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Octave switcher problem on ASM-2
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Uncle Krunkus
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PostPosted: Mon Mar 24, 2008 4:31 am    Post subject: Octave switcher problem on ASM-2
Subject description: Extra mods for my VCOs needed
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I've been trying to iron out some problems with my ASM-2 Sorcerer. Most notably, not being able to get my octave switcher to work properly. This basically means that I can't get it in tune and have any confidence in how CVs actually work.
I've unplugged all other CV ins from the VCO, and tested what is coming out of the octave switcher. For the six different positions I get -4, -3, -2, -1, 0, +1V. But as soon as I plug it into one of the CV ins, it changes. For example the last three values are -.67, 0, +.67V Now I find it really suspicious that the 1V ranges get pulled back to 2/3V. Could there be something about the VCO which is letting it be "loaded" in some way?
I'll attach a schem of the VCO summer section in case that helps.

This is really weird, I thought I'd sorted all this stuff out, but obviously something still hasn't set right in my brain! Laughing

Any help or advice would be greatly appreciated. I feel really stuck and lost and incompetent. Embarassed


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PostPosted: Mon Mar 24, 2008 5:52 am    Post subject: Re: Octave switcher problem on ASM-2 Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
Could there be something about the VCO which is letting it be "loaded" in some way?

The VCO will load your octave switcher some, depending on the switcher's output impedance. Would need to see that schematic to answer the question.

Very Happy

Ian
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Uncle Krunkus
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PostPosted: Mon Mar 24, 2008 7:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Here 'tis. Very Happy

BTW Thanks Ian! Very Happy


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PostPosted: Mon Mar 24, 2008 9:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, that's a high impedance circuit. You need to buffer it before it goes to the VCO. What is happening is that the output impedance is adding to the VCO input resistor, so the input summer is putting out less than it should.

Very Happy

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Uncle Krunkus
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PostPosted: Mon Mar 24, 2008 1:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Can the switcher be re-designed so it doesn't need a buffer? ie: can I make the same thing with lower impedance? I mean, some coarse tune pots are just that, a pot yeah? You can find a number of octaves on one of those. Does the 1V/Oct still work through a 100K summing resistor regardless of what might be hanging off another summing resistor? Man,..... what a nightmare! How come nobody else has been hit with this?
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PostPosted: Mon Mar 24, 2008 1:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

but wouldnt it be far easier for you to just put a buffer infront of your existing design? your great at stripboards you could do it in seconds

this is what elby does for the octaves

http://www.elby-designs.com/synth-modules/octave/octave-cct.pdf

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Uncle Krunkus
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PostPosted: Mon Mar 24, 2008 2:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here is the .pdf I got the switcher idea from. I don't think I've changed the overall impedance that much.
I did just realise that one of the six log CV inputs has a 1M5 resistor on the board instead of a 100K. Now I assumed that this was the fine tune input, but it is in the position (log1) which this pdf shows the octave switcher connected to.
Now the same switcher with a 1M5 would be a completely different impedance match yeah? The fact that mine is putting out .67,0,-.67 shows that the intervals are still proportionally the same, maybe connecting it to the 1M5 input will re-scale it so the trimmers will allow for the droop caused by imperfect impedance matching. Am I barking up the wrong tree? Or have I just had a eureka?


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PostPosted: Mon Mar 24, 2008 2:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Mr Clack wrote:
but wouldnt it be far easier for you to just put a buffer infront of your existing design? your great at stripboards you could do it in seconds

this is what elby does for the octaves

http://www.elby-designs.com/synth-modules/octave/octave-cct.pdf


Jesus - H - Christ - Mr - Clack! Where did you find that?
I think the world has moved on a long way since I got my ASM-2 docs! Laughing
This indeed does exactly what Ian is talking about, and would allow plugging in to any 100K summer input, I think.
I still wonder whether the 1M5 input is the short term solution though. It would settle a few doubts rattling around in my head. Rolling Eyes

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PostPosted: Mon Mar 24, 2008 2:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
I still wonder whether the 1M5 input is the short term solution though. It would settle a few doubts rattling around in my head. Rolling Eyes


When the 1M5 goes to the same summing input as the 100k inputs it has a different sensitivity, so you would have to rescale the octave voltages ... no way out but to build the buffer I'm afraid Laughing

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PostPosted: Mon Mar 24, 2008 2:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
I mean, some coarse tune pots are just that, a pot yeah? You can find a number of octaves on one of those.

Sure you can. Very Happy
But they are not at convenient resistance values. In other words, you *could* design a high impedance octave switcher, but you would have to *simultaneously* calculate all the *different* resistor values. And trim them all.

That's why they invented buffers.

Very Happy

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PostPosted: Mon Mar 24, 2008 4:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Okay, okay, I'm sold. I'll build the buffers onto the back of my octave switchers.

Now I assume that this will be the case for any octave switcher right?
I mean I'm not being singled out for the "Now go to your room and build some buffers!" treatment am I? Crying or Very sad Laughing

BTW I'm gonna make them 8 position switchers while I'm at it. So there! Razz Laughing

BTBTW Anyone want the stripboard layouts when I'm done? Cool

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PostPosted: Mon Mar 24, 2008 4:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
I mean I'm not being singled out for the "Now go to your room and build some buffers!" treatment am I? Crying or Very sad Laughing


Depends .. others might be tone deaf ... that would work just as well Rolling Eyes Laughing

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PostPosted: Mon Mar 24, 2008 4:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Jesus - H - Christ - Mr - Clack! Where did you find that?


http://www.elby-designs.com/synth-modules/octave/octave.htm its in the genie section - seems he may have offered the pcb for a while ( which looks very tidy ) - I never saw it, still on my way to finishing mine (btw does the document on the CD tell you what value pots go fo what controls? there is no info on this on the site. I lost my CD ).

now go to your room and finish your synthwork!

