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?


They got in through the grid circuit of the first tube as I explained in
the original post.

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.

Agreed on its own it is and I have no wish to labour the point.


Cheers

Ian

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.

Cheers

Ian

Indeed for a preamp which makes perhaps 1Vrms output, the change in Idc
is utterly negligible.

Patrick Turner.

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.

Nope. Star ground can mean many inconveniently long leads all running to
the same point.
Its OK in theory, but the OP killed the spikes with a 0.1uF across the
HT sec as I suggested.
The path of noise wasn't in the 0V rail or star point.

I often don't have a star point. I often have a 0V buss using 1.3mm
thick Cu solid wire
the length of the circuit layout, maybe 150mm long. Works fine.

Patrick Turner.


d

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

Patrick Turner wrote:

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?

It disappears as I mentioned in my original post so it clearly enters
into the first stage grid.


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.

snip

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


it is hard to tell but it looks to me more like about 0.75mV rms so I
would like some improvement.

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.

That is exactly where it will be.


Cheers

Ian

On Mon, 03 Dec 2007 14:07:52 GMT, Patrick Turner
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.

Nope. Star ground can mean many inconveniently long leads all running to
the same point.
Its OK in theory, but the OP killed the spikes with a 0.1uF across the
HT sec as I suggested.
The path of noise wasn't in the 0V rail or star point.

I often don't have a star point. I often have a 0V buss using 1.3mm
thick Cu solid wire
the length of the circuit layout, maybe 150mm long. Works fine.

Patrick Turner.

If you aren't too fussed about ultimate noise performance, then no
problem. When I was with Marconi Instruments, we made a doppler radar
speed meter for a train and the first iteration of the board failed to
read below about thirty mph, when theory said it should have been good
down to two. We had missed star-pointing every track around the input
by about two mm. That was all it took to allow massive 1/f ingress.

d

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

"Don Pearce" wrote in message

On Mon, 03 Dec 2007 14:07:52 GMT, Patrick Turner
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.

Nope. Star ground can mean many inconveniently long
leads all running to the same point.
Its OK in theory, but the OP killed the spikes with a
0.1uF across the HT sec as I suggested.
The path of noise wasn't in the 0V rail or star point.

I often don't have a star point. I often have a 0V buss
using 1.3mm thick Cu solid wire
the length of the circuit layout, maybe 150mm long.
Works fine.

Patrick Turner.

If you aren't too fussed about ultimate noise
performance, then no problem. When I was with Marconi
Instruments, we made a doppler radar speed meter for a
train and the first iteration of the board failed to read
below about thirty mph, when theory said it should have
been good down to two. We had missed star-pointing every
track around the input by about two mm. That was all it
took to allow massive 1/f ingress.

OK you got me. What does star-pointing every track around the input mean?

On Mon, 3 Dec 2007 12:11:44 -0500, "Arny Krueger"
wrote:

"Don Pearce" wrote in message

On Mon, 03 Dec 2007 14:07:52 GMT, Patrick Turner
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.

Nope. Star ground can mean many inconveniently long
leads all running to the same point.
Its OK in theory, but the OP killed the spikes with a
0.1uF across the HT sec as I suggested.
The path of noise wasn't in the 0V rail or star point.

I often don't have a star point. I often have a 0V buss
using 1.3mm thick Cu solid wire
the length of the circuit layout, maybe 150mm long.
Works fine.

Patrick Turner.

If you aren't too fussed about ultimate noise
performance, then no problem. When I was with Marconi
Instruments, we made a doppler radar speed meter for a
train and the first iteration of the board failed to read
below about thirty mph, when theory said it should have
been good down to two. We had missed star-pointing every
track around the input by about two mm. That was all it
took to allow massive 1/f ingress.

OK you got me. What does star-pointing every track around the input mean?


Sorry, I meant every ground track and every HT track with decoupling.

d

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