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Tech Tips

Can you use a prepolarized microphone on a LEMO style preamp?

Evro T. Wee Sit

Yes, but let me explain.  If you are like me who grew up adjusting UHF/VHF TV knobs and witnessed the emergence of prepolarized mics, you have probably already considered this question by necessity. But our internet streaming colleagues were probably spared the trouble due to the recent ubiquity and convenience of IEPE powered mics and accelerometers.

First some review. As the name suggests, a condenser microphone is a capacitor whose capacitance changes in proportion to sound pressure. Figure 1 shows how the two parallel plates formed by the diaphragm and backplate are polarized to create the capacitor. An externally polarized mic achieves this with 200 VDC applied through the preamp. In contrast, a prepolarized mic has a thin permanently charged layer on the backplate and thus does not need any external polarization voltage.

Condenser Microphones
Figure 1:  Condenser Microphones

More review but now on the preamps (the pencil shaped device the mic cartridge is screwed on to). There are two common types (at least in the test and measurement world); Integrated Electronic Piezoelectric (IEPE) and the more traditional LEMO style (named after the connector manufacturer that popularized the connector). IEPE preamps cannot provide the 200 VDC polarization voltage while LEMO preamps can. Thus, the common practice which has almost turned into a de facto rule is as follows.

IEPE Preamps must use Prepolarized Mics

LEMO Preamps must use Externally Polarized Mics

Well, that’s mostly right but not quite.

If you look at Figure 2, the connector for IEPE power module and the preamp are both BNC coax. That’s just two conductors. So how can two conductors provide power to the preamp (there are active circuits inside the preamp that require power) and at the same time carry the measured signal? The power for the preamp is a DC voltage while the signal is an AC riding on the DC. That’s pretty clever but not good enough. The IEPE system does not have any more conductors to apply the 200 V external polarization. Thus, an IEPE system MUST use a prepolarized mic and cannot support externally polarized mics. But is the reverse true? Must a prepolarized mic be used with an IEPE system only?

12AL IEPE Power Module IEPE Preamp Connector
Figure 2: IEPE Power Module, IEPE Preamp

No. Look at Figure 3 especially the pinouts. A LEMO style system has enough conductors to power the preamp, carry the measured signal, and it also can supply the 200 V polarization voltage. But this polarization voltage can be switched off. Figure 4 shows a typical LEMO style power module. Set the polarization voltage to 0 V and a prepolarized mic will work. (Another future Tech Tip on what happens if you apply 200 V polarization voltage to a prepolarized microphone.)

LEMO Microphone Preamplifier Lemo Preamplifier Pinouts
Figure 3: LEMO Preamplifier, Pinouts

12AQ Polarization Voltage Set to Zero
Figure 4: Polarization Voltage Set to Zero

So, a better general rule is IEPE preamps can only use prepolarized mics while LEMO preamps can use either externally polarized or prepolarized mics.

Now why would you want to use prepolarized mics with LEMO preamps? I save this for a future tech tip.

Home Tech Tips Can you use a prepolarized microphone on a LEMO style preamp?