THE DISTINCTION BETWEENPROFESSIONAL AND HI-FIEQUIPMENTIn most applications, a variety of auxiliary equipment
will be connected to the P-2200, including: mixers, tape
machines, compressors, graphic equalizers, echo, time
delay, and reverb units, and just about any other audio
electronics imaginable. Regardless of the function of
auxiliary equipment, it will undoubtedly fall into one of
two general categories, professional type or hi-fi type.
The following criteria place most "semi-pro" equipment
in the hi-fi classification.
The distinction between professional and hi-fi equip-
ment is important primarily because it affects the way it
will be used with the P-2200. Brand name, size, panel
colors, durability and subtleties in function are not the
significant differences. What matters is that professional
equipment and hi-fi equipment usually operate at
different input and output levels, and require different
source and load impedances to function properly. The
P-2200 is designed to function well with other pro-
fessional equipment, although it has high enough input
impedance and sensitivity to yield excellent results with
hi-fi type equipment if a few precautions are observed.
(These precautions are outlined in the Installation sec-
tion of the manual.) The following paragraphs explain
how the specific requirements differ for professional and
hi-fi (or semi-pro) equipment.
IMPEDANCE
The inputs of a piece of professional audio equipment
are usually designed to be driven from a low impedance
source,
nominally
150
to
600 ohms, and its
outputs
will
drive low impedance (600 ohm or higher) loads. (Power
amplifier outputs are not considered in this discussion.)
Professional input and output circuits may be
unbalanced, but they are often transformer isolated
(balanced or floating), and use dual conductor shielded
cables, with 3-pin XLR type connectors or Tip/Ring/
Sleeve phone plugs.
The P-2200's inputs are unbalanced due to cost and
adaptability factors. To internally balance the inputs of
the P-2200 would require two matched input transfor-
mers with heavy shielding (to avoid hum pickup from
the P-2200's power transformer). Induced hum in low
level circuits, especially in low level transformers, can
be a problem with any power amplifier, or other high
current device (such as a DC power supply). High quality
external
transformers
with
less
shielding can achieve
the
same
results
with
a
substantial cost
savings.
In
addition,
the user can choose the optimum impedance ratio for
a given situation, increasing the P-2200's adaptability.
Either the "matching transformer box" or "step up
transformer box" described on Pages SIX 3, and SIX 4
are suitable, so long as they are kept several inches
away from the P-2200.
Hi-fi (and semi-pro) equipment generally is designed
to be driven from a 5,000-ohm (or lower impedance)
source, and its output will drive 10,000-ohm (or higher
impedance) loads. Hi-fi input and output circuits are
usually unbalanced, and use single conductor shielded
cables with 2-conductor connectors, either standard
phone plugs or phono plugs (also called RCA or pin
plugs). Occasionally, the inputs of a piece of hi-fi or
semi-pro equipment are professional XLR connectors
which have been converted to a 2-wire, unbalanced
circuit by internally connecting either pin 2 or pin 3
to
pin
1.
The nature of unbalanced, balanced, and floating
circuitry is discussed further in the Appendix 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.
NOTE: THERE IS NO CORRELATION BETWEEN
"BALANCED" OR "FLOATING" AND CIRCUIT
IMPEDANCE.
Low impedance and high impedance are relative
terms. A 150- to 250-ohm microphone is considered low
impedance, whereas a 10,000-ohm mic is considered
high impedance. A 600-ohm line is considered low
impedance, whereas 10,000-ohm, 50,000-ohm or
250,000-ohm lines are all considered high impedance.
Sometimes, mics and lines with an impedance of 600
ohms to about 2000 ohms are considered "medium"
impedance. NOTE: THE IMPEDANCE OF A CIRCUIT
SAYS NOTHING ABOUT ITS LEVEL.
While the exact transition between low and high
impedance is not clearly defined, the distinction is still
important, primarily because the output impedance of a
source determines the length of cable that can be
connected between it and a load before a serious loss
of high frequencies occurs. The losses occur because all
cables, and especially shielded cables, have some
capacitance between their conductors. Some guitar
coil cords may measure as high as 1000 picofarads total
capacitance! A source impedance (such as a high
impedance mixer output) and the capacitance of a
cable form a type of low-pass filter a filter that attenu-
ates high frequencies. This filtering effect, can be
reduced by using low capacitance cable, by shortening
the length of the cable, by using a low impedance source
or by some combination of these methods.
Fig. 32 - The Source's Output Impedance and the Cable
Capacitance act as an "RC Lowpass" Filter which Attenuates
High Frequencies.