Don Pearce wrote:
On Thu, 20 Dec 2007 15:06:33 +0000, Ian Thompson-Bell
wrote:

John Byrns wrote:
In article ,
"Alex" wrote:

If there is no negative feedback circuit in the amplifier, then there is no
reason to suspect coupling via power supply as a cause of the phenomenon.
Negative feedback is not necessary for oscillation to occur because of
coupling through the power supply, although this particular circuit may
not have enough stages to invoke the problem, I missed the schematic
though.

The basic schematic is he

http://i103.photobucket.com/albums/m...icpreschem.jpg

This is a screen shot from the circuit input to my spice simulator so it
does not show the input transformer, for example, nor the PSU itself
(but this is basically a bridge feeding a 470uF reservoir cap).

Cheers

Ian

Does your Spice simulation show the problem?

d


Nope. I even tried setting the op cap to a real type (Nicicon) and it
does not even show any leakage current so I don't expect the simulation
to be good enough to show this problem. Note the 200pF across the output
is to simulate 10 metres of 20pF/m cable because that is a known
possible cause of oscillation in CFs.

Cheers

ian