Opinions on the CA3080 ???

[Harry Bissell]

I've been playing with the CA3080  (old RCA).  I tried it as a preamp
for a guitar pickup
(hex fuzz idea) with voltage variable gain. I do not care about
frequency response here.

What I found was that the unit was prone to oscillation (using the
inverting input) when the coil of the guitar had some series resistance
with it (guitar volume 1/2 way down... usually thats a 250K pot). I got
rid of most of the oscillation with a 20pF feedback cap.

The WIERD thing was that the unit would be dead silent (good) until I
got near (proximity or touching) the input. Then a normal 60Hz hum would
pass...

The thing was sitting on a grounded metal plane. If I disconnected the
ground plane, the hum stopped. Touch the input it starts again... touch
the ground plane (connect, disconnect, whatever...) almost like a toggle
function.

What I did find id that the Gm bias input seemed to be noise sensitive.
A ferrite bead at the Gm pin removed this "effect"

Conclusions: Don't use the 3080 as a preamp. A NE5532 worked a WHOLE lot
better.

But what do YOU think about that ferrite bead ???  Strange isn't it.
BTW I'm driving the Gm input from a trimpot (V+, V-) and a series
resistor to limit the current.

H^)   harry