Hi, I did some "simple" measurements of this SE modified GAR BA2C with my limited equipment and compared it with various preamps, like solid state preamps from AMLs, Cap'is..., tube preamps like Redd47 Style, G9...
The secondary output TX winding of the BA2C with the UTM3590 "gapped" is "solidly" terminated with a 620 ohm resistor. I don't know any source impedance of the DAW... So dear pros, don't tear me to pieces alive...
The maximum gain I could get with a 10mV 1kHz sine wave (not a very accurate measurement!) fed from the DAW is 2.72VAC. That should give a gain of about 49dB. Not bad in my opinion...
Here is a frequency response comparison between this BA2C style pre and an AML EZ573 500 with pink noise, about 40 dB of gain on both preamps. The output shows about 1 VAC on average. The high end tends to roll off above 12 kHz with about 2.5 dB up to 20 kHz. I think it's not bad for a tube preamp.
Noise is not a problem. Very quiet on both preamps... measured with a gain setting of about 45 dB on both devices, upper Graph Noisefloor GAR BA2C Style Mod, lower Graph AML EZ573
To simplify the THD, I measured 1.6% THD with the 620 ohm termination at 0 dBu / 0.775 V output / about 45 dB of gain. And about 2.5% THD at the same 45 dB gain with +4 dBu / 1.23 V output, adjusted with a 1k sine wave from the DAW and measured on the output of the Preamp. Doesn't look too bad either. But I calculated the THD only the first two Harmonics (as example 1K Sinewavefeed shows -18dB, 2K Harmonic shows -54dB, and 4K Harmonics shows about -75dB at the DAW Spectrograph, with 0.775V / 0dB Preamp-output...)
The old RCA BA2C datasheet shows less than 2% THD at 2 dBm, which - if I've calculated correctly into a 620 ohm load - should be about 1 volt...
The THD is not far from what is stated in the old BA2C datasheet.
I am not sure if the UTM inductances with "25H" are really correct. But I cannot measure the inductance properly as my cheap LCR meter is not suitable for that. It even cannot measure correctly EF Cores for Coils.... The gapped UTM 3590 looks like it is working properly in my opinion. And so does it sound.
In the next post I will explain how I made this simple modification. Regards