BHI NEDSP900, Руководство пользователя

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NEDSP900 Development Board
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Appendix B - Balanced Audio inputs
The board has been designed with 2 off balanced input amplifiers, meant for microphones. These are
available on the PCB, but require to be populated with components to enable their use. The inputs are
carried out using standard Molex KK connectors.
Each Microphone input has the possibility of adding Phantom Power up to 48 volts. This is useful when
using professional condenser electret microphones requiring a remote power source.
As standard this section of the PCB is not supplied populated and is left to the user to use this as a
‘Breadboard’ circuit, choosing their own component values to suit the type of input intended to be used.
The circuit shows the values for use with a Magnetic microphone input of around 500 Ohms with an
output of around 5 mV P-P. The gain of the circuit has been set to provide around 26dB in order to bring
the level up to that commensurate with a standard Electret Insert.
PCB as normally supplied PCB Populated ready for use
The circuit diagram is given on page 27 with the Bill of Materials thereafter. The PCB component
positions can be found in the Board Layouts given on Page 16.
Important: Take appropriate Electro-Static precautions before handling the PCB and while
making any additions or changes to the PCB components.
Circuit Explanation
As both circuits are identical, only one circuit Input 1 based around IC3a will be discussed. The
information from this can then be read across to the other circuit.
Phantom Power
PL11 allows the application of up to 48 Volts Phantom power to be applied to the balanced signal pins
via 6.8K Ohm resistors and referenced to Ground. It goes without saying that C36 provides smoothing
for any ripple that may exist on the Phantom supply.