[sdiy] VCA distortion - bad

[Michael Ruberto]

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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>>
>>
>>
>>
>
>--
>uzs159 at uni-bonn.de
>http://www.uni-bonn.de/~uzs159
>
>
>

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