Yamaha P-2200 - Manual (Page 33)

Use of the Input Polarity Switch on the P-2200
The XLR input connectors on the P-2200 are un-
balanced. In one position, the switch beside the con-
nectors attaches pin 2 to pin 1 (ground) leaving pin 3
"hot" (USA standard). In the other position, the switch
attaches pin 3 to pin 1 (ground) leaving pin 2 "hot"
(DIN/JIS standard). If the source feeding the P-2200's
input is unbalanced, the switch must be properly set to
avoid shorting out the source. If the source is balanced,
the P-2200's inputs will unbalance the source. In many
situations, this is acceptable, however, the input polarity
switch must still be set in the position corresponding to
the "hot" pin of the balanced source. If the switch is
set in the wrong position, the signal will be inverted at
the P-2200's output compared to the signal at the
source ("out-of-phase"),
Fig. 49 — Polarity Switch Use
Output Impedance Matching
Within its rated power and voltage limits, the P-2200
acts very much like a perfect voltage source (see
Appendix). Thus, as the impedance of the load goes
down, the total power delivered by the P-2200 goes up.
Figure 4, Page FOUR 1 illustrates this action. Note that
when the impedance of the load falls below 2.5 ohms, the
P-2200's protection circuitry begins to limit the total
amount of power delivered.
For purposes of calculating the total load impedance
that is presented to the P-2200, assume that speaker
impedances do not change with frequency. The Appen-
dix shows various series and parallel combinations of
speakers and the effective loads they present to the
P-2200. Formulas for the power delivered to each
speaker in a parallel or series combination are included.
Fig. 50A - Speakers in Series
Fig. 50B - Equivalent Circuit: Speaker Impedances in Series.
Fig. 50C - Circuit as seen by One Speaker of Series Pair.
Note that a series connection of two speakers degrades
the damping factor (see Page FOUR 7) because each
speaker looks back at the amplifier through the
impedance of the other speaker. Thus the effective out-
put impedance of the P-2200 as seen by one speaker is
equal to the actual output impedance of the other
speaker (see Figure 50).
Also, the impedance of most speakers lowers with
frequency, so that the effective load of two "8 ohm"
speakers in parallel across the output of the P-2200 may
be as low as 2.5 to 3 ohms at certain frequencies. Thus,
speaker loads much lower than 8 ohm nominal im-
pedance could overload the amplifier, especially if the
actual impedance drops far below the nominal impedance.
Figure 51 shows the variation of impedance magnititude
with frequency for one type of speaker system.
Fig. 51 - Free-Air Impedance of Typical " 8 " Loudspeaker.
NOTE: Impedance changes when loudspeaker is installed in a
cabinet.
The impedance of 70-volt speaker transformers also
falls with frequency, especially in lower quality trans-
formers. (Note that a "perfect" transformer would not
have any impedance of its own.) If low efficiency 70-
volt transformers are used, the system will need more
transformers and speakers to achieve the same SPL than