Some mixing boards used input transformers with two primary windings. In parallel, they were 150 Ohm. In series, they were 600 Ohms.
The output transformers had two windings on the output for the same reason.
Some early dynamic studio mics had a three position switch for 150,
600 or high impedance output.** Such transformers had exact matching of the turns number which also allo wed the primary to be used for delivering phantom power to the mic. With th e windings in series, 48VDC is fed to the mid point. The mic would then tak e the exact same current from each half and avoid magnetising the core. **The famous RE20 had 3 low impedances available, by changing wiring - 50, 150 and 250 ohms.
The 50 ohms setting was for use with very long cable runs as might be found with outdoor events.
.... Phil
Ev%20RE-20.pdf
Now *that* is a nice looking microphone. I wonder how well they age? Do they still sound as well at 20 years old as when new?
** Well, it's big and heavy.
** Not as well as a good red.
** Many do, while others make no sound at all.
The model is still made, with updates.
It's a firm favourite with radio announcers and sax players. In both cases, they feel comfortable with it, cos it will not easily disappear up their mouths.
.... Phil
electro-voice-re20
Excellent! Many thanks.
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