Preampifier Designed for Performance

The Preamp and Research Teams collaborated with a common objective: To design and implement a professional balanced microphone preamp. Research was geared towards developing a preamplifier which takes full advantage of the Shure MX-180 Series microphones. The MX-180 Series microphones require a 48VDC external phantom power supply and deliver an electronically balanced signal. Thus, our efforts culminated in a single-stage difference amplifier with phantom power. The final design consists of three major sections: the power supply management, the operational amplifier configuration, and op amp input protection.

Detail 1: Power Section

The 48V DC source is provided by the Astrodyne ASL40-48 Open Frame Power Supply. The 48V is supplied through a 315mA line fuse to the DC to DC converter. (Astrodyne ASD03-48D15H). The 48V supply ground and the DC to DC converter COM terminal are both connected to chassis ground. The 48V is also delivered to the phantom power section of the circuit. The phantom power circuit is isolated from the DC to DC converter by a single pole single throw switch. This feature is to provide compatibility with other microphones which may be damaged by phantom power. The C7 and R14 pair forms a low pass filter with a cutoff frequency of 11.7 Hz. The filter was designed to reduce the effect of any 60Hz ripple remaining in the DC 48V.

Detail 2: Op Amp Section

The difference amplifier uses of a LM833 operational amplifier. The LM833 was chosen for its low-noise and large bandwidth performance characteristics. In order to achieve good common mode rejection, R1, R2, R3, R4, R16, and R17 are all 0.1% tolerance resistors. The op-amp and resistor configuration produces 26dB of voltage gain. C2 and C4 are 47uF electrolytic capacitors for low frequency power supply filtration, while C1 and C3 are 0.1uF ceramic capacitors for high frequency power supply filtration. Together, the set of power rail capacitors provide an AC ground for the op amp.

Detail 3: Op Amp Input Protection

The op amp input protection serves to separate the circuit from the DC 48V phantom power and any associated transients as well as any possible overloading input signals. C5 and C6 serve to block DC and thus prevent the phantom power from affecting the op amp. D1 through D4 are 6.8V Zener diodes. They provide a ceiling of +/-6.8V for each op amp input terminal. R5 and R6 serve to provide high input impedance and dominate over the output impedance of connected microphones.