There has been some discussion of late on the micbuilders’ forum about the need to select the PNP output transistors in microphones. The usual selection criterion is beta (current gain) because that’s easy to measure. Many multimeters have such a function. The concern is that mismatched transistors will cause an imbalance between the positive and negative signals in the balanced mic feed or cause a voltage offset between pins 2 & 3 at the XLR connection.

So I decided to measure a few mics with circuits representative of different types.

Here’s what I found:

MicrophoneVoltage @ Pin 2     V Pin2 - Pin3            Notes

=========================================================================

BM800 stock             40.1V                 -68.2mV

K596 Schoeps           44.0                 10.5mV        Schoeps with K596 input / phase splitter

K596 Fol+3PNP         40.7                   19.6mV        K596 follower + PNP phase splitter + PNP

K596 Fol+3PNP         40.7                -124.5mV            outputs

K596 TL/C                 40.6                  -39.4mV        TL/C = K596 with feedback cap drain to gate

K596 TL/C RK-99       40.8                -123.6mV

K596 TL/C 3-Pat        40.1                 130.3mV

M-P V57 RK-87           33.2                  -21.5mV            Mic-Parts V-57 kit, selected JFET & PNPs

M-P T-84 RK-47           48.3                  -01.1mV            Mic-Parts T-84 kit, transformer output

AKG C414B-XLS         36.0                  -56.2mV

AKG P220 RK-87        39.4                  -01.0mV            Transformer output

No Mic                      49.5                   01.2mV             “48V” is slightly high, and

                                                                                    voltmeters wander a mV or so.

Since all the K596 mics use the 2N5401 PNP output transistors from the factory, we can assume they are randomly selected. The likelihood is they came from the same batch, so perhaps this isn’t worst case, with transistors picked up off the floor, but neither was any individual matching done.

Is this degree of imbalance worrisome? With modern capacitor coupled mic preamps, the voltage balance doesn’t matter. There’s not enough imbalance to cause dangerous currents to flow in transformer input preamps either, so I consider these mics safe to use with any preamp I have, including my old mixers. Worst case 150mV through the 6.8K x 2 phantom power resistors results in under 10 microamps in a transformer primary.

Of note, the Mic-Parts V-57 kit is among the low offset group, so select your PNPs if you can. It can’t hurt, and Matt’s carefully chosen components certainly make for great sounding mics.