Section 5. Gain Structure And Levels; Standard Operating Levels - Yamaha PM4000 Operating Manual

GAIN STRUCTURE AND LEVELS

5.1 STANDARD OPERATING LEVELS

There are a number of different "standard" operating
levels in audio circuitry. It is often awkward to refer to a
specific level (i.e., +4 dBu) when one merely wishes to
describe a general sensitivity range. For this reason,
most audio engineers think of operating levels in three
general categories:
A. MIC LEVEL OR LOW LEVEL
This range extends from no signal up to about
-20 dBu (77.5 mV), or -20 dBm (77.5 mV across
600 ohms = 10 millionths of a watt). It includes the
outputs of microphones, guitar pickups, phono car-
tridges, and tape heads, prior to any form of amplifica-
tion (i.e., before any mic, phono, or tape preamps).
While some mics can put out more level in the presence
of very loud sounds, and a hard-picked guitar can go
20 dB above this level (to 0 dBu or higher), this re-
mains the nominal, average range.
B. LINE LEVEL OR MEDIUM LEVEL
This range extends from -20 dBu or -20 dBm to
+30 dBu (24.5 V) or +30 dBm (24.5 V across 600 ohms =
1 watt). It includes electronic keyboard (synthesizer)
outputs, preamp and console outputs, and most of the
inputs and outputs of typical signal processing equip-
ment such as limiters, compressors, time delays,
reverbs, tape decks, and equalizers. In other words, it
covers the output levels of nearly all equipment except
power amplifiers. Nominal line level (the average level)
of a great deal of equipment will be -10 dBu/dBm
(245 millivolts), +4 dBu/dBm (1.23 V) or +8 dBu/dBm
(1.95 V).
C. SPEAKER LEVEL AND HIGH LEVEL
This covers all levels at or above +30 dBu (24.5V) or
+30 dBm (24.5 V across 600 ohms = 1 watt). These
levels include power amplifier speaker outputs, AC
power lines, and DC control cables carrying more than
24 volts.
NOTE: A piece of consumer sound equipment ("hi-fi
may operate at considerably lower nominal (average)
line levels than +4 dBu. This is typically around -16 dBu
(123 mV) to -10 dBu (245 mV) into 10,000 ohms or
higher loads. Peak output levels in such equipment may
not go above +4 dBu (1.23 V). The output current
available here would be inadequate to drive a 600-ohm
terminated circuit, and even if the professional equip-
ment has a higher impedance input, the output voltage
SECTION 5.
of the hi-fi equipment may still be inadequate. The
typical result is too-low levels and too-high distortion.
This can damage loudspeakers (due to the high fre-
quency energy content of the clipped waveform), and it
can damage the hi-fi equipment (due to overloading of
its output circuitry). There are exceptions, but one should
be very careful to check the specifications when using
consumer sound equipment in a professional applica-
tion.
Let's discuss these levels in the context of a sound
system. The lowest power levels in a typical sound
system are present at the output of microphones or
phono cartridges. Normal speech at about one meter
from the "average" dynamic microphone produces a
power output from the microphone of about one tril-
lionth of a watt. Phono cartridges playing an average
program selection produce as much as a thousand times
this output - averaging a few billionths of a watt. These
signals are very weak, and engineers know that they
cannot be "run around" a chassis or down a long cable
without extreme susceptibility to noise and frequency
response errors. This is why microphone and phono
preamps are used to boost these very low signal levels
to an intermediate range called "line level." Line levels
are between 10 millionths of a watt and 250 thou-
sandths of a watt (¼ watt). These levels are related to
the "dBm" unit of measurement as follows:
-20 dBm =
0 dBm =
+4 dBm =
+24 dBm =
+30 dBm =
+40 dBm =
+50 dBm =
While some console and preamp outputs can drive
lower impedances, primarily for driving headphones,
typical line levels (measured in milliwatts) cannot drive
speakers to useable levels. Not only is the power insuffi-
n
cient for more than "whisper" levels, the console circuits
)
are designed to operate into loads of 600 ohms to 50,000
ohms; they cannot deliver even their few milliwatts of
rated power to a typical 8-ohm speaker without being
overloaded. A power amplifier must be used to boost the
power output of the console so it is capable of driving
low impedance speaker loads and delivering the re-
quired tens or hundreds of watts of power.
Page 5-1
10 microwatts 0.00001 watts
=
1 milliwatt 0.001 watts
=
2.5 milliwatts
0.0025 watts
=
250 milliwatts = 0.025 watts
1000 milliwatts = 1 . 0 w a t t s
10.0 watts
=
= 100.0 watts