Keeping PSU noise out of inputs
 

Nick Gorham wrote:
Ian Thompson-Bell wrote:


Don't forget this is a mic pre not a power amp so current draw is less
than 10mA and as its class A it is also constant.


Just to pick up ona small point, the current draw of a class A single
ended stage is not constant, I think you may be thinking of a push pull
or differential stage.

Provided the stage has its HT decoupled (RC) across the operating
bandwidth then the current draw of a single ended class A stage is in
fact constant. I agree if you consider the stage in isolation of its
decoupling that it is not so, but I don't plan to build a preamp without
decoupling ;-) Even then, the average current is constant.


Ok, it can be constant if you have a infinate anode load or CCS, but
these are special cases.

I agree with a preamp the variation is going to be small, but it sounded
to me like you were thinking it was exactly constant.


See above.

Ian

Keeping PSU noise out of inputs
 

Ian Thompson-Bell wrote:
Nick Gorham wrote:

Ian Thompson-Bell wrote:


Don't forget this is a mic pre not a power amp so current draw is
less than 10mA and as its class A it is also constant.


Just to pick up ona small point, the current draw of a class A single
ended stage is not constant, I think you may be thinking of a push
pull or differential stage.


Provided the stage has its HT decoupled (RC) across the operating
bandwidth then the current draw of a single ended class A stage is in
fact constant. I agree if you consider the stage in isolation of its
decoupling that it is not so, but I don't plan to build a preamp without
decoupling ;-) Even then, the average current is constant.



Fine, please yourself, alter your definition of constant and where the
class A stage starts, and hey presto, you are right.

--
Nick

Keeping PSU noise out of inputs
 

Nick Gorham wrote:
Ian Thompson-Bell wrote:
Nick Gorham wrote:

Ian Thompson-Bell wrote:


Don't forget this is a mic pre not a power amp so current draw is
less than 10mA and as its class A it is also constant.


Just to pick up ona small point, the current draw of a class A
single ended stage is not constant, I think you may be thinking of a
push pull or differential stage.


Provided the stage has its HT decoupled (RC) across the operating
bandwidth then the current draw of a single ended class A stage is in
fact constant. I agree if you consider the stage in isolation of its
decoupling that it is not so, but I don't plan to build a preamp
without decoupling ;-) Even then, the average current is constant.



Fine, please yourself, alter your definition of constant and where the
class A stage starts, and hey presto, you are right.


Sorry to have upset you but this was in the context of the power supply
for the preamp. I did agree with you that the stage in isolation does
not draw a constant current. But looking from the power supply into the
preamp (with its decoupling) which was the context of the comment, it is
correct. I am not altering anything, just trying to be accurate. From
the point of view of PSU design, the current draw from a preamp is constant.

Cheers


Ian

Keeping PSU noise out of inputs
 

Ian Thompson-Bell wrote:
Nick Gorham wrote:

Ian Thompson-Bell wrote:

Nick Gorham wrote:

Ian Thompson-Bell wrote:


Don't forget this is a mic pre not a power amp so current draw is
less than 10mA and as its class A it is also constant.


Just to pick up ona small point, the current draw of a class A
single ended stage is not constant, I think you may be thinking of a
push pull or differential stage.



Provided the stage has its HT decoupled (RC) across the operating
bandwidth then the current draw of a single ended class A stage is in
fact constant. I agree if you consider the stage in isolation of its
decoupling that it is not so, but I don't plan to build a preamp
without decoupling ;-) Even then, the average current is constant.



Fine, please yourself, alter your definition of constant and where the
class A stage starts, and hey presto, you are right.


Sorry to have upset you but this was in the context of the power supply
for the preamp. I did agree with you that the stage in isolation does
not draw a constant current. But looking from the power supply into the
preamp (with its decoupling) which was the context of the comment, it is
correct. I am not altering anything, just trying to be accurate. From
the point of view of PSU design, the current draw from a preamp is
constant.

Assuming that the decoupling is done with zero ESR caps, and there is no
inductance in either the cap or resistor, both of which will be false.

Given the assumed wonderful decoupling, maybe its worth thinking how the
switching spikes got to the valve?

My point was in the context of your statement

"Don't forget this is a mic pre not a power amp so current draw is less
than 10mA and as its class A it is also constant."

That on its own is false. You could apply your logic to say a class B PP
stage has constant current use, as if you integrate the current over a
long enough period then the average will be a single value.


--
Nick

Keeping PSU noise out of inputs
 

On Mon, 03 Dec 2007 12:41:34 +0000, Nick Gorham
wrote:

Assuming that the decoupling is done with zero ESR caps, and there is no
inductance in either the cap or resistor, both of which will be false.

Given the assumed wonderful decoupling, maybe its worth thinking how the
switching spikes got to the valve?

