"Serge Auckland" wrote in message ...
"Fred" wrote in message ...

"Ian Thompson-Bell" wrote in message ...
West wrote:
"Ian Thompson-Bell" wrote in message
...
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 would suspect the spikes are from switching transients from those fast
recovery diodes (are they hex-freds?). Try a snubbing cap across each diode
and I'll bet you a dollar to a cup of coffee that you will see a difference.
Keeping my fingers crossed for you.

west



I tried a 0.1uF straight across the HT transformer secondary and that did the trick.

Ian

A few years ago someone here asked if bypassing fast recovery rectifiers like
hexfreds was necessary. Nelson Pass surprised everyone when he dropped in
and responded with the advice to snub them with a .05 cap in series with a 100
ohm 1/2W resistor. It just makes things even quieter, in a way that can only be
appreciated when heard.

In a mic preamp, every possible source of noise needs to be attacked. That's a
big part of what differentiates the $1000 per channel professional mic pre's from
everything else. They don't always measure all that much better, but you can
hear the difference in a heartbeat.

Fred

Sorry Fred, but if they sound better, then you should be able to measure what the difference is. If you can't, then they don't.

S.

You can sometimes measure some differences with the right test equipment
(Audio Precision or a computer with the right (expensive) hardware & software),
but not necessarily with your run of the mill THD+noise measurements. The
spectrum of the distortion and the noise is more different than the amount,
so which spectrum is going to sound better? Measurements won't tell you
that.

But your ears will tell you, like I said, in a heartbeat.

Check with the guys over at RAP and see if they use measurements to tell
the difference between a great mic pre and a pretty good one (they don't; they
use their ears, just like you and I do).

Fred

You can sometimes measure some differences with the right test equipment
(Audio Precision or a computer with the right (expensive) hardware & software),
but not necessarily with your run of the mill THD+noise measurements. The
spectrum of the distortion and the noise is more different than the amount,
so which spectrum is going to sound better? Measurements won't tell you
that.

But your ears will tell you, like I said, in a heartbeat.

Some amazing lug 'oles some have then!...

Check with the guys over at RAP and see if they use measurements to tell
the difference between a great mic pre and a pretty good one

That can mean different things to different people...

--
Tony Sayer

"Fred" wrote in message
...

"Serge Auckland" wrote in message
...
"Fred" wrote in message
...

"Ian Thompson-Bell" wrote in message
...
West wrote:
"Ian Thompson-Bell" wrote in message
...
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 would suspect the spikes are from switching transients from those
fast
recovery diodes (are they hex-freds?). Try a snubbing cap across each
diode
and I'll bet you a dollar to a cup of coffee that you will see a
difference.
Keeping my fingers crossed for you.

west



I tried a 0.1uF straight across the HT transformer secondary and that
did the trick.

Ian

A few years ago someone here asked if bypassing fast recovery rectifiers
like
hexfreds was necessary. Nelson Pass surprised everyone when he dropped
in
and responded with the advice to snub them with a .05 cap in series with
a 100
ohm 1/2W resistor. It just makes things even quieter, in a way that can
only be
appreciated when heard.

In a mic preamp, every possible source of noise needs to be attacked.
That's a
big part of what differentiates the $1000 per channel professional mic
pre's from
everything else. They don't always measure all that much better, but
you can
hear the difference in a heartbeat.

Fred

Sorry Fred, but if they sound better, then you should be able to measure
what the difference is. If you can't, then they don't.

S.

You can sometimes measure some differences with the right test equipment
(Audio Precision or a computer with the right (expensive) hardware &
software),
but not necessarily with your run of the mill THD+noise measurements. The
spectrum of the distortion and the noise is more different than the
amount,
so which spectrum is going to sound better? Measurements won't tell you
that.

But your ears will tell you, like I said, in a heartbeat.

Check with the guys over at RAP and see if they use measurements to tell
the difference between a great mic pre and a pretty good one (they don't;
they
use their ears, just like you and I do).

Fred

You may Fred, I prefer to put my trust in Audio Precision.

S.


--
http://audiopages.googlepages.com

adding a bypass cap works up to a point. you seem to have removed all the
ripple a cap could remove.

From the waveform, it sounds like a bad isolation issue: adding caps will
help but you'll need much more than nanofarads to get that slow ripple out.
Use bigger caps if you want to keep it to simple cap patching (something
like 1100uF or 2200uF is nice.

Personally, I prefer the capacitor array method to the simple addition of
single poles.

Power supply would go as such:

XFO - DIODE BRIDGE(tube rect for purists..)- Pi Filter (Shunt cap +choke +
shunt cap)

At equal main's frequency attenuation, the tradeoff is the filter's
inductance vs instant current draw. I usually lift off that limit by using
bigger chokes and a few (BIG) tank capacitors right after the pi filter (I
like arrays of 1100uF,2x2200u,3300u...you can make a nice little square with
1-sided pcb routing if you put 2 together...little footprint, nice instant
draw compensation).

