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

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