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Simple X-fader/Panning PCB project for DIY Serge Gator
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macumbista



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PostPosted: Mon Nov 28, 2011 9:22 am    Post subject: Simple X-fader/Panning PCB project for DIY Serge Gator Reply with quote  Mark this post and the followings unread

I've assembled almost everything I need to do a Serge Gator clone from the CGS collection, the one thing missing would be the X-fader/VC gain module.

I've looked at two projects out there, Fonik's X-Pan-Fade from the Triple Resonator module, and M. Bareille's SSM2164-based Quad VCA. The former may not be exactly what I am looking for (as well as unavailable), the latter is a bit large and expensive. Does anyone know a simple X-Fade or stereo panning project PCB that is approximately the same size format as the CGS modules?

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ericcoleridge



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PostPosted: Mon Nov 28, 2011 10:27 am    Post subject: Re: Simple X-fader/Panning PCB project for DIY Serge Gator Reply with quote  Mark this post and the followings unread

I've been looking for solutions to the same problem--and I know of at least two other possibilities, the second one being the one I'm leaning towards (small enough to squeeze into a Serge style quad vca/panner):

Yu Synth VCpanner

Vactrol Panner

Also, here's a voltage controlled x-fader:

Bergfotron Crossfader


macumbista wrote:
I've assembled almost everything I need to do a Serge Gator clone from the CGS collection, the one thing missing would be the X-fader/VC gain module.

I've looked at two projects out there, Fonik's X-Pan-Fade from the Triple Resonator module, and M. Bareille's SSM2164-based Quad VCA. The former may not be exactly what I am looking for (as well as unavailable), the latter is a bit large and expensive. Does anyone know a simple X-Fade or stereo panning project PCB that is approximately the same size format as the CGS modules?
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richardc64



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PostPosted: Mon Nov 28, 2011 11:49 am    Post subject: Reply with quote  Mark this post and the followings unread

My way is simpler.

http://www.sdiy.org/richardc64/mmvca/index.html

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PostPosted: Mon Nov 28, 2011 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
My way is simpler.

http://www.sdiy.org/richardc64/mmvca/index.html


Oh that *is* elegant! Response is linear, isn't it? Maybe better for CV crossfade than for stereo audio output...

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PostPosted: Mon Nov 28, 2011 4:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Haha thanks for the reminder i must post the schematic
as i said i would, my apologies , completely forgot about it Embarassed . will try and get it posted before the weekend.

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PostPosted: Mon Nov 28, 2011 7:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
My way is simpler.

http://www.sdiy.org/richardc64/mmvca/index.html


Yeah, this would be really nice for a lot of different applications. I'm always looking for a small parts count cross fader for all kinds of stuff. I'll have to try making a little layout for it.
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PostPosted: Mon Nov 28, 2011 9:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
My way is simpler.

http://www.sdiy.org/richardc64/mmvca/index.html


Quote:
Yeah, this would be really nice for a lot of different applications. I'm always looking for a small parts count cross fader for all kinds of stuff. I'll have to try making a little layout for it.


It does indeed look good
I think i could get 2 on a smallish board, would make a nice ancillary module might try and test out the lm13700 version
as long as it's ok with richardc64 ?

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richardc64



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PostPosted: Mon Nov 28, 2011 10:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Never seen my stuff called "elegant" before. I'll have to remember to keep a pinky raised whenever I post Laughing

Yeah, it's linear

diablojoy wrote:

It does indeed look good
I think i could get 2 on a smallish board, would make a nice ancillary module might try and test out the lm13700 version
as long as it's ok with richardc64 ?


Go for it. Might need a different current source than what my untested schema shows.

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macumbista



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PostPosted: Tue Nov 29, 2011 2:55 am    Post subject: Reply with quote  Mark this post and the followings unread

diablojoy wrote:

It does indeed look good
I think i could get 2 on a smallish board, would make a nice ancillary module might try and test out the lm13700 version
as long as it's ok with richardc64 ?

richardc64 wrote:

Go for it. Might need a different current source than what my untested schema shows.


If you work that one out, please let me know. I have a ton of dual OTAs sitting around. Can you explain why the LM13700 version needs a different current source than the 3080? Couldn't the op-amp buffer used in the single arrangement also be used in the dual? This part is a bit beyond my understanding...

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PostPosted: Tue Nov 29, 2011 9:04 am    Post subject: Reply with quote  Mark this post and the followings unread

macumbista wrote:

If you work that one out, please let me know. I have a ton of dual OTAs sitting around. Can you explain why the LM13700 version needs a different current source than the 3080? Couldn't the op-amp buffer used in the single arrangement also be used in the dual? This part is a bit beyond my understanding...