You got a schematic somewhere of that PSU? Tweaking the pi filter should get
rid of the problem

GLM

"Serge Auckland" wrote in message
...
"Fred" wrote in message
...

"Ian Thompson-Bell" wrote in message
...
West wrote:
"Ian Thompson-Bell" wrote in message
...
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 would suspect the spikes are from switching transients from those
fast
recovery diodes (are they hex-freds?). Try a snubbing cap across each
diode
and I'll bet you a dollar to a cup of coffee that you will see a
difference.
Keeping my fingers crossed for you.

west



I tried a 0.1uF straight across the HT transformer secondary and that
did the trick.

Ian

A few years ago someone here asked if bypassing fast recovery rectifiers
like
hexfreds was necessary. Nelson Pass surprised everyone when he dropped
in
and responded with the advice to snub them with a .05 cap in series with
a 100
ohm 1/2W resistor. It just makes things even quieter, in a way that can
only be
appreciated when heard.

In a mic preamp, every possible source of noise needs to be attacked.
That's a
big part of what differentiates the $1000 per channel professional mic
pre's from
everything else. They don't always measure all that much better, but you
can
hear the difference in a heartbeat.

Fred
Sorry Fred, but if they sound better, then you should be able to measure
what the difference is. If you can't, then they don't.

S.



--
http://audiopages.googlepages.com

In article , GLM labbe-
scribeth thus
adding a bypass cap works up to a point. you seem to have removed all the
ripple a cap could remove.

From the waveform, it sounds like a bad isolation issue: adding caps will
help but you'll need much more than nanofarads to get that slow ripple out.
Use bigger caps if you want to keep it to simple cap patching (something
like 1100uF or 2200uF is nice.

Personally, I prefer the capacitor array method to the simple addition of
single poles.

Power supply would go as such:

XFO - DIODE BRIDGE(tube rect for purists..)- Pi Filter (Shunt cap +choke +
shunt cap)

At equal main's frequency attenuation, the tradeoff is the filter's
inductance vs instant current draw. I usually lift off that limit by using
bigger chokes and a few (BIG) tank capacitors right after the pi filter (I
like arrays of 1100uF,2x2200u,3300u...you can make a nice little square with
1-sided pcb routing if you put 2 together...little footprint, nice instant
draw compensation).

You got a schematic somewhere of that PSU? Tweaking the pi filter should get
rid of the problem

GLM

You can get high voltage switch mode power supplies these days
y'know;-)..
--
Tony Sayer

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?

The amp input should have not more than say 2k7 series R from input 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. See a sample at
http://www.turneraudio.com.au/preamp...ated-2006.html

The most sensitive inputs are at the rear of the amp and as far away as
possible from the PSU.
Noise is no problem, despite the phono stage having suitability for MC
at 0.3mV input.
But I have a 1k load match resistance with 0.1uF across it to shunt
noise and reduce cartridge distortion content
from the MC. The MC itself is less than 20 ohms R so when in use the
gain may be turned up
to a much higher level than will actually be used before any hiss of hum
is heard.

The noise is low even without the MC cart connected, because what is
heard at high gain is mainly the
sound of the noise of the 1k resistance, a fraction of a uV.

If I had a 47k across the input without a cartridge connected, noise
would be about 7 times greater
since noise varies at the square root of the series R connected.
But an MM cart would shunt the 47k, and reduce the noise,
and anyway the MM has maybe 20 times the MC signal so noise isn't going
to be a problem.

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

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

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
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.

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.

Ian Thompson-Bell wrote:

Nick Gorham 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

Maybe some snubbers or LCR filtering on the secondaries.


Yes, I was thinking maybe small series inductors for each diode - is
that the sort of thing?

I tried the series L and it made no difference.

Patrick Turner.

Are you using DC heaters? you may find the spikes are from the heater
supply


Definitely not heaters - at present they are supplied by a battery.

Ian

GLM wrote:
adding a bypass cap works up to a point. you seem to have removed all
the ripple a cap could remove.

From the waveform, it sounds like a bad isolation issue: adding caps
will help but you'll need much more than nanofarads to get that slow
ripple out. Use bigger caps if you want to keep it to simple cap
patching (something like 1100uF or 2200uF is nice.


The slow ripple is definitely not via the HT - one a scope after the RC
filters it is as quiet as a mouse.

Personally, I prefer the capacitor array method to the simple addition
of single poles.

Power supply would go as such:

XFO - DIODE BRIDGE(tube rect for purists..)- Pi Filter (Shunt cap
+choke + shunt cap)

At equal main's frequency attenuation, the tradeoff is the filter's
inductance vs instant current draw. I usually lift off that limit by
using bigger chokes and a few (BIG) tank capacitors right after the pi
filter (I like arrays of 1100uF,2x2200u,3300u...you can make a nice
little square with 1-sided pcb routing if you put 2 together...little
footprint, nice instant draw compensation).


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.


Cheers

Ian

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.

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?

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

These look like decaying 50KHz sine waves

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.

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.

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

Ian

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.

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.

--
Nick