Cables from high impedance sources (5000 ohms and
up), should not be any longer than 25', even if low
capacitance cable is used; shorten the cables if the
impedance is higher. For low impedance sources of 600
ohms or less, cable lengths to 100' are relatively effective.
For very low impedance sources of 50-ohms or less,
cable lengths of up to 1000 feet are possible with
minimal loss. However, the frequency response of the
source, the desired frequency response of the system,
and the amount of capacitance and resistance in the
cable all play a role in any potential high frequency
losses. Thus, these values are meant as guide lines, and
should not be considered fixed rules.
For short runs and in smaller systems with fewer
components, the performance of an unbalanced circuit
may be adequate. In a long cable run, a balanced or
floating circuit tends to reject hum and noise pickup
better than an unbalanced circuit, and in complex
systems, with several components separated by some
distance and running on different AC outlets, balanced
or floating circuits make proper grounding much easier.
In any given situation, the decision to use a hi-fi
(semi-pro) device or a professional one should be based
on the specifications of the inputs and outputs of that
device and on the requirements of the application.
OPERATING LEVELS
Nominal professional line level is u4dBm or
+8dBm; that is, the average program level is approxi-
mately 1.23V rms (+4dBm), or 1.95V rms (+8dBm)
terminated by a 600-ohm line. The peak level may
extend to about +24dBm (12.3V rms). The line (high
level) input of professional audio equipment is
designed to accept levels on this order of magnitude
without overdrive (clipping distortion); most pro-
fessional equipment can be driven to full output by
n4dBm input (source) levels, although a few
units r8dBm (1.95V rms) at their input to
yield full output. See the discussion of "Gain Overlap"
on Page FIVE 4.
Hi-fi type equipment operates at considerably lower
line levels than professional equipment (with exceptions),
usually at -16dB (0.123 volts) nominal levels. Notice we
use the expression "dB," not "dBm." This is because
"dBm" denotes a power level (relative to 1mW, or
0.775V rms across a 600-ohm impedance), whereas "dB"
denotes a voltage level (as defined in this manual) rela-
tive to 0.775V rms. This is a subtle distinction, and is
explained in greater detail in the Appendix on Page
EIGHT 1 , and on Page THREE 1 of the specifications.
The nominal -16dB (0.123 volts) level of hi-fi
equipment is equal to 123mV rms ( 1 2 3 one-thousandths
of a volt) across a 10,000-ohm or higher impedance line.
Peak program levels may reach or slightly 4dB
(1.23V rms across a high impedance line). Note that a
hi-fi unit capable of +4dB (1.23 volts) maximum output
into a high impedance, does not possess adequate drive
for 600-ohm circuits with nominal +4dBm level require-
ments. Thus, hi-fi equipment is usually incapable of
driving professional equipment to its full rated output,
at least not without first reaching a high level of
distortion. Moreover, when the output of hi-fi equip-
ment (which is almost always meant to be operated
into a high impedance) is connected directly to the low
impedance input of professional equipment, the hi-fi
unit "sees" a partial short circuit. This may overload the
hi-fi output, or it may simply drop the output level by a
few dB, depending on the circuitry. The P-2200's input
sensitivity and input impedance are high enough to allow
its use with some hi-fi or semi-pro equipment, however
it's a good idea to check the specifications for each
situation. The point of this discussion, is that impedance
and level are extremely important considerations when
connecting audio equipment.
DYNAMIC RANGE
Every sound system has an inherent noise floor
which is the residual electronic noise in the system
equipment (or acoustic noise in a room). The effective
dynamic range of a system is equal to the difference
between the peak output level of the system and its
noise floor.
A concert with sound levels ranging from 30dB SPL
to 120dB SPL has a 90dB dynamic range. The electrical
signal level in the sound system (given in dB of voltage)
is proportional to the original sound pressure level (given
in dB SPL) at the microphone. Thus, when the program
sound levels reach 120dB SPL, maximum electrical
levels (at the mixer's output) might reach +24dB (12.3
volts), and maximum power output levels (at the
P-2200's output) might reach 230 watts into an 8-ohm
load. Similarly, where sound levels drop to 30dB SPL,
minimum electrical levels will drop to -66dB (0.388
milli-volts) and power levels will drop to 230 nano-watts
(230 billionths of a watt; these levels are not uncom-
mon). The program still has an electrical dynamic range
of 90dB: [+24dB (12.3 volts)] - [-66dBm (0.388
micro-volts)] = 90dB. This dB to dB correspondence is
maintained throughout the sound system, from the
original source at the microphone, through the
electrical portion of the sound system, to the speaker
system output. A similar correspondence holds for any
other type of sound system, a recording studio system,
disco system or a broadcast system.
Generally, the average electrical line level in the
above sound system is +4dB (1.23 volts) corresponding
to an average sound level of 100dB SPL. This average
level is usually called the nominal program level. The
difference between the nominal and the highest (peak)
levels in a program is the headroom. In the above
example, the headroom is 120dB SPL -100dB SPL =
20dB (not 20dB SPL). Similarly, the electrical head-
room is [+24dB (12.3 volts)] - [+4dB (1.23 volts)] =
20dB (not 20dBm, see Appendix). This corresponds to a
power headroom which is also 20dB.
In the above example, if the system had an
electronic noise floor of -56dB (1.23 millivolts), and
a peak output level of +18dB (6.16 volts), its dynamic
range would only be 74dB. If the original program has
a dynamic range of 90dB, then 16dB of the program is
lost in the sound system. There may be extreme clipping
of program peaks, some of the low levels may be buried
in the noise, or some of the program may be lost in both
ways. Thus, it is extremely important to use wide
dynamic range equipment, like the P-2200 and Yamaha
PM-Mixers, in a professional sound reinforcement system.
In the special case of a tape recorder, where the
dynamic range is limited by the noise floor and
distortion levels of the tape itself, one way to avoid
these program losses due to clipping and noise is to
"compress" the program's dynamic range (see Page
SEVEN 3). A better way is to apply special "noise
reduction equipment" which allows the original program
dynamics to be maintained throughout the recording
and playback process. This improvement in the dynamic
range of recorded material again demands wide dynamic
range from every piece of equipment in the recording/
playback chain, including the power amplifier.