Mine too, actually. But no, the op-amp and diode trick supplies the 3080 Iabc (the control current), AND -V. In the LM13700 -V is common to both OTAs, so this method can't be used. That's why I show the simple one-transistor current source in the untested '13700 version, but that might be too simple, and something better -- like a current mirror -- could be needed.

Someone more knowledgeable could perhaps offer some advice, if we could get that "someone's" attention.

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emdot_ambient



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PostPosted: Tue Nov 29, 2011 12:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, you've got my attention........however all this is Greek to me geek
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macumbista



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PostPosted: Tue Nov 29, 2011 1:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
the op-amp and diode trick supplies the 3080 Iabc (the control current), AND -V. In the LM13700 -V is common to both OTAs, so this method can't be used.


That's interesting, since I was recently asking about the differences in OTAs in another thread. I was assured there wasn't much, now you're here to prove them wrong!

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PostPosted: Tue Nov 29, 2011 4:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
That's why I show the simple one-transistor current source in the untested '13700 version, but that might be too simple, and something better -- like a current mirror -- could be needed.

seems I will need to do a bit more studying on this, always something more to learn i have quite a few of 3080 and lm13600/13700 but would rather keep the 3080's back for other ccts as the lm13700 are far more common
time will tell if i can figure it out.

Eric : seems i never actually drew up a proper schematic up for that vactrol pan on my computer so if i cant find the bit of paper i scrawled it on by the weekend i will have to backtrace the cct board though i think i remember most of it anyway.
oh well at least Its a very simple cct

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frijitz



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PostPosted: Wed Nov 30, 2011 1:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi all --

Richard asked me if I could comment on this discussion, especially re the OTAs.

First, this is really a brilliant design. I'm surprised it hasn't been pick up on before. I think lots of folks would be interested if boards were available. People seem to be getting more and more interested in modules with lots of features.

Comments on the original design: The reason for the oddball resistors around A4 is most likely that the left side of the 47k sees a finite impedance. If this point were buffered then the three odd resistors could all be 47k.

The unknown R below the Init control is there so the input CV (eg, envelope) can be burried below zero. I think it could actually be omitted, depending on how you like to use the CV In. IOW the Init pot can go across the +/- supply.

On the OTAs: When the 3080s were going away I suggested everybody stock up on them because of all the designs that use the Jung trick of driving through the power pin. I have a pretty good supply myself and I could probably let a few go for projects like this.

The question of whether the same drive configuration could be used on the 13600/13700 has been asked but AFAIK never resolved. Would it still work if only half the chip were used, or if the two halves were connected in parallel???

The untested current source would probably work, but I would prefer an active (opamp) summer approach. See the pt. 2 drawing here:
http://home.comcast.net/~ijfritz/Chaos/ch_cir6_chaotica.htm (lower left)
So at the cost of a dual opamp you could make the circuit a bit more precise. A matter of taste, for sure.

Very Happy

Ian

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richardc64



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PostPosted: Wed Nov 30, 2011 7:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Whoa! First "elegant", now "brilliant"...

Most brilliant, IMNSHO, is that it doesn't need switches to select the different functions. Less brilliant is that, since it uses just one OTA, it can't function as two normal VCAs, like the more common panning method.

frijitz wrote:
Comments on the original design: The reason for the oddball resistors around A4 is most likely that the left side of the 47k sees a finite impedance. If this point were buffered then the three odd resistors could all be 47k.


Not sure what that means scratch

Quote:
The unknown R below the Init control is there so the input CV (eg, envelope) can be burried below zero. I think it could actually be omitted, depending on how you like to use the CV In. IOW the Init pot can go across the +/- supply.


I think I remember now; without the "unknown resistor" the init pot had to be rotated far off-center to make a single input come out at roughly equal levels on both outputs. Rather than figure out why that should be, I went for the expedient fix.

Quote:
The question of whether the same drive configuration could be used on the 13600/13700 has been asked but AFAIK never resolved. Would it still work if only half the chip were used, or if the two halves were connected in parallel???


THAT's something that might be worth exploring. Wish I had the facilities.

Quote:
The untested current source would probably work, but I would prefer an active (opamp) summer approach. See the pt. 2 drawing here:
http://home.comcast.net/~ijfritz/Chaos/ch_cir6_chaotica.htm (lower left)
So at the cost of a dual opamp you could make the circuit a bit more precise. A matter of taste, for sure.


