In designing a semiconductor preamp you usually aim for a low collector
current in the first stage as one method of minimising noise.

What is the situation for tubes? RDH talks about triode input noise being
proportional to 1/gm but does not mention any dependency on anode current.
I would have expected shot noise to be pretty much proportional to anode
current.

Cheers

Ian

In article , Ian Bell
wrote:
In designing a semiconductor preamp you usually aim for a low collector
current in the first stage as one method of minimising noise.

Which "you" do you mean?... :-)

The noise level from a "semiconductor" preamp may be lower with a higher
current. Depends entirely on the device type, impedances, etc, etc.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html

Jim Lesurf wrote:

In article , Ian Bell
wrote:
In designing a semiconductor preamp you usually aim for a low collector
current in the first stage as one method of minimising noise.

Which "you" do you mean?... :-)

The noise level from a "semiconductor" preamp may be lower with a higher
current. Depends entirely on the device type, impedances, etc, etc.

Slainte,

Jim


Jim, I expected better of you. Sometimes I wish people would just read the
question and try to answer it rather than critique it. I have had several
responses to this post in rec.audio.tubes and not one has actually
addressed the question. I am looking for some guidance on low noise tube
preamp design and particularly the effect of anode current. Can you help?

Cheers

Ian

On Sat, 16 Jun 2007 16:47:03 +0100, Ian Bell
wrote:

Jim Lesurf wrote:

In article , Ian Bell
wrote:
In designing a semiconductor preamp you usually aim for a low collector
current in the first stage as one method of minimising noise.

Which "you" do you mean?... :-)

The noise level from a "semiconductor" preamp may be lower with a higher
current. Depends entirely on the device type, impedances, etc, etc.

Slainte,

Jim


Jim, I expected better of you. Sometimes I wish people would just read the
question and try to answer it rather than critique it. I have had several
responses to this post in rec.audio.tubes and not one has actually
addressed the question. I am looking for some guidance on low noise tube
preamp design and particularly the effect of anode current. Can you help?

Cheers

Ian

I really wouldn't be too upset, Ian. What you are asking is a bit like
"I want to build the tallest possible tower of people, and I want to
use midgets. Do you have any info?". Now while what you are doing is
obviously quite interesting, it frankly isn't what you do if you
actually want low noise. So accept that if you genuinely want low
noise, there are vastly easier and better ways of doing it.

There is a good reason for the lack of info about your proposed
method.

d

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

Don Pearce wrote:

On Sat, 16 Jun 2007 16:47:03 +0100, Ian Bell
wrote:

Jim Lesurf wrote:

In article , Ian Bell
wrote:
In designing a semiconductor preamp you usually aim for a low collector
current in the first stage as one method of minimising noise.

Which "you" do you mean?... :-)

The noise level from a "semiconductor" preamp may be lower with a higher
current. Depends entirely on the device type, impedances, etc, etc.

Slainte,

Jim


Jim, I expected better of you. Sometimes I wish people would just read the
question and try to answer it rather than critique it. I have had several
responses to this post in rec.audio.tubes and not one has actually
addressed the question. I am looking for some guidance on low noise tube
preamp design and particularly the effect of anode current. Can you help?

Cheers

Ian

I really wouldn't be too upset, Ian. What you are asking is a bit like
"I want to build the tallest possible tower of people, and I want to
use midgets. Do you have any info?". Now while what you are doing is
obviously quite interesting, it frankly isn't what you do if you
actually want low noise. So accept that if you genuinely want low
noise, there are vastly easier and better ways of doing it.


I have no problem with that. However, what I *want* to do is design and
build a tube mic pre. I would like it to be as noise free as possible but I
accept that will not be the lowest noise that can be achieved by using
other means. It does not need to be the ultimate in low noise design but I
don't want it to be any noisier than it has to be.

A fairly standard approach is to use a step up transformer followed by a
couple of tube stages of amplification and possibly an output transformer.
In principle, noise in the first tube stage will not be a significant
contributor to overall noise if the equivalent input noise resistance of
the tube input Req is a half or preferably one quarter the resistance of
the source reflected into the secondary of the transformer (see RDH).
According to RDH, you can express the shot noise of a triode in terms of an
equivalent noise resistance connected between grid and cathode of a
noiseless triode. For triodes this resistance is approximately 2.5/gm at
the operating point. So apart from the possible variation of gm with anode
current, the noise added by a triode seems to to fairly independent of
anode current. This seems strange to me since shot noise is caused by
random fluctuations in anode current so I would expect higher anode
currents to produce higher noise. Indeed, RDH notes that shot noise is
produced in the anode circuit and refers it to the more convenient grid
simply by noting that the incremental grid voltage and anode current are
related by gm so that Eg = Ia/gm. The leap from this to Req =2.5/gm for a
triode is not explained.

