Serge Auckland wrote:
"Ian Thompson-Bell" wrote in message
...
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.

Nothing global but there is an unbypassed cathode resistor in the first
stage and of course the CF has 100% feedback.

I do not know how you are measuring the noise, but if you are using a
multimeter, it is possible to have some beat frequency between the 60Hz
grid
and the harmonics of the switching cycle of the dual slope ADC which the
multimeter is based on. If you have a residual 60Hz ( I am assuming you
are
in the USA) on the preamp output and if the ADC in the multimeter is not
absolutely synchronised with the grid (which is not), then you will have
sort of "product detector" delivering the beat frequency.

Good idea but not the cause I think. I am using a purely analogue meter
(Ferrograph RTS2).

And by the way, I am in the UK.

Cheers

Ian

Ah Ha! My RTS2 does much the same on its lower voltage (greatest
sensitivity) setting. Even with the input shorted, the needle fluctuates by
about +- 10% full scale. I have changed all the electrolytic caps, and
cleaned every contact, but the effect's the same. I use my Levell meter for
low-noise measurements but I suppose I should sort out the RTS2 sometime.
Have you checked whether the RTS2 needle still moves with the input shorted?


I just tried that and it sits rock solid on the -60dB range at 20uV and
that's just pick up from the unscreened part of the probe.

However, I just reconnected my scope and switched it to dc (this is on
the 1mV/division range) and the output noise sits on a couple of mV
positive dc. I am just wondering if this is leakage from the output
electrolytic. The load is a 10K resistor so 2mV represents about 20uA if
my calculations are correct. I am just going to try another one and also
a 0.47uF non electrolytic to see if there is any difference.

Well, the second electrolytic, same make (Jamicon) same value (4.7uF,
450V) is even worse - it sits at +5mV.

Now tried with 0.47uF non electrolytic and that seems to have pretty
much cured it. There is still a very small movement on the Ferrograph
meter but it is hard to tell if it is regular or just a bit of very low
frequency shot noise. And the scope shows the noise straddling 0V as it
should.

So now all I need to do is decide how to get a large enough output non
electrolytic coupling cap to avoid losing bass response.

Thanks for all the input.

Cheers

ian