Tube current and Noise
 

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

Tube current and Noise
 

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

--
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Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html

Tube current and Noise
 

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

Tube current and Noise
 

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

Tube current and Noise
 

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

Tube current and Noise
 

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

Tube current and Noise
 

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

Tube current and Noise
 

Don Pearce wrote:

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


I plan to. I had hoped some of the leading lights on this group would have
done this already. I find it hard to believe I am doing pioneering
experiments. Crikey, I might even publish the results on a web site ;-)

Ian

Tube current and Noise
 

Eeyore wrote:



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.


Er, surely it means there are more of them to create noise?

Ian

Graham

Tube current and Noise
 

On Sun, 17 Jun 2007 08:05:40 +0100, Ian Bell
wrote:

Eeyore wrote:



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.


Er, surely it means there are more of them to create noise?

Ian

Graham

What matters is never the absolute magnitude of the noise, but the
ratio of the signal to the noise. Signal, being coherent, add in a
"6dB per doubling" fashion, while noise, being random, adds in a "3dB
per doubling" fashion.

d

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