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 Forum index » DIY Hardware and Software » Thomas Henry designs
dual 8038 VCO cross-talk
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fonik



Joined: Jun 07, 2006
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PostPosted: 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...

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bubblechamber



Joined: Nov 04, 2006
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PostPosted: Wed Sep 26, 2007 10:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Wed Sep 26, 2007 11:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

sounds consistent. so i will do a better neat and decend job and solder in some decoupling caps. hope that will help...
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etaoin



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PostPosted: Tue Oct 09, 2007 1:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Matthias, did you solve this?
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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Oct 09, 2007 9:06 am    Post subject: Reply with quote  Mark this post and the followings unread

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...

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Thomas Henry



Joined: Mar 25, 2007
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PostPosted: Wed Oct 10, 2007 8:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Thu Oct 11, 2007 4:40 am    Post subject: Reply with quote  Mark this post and the followings unread

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).

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Thomas Henry



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PostPosted: Fri Oct 12, 2007 6:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

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
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PostPosted: Sat Oct 13, 2007 3:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thomas Henry wrote:
I probably came across as a bit heavy-handed in my last post,...

not at all! Cool

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.

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