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 Forum index » DIY Hardware and Software
CA3080 VCA design
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funkyfarm



Joined: Jan 21, 2007
Posts: 583
Location: France
Audio files: 3

PostPosted: Fri Jan 16, 2009 7:33 am    Post subject: CA3080 VCA design
Subject description: audio signal to +input or -input ?
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Hi,

I've once build the AMS-100 VCA/VCD, which is a simple CA3080 based design (maybe the simplest ?).

ARP ODYSSEY VCA is a *very* similar design, based around 3080 too.

Main differences :
output buffer (transistor vs modern IC)
input resistor (100k vs 1M ?)
R12 is the AMS-100 vca gain resistor (no trimmer here)

But what seems strange to me (a real neophyte) is input configuration of OTA :

ARP input signal goes to PIN 2 (-input)
AMS-100 input signal goes to PIN 3 (+input) of IC1. (main vca)


Saint Eric seems to modify Oddyssey here in Europe.
http://www.sainteric.eu/?q=node/15
He is working on a "new improved VCA".

Assuming their new design will still be 3080-based, I wonder if they should change the VCA input impedance/resistor or if they'd better swap pin 2 and pin 3 of CA3080 OTA Very Happy

(AMS-100 VCA is way cool, quiet , with no click and all...)


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yusynth



Joined: Nov 24, 2005
Posts: 1314
Location: France

PostPosted: Fri Jan 16, 2009 8:26 am    Post subject: Reply with quote  Mark this post and the followings unread

The value of the input resistor is mostly linked to the maximum signal level that must be handled without distorsion. Therefore if the VCA are implemented in different designs the value of the input resistor is determined by the output level of the VCOs or of the VCFs...

The fact one uses the inverting or non-inverting input just defines if the VCA operates with a signal inversion (180° phase shift) or without (altough you must consider what comes after the CA3080 too). For audio signals this is not really relevant , it is more important to consider if the VCA is to be used for controlling CVs. In such a case a non-inverting VCA is preferred.

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