Recording - June 2017 - 64

HOW
MICS
WORK
A condenser mic with a head amp included usually has a
resistive output impedance, which means it just acts like a resistor,
typically 150-200Ω. A microphone with such a head amplifier can
typically drive a reasonable length of cable with no losses in the
audio range-a mic with a 200Ω resistive output impedance,
connected to a 100' mic cable, will typically form a lowpass filter
with a cutoff frequency of about 100 kHz, and frequencies above this
will be rolled off. (As the late Michael Flanders once wrote, "The ear
can't hear as high as that / Still it ought to please any passing bat.")
Early condenser mics all used vacuum tubes for head amps
(there were no other amplifying devices back
then). In the 1960s, however, manufacturers
began using newly-developed field effect transistors, or FETs, for head amplifiers. The FETbased amplifiers ran cool, so they wouldn't
cook the capsules' diaphragms, and FETs
didn't get old and need replacing the way
tubes did. It was also easier to power them
(I'll talk about that in a minute); as a result,
by the end of the 1970s FETs had pretty well
driven tubes out of the head amplifier market.
Until the backlash. Recording engineers had
liked the old condenser mics with tube head
amps, and now they paid top dollar for them.
Seeing an opening, in the 1980s manufacturers
introduced new microphone models with tube head amplifiers, and
these days, you pays your money and you takes your choice.

Phantom power

Head amplifiers need electricity to power them. A vacuum
tube needs a high-voltage supply (say, 100-250V) feeding
the tube's plate, as well as a filament supply (typically 6-12V)
to keep the elements of the tube hot (tubes aren't called thermionic devices for nothing; they can be as hot as 1,000° F.
inside). There also needs to be a source of voltage to charge
up the capsule-in the last segment we postulated a battery,
but these days it's usually a power supply.
In fact, everything in a tube condenser mic (capsule and head
amp) is powered by an external supply. This supply is typically in
a separate box, and it connects with the actual microphone via a
multi-conductor cable and plug. The audio signal usually shows
up on a separate XLR connector on the power supply box.
When manufacturers switched to the new FET-driven head
amplifiers, they didn't need a supply to provide filament voltage
64

RECORDING June 2017

any more (FETs don't have filaments), and instead of the high-voltage
supply that drove the tube's plate, they could get along with
more modest supplies (on the order of +25-50V). The first FET
mics still used separate power supplies or, occasionally, batteries-
until the engineers at Neumann figured out another way.
They took advantage of a trick that telephone technicians had
used for years. Telephones are differential devices, meaning they
only respond to differences in voltage between their two connecting
wires. So the linemen would carry special handsets with them that,
when clipped to the line, would put the same voice signal on both
of the wires, with the same polarity, when he talked into the handset.
The customers' phones would ignore that signal, because there was
no difference in voltage between the two wires. If another lineman
clipped one of those special handsets to the line, however, he could
hear whatever the first was saying-the handset would extract the
first tech's signal from the wires. The technique was called phantom
signaling, because the linemen's conversation floated on the wire like
a phantom while the customers' conversations went on as though
nothing else was present.
Neumann's engineers realized that the same technique could be
adapted to power microphones. In the 1960s mic preamps (alone or
in consoles) usually used transformers at their inputs, and transformers
are differential in the same way as telephones; they respond only
to differences in their input voltages. If you put
8
something on the two input wires simultaneously,
the transformer will ignore it (that's why they
reject noise, by the way).
So Neumann's engineers took the output of
a +48V DC supply and connected it to the mic
input lines via a pair of well-matched resistors
(Figure 8). Because the DC voltage appeared
simultaneously on the two inputs, the trans9
former on the preamp input ignored it-but a
couple of matched resistors in the microphone
could "pick off" the DC, and that could be
used to polarize the condenser capsule and to
run a FET-powered head amplifier.
They called their new scheme phantom
power in tribute to the telephone techs' communication trick. A
side note: engineers at Microtech Gefell, which is Neumann's
"separated-in-childhood" sibling, found a way to use standard
+48V phantom power to run a tube microphone. So far it
hasn't been copied-perhaps because it's well protected by
patents. Phantom power also works on transformerless mic
inputs; they usually use capacitors to keep the DC off their input
transistors (Figure 9).
So that's the story of how condenser mics work: how their
capsules (really capacitors) detect sound, why the capsules need
head amplifiers, and how those head amplifiers get the juice they
need to work. Next issue we'll take a look at the other variety of
mic often found in studios-the dynamic. (What about ribbons?
Well, it turns out that ribbon mics are actually a variety of dynamic
mic. Really? Yes; stay tuned.)
Paul J. Stamler (stamler@recordingmag.com) is an audio
engineer, educator, musician, and collector of vintage and
historical audio recordings, living and working in St. Louis.



Table of Contents for the Digital Edition of Recording - June 2017

Contents
Recording - June 2017 - Intro
Recording - June 2017 - Cover1
Recording - June 2017 - Cover2
Recording - June 2017 - 1
Recording - June 2017 - 2
Recording - June 2017 - 3
Recording - June 2017 - 4
Recording - June 2017 - 5
Recording - June 2017 - Contents
Recording - June 2017 - 7
Recording - June 2017 - 8
Recording - June 2017 - 9
Recording - June 2017 - 10
Recording - June 2017 - 11
Recording - June 2017 - 12
Recording - June 2017 - 13
Recording - June 2017 - 14
Recording - June 2017 - 15
Recording - June 2017 - 16
Recording - June 2017 - 17
Recording - June 2017 - 18
Recording - June 2017 - 19
Recording - June 2017 - 20
Recording - June 2017 - 21
Recording - June 2017 - 22
Recording - June 2017 - 23
Recording - June 2017 - 24
Recording - June 2017 - 25
Recording - June 2017 - 26
Recording - June 2017 - 27
Recording - June 2017 - 28
Recording - June 2017 - 29
Recording - June 2017 - 30
Recording - June 2017 - 31
Recording - June 2017 - 32
Recording - June 2017 - 33
Recording - June 2017 - 34
Recording - June 2017 - 35
Recording - June 2017 - 36
Recording - June 2017 - 37
Recording - June 2017 - 38
Recording - June 2017 - 39
Recording - June 2017 - 40
Recording - June 2017 - 41
Recording - June 2017 - 42
Recording - June 2017 - 43
Recording - June 2017 - 44
Recording - June 2017 - 45
Recording - June 2017 - 46
Recording - June 2017 - 47
Recording - June 2017 - 48
Recording - June 2017 - 49
Recording - June 2017 - 50
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Recording - June 2017 - 53
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Recording - June 2017 - 55
Recording - June 2017 - 56
Recording - June 2017 - 57
Recording - June 2017 - 58
Recording - June 2017 - 59
Recording - June 2017 - 60
Recording - June 2017 - 61
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Recording - June 2017 - 64
Recording - June 2017 - 65
Recording - June 2017 - 66
Recording - June 2017 - 67
Recording - June 2017 - 68
Recording - June 2017 - 69
Recording - June 2017 - 70
Recording - June 2017 - 71
Recording - June 2017 - 72
Recording - June 2017 - Cover3
Recording - June 2017 - Cover4
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