by Rupert Neve
In former years, before the introduction of solid state amplifiers, transformers were necessary to step up to the very high input impedance of tubes, and to provide a balanced input for the microphone line. An input impedance of 1,000 or 1,200 ohms became established for microphones having a source impedance of 150 or 200 ohms. Thus microphones were not heavily loaded.
Condenser microphones worked off high voltage supplies on the studio floor which polarized the diaphragms and powered a built-in pre-amplifier.
More and more microphones were needed as Pop music gained ground and this led to the popular and efficient method of 48 volt “Phantom” powering that was built into the multi-channel recording Console in place of numerous bulky supplies littering the studio, a miniature pre-amplifier now being fitted inside the microphone casing.
The 48-volt supply was fed to the microphone through balancing resistors so it was impossible for this voltage to actually reach the microphone, resulting in low polarizing volts and virtual starvation of the little pre-amp inside the microphone. Nevertheless amazingly good microphones were designed and made, becoming the familiar product we use today.
If a low value resistive load is connected to the output of an amplifier, that amplifier has to produce power in order to maintain a voltage across that load. Obviously if we want more voltage (output from the microphone) we need to provide a larger supply for the amplifier or settle for a lighter load.
A microphone is a voltage generator, not a power amplifier. Most microphones give their most accurate performance when they are not loaded by the input impedance of a traditional preamplifier.
If the microphone uses an electronic circuit (transformerless) output, a low value of load impedance will likely stress the little microphone pre-amplifier, causing slew rate and compression at high levels.
On the other hand, a high value of load impedance allows the microphone to “breathe” and give of its best, this being particularly advantageous with very high level percussive sounds.
If the microphone has an inductive source (such as would be the case if it has a transformer output) a low value of load impedance causes the high frequencies to roll off due to leakage inductance in the transformer in addition to the above amplifier distortion. (This can be an advantage with some microphones!)
For this reason I have provided a high value of input impedance that will load microphones to the smallest possible extent and makes the best possible use of that limited “Phantom” 48 volts supply.