Violinist said:
and of course some question is rising, because if APEX205 and RB500 are identical, I suppose that the issue of one is automatically the issue of the second one.
I am not sure they are identical. Apparently there are several variations, and one "badge-engineer" might want a different set of compromises than an other.
I'm referring to the "hiss" described in the article and specifically at the phrase "but they're fairly noisy mics".
Did you get same issue with?
Definitely, if used without any precaution on a standard preamp. The worst match I had was with a GT "The Brick", which has an actual input impedance of 500r; in that case, the mismatch between the 2500r source impedance and the 500r input Z creates a loss of 15dB, which impacts directly the S/N ratio.
I have also some technical doubt, related to the issue above: How can be generated a so strong "hiss" by a passive circuit that hasn't any other component than a transformer and a "quite common" motor?
This question needs to be answered in two parts:
First, a resistor has intrinsic noise, because it is at room temperature, which agitates the electrons. This noise has been measured by Johnson and quantified by Nyquist. Noise is proportional to the sq root of the resistance, the sq root of the BW and the sq root of the temperature (in °K i.e. room temperature is 300°K).
A 200r resistor at room temperature produces a noise voltage of -129.7dBu in a 20Hz-20kHz BW. My RB500, with its 2500r impedance (which is mainly resistive except for a slight hump at about 200Hz) produces a self-noise of -117.8dBu. That's 11dB more self-noise of electric origin. Why have they done that? Probably because customers complained about the low level of the first versions, that were indeed about 250r. But it is not without consequences.
Second, The actual S/N ratio depends on the sensitivity of the microphone, i.e. the voltage delivered for a certain SPL level, on what actual level is fed to the preamp, and on the accumulation of all noises. Room noise is related to the actual surface of the diaphragm, electrical noise is the accumulation of self-noise due to the resistance of the element, preamp noise is the result of the input circuit's intrinsic noise performane AND the actual amount of gain needed to elevate the tiny microphone signal to the system's operating level.
As you see, it's a quite complex subject, but it's easy to demonstrate that, all oter parameters being equal, two mic ribbons differeng only by the ratio of their transformers (and as a consequence, their impedance) will produce different results in terms of noise.
Is that an issue that may be generated by the transformer itself?
The xfmr, with its unavoidable losses, will just make matters slightly worse.