[sdiy] Weird MFOS VCO max CV problem

[Harry Bissell]

you might be getting to a point where opamp 'phase reversal' is a problem. This CAN happen with
TL07x opamps when the common mode range is exceeded in a negative direction.  The opamp output can
go to the opposite rail and latch up.

I had this issue with an LM324 I had designed into an envelope generator. (it phase reverses in the opposite
direction, but the same overall effect). The odd thing is that the generator worked for several YEARS before
it had the issue... then I replaced the opamps with other units from the very same lot and it failed every time.
I replaced it with a TH074, which works all the way to the positive rail... fixed 

There were some discussions about using a TL07x as an inverting integrator  in an OTA filter, many people had hang-up
issues at power up. Using the same part a voltage follower (with the integtator cap to ground) worked perfectly
every time...

OR I might be totally wrong about ~this~ particular issue, just guessing...

H^) harry

----- Original Message -----
From: Ove Ridé <nitro2k01 at gmail.com>
To: Travis Thatcher <recompas at yahoo.com>
Cc: synthdiy <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 09 Apr 2012 16:53:09 -0400 (EDT)
Subject: Re: [sdiy] Weird MFOS VCO max CV problem

I've had a similar problem with a VCF. I don't know what the cause is
at this point, but I suspect it may have to do with over-spec control
current to a ca3080, which is an OTA. Does MFOS VCO utilize any OTAs,
such as the ca3080, ca3280 or LM/NJM13x00?

On 9 April 2012 19:26, Travis Thatcher <recompas at yahoo.com> wrote:
>
>
> I've had a MFOS VCO in my rig for some time now and its more or less
> been functioning well.  I finished it as a build a friend started and
> had some initial problems but after going through the circuit everything seemed to be okay. The problem I'm having is that if you feed it a CV
> above a certain threshold, when the pitch is high, the VCO just turns
> off. I suspect the summing op amp for CV is just getting stuck in a
> state where its hitting the rails, but I've tried a few different chips
> to be sure that's not the issue.  Not really sure where else to poke
> around to troubleshoot this. Has anyone else ever had this problem?
>
> Travis Thatcher
> ___________________________________
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>
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-- 
/Ove

Blog: <http://blog.gg8.se/>

"Here is Evergreen City. Evergreen is the color of green forever."
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Harry Bissell & Nora Abdullah 4eva