5
LI0083 – Revision 22 June 2017
Microphone Conditioner
Input and output
The 12AU has two microphone inputs, a LEMO input for externally polarized microphones and a
CCP input for prepolarized microphones.
The LEMO input has a 7-pin LEMO 1B socket for a microphone preamplifier such as the
G.R.A.S. Preamplifiers 26AM, 26AC, and 26AK. Fig. 1 shows the wiring diagram of this input
socket which is also compatible with a range of microphone preamplifiers from other suppliers
such as Norsonic, L&D and Brüel & Kjær. The input impedance is 100
k
Ω
.
Fig. 1.
7-pin LEMO female socket 1B (external view).
The CCP input has a standard BNC socket.
The Microphone Output is a standard BNC socket for direct use with analyzers, voltmeters, oscil-
loscopes etc. The output is floating (2 k
Ω
//100 nF to common ground). The output impedance is
100
Ω
.
Input for Externally Polarized Microphone or Pre-polarized Microphone (CCP)
The 12AU has a 7 pin LEMO socket for a microphone with a LEMO type preamplifier and a BNC
socket for a CCP microphone set.
The polarization voltage can be set to 200 V or 0 V, a green LED on the front indicates the
currently selected polarization voltage.
The preamplifier voltage supply is ±15 V DC. The CCP supply is 4 mA.
Bandwidth, Filters and Noise
The bandwidth of the
Microphone Conditioner
is from 1 Hz to 100 kHz (–3 dB). A 20 Hz
(Butterworth 3. order) high-pass filter can be selected remotely. A front-panel LED lights up when
the filter is switched on.
The noise floor relative to input, with the input shorted, is (
≥
20 dB gain) <1.5 μVrms
(20 Hz - 20 kHz). With the input loaded with a 20 pF dummy microphone, the noise is less than
5 μVrms (20 Hz – 20 kHz).
