Input is BJT biased at about 0.6V/6K8= 0.073mA.
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1) The emitter impedance is about 28Ω/0.073 or about 380Ω. This acts in series with the source. We want to transform the source to much higher than 380Ω.
2) Assuming Hfe is 100, ignoring feedback and bias, the shunt impedance is 38K, and we want to transform the source to much less than 38K.
In fact we have another 40K of shunt resistance. So the source should be transformed to much less than 38K||40K= 20K.
Some of your shunt feedback resistors (which act in series with the source) are much higher than 380Ω, but the highest-gain settings are very low Ω and I will ignore this for a moment.
Simply taking a geometric mean of 380Ω and 20KΩ suggests best noise performance around 2,750Ω. This does assume Hfe=100, and will shift a little with a large shift in Hfe.
What is the data-sheet claim for the actual NTE159 you propose? And since the NTE159 sheet is lean, what is the plot for any similar transistor? (I picked 2N3904 at random.)
The one-line noise spec for NTE159 happens to be very similar to the proposed operating condition. We don't know if this is the "perfect" condition, or just a handy test-jig, but it is reasonably low and probably good. Taking for granted that 1KΩ is "good", then we want to transform 150Ω to 1KΩ. We want a 1:2.58 transformer. Since the input impedance is really about 38.4K, the mike will see about 5.76K (in parallel with any Phantom resistors). This is much-much more than 150Ω so the mike will be lightly-loaded and we will see full output voltage.
How bad do things get if, instead of 0.1mA we run at 0.073mA, or can't find a 1:2.58 transformer? The 2N3904 data won't be exact-right, but the trends will be similar.
Indeed 0.1mA does seem to be as good as it gets, worsening slowly at other currents, and requiring a different value of source resistance for best noise performance. 0.073mA may be imperceptably worse than 0.1mA, but I doubt you can prove it on the bench, and this bias scheme favors low current in the input device.
For the 3904 working at 0.1mA, it seems that the best source impedance is actually 600Ω. For 0.050mA, about 1200Ω. So for 0.073mA we might guess 850Ω. Now we come up with a 1:2.38 transformer. BUT we can go as far as 250Ω or 2500Ω with a barely perceptable 1dB rise in noise figure. So transformers from 1:1.3 to 1:4.1 will all give "the same" noise. (Very different gain, but gain is cheap, which is why we hardly-ever want to match signal impedances.)
I have here a 150Ω:600Ω 1:2 transformer. After verifying that the unloaded input impedance really is above 150Ω at the lowest frequency of interest, I can use it and expect the noise to be within a few tenths of a dB of the very best I can get. This is so very close to optimum that we should consider transformer losses. A typical 150:600 tranny will add about a dB of resistance noise. I also have some 2K:10K trannies, a 1:2.24 ratio. which is "even closer" to the "ideal 1:2.38 ratio". But these have winding resistance in the hundreds of ohms, which will badly contaminate a 150Ω source with noise.
Look at the "obvious" 150Ω:38KΩ transformation. At 0.073mA the transistor noise figire is 14dB, awful bad. The voltage ratio is 1:16, so we get lots of cheap gain. Except the heavy load will drop the mike voltage to half of its rating: we really only get 1:8 of voltage gain. While this is 4:1 better than the 1:2 transformer, the noise figure sucks. Gain is cheap. Find another 1:4 of gain in the tubes/transistors (you probably have a ton of excess gain already).
I have ignored your feedback network. In the high-gain position, 82Ω is pretty negligible compared to the 850Ω optimum. But 910Ω is not negligible, and noise will rise. This may not matter much. If it does, if you have excess hiss at medium and low gains, you should reduce the 18K resistor (maybe as low as 2K) and scale the gain-set resistors accordingly (which will force the 220uFd much larger).
_*_ Screw-up! I missed the 68Ω resistor in the output emitter. So the input device current is more like 0.150mA, isn't it? Argh, I'm such a dumdum. No great matter: the noise curves of a high-Hfe BJT are so flat around optimum that even a 2:1 shift of bias current hardly affects the noise. You might prefer to aim for 300Ω source impedance for best result on paper. And it might be wisest to stick with readily available 1:2 150Ω:600Ω transformers because noise will not rise enough to measure. (also I suspect the 3904 curves are pessimistic about Hfe and leakage; that for a typical device the bottom of the noise curve runs to somewhat higher impedance with slightly lower noise at the bottom.)