Just out of curiosity, are there any decent circuits for mic preamps that feed directly into the tube circuit?
I've seen some examples of transformerless outputs (WE circuits shown by
@emrr in one of his threads).
It would be super nice to have all tubes and no transformers. Just dreaming))
There are a few reasons. You need to supply phantom power. Noise may be an issue but less so.
I did both SE and BAL input MC Cartridge input Phonostages (that means max. input is ~ 2.5mV with 0.5mV @ 0dBVU) using only tubes. It does take selected low noise tubes like 6922, which are a LOT quieter than most 12AX7 etc.
They can be as much as 20dB quieter than 12AX7, if operated correctly (low anode voltage, high anode current).
Given that common Mic's average 10mV/94dB signal and usually do not get better than 20dB(A) self noise, with selected tubes it's fairly easy to match the noise.
But personally I would use a solid state "electronic transformer" instead. Selecting low noise tubes and finding good ones get's time consuming and expensive quickly. More expensive than transformers.
In effect make a "flying" Sziklai Pair Pre-Pre with (say) 14/20/26dB Gain and use 2N4403 as input (could also use low noise P-Channel J-Fet's, but not sure there is an advantage here) and nice N-Channel Mosfets for the second stage.
Use current mirrors against a negative voltage derived from the heater voltage and a resistor to ground as output and another current mirror/splitter (to ~ 55V) to bias the circuit.
Voila you have a balanced in, single ended out, direct coupled input / output "electronic transformer" that handles phantom power injection just fine and takes the place of Mic-Transformer in any tube Preamp.
It will sound cleaner than a transformer at high levels and probably not THAT different from a transformer at lower levels.
Next? How to make an se in, bal out step-down "electronic transformer" for the output...
Thor