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.

Cheers

Ian

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

--
Surf my web pages at, http://fmamradios.com/

Ian Thompson-Bell:

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).

At a wild guess the noise must track a similar periodic
variation in gain or current somewhere. Only way I can think
of that happening is if the amp is oscillating.

First thing I would do is measure the electrode voltages of
all the valves, looking for some periodic change that the
noise tracks.

Same circuit you showed us before?

Any help appreciated.

I'd rather discuss.

cheers, Ian

John Byrns wrote:
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?



Not yet but that is probably my next test.

Ian

Ian Iveson wrote:
Ian Thompson-Bell:

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).

At a wild guess the noise must track a similar periodic
variation in gain or current somewhere. Only way I can think
of that happening is if the amp is oscillating.


I was wondering that too. It is also noisier than I expected. Separate
measurements on the input stage gave it a 50uV noise. This is connected
to a CF and the noise at the output is 100uV - respectable but more than
I expected - assuming noise powers add this implies the CF is producing
86uF of the noise itself.

First thing I would do is measure the electrode voltages of
all the valves, looking for some periodic change that the
noise tracks.

That's the odd thing. I can see this variation on my VTVM but not on my
scope connected to the same point.


Same circuit you showed us before?


Pretty much - regular CC stage RC coupled to CF - no rocket science.

heers

Ian

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

--
Surf my web pages at, http://fmamradios.com/

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

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.

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.

Something of this sort could easily cause the problem.

Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

"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?

S.


--
http://audiopages.googlepages.com

"Ian Thompson-Bell" wrote in message
...
Good idea but not the cause I think. I am using a purely analogue meter
(Ferrograph RTS2).

Well, the phenomenon remains unexplained -- a mystery.


And by the way, I am in the UK.

Ooops! I did not look at your e-mail address.
And I am in Australia.
Regards,
Alex

Cheers

Ian