[Phil Allison[_4_]]

On Tuesday, July 10, 2018 at 3:47:20 PM UTC+10, Sean B wrote:
Thanks!

The only other thing I don't get is the function of Zeners D3 and D4.
Any ideas?



** PNP transistors T2 & T3 need protection against C-E reverse polarity, which happens when one mic signal line ever shorts to ground. Such a short allows C9 to momentarily supply quite high current, enough to cause immediate damage. Zeners D3 & D4 prevent this by forward conduction. When the mic is first powered up - the same zeners provide voltage limiting in breakdown mode.

FYI: The Rode NT-2 uses 1N4148 diodes in the same position providing only reverse breakdown protection.



The test point voltages seem to indicate that there is a Vce of only
1 volt or so across the output BJTs.

Seems kind of low for good linearity.


** It does, but the JFET input stage has little voltage swing available and poorer linearity than emitter followers - so JFET distortion likely dominates.

Max output from the pre-amp is given as 1V rms which corresponds to a SPL around 132dB at the capsule ( calculated using a typical capsule sensitivity figure of 13mV per Pascal ) which is plenty of headroom for a studio mic.




..... Phil