Recording - October 2015 - 68

By Paul J. Stamler

Part 10: Beyond The Boost-Mic Preamps II
Figure 1a

Figure 1b

Last time we talked about the physics of mic preamps: how they change signal levels,
and what lurks in the shadows when we do (clipping and hiss). But there's more to preamps than just providing gain, and it's time to look at these handy extra features now.
Power games
What else do preamps do? Well, they provide phantom power to power condenser
(and some ribbon) mics. They do that by including a source of +48V of DC (direct current) that connects to the Hot and Cold input terminals of the preamp (we talked about
them last month) through a pair of well-matched resistors. The phantom power can be
turned on and off; sometimes this can be done for each channel on a console. On cheaper consoles, and on channel strips and interfaces, all channels of phantom power get
switched on and off at once.
I've mentioned this before, but I'll say it again: never plug in or unplug either end of
a cable with phantom power turned on! Most of the time, nothing will happen (aside
from a pop in the audio), but if you happen to have a defective cable, you can destroy
an expensive microphone or console. So just don't do it.
A question of balance
Here's something else important to know about mic preamps: Their mic inputs are balanced. What does that mean?
68

RECORDING October 2015

Let's take a short detour. If you play
electric guitar, you know that the cord
connecting the guitar to the amp has two
conductors: the Hot conductor, which is
wired to the tips of the 1/4" plugs, and
the Ground conductor, which is wired to
the shafts (sleeves) on the plugs. The Hot
wire carries the varying signal from the
pickups, while the Ground is defined as
maintaining a constant voltage of zero
volts. When you plug the cord into an
amp, the Hot signal goes to the amplifying circuit, while the Ground signal connects to the sleeve of the input jack and
thence to the amp's chassis, which is
grounded through the wall plug.
You also know that, if you lay your guitar cord near an AC power line, or something else which generates hum, the cable
will pick up the hum and you'll hear it
through the amp, which amplifies the hum
just like it was a signal from the guitar.
Microphone inputs on mic preamps
are different. They are differential, which
means they will amplify the difference
between the Hot connection (pin 2 on
the XLR connector) and the Cold connection (pin 3). Some mic preamps do the
differential amplifying thing by using a
transformer at the input, while others do
it with an electronic circuit that's transformerless, but they all do it-mic input
circuits are always differential.
Here's how that makes a difference
(pardon the expression). Most microphones put out a balanced signal, meaning that the changing voltage on the Hot
pin (2) is matched by a mirror-image signal on the Cold pin (3), with the polarity
inverted. When Pin 2 goes up, Pin 3
goes down; the differential circuit in the
preamp sees that they are different, and
amplifies that difference (Figure 1a).
But what happens if you lay the mic
cable in a hum field, like next to a power
cable, or in front of a refrigerator motor?
As we talked about last month, the Hot
and Cold conductors in the cable are
twisted tightly around each other (extratightly in a Star-Quad cable), and the
same hum signal gets induced in the Hot
and Cold wires. Figure 1b shows the



