2012 7:43 am    Post subject: Reply with quote  Mark this post and the followings unread

yeah the one by marc is very similar to the Arp lfo you linked to, just that marc's has voltage control and a few of output waves.

the MG1 doesnt use your typical integrator/comparator type LFO, but it would appear C25 is the cap for the LFO that would need shorting/discharging to reset.
the only issue with this is that i've never used a sync/reset feature on an lfo like this, so i'm not sure if it will provide the same effect, but since the cap sweeps from +/-1.6v, then discharging to ground should allow it to reset and climb from ground to its upper voltage (+1.6v) before the lfo output goes low and the cap discharges on the downward slope of the triangle.
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steenamaroo



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PostPosted: Mon Aug 06, 2012 8:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you Bod. You've been such a help.

"i've never used a sync/reset feature on an lfo like this"
Perhaps there's a good reason they didn't implement it here.


I just manually shorted the +1 side of that cap to ground, and it seems to do the same as my previous attempts; It holds the LFO at whatever point it's at, then releases it from that same point.

Basically, it acts as a pause button.

Last edited by steenamaroo on Mon Aug 06, 2012 8:54 am; edited 2 times in total
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bod



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PostPosted: Mon Aug 06, 2012 8:47 am    Post subject: Reply with quote  Mark this post and the followings unread

no worries mate, glad to be of help!

the only reason ive not used an LFO like this is that most designs of this nature produce a curved triangle shape more like an A/R envelope, but i've never dug about in an MG1 with a scope so dont take that as a given in this case!
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bod



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PostPosted: Mon Aug 06, 2012 8:56 am    Post subject: Reply with quote  Mark this post and the followings unread

steenamaroo wrote:
I just manually shorted that cap to ground, and it seems to do the same as my previous attempts; It hold the LFO at whatever point it's at, then releases it from that same point.

Basically, it acts as a pause button.


now that could be to do with the design, but..... marc's LFO has a hold function that works from just the gate input, but the reset function only works in my experience if you feed a transistor a 1-10ms pulse that bridges the cap (marc's switches between gate and trig so you can choose the effect), so if you are just touching the cap with a wire to discharge it, this maybe why it only holds the cv.

but, as i said, it could just be the design.
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steenamaroo



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PostPosted: Mon Aug 06, 2012 9:27 am    Post subject: Reply with quote  Mark this post and the followings unread

I understand bod (and the pm). Cheers for that.

When testing, I forgot that earth was broken between keyboard PCB and control PCB, so here are the real results. :p

A sustained short of c25 to ground pauses the LFO at the same point every time. Smile but whether the LFO rises or falls on release is dependant on when I interrupted it, if that makes sense.


I'll look into obtaining a very short trigger from gate and see how it goes from there.

Last edited by steenamaroo on Tue Aug 07, 2012 6:41 pm; edited 1 time in total
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bod



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PostPosted: Mon Aug 06, 2012 9:44 am    Post subject: Reply with quote  Mark this post and the followings unread

thats exactly what happened to me with the integrator/comparator circuits when i used a gate, so with any luck the pulse/trigger feeding a transistor should be perfect!
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steenamaroo



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PostPosted: Mon Aug 06, 2012 9:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Aww that's very encouraging!

I'll post back when when i get a proper trigger to try it with.

Thanks again. Smile
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steenamaroo



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PostPosted: Tue Aug 07, 2012 6:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Back again.

I got a basic trigger setup up and wired it all together.

Tested across the transistor with a tone continuity meter first, and sure enough. bip, bip, bip every time I hit a key. Smile

So, nearly there...
I put it all in place to test it out and the trigger itself does work.

The direction problem still remains though I'm afraid.

I wonder could the LFO be reset to its peak rather than 0.
That would solve the problem in a round about way, and still be very usable.

Here's what I currently have.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
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umschmitt



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PostPosted: Tue Aug 07, 2012 7:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey. The problem of resetting the LFO at peak instead of zero is that the peak of the triangle wave corresponds with the lower part of the square… (see diagram) It still can be done (I think), for instance with a double transistor arrangement and a switch to choose between +tri/-squ and -tri/+squ. Then your LFO starts low or high at choisce…
These were my two (2) cents.