Lack of star point grounding would be my number one candidate.

d

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

Keeping PSU noise out of inputs
 

Don Pearce wrote:
On Mon, 03 Dec 2007 12:41:34 +0000, Nick Gorham
wrote:


Assuming that the decoupling is done with zero ESR caps, and there is no
inductance in either the cap or resistor, both of which will be false.

Given the assumed wonderful decoupling, maybe its worth thinking how the
switching spikes got to the valve?


Lack of star point grounding would be my number one candidate.

d


Agreed, but would 100% decoupling not have removed that?

--
Nick

Keeping PSU noise out of inputs
 

On Mon, 03 Dec 2007 13:24:16 +0000, Nick Gorham
wrote:

Don Pearce wrote:
On Mon, 03 Dec 2007 12:41:34 +0000, Nick Gorham
wrote:


Assuming that the decoupling is done with zero ESR caps, and there is no
inductance in either the cap or resistor, both of which will be false.

Given the assumed wonderful decoupling, maybe its worth thinking how the
switching spikes got to the valve?


Lack of star point grounding would be my number one candidate.

d


Agreed, but would 100% decoupling not have removed that?

No. If the decoupling is feeding the spike current into a point which
is slightly elevated above true ground by a finite path length, it
will generate an EMF that becomes a noise signal. Remember decoupling
doesn't remove noise; it turns it from a voltage to a current.

d

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

Keeping PSU noise out of inputs
 

Don Pearce wrote:
On Mon, 03 Dec 2007 13:24:16 +0000, Nick Gorham
wrote:


Don Pearce wrote:

On Mon, 03 Dec 2007 12:41:34 +0000, Nick Gorham
wrote:



Assuming that the decoupling is done with zero ESR caps, and there is no
inductance in either the cap or resistor, both of which will be false.

Given the assumed wonderful decoupling, maybe its worth thinking how the
switching spikes got to the valve?


Lack of star point grounding would be my number one candidate.

d


Agreed, but would 100% decoupling not have removed that?


No. If the decoupling is feeding the spike current into a point which
is slightly elevated above true ground by a finite path length, it
will generate an EMF that becomes a noise signal. Remember decoupling
doesn't remove noise; it turns it from a voltage to a current.

d


True, but I suspect you see the point I was trying (and failing) to make.

--
Nick

Keeping PSU noise out of inputs
 

Ian Thompson-Bell wrote:

Patrick Turner wrote:

Ian Thompson-Bell wrote:
As I have mentioned I am designing a 6AU6 based mic preamp. Thanks to
Patrick et al I now have hum free HT rails. However, I still find some
50Hz hum and 10mS period PSU spikes in the output. By shorting the grid
of the first stage these both disappear so that is where they are
getting in. By turning off the HT they also both disappear so it seems
the source is the PSU. So the question is what should I do to stop it. I
tried grounding the mains transformer metal clamp and this helped a bit
but there is no connection of this transformer for a screen of any kind.
I am also using fast recovery diodes in the HT supply because I have
read they produce less spike interference. Any help appreciated.

For a not very good picture of the waveform at the preamp output look he

http://s103.photobucket.com/albums/m...6CFopnoise.jpg

Cheers

Ian

I am familiar with noise spikes in amplifier outputs.

If they dissappear when the input is shorted to ground when a shorted
RCA plug is
connected, the effect of the low impedance path to 0V shunts the high
impedance
input of the preamp which is sensitive to pick up of diode switching
pulses.

When the input is connected to a mic transformer or other proposed
source,
what is the noise problem like then?


It is as shown in the photo referred to i my original post i.e a bit of
50Hz with some 100Hz spikes on it.

Ok, you need to identify where the noise is being picked up.

With the input grid taken to the 0V rail with a short 25mm lead , what
then?

If noise persists, shunt the next stage's grid. What then?

By process of logic and analysis you can work it all out like we have to
do.



The amp input should have not more than say 2k7 series R from input to
the grid.

At present the transformer secondary is connected directly to the grid.

The cable from RCA active terminal should be well shielded to the grid
connection, and any series R should have short leads.
The power supply for a sensitive preamp should be all be within a steel
box mounted at least 400mm away from the
amp input, which means a chassis 500mm long is OK with about a 120mm
high x 200mm long x 150mm wide box will do
for the PSU.

It will eventually be fitted into a steel 2U high 19 inch rack mounting
case so it should be possible to build the PSU in a screened box this
far away from the inputs.

snip

Fast diodes don't reduce the spikes any more than plain old slow Si
diodes
such as 1N4007.


So why do I keep seeing people recommending them for tube supplies?

Mainly because the people making such recomendations are
grossly inexperienced and have designed nothing and built almost almost
nothing.
Good with a keyboard, that's all.

