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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Wed Sep 26, 2007 8:53 am Post subject:
dual 8038 VCO cross-talk Subject description: possible reasons |
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hi all,
as you might already know i built a dual 8038 VCO recently. the layout is that way, that all ICs are placed in a row utilizing one half of each IC for each VCO.
now i observe a cross-talk between both VCOs, audio and cv. i wonder if the described usage of the ICs might be the cause? any suggestions or experiences?
BTW i like the sound of the VCOs very much and it would be a shame if i would have to build it again, which would have to wait for a couple of weeks... _________________
cheers,
matthias
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Big Boss at fonitronik
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bubblechamber
Joined: Nov 04, 2006 Posts: 279 Location: NYC
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Posted: Wed Sep 26, 2007 10:59 pm Post subject:
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i've had cross talk in twin oscillators coming from a shared ground bus. it cleared up when I stopped sharing the power and ground between the 2.
david |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
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Posted: Wed Sep 26, 2007 11:55 pm Post subject:
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sounds consistent. so i will do a better neat and decend job and solder in some decoupling caps. hope that will help... _________________
cheers,
matthias
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Big Boss at fonitronik
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etaoin
Joined: Jun 30, 2005 Posts: 761 Location: Utrecht, NL
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
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Posted: Tue Oct 09, 2007 9:06 am Post subject:
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not yet! i am currently struggling with a loose contact or hairline crack on my wasp vcf proto pcb!
i will try decoupling caps next week. stay tuned... _________________
cheers,
matthias
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Big Boss at fonitronik
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Thomas Henry
Joined: Mar 25, 2007 Posts: 298 Location: Southern Minnesota
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Posted: Wed Oct 10, 2007 8:59 pm Post subject:
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Hi all,
I've been really swamped with school work lately. But I just had to jump into this thread.
My experiences over the last thirty years have made it very plain to me that dual VCOs are inherently dangerous as a general rule. The tiny currents involved mean that any sort of variation through the power supply or through messy wiring or parallel traces on a PCB will lead to tuning disorders, unintentional syncing, and more.
I really think that VCOs should only be built as single units.
However, if you're going to risk a dual, I would recommend:
1. Don't share dual or quad op-amps between the units, and
2. Be sure to use a star-configuration for wiring the power supply lines. This is extremely important, even more important than the decoupling caps. VCOs in any system have a tendency to lock or interact, but a star power supply connection will minimize it.
Anyway, that's my two-cents worth,
Thomas Henry |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Thu Oct 11, 2007 4:40 am Post subject:
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thank you thomas for your valuable comments.
so i think i will do the following: i will design a layout for a dual 3038 VCO whereby the VCOs are only sharing the matched transistor array (which is a passive device anyways) and the power connection header...
so STOP everybody! hope i am able to achieve it next week (i am on vacation this week and have no access to the machine storing the files). _________________
cheers,
matthias
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Thomas Henry
Joined: Mar 25, 2007 Posts: 298 Location: Southern Minnesota
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Posted: Fri Oct 12, 2007 6:34 pm Post subject:
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Hiya Fonik,
I probably came across as a bit heavy-handed in my last post, and I hate it when the so-called "old-timers" lay down pronouncements. I should have given some reasons for my prejudices.
But I remember building a three VCO unit back in 1982 or thereabouts for a friend. The idea was have a lush sound with the VCOs slightly out of tune with each other for a natural chorusing effect.
I spent hours, days, even months trying to debug the system and keep the VCOs from syncing to each other. I can't remember all the details, but I do recall that the star configuration to the power supply was a major part of it.
Also, much later I remember doing a shared quad op-amp on another circuit and the square wave interacted with the triangle wave within the package.
So anyway, I don't usually do dual units myself nowadays, but you might consider these points as you lay out a PCB.
Then again, I used to do PCBs by hand. Perhaps the newer software has clever ways to route the traces to avoid these problems.
Thomas Henry |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Sat Oct 13, 2007 3:14 pm Post subject:
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Thomas Henry wrote: | I probably came across as a bit heavy-handed in my last post,... |
not at all!
Quote: | Then again, I used to do PCBs by hand. Perhaps the newer software has clever ways to route the traces to avoid these problems. |
i tried some demo layout software with autoroute functionality. i personally don't think it makes things easier, the software is not clever at all. cleverness is probably related to creativity which is hard to define by rules a software may need? don't know.
i just try to follow the schematic/circuitry. _________________
cheers,
matthias
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Big Boss at fonitronik
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