By Allen H. Burdick
To perform valid noise level measurements, two specific conditions must be established:
- A 150 Ohm source impedance
- A 20 kHz measurement bandwidth
The microphone preamplifier must have the proper source impedance at its input. If it does not, the preamp will amplify the noise of its internal bias resistors, resulting in much higher noise voltage. Typically, the easiest way to terminate an input on the Benchmark MP-4 is to engage the 20 dB attenuator, since that presents a 150 ohm source to the input stage. When measuring other preamplifier systems that do not have an internal attenuator, a 150 ohm metal film resistor connected between pins 2 and 3 inside an XLR type connector shell, such as the Neutrik NC3MX or Switchcraft A3M, will do the job.
The use of an carbon film resistor is usually ok, but the carbon composition resistor should never be used due to a phenomena known as "excess noise" found in these resistors. This noise is created by electrical currents having to jump over small but real physical spacing between the carbon particles which are held apart by the resistors binder material. These days, the discrete carbon comp resistor is almost unavailable. However, the thick film chip resistor and thick film resistor SIP or DIP packs are in fact carbon composition resistors and should be avoided.
The second condition that must be met is the limitation of the measurement bandwidth to 20 kHz. Manufacturers never specify, nor do we really want to measure, the noise outside the 20 kHz audio bandwidth. Be sure the test instrument that you are using has an internal filter for that purpose. Audio Precision markets a 20 kHz "brick wall" filter for their Systems 1 and 2.
The output noise of the microphone preamp will be the noise of the source resistance, -130.82 dBu for 150 ohms, plus the gain of the preamp, plus the noise figure of the preamp in dB. The expected noise of a preamp operating at 50 dB of gain would then be -130.82 dBu, plus 50 dB, plus the noise figure of the preamp, which is < 1 dB for the Benchmark MP-4, or a total of approximately -80 dBu. Using bipolar 15 volt power the MP-4 has an output clip point of +27 dBu thus yielding a preamp dynamic range of 107 dB.
The only way to get lower noise and therefore greater dynamic range (with a given power supply voltage source) is to operate both the mic and the preamp at much lower (i.e. cryogenic) temperatures. Unfortunately, at this point in the history of man, this is not possible.