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.

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.

Regards,
Alex

"John Byrns" wrote in message
...
In article ,
Ian Thompson-Bell wrote:

The 6AU6 based mic pre I am designing has an interesting quirk. I am
testing various valves for low noise with the first stage input grid
grounded. As expected different valves exhibit different noise levels
and susceptibility to hum. To eliminate hum from the readings my
millivoltmeter has an LF cut switch. When this filter is used I noticed
a small but regular variation in the indicated noise level. It varies by
about 10% over a regular 2 second period. We are talking very small
values here as the indicated noise is only a tenth of a millivolt. But
it is very regular and I have tried half a dozen different valves all
with the same result. So the question is what is it? Both stages of the
preamp are decoupled with 470uF caps via 10K resistors (time constant
4.7 seconds).

Any help appreciated.

Since you mention the power supply time constants, have you tried
changing them to see if the period of the "oscillation" changes, and
ruling that path in or out?


Regards,

John Byrns

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