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This design is no longer in production.

The MI02 is a microphone amplifier and power supply that can be installed in a PC's half length slot, providing a microphone front end for an audio processing card.

The microphone amplifier has been designed for use within the 20 Hz - 20 KHz bandwidth, where it provides a linear gain and phase response with low distortion and noise. It has a high impedance, balanced input and offers gains from -20 dB to 40 dB in 10 dB steps.

The power supply, which may be powered from either the PC or from an external 5 V / 3 A power supply, is a four output flyback converter that provides isolation between the PC's ground and the microphone amplifier's ground so that ground loops between the MI02 and the signal processing card can be avoided.
Two grounding jumpers selects the ground connections for the isolated part of MI02.

All signals to and from the MI02 is routed via a 15 pole SUB-D interface connector, and a jumper in this connector acts as an on switch for the MI02, so that it will only consume power from the PC when it is in use.

Each output from the converter is regulated by a linear voltage regulator to provide the following stable, noise free voltages:

+28 Vcondenser microphone power supply.
+120 Vcondenser microphone power supply or phantom power.
+200 Vcondenser microphone polarization voltage.
+48 Vcondenser microphone phantom power.
The power supply is designed to provide the voltages necessary to drive a 48 V phantom powered microphone or a Brüel&Kjær condenser microphone.
MI02 can not provide power for the heating element built into some microphones.

MI02 photo

MI02 block diagram and PCB.

Fig 1 : MI02 block diagram.
Fig 1: MI02 block diagram.

Fig 2 : MI02 PCB shown from component side.
Fig 2: MI02 PCB shown from the component side.

MI02 Power supply technical specification

All voltages are referred to MI02 ground unless otherwise noted.
Maximum load current is the current that may be supplied from the output during normal operation.
All outputs are short-circuit proof to ground, but exceeding Maximum load current may prevent other parts of the MI02 circuitry from operating.
Short circuit current is the maximum current the output will supply if it is shorted to ground.
The MI02 power supply outputs may not be connected to any external voltages or to each other.

Output noise is measured with an AC coupled noise analyzer having a 100 KΩ input resistance.
The 28 V and 120 V outputs are measured on the MI02 terminals.
The two 48 V outputs are tied together for noise measurement.
The 200 V output is measured via a buffer amplifier having a 1 GΩ input resistance.
For noise measurements, all four outputs are loaded with a resistor giving the stated current for the measurement.
Measured noise voltages are corrected for the voltage division between the power supply output resistance and the load resistance.

Discharge time at no load (all internal voltages < 24 V)10 minutes
Input voltage5 V ± 10 %
Current consumption0.5 A - 2 A
Peak current drawn from power supply3 A
28 V output (pin 4)
Unloaded output voltage 27.8 V ± 7 %
DC output resistance10 Ω ± 10 %
Maximum load current20 mA
Short circuit current< 150 mA
Line regulation< 1 mV / V
Output noise 22 Hz - 22 KHz RMS at 20 mA loadtyp -86 dBu
Output noise CCIR 468-2 Q-peak at 20 mA loadtyp -78 dBu
48 V phantom voltage output (pins 1 and 9)
Unloaded output voltage47.4 V ± 7 %
DC output resistance (each pin)6810 Ω ± 0.5 %
Maximum load currentNA
Short circuit current (each pin) < 7.5 mA
Line regulation< 1 mV / V
Output noise 22 Hz - 22 KHz RMS at 2 mA loadtyp -85 dBu
Output noise CCIR 468-2 Q-peak at 2 mA loadtyp -78 dBu
120 V output (pin 12)
Unloaded output voltage118 V ± 10 %
DC output resistance10 KΩ ± 6 %
Maximum load current5 mA
Short circuit current< 20 mA
Line regulation< 1 V / V
Output noise 22 Hz - 22 KHz RMS at 5 mA loadtyp -79 dBu
Output noise CCIR 468-2 Q-peak at 5 mA loadtyp -73 dBu
200 V polarization voltage output (pin 5)
Unloaded output voltage200 V ± 7 %
DC output resistance20 MΩ ± 2 %
Maximum load currentNA
Short circuit current< 11 µA
Line regulation< 1 V / V
Output noise 22 Hz - 22 KHz RMS at 1 uA loadtyp -97 dBu
Output noise CCIR 468-2 Q-peak at 1 uA loadtyp -87 dBu

MI02 Amplifier technical specification

All voltages are referred to MI02 ground unless otherwise noted.
Specification is for 20 Hz - 20 KHz with 40 Ω source resistance and 100 KΩ load resistance unless otherwise noted.
Absolute maximum ratings:
DC input voltage0 V to 250 V
Differential AC input voltage
 Gains -20 dB and -10 dB200 V pp
 Other gains25 V pp
Common mode AC input voltage25 V pp
DC voltage applied to the output0 V to 12 V
Operational characteristics:
Differential input resistance110 KΩ
Common mode input resistance
 Gains -20 dB and -10 dB260 KΩ
 Other gains240 KΩ
Differential input capacitance< 200 pF
Common mode input capacitance< 100 pF
AC output resistance (1 KHz)10 Ω
AC output resistance< 30 Ω
Minimum output load resistance300 Ω
Maximum output voltage (600 Ω load)18 dBu
Maximum input voltage
 Gain -20 dB38 dBu
 Gain -10 dB28 dBu
 Gain 0 dB18 dBu
 Gain 10 dB8 dBu
 Gain 20 dB-2 dBu
 Gain 30 dB-12 dBu
 Gain 40 dB-22 dBu
Input referred noise 22 Hz - 22 KHz RMS with 600 Ω source resistance
 Gain -20 dB< -85 dBu
 Gain -10 dB< -89 dBu
 Gain 0 dB< -106 dBu
 Gain 10 dB< -111 dBu
 Other gains< -113 dBu
Input referred noise CCIR 468-2 Q-peak with 600 Ω source resistance
 Gain -20 dB< -75 dBu
 Gain -10 dB< -79 dBu
 Gain 0 dB< -96 dBu
 Gain 10 dB< -101 dBu
 Other gains< -103 dBu
 Gain -20 dB> 20 dB
 Gain -10 dB> 30 dB
 Gain 0 dB> 40 dB
 Gain 10 dB> 50 dB
 Other gains> 60 dB
Gain accuracy at 1 KHz
 Gain 0 dB± 0.2 dB
 Other gains± 0.5 dB
Linearity± 0.3 dB
Input - output phase
 Gain 30 dB± 10 deg
 Gain 40 dB± 20 deg
 Other gains± 5 deg
THD+N (500 KHz measurement bandwidth)
 Gains 0 dB and 10 dB< 0.005 %
 Gain 40 dB< 0.02 %
 Other gains< 0.01 %

All specifications subject to change without prior notice.PP 921005

Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
Tel/Fax: (+34) 928 152 807
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