[sdiy] 3080/13700 OTA question

[Tim Stinchcombe]

Hi Ian, Tim,

> Clearly an error on the drawing.  My guess would be that the 
> 10k and 2.2M 
> resistors are swapped.  this would give you envelope time up to a few 
> seconds and 1.5 mA of Iabc.

Intriguing! It always fascinates me to see different approaches to the same
problem, and it hadn't occurred to me to look at the time constants!

However, if Fig 4.21c is what it is meant to be used with, we see that the
attack and decay isn't generated from a single pulse per key press, but from
a PWM oscillator running at 400Hz, and so the much smaller time constant
resulting from the 10k res + 1u cap might actually be what is required?!

But another anomaly then arises: fig4.21c shows +/-12V supplies, whereas
figs 10.17/10.18 seem to imply +/-15V. Would be really nice to see the
original set of schematics from the magazine (June 76 Popular Electronics if
anyone has them...)!

Tim
__________________________________________________________
Tim Stinchcombe 

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