Don't believe ANYTHING YOU READ on the net fully until you prove its
true for yourself.




Spike pulses can be actually short time bursts of RF junk noise.

These look like decaying 50KHz sine waves

Sounds about right for power transformer resonances.

So anything that can act as an antenna can pick up the transmissions.
The steel box should provide enough magnetic and electrostatic shielding
to
prevent spikes. Often the means of spike production is due to the diode
switching
currents exciting the resonant circuits of the power transformers
consisting of
stray interwinding capacitances and leakage inductances. C = 400pF and
LL = 100mH
will resonate, and the noise may appear in the mains input leads and all
chassis and earth.

So 50KHz would be not an unreasonable grequency to see.

Exactly.

Using 0.05uF x 2kV rated cap betaeen active and neutral and between both
of them and the case lead
may help as well as across the HT secondary.


I tried a 0.1uF directly across the HT secondary and that seems to have
killed it stone dead.

Use the lowest value cap to kill the spikes; you don't want to load
the HT secondary unecessarily.

The capacitance added just moves the resonance to a lower F,
but one low enough to not be able to be "broadcast and received".

In any LC resonance network Fo will reduce by a factor 1/3.16 if C is
increased 10 times.
So increasing C by 100 times gives Fo 1/10.


You should always use a shielded and RF filtered IEC input plug.
However, I have found such pulses are often RF energy bursts at around
100kHz, indicatiing
C and L resonances. One should also have 0.01uF x 2kV ceramic caps
across each diode
used, which means 4 must be used on a bridge. Usually the boxed PSU
"contains" the pulses,
and using pulse filtration isn't needed.

I usually have large value electros on all the rails to 0V for the
preamp

Yes I have found a local supplier of reasonably priced 470uF 400V caps
which I am now using in the CRCRC filter.

including the dc heater supply. But the electros are maybe 300mm away
from the stages, with a wire carrying
the dc supply over to an anode or load resistance point, and from this
point I will have a 2uF plastic cap
to 0V so that the ESR and series L of the electros is well shunted at HF
and all done right near
where the supply is applied.

OK


If possible use some series R say 100 ohms between bridge output and
first filter caps.
Bypass C1 with say 0.47uF 630V plastics.
With 0.05 across the HT winding and the series R the RF pulses should be
well attenuated.

Treat the lower voltage rectifiers for heater supplies the same way if
possible.

Keep mains power leads well away from amp input terminals and mic leads
etc.

Patrick Turner.

Thanks for the input Patrick

The noise wave form shown has some LF hum. If its below 0.25mV with gain
up you are doing well.

But remember a following power amp with gain of 20x would make 0.25mV
become 5mV, and unacceptable.

Hence the gain control pot/attenuator should be placed between the last
preamp gain stage and the
output cathode follower. If the hum at full volume with a shorted input
grid to 0V
is less than 0.25 mV then all would be well because full volume will
never be used.
The gain control should be logarithmic and gains set up so that normal
listening
is achied at the 12 o'clock ( -20dB ) position.
Noise at the preamp output should then be no more than 0.025mV, and this
will become only 0.5mV at the power amp
output.

Patrick Turner.










Ian

Keeping PSU noise out of inputs
 

Ian Thompson-Bell wrote:

Nick Gorham wrote:
Ian Thompson-Bell wrote:


Don't forget this is a mic pre not a power amp so current draw is less
than 10mA and as its class A it is also constant.


Just to pick up ona small point, the current draw of a class A single
ended stage is not constant, I think you may be thinking of a push pull
or differential stage.

Provided the stage has its HT decoupled (RC) across the operating
bandwidth then the current draw of a single ended class A stage is in
fact constant. I agree if you consider the stage in isolation of its
decoupling that it is not so, but I don't plan to build a preamp without
decoupling ;-) Even then, the average current is constant.


Ok, it can be constant if you have a infinate anode load or CCS, but
these are special cases.

I agree with a preamp the variation is going to be small, but it sounded
to me like you were thinking it was exactly constant.


See above.

Ian

Class A output stages do have varying current draw.

Its a small variation, and the variation in Idc draw can be measured for
between idle and max clipping signal and the change in Idc can be used
to calculate
2H distortion. Its how the old guys did it.

But even with a power triode and with NFB to ensure the wave has less
than 1% THD at clip,
you can see 5% Ia change over the range of output signal voltage.
Certainly its seen with multigrids used as such or connected as triodes
or in UL.

The other mechanism which alters Idc draw id the varying mains voltage
and varying B+ voltage.

Both aspects mentioned are not of any concern providing adequate
filtering is done
and the supply FC filter resonant frequency is less than 5 Hz.

Patrick Turner.