"Phil Allison" wrote in message
...

"Jim Lesurf"

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.


** Shame it has got almost nothing to do with that.

It is only possible to do so much with a STEP UP input transformer
before serious problems arise with frequency response. A step up
voltage ratio of 1:10 or 1:15 is the limit where flat response over
the whole audio band is required.
A step up ratio no more than 1:5 is the norm for studio grade
equipment.

Attempting to go for high ratios ( ie for better s/n) results in the
HF response becoming a function of the mic's actual impedance in an
alarming way as the optimum output side RC loading network becomes
very source impedance dependant.

EG: With a low mic impedance, say 50 to 100 ohms, HF response will
peak at 14kHz by many dBs. With a higher mic impedance, say 600 ohms,
response droops in the upper audio band by many dBs. Only with a 200
to 250 ohm mic will flat response be maintained, +/- 1 dB, as
intended.

Using a step up ratio of 1:10 means the input noise voltage of the
valve is effectively diminished by that same factor - making EIN
figures of circa 0.3 uV ( rel 200 ohms) possible with valves like the
12AX7. This compares fairly well with good SS designs.

The wise engineer knows to use high output condenser mics with valve
input stages for a top class result and to use dynamics only where the
SPL is high enough to overcome system noise.


Interesting....

(It's a pity this guy's *condition* means most of his posts are like
they are - perhaps it would be worth making the necessary allowances to
see what he has to say...??)