[sdiy] 3080/13700 OTA question

[Tim Stinchcombe]

Hi Tim,

> I tried to implement the 3080 VCA from Lancaster's Music 
> Modules as seen in the CMOS Cookbook. It has a simple 
> envelope generator that idles at the negative rail and rises 
> to 0 volts for full on. A resistor divider brings the voltage 
> range down to about 2.8 volts above the negative rail.

It won't quite work like that - pin 5 voltage will be 'clamped' by the diode
at the Iabc input, so will be around 0.7V or so (double this for a LM13700,
as it has two diodes there). Still, that means the max current is of the
order of 15-0.7/2.2meg = 6.5uA, hardly very much. (I assume the 'pull down'
680k to the negative rail is simply to ensure that the VCA switches fully
off.)

> What am I missing here? 

Not much I don't think, but it looks as though the entire circuit is working
with mV-sized signals: there are no resistive dividers on the input of the
3080, and we are told the 'bias' side is approx 45mV, and the input from the
divider circuit is 'max 90mV' (indeed at Fig 6-2 we again see some large
voltage division going on). So lets say the differential input to the CA3080
is 45mV, and take into account we have 6 of them feeding the same 22k at the
output, then the max voltage there is of the order:

   .045*19.2*0.0000065*22000*6 = 0.75V.

Unfortunately there doesn't seem to be any detail for the '3080 combiner' in
Fig 10-18, so it is not clear how this voltage would be used exactly, but it
may not be 'out of sorts' with the entire circuit.

My conclusion: too much missing to be able to say if the component values
given are OK; however the general idea of the cap charge/discharge for
generating an envelope looks sound enough, bearing in mind my comment about
the diode drop above, so if you were to decrease the 2.2meg to get a
healthier current into the 3080, I see no reason why it wouldn't work! (33k
would give Iabc max at about 400uA, though of course this will now affect
the charge/discharge rates to a larger extent.)

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk