[sdiy] SSM2018 VCA

[Tom Wiltshire]

Tim,

>> The control law is -30mV/dB, so 4V gives you -100dB, and -1.3V gives
>> you 40dB gain.
>
> ...but a quick look at these numbers, and the arithmetic doesn't  
> stack up: a
> CV range of 4-(-1.3) = 5.3V over 140dB = 5.3/140 = 0.038, i.e is  
> more like
> 38mV/dB, way off the quoted 30mV/dB.

Yes, I think you've hit the nail on the head. One place it says one  
thing, and somewhere else it says something else.

> - looking at the bottom of the left-hand column on page 9 we can  
> infer *3V*
> equates to -100dB, i.e. total range of 3-(-1.3)=4.3V for 140dB, or  
> 4.3/140 =
> 0.031V/dB, that looks more like it!

If we assume then that 3V gives -100dB, we can also assume 1.5V gives  
-50dB and 1V gives about -33dB.

Now, the problem I've got with my circuit is that I can't hear  
anything coming through at -50dB (I'd have expected to be able to  
hear *something*) and I need to reduce the CV to 1V to really start  
to hear the signal. Does this sound normal?

I think what I'll do is reduce the CV range to about 2.5V (easy to  
get to from my 0-5V input) and add an offset to use the CV from about  
2V to -0.5V. That should give me a decent "off" to "on" range, even  
if it is *nothing like* the standard AD numbers. We'll see!

Thanks,
Tom