[sdiy] VCA distortion - bad

[Jay Schwichtenberg]

Michael,

Have you made sure that you don't have a bad 3080? I've fried a number of
CA3080s and LM13X00 with wrong resistors giving them to much Iabc or
shorting scope probes. At least for me I've fried more OTAs than any other
chip.

Tried measuring things? You might just setup some DC tests and do
measurements. Remember that OTAs are usually current devices and you measure
current in series, voltage in parallel.

Good luck.
Jay

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Michael Ruberto
> Sent: Saturday, June 14, 2003 8:15 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] VCA distortion - bad
>
>
> I have not been able to get rid of the distortion entirely. I have
> determined that the 3080 is definately the cause. here's something
> interesting, I changed the input resistor back to it's original
> value -100K
> and found that now the signal is only distorted when the CV is more than
> half maxed. attenuating the CV level only results in a weak (with
> distortion) or no output from the VCA. I suspect the problem lies in the
> Iabc, too much current. I think Scott mentioned that earlier. however I
> cannot seem to correct it by changing Rf. In Hutchins' design he has the
> negative pin of the 3080 (pin 4) going to the CV buffer amp (I
> think that's
> correct) output. then there is a diode from the buffer output to it's
> inverting input and another diode from that point to pin 5 on the
> 3080. is
> there a way to limit the current through these diodes?
>
> btw, the reference material has helped alot!!!
>
> ~M
>
>
> >From: René Schmitz <uzs159 at uni-bonn.de>
> >To: Michael Ruberto <frankentron at hotmail.com>
> >CC: synth-diy at dropmix.xs4all.nl
> >Subject: Re: [sdiy] VCA distortion - bad
> >Date: Fri, 13 Jun 2003 12:57:37 +0200
> >
> >Hi Michael,
> >
> >It could be that the triangle is clipped at the opamp after the OTA.
> >Does the clipping depend on the CV? If so, then you should lower the CV
> >instead. Or change the gain at the output opamp.
> >
> >Cheers,
> >  René
> >
> >Jim Patchell wrote:
> >>     You need to limit the signal at the input of the CA3080 to about
> >>10mV...so you need to let us know what your input voltage
> is...but, with
> >>the values you show...200K-220, 10 volts would attenuate down to 10mV.
> >>(the attenuation ratio of 200K to 220 ohms is about a factor of 1000).
> >>
> >>     Several people have posted analysis of the OTA recently...mine is
> >>here:
> >>
> >>http://www.silcom.com/~patchell/SecretsOfOTAs.pdf
> >>
> >>     Another list member also wrote a very nice article on the
> OTA...I am
> >>sure he will post his url himself...I hope (since I can't
> rembmer it, lost
> >>all my bookmarks).
> >
> >You mean the article from Achim Gratz, I have linked it from my homepage.
> >
> >>Michael Ruberto wrote:
> >>
> >>>I am using Bernie Hutchins' 3080 VCA circuit in my processor but I am
> >>>getting distortion causing my triangle waves to become flat on top. I
> >>>have changed the input resistor from 100K to 200K compared to the
> >>>electronotes version of the circuit. should I have also
> changed the 220
> >>>ohm bias resistor following the input resistor? is there some
> calculation
> >>>to determine these values?
> >>>the only other thing I changed from the original design was
> the feedback
> >>>resistor in the output amp, I don't have 560K so I used the
> closest I had
> >>>- 470K. I greatly appreciate any advice...
> >>>
> >>>~M
> >>>
> >>>_________________________________________________________________
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> >>
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> >>
> >
> >--
> >uzs159 at uni-bonn.de
> >http://www.uni-bonn.de/~uzs159
> >
> >
> >
>
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