AW: 13600 (was VCA chips)

[jorgen.bergfors at idg.se]

Yes, the load resistor was the same and the gain (output amplitude) was also 
the same. For 10 V p-p input I got 4,4 V p-p output at full control voltage 
(both circuits).  The last bit surprised me as the input is different. You 
would assume that the version with 500 ohms through the pot to ground would 
have attenuated the signal more.
By the way, the "normal" circuit was the same as in the data sheets for the 
LM13600. I just downloaded them from the NS website. 
Regarding your second question: I'm not sure how to measure the diode 
impedance. Maybe I can measure the voltage drop across the different resistors 
and calculate it. I will go home now, and make some more experiments. The Audio 
Precision is promised to arrive tomorrow. Then I can measure the distortion.

Thank you for your support so far
/Jorgen





MIME:Juergen.Haible at nbgm.siemens.de on 98-09-03 13.55.35
To:  synth-diy at mailhost.bpa.nl @ SMTP ,  Jorgen Bergfors at IDGSE
cc:  
Subject: AW: 13600 (was VCA chips) 

Interesting results, and quite surprising. Well, maybe not that
surprising, as this Serge article is the one and only reference
on this I'm aware of. Might as well be that I have a blurred 
memory about this article - someone should dig it up, just
to be sure.

The version which I have built can be found at
http://www.synthfool.com/diy/hjmp2.gif
but honestly I don't remember the exact measurements results.
Note the the 1k normalisation at the input. As the input is DC
coupled, and the OTA input pins are not exactly at 0V, a varying
driving impedance will unbalance the VCA. Not the most
elegant circuit for sure, but it works for the purpose it was
intended.

Just a few questions about the measurements: Did you have
the same gain / the same load resistor in both cases ? Is the
diode impedance low, compared with 1k ?

JH.