There is a good reason for the lack of info about your proposed
method.


I had hoped some of the contributors to this group would have a greater
depth of knowledge and would have helped me determine the optimum first
stage anode current.

Ian

On Sat, 16 Jun 2007 21:59:54 +0100, Ian Bell
wrote:


I had hoped some of the contributors to this group would have a greater
depth of knowledge and would have helped me determine the optimum first
stage anode current.

Ian

Well, this is something you can quite easily measure for yourself,
then let us know what you find. Put a pot in place of the cathode
resistor (or at least part of it), bypass it with a good fat cap, then
tweak and measure the noise, current and gain.

d

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

Ian Bell wrote:

So apart from the possible variation of gm with anode
current, the noise added by a triode seems to to fairly independent of
anode current. This seems strange to me since shot noise is caused by
random fluctuations in anode current so I would expect higher anode
currents to produce higher noise.

Not because of the shot effect for sure.

A higher anode current means the relative size of the unit electronic charge is
actually *smaller* in influence.

Graham

In article , Ian Bell
wrote:
Don Pearce wrote:




I have no problem with that. However, what I *want* to do is design and
build a tube mic pre. I would like it to be as noise free as possible
but I accept that will not be the lowest noise that can be achieved by
using other means. It does not need to be the ultimate in low noise
design but I don't want it to be any noisier than it has to be.

A fairly standard approach is to use a step up transformer followed by a
couple of tube stages of amplification and possibly an output
transformer.

What you really need to do is to build a valve stage and measure its In and
En values. i.e. measure the output noise with a short and then an o/c (or
large resistor value) at its input. From this you can work out the source
impedance that will give optimum SNR. Knowing the actual source impedance
for your microphones this then allows you to decide the turns ratio of the
transformer.

With a short on the input the noise only comes from En. With an open it
comes from both En and In through the effective input/source impedances in
parallel. The ratio of the two gives you the source/input resistance in
parallel that provides maximum SNR.

If you wish, do the above for different bias resistor values, etc. If you
were using a solid state device I could advise on that. But afraid I don't
have the experience with valves, so you'll need advice from someone who
has. Ideally someone who understands the engineering rather than someone
who tweaks but never measures or analyses. :-)


In principle, noise in the first tube stage will not be a significant
contributor to overall noise if the equivalent input noise resistance of
the tube input Req is a half or preferably one quarter the resistance of
the source reflected into the secondary of the transformer (see RDH).


I'd suggest you use the above approach is it will tell you the actual
optimum rather than giving you a vague 'rule of thumb'. :-)

According to RDH, you can express the shot noise of a triode in terms of
an equivalent noise resistance connected between grid and cathode of a
noiseless triode. For triodes this resistance is approximately 2.5/gm at
the operating point. So apart from the possible variation of gm with
anode current, the noise added by a triode seems to to fairly
independent of anode current. This seems strange to me since shot noise
is caused by random fluctuations in anode current so I would expect
higher anode currents to produce higher noise. Indeed, RDH notes that
shot noise is produced in the anode circuit and refers it to the more
convenient grid simply by noting that the incremental grid voltage and
anode current are related by gm so that Eg = Ia/gm. The leap from this
to Req =2.5/gm for a triode is not explained.

Comment as above. In this context I'd avoid modelling the noise in terms of
a 'resistance' as the In and En approach is easier to measure and use.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html

In article , Ian Bell
wrote:
Jim Lesurf wrote:

In article , Ian Bell
wrote:
In designing a semiconductor preamp you usually aim for a low
collector current in the first stage as one method of minimising
noise.

Which "you" do you mean?... :-)

The noise level from a "semiconductor" preamp may be lower with a
higher current. Depends entirely on the device type, impedances, etc,
etc.

Slainte,

Jim


Jim, I expected better of you. Sometimes I wish people would just read
the question and try to answer it rather than critique it.

I was not doing a "critique" on your question. Just pointing out that what
you'd said above - which was not your question - was incorrect. :-)


I have had several responses to this post in rec.audio.tubes and not one
has actually addressed the question. I am looking for some guidance on
low noise tube preamp design and particularly the effect of anode
current. Can you help?

Not really. So far as I know if you require low noise then the sensible
approach is to use solid-state devices designed for such purposes. I'd
expect the noise level of a valve to be higher if only because of the
higher levels of thermal noise. So I'm afraid I've never even tried
designing a low noise valve amplifier myself.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html