"Wiebe Cazemier" wrote in message
...
On Tuesday 11 April 2006 11:59, Wiebe Cazemier wrote:
These NTC's get a very low resistance once they are warm. In believe it
was
in the order of 0.02 Ohms or something. The only thing I don't understand
is,
that they don't feel warm, even when the amp has been on for a few
minutes.
And, when the entire casing of the amp has warmed up, therefore the NTC's
as
well, and I turn it off and back on immediatly, there is nothing that
indicates a high inrush current. Even my bench power supply has a higher
inrush current, because sometimes one of my computer resets when I turn
it on
(and sometimes even when I turn it off...)

This reasoning has me doubting a bit if the resistance really does get
very
low. Also because perhaps in most devices, a 600VA transformer normally
has a
higher minimal/standy current than what I'm using it for (the amp in idle
only pulls about 25 watts of the mains).

I'll see if I can measure the resitance when they're cold, and when the
amp
has been warmed up.

I have results. The NTC's are about 9 Ohms total when cold. And they don't
decrease by any usable amount when warm (well, as far as they get warm)...
On
to find a better solution...

About this. Ever since the beginning, there is a slight 50 Hz hum in the
output, almost inaudible, you have to put your head against the speaker to
hear it. There are no harmonics, it's just 50 Hz. Could this be caused by
these NTC's, that the supply ripple is getting to high? How much ripple
can an
amp with good common mode rejection ratio handle?

The filterbanks are 4x4700 µF, positive ripple is about 300 mV, negative
260.
Transformer is 25-0-25, so DC is about +35 and -35. That ripple does seem
quite high. The difference in ripple can be explained because the positive
rail is used to drive 4 relays.

The hum is not present when the amp is powered from my bench supply.



The NTC thermistors I've salvaged from a whole range of scrap PC monitors
have ranged from a few hundred Ohms to as high as 12k at room temperature,
the running resistance is usually 5 Ohms or less.