“Phantom powering” is the most common way to power a microphone. The technique supplies 48 V provided through two 6.81-kΩ resistors in a differential input line.1 This idea explains an improved way to use phantom power to run ultrasound microphones requiring long cables.
Typically, the microphone should incorporate a signal-splitting circuit—dc blocking capacitors or a transformer—to separate the phantom power from the audio signals. The capacitors/transformer pass the audio on the differential pair while blocking dc power. The designer must select components that do this without degrading the audio signal.
Another approach used in variations for some time now, the Shoeps circuit (Fig. 1), employs the 6.81-kΩ resistors—Rf1 and Rf2—as load resistors for direct-coupled emitter followers with pnp transistors Q2 and Q3.2 The input amplifier stage uses a JFET (Q1) with a very high input impedance.
The input stage not only acts as an impedance converter for the cartridge, it also performs phase-splitting, turning the input signal from the cartridge into two paraphase outputs. The output stages act as current amplifiers with a unity voltage gain.
Blocking capacitors C5 and C6 are placed between the outputs of the impedance converter and the inputs of the voltage followers. Because of the high input impedance of the voltage followers, the value of these capacitors is small, so quality film capacitors can be used.
However, using a voltage follower with a current-setting resistor has a limitation: asymmetrical transient response with a capacitive load. A long cable creates a load capacitance that will charge slowly through the current-setting resistor and discharge fast through the pnp device.
The average current through each 6.81-kΩ resistor is about 4 mA. This current will charge cable capacitance (C21 + C23) at the rate of (4 mA)(C21 + C23). So, an input sine wave with amplitude Vp would be output as a triangle-wave if the frequency is greater than (4 mA)(C21 + C23)/2π Vp.
The addition of npn emitter followers Q4 and Q5 speeds up the charging of the cable capacitance, eliminating this slew-rate limiting and lowering total harmonic distortion (THD). Figure 2 compares the two techniques. The square nonlinearity of the impedance converter JFET causes the gradual rise of distortion with the input amplitude.
1. IEC 61938 Audio, Video, and Audiovisual Systems—Interconnections and Matching Values—Preferred Matching Values of Analogue Signals, clause 7.4
2. J. Wuttke, Mikrofonaufsatze, Schalltechnik Dr.-Ing. Schoeps, 2000, p. 83; www.schoeps.de/D-2004/PDFs/Mikrofonbuch_komplett.pdf