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steenamaroo



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PostPosted: Tue Aug 07, 2012 7:47 am    Post subject:   Reply with quote  Mark this post and the followings unread

Hi, and thanks.
That sounds interesting.

Can you elaborate at all? This is effectively my first electronics lesson.
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umschmitt



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PostPosted: Tue Aug 07, 2012 8:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Disclaimer : I haven't gone myself much further than the first electronics lesson… That being said, here we go :
* Triangle down / square up : cap to the base of an NPN transistor that has its emitter tied to V- and its collector to pin 6 of U15B. I guess a limiting resistor would be welcome somewhere, along with a proper cap value… I've already built something similar so it might work.
* Triangle uo / square down : here I'm just guessing. Same arrangement with an NPN, but with collector tied to V+ and emitter to pin 6.
A good location for the switch would be between transis and pin 6. Extra option : with a 3 position switch you get the possibility of no reset at all (nothing tied to pin 6).
Now there's a lot of experimented people on this board who will certainly correct / object most of the crap I just elaborated (see top of post). But that's roughly the idea !

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steenamaroo



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PostPosted: Tue Aug 07, 2012 8:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, I think I follow, and thanks for your patience.

I'll go away and footer, then report back.

Last edited by steenamaroo on Tue Aug 07, 2012 4:30 pm; edited 1 time in total
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steenamaroo



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PostPosted: Tue Aug 07, 2012 4:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've done it. Not in the most elegant way, but I've done it.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Now when I tap a key, c25 sees 1.6V and resets to the peak perfectly.

I guess now I have to find or create a 1.6 volt supply from somewhere inside the synth, instead of using a battery.


Umschmitt, I get the feeling your description would give the same results? I just didn't really understand it, through no fault of yours.
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umschmitt



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PostPosted: Wed Aug 08, 2012 11:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Yep it was the idea… but with +12V instead. (not sure if it'd be a problem ; I suspect that what's requested here is a quantity of electrons to charge the capacitor,delivered as quick as possible - heard as immediate. Any experts around ?)
Anyway. An elegant way to get 1.6V is to tie : 12V > 1K resistor > +red LED- > gnd. At the anode of the LED you have the requested voltage. (In this case I would go for a green LED, which has a voltage of 1.8V, so you're sure it's above what's needed to fold the wave)
Same applies on the negative side : gnd > +LED- > 1k R > -15V. Here the proper voltage is at the cathode of course.
The value of 1K ohms comes out of nowhere and is to be taken as an example, the resistor is needed in order not to burn the LED, and I guess it is small enough to let "fill" or "empty" the cap fast enough. (theoretically down to 330 ohms should do)
Once again, please someone double check my babbling.

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steenamaroo



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PostPosted: Wed Aug 08, 2012 11:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Schmitt, thank you so much. That is a fantastic help.
I've been (blindly) looking at other solutions that seemed convoluted and complicated.
What you describe sounds perfect.

I'll get to work and post back some photos and audio clips when i get the final job done.
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steenamaroo



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PostPosted: Thu Aug 09, 2012 2:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just had a little afterthought.

Unless I've got this wrong, I could put a 10k pot between 1.6v and -1.6v, with the wiper as the output.

Instead of using a switch to pick + or -, I could use the pot to pick any point of the LFO as the reset point!

I have a few parts in the post, so it'll be a few days before I report back.
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steenamaroo



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PostPosted: Mon Aug 13, 2012 8:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi again.

I've scratched the idea of a negative rail.

I suppose it makes sense, but using an LED with 1.6 voltage drop will give me -10.4V; Not -1.6.

I should have known that,lol.

Anyway, the positive side of things (which was the main goal) is working a charm, even if the real voltage drop of the LEDs turned out to be closer to 1.2.

I've only just finished putting everything back together, but so far so good.

Thank you all.
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