Table of Contents for the Digital Edition of Recording - October 2015

Contents
Recording - October 2015 - Intro
Recording - October 2015 - Cover1
Recording - October 2015 - Cover2
Recording - October 2015 - 1
Recording - October 2015 - 2
Recording - October 2015 - 3
Recording - October 2015 - 4
Recording - October 2015 - 5
Recording - October 2015 - Contents
Recording - October 2015 - 7
Recording - October 2015 - 8
Recording - October 2015 - 9
Recording - October 2015 - 10
Recording - October 2015 - 11
Recording - October 2015 - 12
Recording - October 2015 - 13
Recording - October 2015 - 14
Recording - October 2015 - 15
Recording - October 2015 - 16
Recording - October 2015 - 17
Recording - October 2015 - 18
Recording - October 2015 - 19
Recording - October 2015 - 20
Recording - October 2015 - 21
Recording - October 2015 - 22
Recording - October 2015 - 23
Recording - October 2015 - 24
Recording - October 2015 - 25
Recording - October 2015 - 26
Recording - October 2015 - 27
Recording - October 2015 - 28
Recording - October 2015 - 29
Recording - October 2015 - 30
Recording - October 2015 - 31
Recording - October 2015 - 32
Recording - October 2015 - 33
Recording - October 2015 - 34
Recording - October 2015 - 35
Recording - October 2015 - 36
Recording - October 2015 - 37
Recording - October 2015 - 38
Recording - October 2015 - 39
Recording - October 2015 - 40
Recording - October 2015 - 41
Recording - October 2015 - 42
Recording - October 2015 - 43
Recording - October 2015 - 44
Recording - October 2015 - 45
Recording - October 2015 - 46
Recording - October 2015 - 47
Recording - October 2015 - 48
Recording - October 2015 - 49
Recording - October 2015 - 50
Recording - October 2015 - 51
Recording - October 2015 - 52
Recording - October 2015 - 53
Recording - October 2015 - 54
Recording - October 2015 - 55
Recording - October 2015 - 56
Recording - October 2015 - 57
Recording - October 2015 - 58
Recording - October 2015 - 59
Recording - October 2015 - 60
Recording - October 2015 - 61
Recording - October 2015 - 62
Recording - October 2015 - 63
Recording - October 2015 - 64
Recording - October 2015 - 65
Recording - October 2015 - 66
Recording - October 2015 - 67
Recording - October 2015 - 68
Recording - October 2015 - 69
Recording - October 2015 - 70
Recording - October 2015 - 71
Recording - October 2015 - 72
Recording - October 2015 - 73
Recording - October 2015 - 74
Recording - October 2015 - 75
Recording - October 2015 - 76
Recording - October 2015 - 77
Recording - October 2015 - 78
Recording - October 2015 - 79
Recording - October 2015 - 80
Recording - October 2015 - Cover3
Recording - October 2015 - Cover4
https://www.nxtbook.com/musicmaker/musicmaker/recording_202108
https://www.nxtbook.com/musicmaker/musicmaker/recording_202107
https://www.nxtbook.com/musicmaker/musicmaker/recording_202106
https://www.nxtbook.com/musicmaker/musicmaker/recording_202105
https://www.nxtbook.com/musicmaker/musicmaker/recording_202104
https://www.nxtbook.com/musicmaker/musicmaker/recording_202103
https://www.nxtbook.com/musicmaker/musicmaker/recording_202102
https://www.nxtbook.com/musicmaker/musicmaker/recording_202101
https://www.nxtbook.com/musicmaker/musicmaker/recording_202012
https://www.nxtbook.com/musicmaker/musicmaker/recording_202011
https://www.nxtbook.com/musicmaker/musicmaker/recording_202010
https://www.nxtbook.com/musicmaker/musicmaker/recording_202009
https://www.nxtbook.com/musicmaker/musicmaker/recording_202008
https://www.nxtbook.com/musicmaker/musicmaker/recording_202007
https://www.nxtbook.com/musicmaker/musicmaker/recording_202006
https://www.nxtbook.com/musicmaker/musicmaker/recording_20205
https://www.nxtbook.com/musicmaker/musicmaker/recording_202004
https://www.nxtbook.com/musicmaker/musicmaker/recording_202003
https://www.nxtbook.com/musicmaker/musicmaker/recording_202002
https://www.nxtbook.com/musicmaker/musicmaker/recording_202001
https://www.nxtbook.com/musicmaker/musicmaker/recording_201912
https://www.nxtbook.com/musicmaker/musicmaker/recording_201911
https://www.nxtbook.com/musicmaker/musicmaker/recording_201910
https://www.nxtbook.com/musicmaker/musicmaker/recording_201909
https://www.nxtbook.com/musicmaker/musicmaker/recording_201908
https://www.nxtbook.com/musicmaker/musicmaker/recording_201907
https://www.nxtbook.com/musicmaker/musicmaker/recording_201906
https://www.nxtbook.com/musicmaker/musicmaker/recording_201905
https://www.nxtbook.com/musicmaker/musicmaker/recording_201904
https://www.nxtbook.com/musicmaker/musicmaker/recording_201903
https://www.nxtbook.com/musicmaker/musicmaker/recording_201902
https://www.nxtbook.com/musicmaker/musicmaker/recording_201901
https://www.nxtbook.com/musicmaker/musicmaker/recording_201812
https://www.nxtbook.com/musicmaker/musicmaker/recording_201811
https://www.nxtbook.com/musicmaker/musicmaker/recording_201810
https://www.nxtbook.com/musicmaker/musicmaker/recording_201809
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201808
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201807
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201806
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201805
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201804
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201803
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201802
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201801
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201712
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201711
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201710
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201709
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201708
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201707
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201706
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201705
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201704
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201703
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201702
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201701
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201612
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201611
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201610
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201609
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201608
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201607
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201606
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201605
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201604
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201603
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201602
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201601
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201512
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201511
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201510
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201509
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201508
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201507
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201506
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201505
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201504
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201503
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201502
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201501
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201412
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201411
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201410
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201409
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201408
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201407
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201406
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201405
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201404
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201403
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201402
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201401
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201312
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201311
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201310
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201309
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201308
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201307
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201306
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201305
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201304
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201303
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201302
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201301
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201212
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201211
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201210
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201209
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201208
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201207
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201206
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201205
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201204
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201203
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201202
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201201
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201112
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201111
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201110
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201109
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201108
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201107
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201106
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201105
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201104
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201103
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201102
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201101
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201012
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201011
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201010
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201009
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201008
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201007
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201006
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201005
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201004
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201003
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201002
https://www.nxtbook.com/nxtbooks/musicmaker/recording_201001
https://www.nxtbook.com/nxtbooks/musicmaker/recording_200912
https://www.nxtbook.com/nxtbooks/musicmaker/recording_200911
https://www.nxtbook.com/nxtbooks/musicmaker/recording_200910
https://www.nxtbook.com/nxtbooks/musicmaker/nxtd
https://www.nxtbookmedia.com