Half Hertz very low level oscillation?
 

"Ian Thompson-Bell" wrote in message
...
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).

Then even a crazier idea!
If you are located close to a strong TV station, you might have direct RF
rectification on the detector of your meter (RF induced on the leads of the
instrument) or even worse, on the non-linearities of your preamp, if the
input is not well shielded.

If it is the case then you might have infra-low beat between the crystal
stable 50.0000Hz frame sync of the TV station and the hum from the grid,
which might be fractions of a Herz off.

Regards,
Alex

And by the way, I am in the UK.

Cheers

Ian

Half Hertz very low level oscillation?
 

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

Half Hertz very low level oscillation?
 

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

--
Pearce Consulting
http://www.pearce.uk.com

Half Hertz very low level oscillation?
 

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

Incidentally, from the simulation point of view, you absolutely can't
represent a transformer and bridge rectifier by a voltage source.
Nothing could be further from reality, and you may need to be more
realistic if the problem is to show itself in simulation.

d

--
Pearce Consulting
http://www.pearce.uk.com

Half Hertz very low level oscillation?
 

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

Half Hertz very low level oscillation?
 

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

Half Hertz very low level oscillation?
 

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

Incidentally, from the simulation point of view, you absolutely can't
represent a transformer and bridge rectifier by a voltage source.
Nothing could be further from reality, and you may need to be more
realistic if the problem is to show itself in simulation.

d


I agree. I was not expecting the problem to show itself by simulation. I
just posted the simulation schematic because that was easiest.

Cheers

Ian

Half Hertz very low level oscillation?
 

"Alex" wrote in message


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.

Where did you get that idea?

If anything, negative feedback reduces the effects of coupling through the
power supply.