3–4 Dual Microphone Supply Type 5935 Brüel&Kjær
User Guide
Chapter 3 — Operation
Grounding Considerations
Caution – there is no indication that the batteries are being charged when the
On/Off switch is in the Off position and an external supply is connected.
3.2 Grounding Considerations
The signal path in the Type 5935 is referenced to the chassis of the unit. The
output connectors, the ground for the preamplifiers and the heater input also use
this reference ground. However, the negative terminal of the external supply, pin 2,
is isolated from this ground. This arrangement is used to reduce the noise level in
the signal path. The DC/DC converter uses the switched mode principle with a
switching frequency of 150 kHz. It is not desirable to have harmonic components of
the switching frequency in the signal chain, as this would add unwanted noise, so
the external supply circuitry is kept separate from the amplifier circuitry.
Under certain circumstances it may be required to link the external supply ground
and the signal ground. This is not dangerous, but may result in the noise levels
being out of specification. An example where this linking may be used is in a
mobile situation, where the same 12 VDC battery is used to feed both the heater
supply and the external supply.
3.3 Polarization Voltage
The Type 5935 provides the necessary polarization voltages necessary for all the
Brüel& Kjær range of microphones. The microphone receives this voltage via the
preamplifier. It is important to set the correct polarization voltage for the micro-
phone capsule being used. This is done with the polarization switch found by re-
moving the bottom cover. Fig. 3.3 shows the position of the switch.
The polarization voltage switch has three positions, 0V, 28V and 200 V. Choice of
polarization voltage depends upon the microphone used. Refer to the microphone’s
calibration chart to find the correct polarization voltage to be used.
Note that, due to high impedances giving slower charge flow, it takes at least one
minute for the microphone capsule and polarization power supply to become stable.
Therefore it is necessary to allow the system to rest after power is applied or after
the preamplifier is attached.
The same polarization voltage is applied to both channels simultaneously. It is not
possible to have capsules with different polarization voltages working at the same
time.
3.4 Output Connection
All the outputs from the Type 5935 appear on unbalanced BNC connectors. The
outputs are all very low impedance and are designed to drive into the input circuit-
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