[BC] Mic impedance question...

[RichardBJohnson at comcast.net]

Many microphones, all the dynamic ones including ribbon microphones, and capacitor microphones as well have a basic frequency response that rises 6 dB per octave. This is because the voltage output of these devices depends upon the rate-of-change of the input audio. Since the rate-of-change of a 2 kHz signal is twice that of a 1 kHz signal, the output of the active element will be twice that of 1 kHz when a 2 kHz signal is received.

To compensate for this, there is normally an inductance placed between the microphone element and its load. This will produce a compensating 6 dB per octave reduction in frequency response. The inductance is normally the leakage inductance of the microphone's built in transformer. When the microphone is loaded with its designed load impedance, the two frequency-response deltas cancel and one has a reasonable flat frequency response within a specified range.

Capacitor microphones also have an increase in output with frequency. However, they normally have a built-in preamplifier with the required compensating frequency response. Such microphones may operate into any reasonable impedance although the impedance specified in its data-sheet will usually produce the best overall results.

Typical dynamic microphones are designed to operate into a 250 ohm impedance. The RCA 77-DX http://www.coutant.org/rca77dx/index.html had a built-in tapped transformer to accommodate many loads.

The input circuitry of early vacuum-tube amplifiers consisted of a transformer that coupled into the grid of a low-level amplifier.  http://www.tangible-technology.com/tubes/1566/1566.html A careful look at the schematic of a high-fidelity design will show that the high impedance side of the transformer is terminated with a resistor. This is so the low-impedance input will reflect the correct termination for a microphone. In this schematic, the load resistor is 100 kilohms and there is an additional base-boost resistor-capacitor network in series to compensate for the small transformer's lack of low-frequency response. Such compensating networks were commonplace it the days of high-fidelity broadcast equipment design.

The bottom line is look at the microphone specification. You should choose a preamplifier with the correct input impedance and not terminate your microphone with a resistor. Use of a resistor may reduce the S/N at least 6 dB even if the microphone is designed for that load impedance.

If you do not like this response then just delete it. I will not be answering questions about spelling, punctuation,  my qualifications to write this, or other attempts to discredit me. I have too much work to do, working with anti-matter, http://en.wikipedia.org/wiki/Positron_emission_tomography ,  to defend myself from cretins.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Glen Kippel" <glen.kippel at gmail.com>