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PostPosted: Mon Mar 24, 2008 5:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Uncle Krunkus wrote:
I mean I'm not being singled out for the "Now go to your room and build some buffers!" treatment am I? Crying or Very sad Laughing


Depends .. others might be tone deaf ... that would work just as well Rolling Eyes Laughing


Sometimes I think being tone deaf could be a blessing in disguise! Laughing
A very underated source of ignorance/bliss. Very Happy

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PostPosted: Mon Mar 24, 2008 6:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Mr Clack wrote:
Quote:
Jesus - H - Christ - Mr - Clack! Where did you find that?


http://www.elby-designs.com/synth-modules/octave/octave.htm its in the genie section - seems he may have offered the pcb for a while ( which looks very tidy ) - I never saw it, still on my way to finishing mine (btw does the document on the CD tell you what value pots go fo what controls? there is no info on this on the site. I lost my CD ).

now go to your room and finish your synthwork!


Yes, I know where you got it! It was a rhetorical question! "Why I outta!" Laughing

Anyway, I've had to work out alot of it myself, and I'm sure there are plenty of things I could put some extra work into. It is a big project, and improving it could take some time. I'm not sure where I got the values for my pots, but if you need any specifics, just ask.
Oh, look, I just found this. Doesn't include everything. There are other pots I've included which aren't listed. Like levels for the ring mod inputs etc. Have you worked out a front panel yet? I can send you a pic of mine if you want some inspiration.
All up, it is a really nice sounding synth, and I know there is more in there for me to wring out of the basic design.


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PostPosted: Mon Mar 24, 2008 6:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

One more question,
Can I run the buffer op-amps off the reference +/-10V which is already going to the old switchers? I only want -5 to +2 so I assume it should be fine.

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PostPosted: Mon Mar 24, 2008 6:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
One more question,
Can I run the buffer op-amps off the reference +/-10V which is already going to the old switchers? I only want -5 to +2 so I assume it should be fine.


Probably, but are you sure you want to do that? feeding a work horse from a reference voltage I mean ... You'd get away with it I guess, but ... erm ... anyway, I'm biting my tongue Laughing

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PostPosted: Tue Mar 25, 2008 12:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
BTW I'm gonna make them 8 position switchers while I'm at it.


Well, since most rotary switches are 12-position anyway, why not 12? Very Happy

I use this one myself:
http://www.yusynth.net/Modular/EN/STANDARDS/index.html
12 octaves plus 12 semitones.

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PostPosted: Thu Mar 27, 2008 7:57 am    Post subject: Reply with quote  Mark this post and the followings unread

In reply to both Jan and Etaoin (how do you pronounce that?)
I understand your concerns and advice, but this octave switcher is for a self contained ASM-2. It's not a modular. It has it's own PSU, and dedicated reference, so I'm not that concerned with running clean power to the "workhorse" if you know what I mean. And this octave switcher, or rather two of them, will hang off the back of the actual rotary switch. It's a special feature, not a separate module. Small, simple, accurate, that's the need.
And with that I submit my solution: -

It's designed to solder directly onto a 12 position rotary. Across the position 1 (-8V), Pole, and position 6 (-3V) pins. I actually went for the full 9 position version, which means 10 connections to the switch. All but the aforemention 3 must be made with solid strand wire.


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PostPosted: Mon Mar 31, 2008 6:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well,
while I was exploring the process of adding the octave switcher buffers, I went back and had a look at the Elby site. I downloaded the latest set of schems for the ASM-2 and I've created a whole new load of work for myself.
It seems my PCB (which was really a prototype (Laurie actually told me to wait for a tested version, and I said I'd take my chances, it has taught me alot!)) doesn't have sync buffers! No wonder I've always found the sync to sound a bit, well,... not there!
I also found 4 resistors in each VCO which have had their values modified. I've only checked the VCOs so far!
So my job at the moment is to get the VCOs up to speed. They actually sounded quite good before, so I'm expecting them to sound even better.
I have to put the stripboard octave switcher with buffer on the back of the switch, build a second stripboard with the sync buffer on it which will plug into the VCO with risers, and change the value of 4 resistors. And all of that on the other VCO as well!
I'll take some photos when I'm done.

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PostPosted: Sat Apr 12, 2008 4:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Okay,
The sync buffers are finished, and so too are the new octave switchers.
And here they are.
Purty aren't they?

I need to update the stripboard layouts I posted before anyone uses them. (minor changes)

I'll get these babies into place and see if I can't teach that Klee sequencer a thing or two! Laughing


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PostPosted: Sat Apr 12, 2008 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Do not build this octave switcher.
It's been trolled to output the opposite voltage.
I should have known, I was close to asking, isn't that an inverting buffer followed by a non-inverting buffer? Doesn't that mean the output will be upside down?

Anyway, I'm gonna replace the -10V into the divider with +10V, and it should be fine.

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PostPosted: Sat Apr 12, 2008 8:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
It's been trolled to output the opposite voltage.


Oops Laughing

almost as smart as those patriots being programmed to their current location by default.

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PostPosted: Sat Apr 12, 2008 4:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well,....
trolled is a bit harsh, I'd had a few beers and was overcome with frustration. Laughing
It's not hard to fix really, I've got the +ve and -ve rails side by side, so I'll just feed +10V to the resistor chain instead of -10V. That should do it!
I'll update the stripboard layout once it's tested.

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PostPosted: Sun Apr 13, 2008 9:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Okay, they work perfect now!
I've also updated the stripboard layouts a few posts back.

As well as changing the polarity of the resistor chain to +ve I also had to change the offset to -ve. After putting those two summed voltages through the inverting stage U101B you get the right voltages for the right positions on the rotary.

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