Operating Levels; Channel Inputs - Yamaha PM-1000 Manual

TAREE 2
The nature of balanced, floating and unbalanced
circuitry is detailed in the Specifications section of this
manual. For the purpose of this discussion, the most
significant point is that an unbalanced circuit is some-
what more susceptible to hum and noise, especially if
there is any irregularity in the grounding system. It
should be noted that whether a circuit is unbalanced,
balanced or floating has no direct correlation with its
impedance. Low impedance and high impedance are
relative terms. A-150 ohm microphone is considered low
impedance, whereas a 2,000-ohm mic is considered high
impedance. A 600-ohm line is considered low impedance,
whereas 5,000-ohm, 50,000-ohm, or 100,000-ohm lines
are all considered high impedance.
For audio transmission over long distances, low
impedance lines provide the best results. High impedance
lines attenuate the higher frequencies, however in short
cable runs, under 10 feet, there is virtually no difference
between the results obtained with low and high imped-
ance lines. Also, provided the grounding is correct, there
is little difference between unbalanced and balanced or
floating circuitry. In any case, the specific requirements
of the PM-1000's input and output circuits should be
considered in setting up the system.
OPERATING LEVELS
Professional line level is nominally between OdBm
and *4dBm; that is, the average program level is approxi-
mately .775Vrms to 1.23Vrms terminated by a
600-ohm line. The peak level may extend up to about
*24dBm or higher (12.3Vrms). The high level input
(not mic input) of professional audio equipment is
designed to accept levels in this power range without
overdrive (clipping distortion); most professional equip-
ment can be driven to full output by nominal +4dBm
input levels.
Hi-fi type equipment operates at considerably lower
levels than professional equipment, approximately
-16dB nominal. This is equal to about 120mV across a
10,000-ohm or greater (high impedance) line. Peak
program levels may reach or slightly exceed +4dB,
1.23V — across a high impedance line. Thus, hi-fi equip-
ment is usually not capable of driving professional
equipment to its full output, or at least not before the
hi-fi output reaches a high level of distortion. More-
over, when the output of hi-fi equipment, which is
meant to be terminated in a high impedance, is con-
nected directly to professional equipment, the low
impedance acts as a partial short-circuit. This serves to
overload the hi-fi output.
The PM-1000's channel inputs are correctly matched
to professional equipment; they will not overload low
impedance microphones and lines. But the same inputs
can overload some hi-fi equipment. To prevent possible
overload, and to obtain the best frequency response
with hi-fi equipment, a matching transformer may be
installed. This converts a high impedance to a low
impedance. Approximately 10dB to 20dB of voltage
level is lost in the transformer, but this level can be
recovered with the console's Input Level switches.
Because the signal is being attenuated and then am-
plified again, some increase in the signal-to-noise ratio
will occur, but it is usually not objectionable.
At the other end of the system, the PM-1000's out-
puts are best suited to driving professional equipment
(with the exception of Master Out, as explained on pg. 3
of this section). When driving hi-fi equipment, care must
be taken to avoid overdrive of the hi-fi input. An excel-
lent method to avoid overdrive is to connect a 20dB or
24dB T-pad across the hi-fi input. The pad attenuates the
console's nominal +4dBm output to about -16dB, a good
match for average hi-fi input sensitivity. This allows
both the console's faders and the auxiliary equipments'
controls to be operated in the range that yields maxi-
mum headroom, lowest signal-to-noise characteristics,
and a wide range of physical control. (Pad construc-
tion is illustrated in the Installation secton).
CHANNEL INPUTS
The PM-1000's channel inputs are transformer-
isolated. A very broad range of level control, by means
of attenuation and gain trim, is provided by the Input
Level switches. Virtually any microphone or line source
can be accommodated. Only very high level inputs, such
as the speaker output from power amplifiers or electric
instrument amplifiers, will require external attenuation
pads.
The inputs are designed for low impedance sources,
typically 150-ohm to 250-ohm microphones and 600-
ohm lines. Low impedance cables yield superior trans-
mission over long distances; many such cables can be
run 2,000 feet or more without severe high frequency
loss and without hum pick-up, although microphone
manufacturer's specifications should be consulted.
When a high impedance microphone (or auxiliary
electronic device) is used, a high-to-low impedance
matching transformer should be installed at the remote
(mic) end of the cable. Condenser microphones that
contain integral preamplifiers can be phantom powered
from the console's 48V supply. This power is trans-
mitted through standard 2-conductor shielded cables,
along with audio signal; the circuitry is designed so that
non-condenser type microphones should not be damaged.
SUBMIXER INPUTS
Sub In provides a convenient means to expand the
input capability of the PM-1000 by allowing another
console or mixer to be ''chained'' to the PM-1000.
When more input channels are required, an auxiliary
mixer (or several for that matter) can accept the extra
inputs, pre-mix them, and feed them to the console's
mixing buses through the Sub In jacks. Sub In may also
be used to accept feeds from remote studios or other
remote sources, as well as from tape recorders and
echo/reverb devices.
Since the four Sub In jacks apply audio directly to
the program mixing buses, the level and impedance of
the incoming audio must be correct. Sub In has a
nominal input sensitivity of -20dB, so professional
equipment with nominal +4dBm output levels will
have to be padded 20 or 24dB. Some mixers, such as
the Yamaha PM-400B, have a -20dB (medium level)
output in addition to a +4dBm output, and can there-
fore be connected directly to Sub In without padding.
Many hi-fi type mixers, as well as hi-fi tape machines,