I built a MC preamp board today and before installing the ZTX851 transistors I decided to test the ZTX689B "in-socket."
The MC preamp, with the ZTX851, shows that the rbb' of the ZTX851 is around 1.75Ω.
(Ein about -141 dBu, gain = 62 dB, Rsource 10Ω, Rg 1Ω produces a total Rnv of about 14.5Ω. Ic = 5.5 mA nominal)
Given the low noise of the MC preamp it makes a good test-bed for the ZTX689B.
The ZTX689B, and its PNP complement the ZTX789A, have much higher gain.
The ZTX689 has an hfe of 500 minimum. Cob is lower at 16 pF typical.
The maximum collector voltage is 20V compared to the 60V of the ZTX851.
I didn't expect that the ZTX689B would perform as well as the ZTX851 but the higher gain, and lower Cob, might make it a good choice for mic preamps with their higher source impedance in the 50-300Ω range.
The ZTX689B has an estimated rbb' of about 5.5Ω based on the same test conditions.
(Ein about -139 dBu, gain = 62 dB, Rsource = 10Ω, Rg 1Ω for a total Rnv of about 22Ω.)
This works out to be about 600pV√Hz.
The ZTX689B is a strong contender for mic preamps.
It's 1/f performance appears just as good as the ZTX851.
With higher source impedances the lower rbb' of the ZTX851 may be less of an advantage compared to the higher gain of the ZTX689B.