"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.



........ Phil