T.Henry VCA-1

Wobuse · Joined: Dec 07, 2011 Posts: 34 Location: Lyons,il

I put one of Thomas Henry's VCA-1's together using a PCB board I etched ,from Scott's site.
It seemed to be working fine till I took a closer look(and listen).[img]

Here is what my sine should look like .....

Ive tryed every thing I could think of ,The power voltages are in the right spots,Ive swapped chips,, attenated the inputs,,still the top of my sine is chopped off.
Any advice would be much appreciated.[/img][img][/img]

elmegil · Posted: Thu Nov 26, 2015 9:29 pm Post subject:

First suspect for this kind of circuit would always be the 3080.

You've swapped chips, but were they all from the same batch? Same supplier? They could all have the same fault; I had to throw out 16 3080's I got a long while back when they turned out to all be bogus.

The way I'd test is is another smaller simpler circuit: the Unity Gain Voltage Follower from page 3 of the CA3080 datasheet. In fact that was how I tested and found my problem. I fit it on a 2 x 3 piece of proto board from Radio Shack.

The other thing to look at is, what is the voltage at the output of the input summer? On Thomas' schematic, IC2a, pin 1. That will tell you whether your sine is OK or not by the time it gets to the 3080.

I assume you've adjusted the offset trimmer?

Does the waveform amplitude change at all as you turn the "Initial" pot? That would be useful to know; You say you've attenuated the inputs, but it's not clear to me what state that was in or whether the 3080 had any response to the control voltage coming through Q1.