Product Review - Magneto Audio Labs VariOhm

Article Preview :: Microphone Impedance Converter


Reviews : Preamp


If you’re searching for the perfect match of mic and preamp, this useful gadget could help you achieve it...
Hugh Robjohns
In the days of yore — and we’re talking 40 or 50 years ago, really — microphones used to be designed to operate in a ‘matched impedance’ environment with their preamplifiers, and usually at quite low impedances, such as 30Ω. This mode of connectivity is all about maximising the transfer of power from the source to the destination, and essentially came from the practices employed in the telephone industry upon which pro audio was conceived and developed. The benefit of the impedance‑matched interface in the telecoms world at that time was that the power generated by a telephone’s microphone was passed in the most efficient way possible to another telephone’s earpiece in a predominantly passive system.
However, eventually common sense prevailed in the professional audio world, and microphone interfaces evolved into a matched‑voltage configuration. We’re not interested in passing power around the place — everything is powered and active anyway — and all we need to do is make sure the alternating voltage generated by the microphone, which represents the audio signal, is passed as accurately as possible to the microphone preamplifier. To that end, we generally use output devices with low source impedances, and detect the voltage generated by the source with a relatively high‑impedance destination. The general rule of thumb is that the destination should have about 10 times the impedance of the source. Consequently, the vast majority of microphones have output impedances of 150‑200Ω, and most microphone preamps have an input impedance of 1500‑2400kΩ.
Why Change Impedance?
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