Yeah, that does look better. Besides being more precise it's simple enough to make trying to apply the Jung trick to the '13700 unnecessary.

Thanks so much for your insights!

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frijitz



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PostPosted: Wed Nov 30, 2011 9:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
Most brilliant, IMNSHO, is that it doesn't need switches to select the different functions.

Exactly!

Quote:
frijitz wrote:
Comments on the original design: The reason for the oddball resistors around A4 is most likely that the left side of the 47k sees a finite impedance. If this point were buffered then the three odd resistors could all be 47k.

Not sure what that means scratch


Hmmm... Let's try looking at it from a different angle. The outputs are not linear functions of the input pot settings, ie the gains of the two output amp circuits depends on the input pot settings. If you buffer the pot wipers, then the behavior is ideal. But maybe that's not the main issue here, especially if you do all the setup with the input pots full up or full down. So the needed R values might need a bit more explaining.

Quote:
I think I remember now; without the "unknown resistor" the init pot had to be rotated far off-center to make a single input come out at roughly equal levels on both outputs. Rather than figure out why that should be, I went for the expedient fix.

That's odd. It was OK at the endpoints but not the middle?

Very Happy

Ian

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macumbista



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PostPosted: Thu Dec 01, 2011 7:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey Ian, thanks for checking in on this! I may just try prototyping this in the next week with your op-amp summer, I'll post back if I do.
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PostPosted: Thu Dec 01, 2011 9:27 am    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
richardc64 wrote:
I think I remember now; without the "unknown resistor" the init pot had to be rotated far off-center to make a single input come out at roughly equal levels on both outputs. Rather than figure out why that should be, I went for the expedient fix.

That's odd. It was OK at the endpoints but not the middle?


I should have written "...had to be rotated too far clockwise..." The unknown resistor allowed the 2 outputs to be equal when the init pot was centered. In other words, without the resistor the CV was "buried" over much, much more than half of the pot's rotation.

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PostPosted: Fri Dec 02, 2011 10:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
...But no, the op-amp and diode trick supplies the 3080 Iabc (the control current), AND -V. In the LM13700 -V is common to both OTAs, so this method can't be used.

So I finally tested the power-pin biasing on the LM13700. Using one half of the pair works exactly the same as the original. Using the two halves in parallel also works. I've started a new thread on the circuits.
http://home.comcast.net/~ijfritz/Chaos/ch_cir5_twc.htm

Very Happy

Ian
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PostPosted: Fri Dec 02, 2011 11:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks Ian is really good of you to do that
will read the new thread with interest.

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PostPosted: Sun Dec 04, 2011 8:22 am    Post subject: Reply with quote  Mark this post and the followings unread

OK, my eyes are starting to swim a bit here, let me see if I understand what is being suggested. The op amp summer shown in the attachment below (Picture1.png) should replace the transistor arrangement in the lower left of this image:

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

Where "c" in Ian's drawing connects where the 10K resistor enters the LM13600. Right? What pin on the IC is that, Richard? In your first, tested VCA layout you are using both 4 and 5 for the biasing. So in the second, you use which? Or does it depend which half of the OTA IC you are aiming at?

Help me connect the last dots please!

Also, do I understand you correct Ian when you say this arrangement has been tested now? Thanks for sharing your time with us!


Picture 1.png
 Description:
Ian Fritz op-amp summer from http://home.comcast.net/~ijfritz/Chaos/ch_cir6_chaotica.htm
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Picture 1.png



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PostPosted: Sun Dec 04, 2011 8:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Last question: the CV for this VCA would be bipolar, or positive only?
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PostPosted: Sun Dec 04, 2011 8:57 am    Post subject: Reply with quote  Mark this post and the followings unread

macumbista wrote:
So in the second, you use which? Or does it depend which half of the OTA IC you are aiming at?

Pin 1 or 16, depending on which half of '13700 is used.

Quote:
Last question: the CV for this VCA would be bipolar, or positive only?

The original, with the Jung current source, was intended to function with either, with proper adjustment of the "initial" pot. As for Ian's method, I think I might've gotten in a bit over my head, here.

Maybe I should've kept quiet.

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PostPosted: Sun Dec 04, 2011 10:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Also, do you think there is any way this circuit to could overall voltage-controlled gain (like the Serge X-fader)? Maybe more CV summing tricks? Or another VCA needed? Hmmm....
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PostPosted: Tue Dec 06, 2011 10:03 am    Post subject: Reply with quote  Mark this post and the followings unread

This circuit shouldn't be any different with am LM13700, should it? See again my thread on differences between OTAs...
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