Power Amplifiers in Bridge
Mode - Page 2
Therefore, with many boxes, if you pack the trapezoidal enclosures
tightly, you still have severe overlapping of the high frequency
horns, resulting in diminished high frequency output directly
on-axis. Often the boxes must be splayed farther apart to minimize
the overlap of the horns.
In many auditorium applications where you need coverage for
both the near and far field seats, you can address the room
with two sets of loudspeakers. The near seats are in the near
field and the seats of course are in the far field. You can
solve the problem of excessive overlapping and still address
the near and far field requirements by employing an array of
three loudspeakers. In my example I am going to use three HDH-244T
enclosures. The three speakers would be flown in the center,
directly above the front edge of the stage or sanctuary platform.
The center HDH-244T would be flown right side up, with the
horn on the top, and it would have some downward angle to it.
The two outside HDH-244T loudspeakers would then be flown upside
down, with their horns on the bottom, and they would have more
downward angle. With most of our Peavey enclosures, this downward
angle would be twenty to twenty-five degrees greater than that
the center loudspeaker.
Now the loudspeakers would be "Arrayed" properly, but we must
do something to allow the center speaker to truly take care
of the Far Field seats. The inverse square law says that the
sound coming from a loudspeaker is reduced in level directly
proportional to the inverse of the square of the distance away
from the source. That sound coming directly from the loudspeaker
is called the direct field. The inverse square law can be simplified
if we relate it to the decibel scale. The direct field emanating
from a loudspeaker drops in level or diminishes, at a rate
of -6 dB, every time you double the distance away from the
loudspeaker. Therefore in our application, in order for the
Far Field loudspeaker to provide adequate coverage, the Near
Field loudspeaker system must be turned down in level by -6
dB, so that the Far Field speakers can have a chance to provide
the proper level to the farthest seats.
Also if the Near Field speakers are not reduced in level, the
gain of the entire system, before feedback occurs, will be
limited by the Near Field speakers themselves, because the
Near Field speakers will react with the open microphones causing
the system to feedback before the Far Field speakers can reach
the level necessary to provide the desired SPL to the farthest
seats.
In the past, if you wanted to use one power amplifier for this
application, the two Near Field loudspeakers would be put on
one channel of a power amplifier, and the gain or level control
of that amp would be turned down -6 dB. The single Far Field
speaker would be on its own channel and that channels level
control would be set wide open. We would now have a situation
where our gain structures would be set properly for the Near
and Far Field components of the array. Suppose we were to use
a CS-800X in this situation. When the Far Field loudspeaker
received 200 Watts from the power amp, each of the Near Field
speakers would received 50 Watts (-6 dB equals 1/4 Power).
I am basing this exercise on the condition that each of the
individual loudspeakers has an impedance of eight ohms. Two
hundred Watts may or may not be adequate power depending on
the room and the type of music to be reproduced. Also I would
worry that some individual, who did not understand the need
for limiting the level of the Near Field coverage loudspeakers
by -6 dB, would come along and turn the power amps' Near Field
channel up all the way. This would completely change the performance
capability of the system as a whole.
I can show you how to get +6 dB more performance out of this
system with no more financial investment, while having the
system configured to prevent anyone from changing the calibration
of the Near and Far Field gain structure. The secret to this
application is to operate the CS-800X in bridge mode. Connect
the Far Field HDH-244T across the two red binding posts of
each channel, i.e., in normal bridge mode operation. Then you
would connect one of the Near Field HDH-244T's, across the
A channels' red and black binding posts. Next, the second Near
Field HDH-244T would be connected across the B channels' red
and black binding post; however, (note) the second Near Field
HDH-244T, that is connected to the B channel output, would
have the loudspeaker leads reversed. This is very important,
since the B channel is out of phase with the A channel, you
don't want the second Near Field speaker to be out of phase,
so reversing the loudspeaker leads will maintain the proper
polarity. In this application I would recommend reversing the
loudspeaker leads at the speaker itself.
This would prevent someone from noticing the different color
wires on the A & B channels' red and black banana posts, thinking
they are "correcting" someone else's error by changing them
around. In this application with a CS-800X, the Far Field HDH-244T
would automatically have +6 dB more gain than the Near Field
HDH-244T loudspeakers. When the Far Field loudspeaker receives
800 Watts from the power amplifier, each of the Near Field
speakers would receive 200 Watts. I have shown you a way in
this application to obtain +6 dB more performance in both the
Near and Far Field, with no more investment in dollars. Typically,
you would have had to quadruple the number of loudspeakers
to obtain +6 dB more system performance.
The CS-800X can operate bridge mode into a four-ohm load. It
can operate in stereo down to two ohms on each channel. In
the above application we are connecting a single eight ohm
loudspeaker across the two red binding posts of channels A & B,
and then an additional eight-ohm speaker is connected to channel
A, and another to channel B. As far as the amplifier is concerned,
it's as if we had a four-ohm load hooked up in bridge mode.
Don't try to make this application anymore complicated than
it already is. Many times when I have explained this approach
to someone, they immediately try to figure out how to utilize
even more loudspeakers in the same application. In some rooms,
such as a slice of pie shaped auditorium or church, where the
farthest distance can be to the left and right rear corners,
the same three loudspeakers can be employed in a different
configuration. That is, the center loudspeaker is flown upside
down with more of a downward angle, while the two outside speakers
are flown right side up with less of a downward angle. The
center speaker is now covering the Near Field, so it requires
the -6 dB of attenuation, and the two outside loudspeakers
are now playing to the Far Field, so they do not need the -6
dB reduction in gain. With this type of array, there is no
way to employ the above outlined application.
Here is one more application that can work, however, with the
CS-X amplifiers. Perhaps we are in a room that is long and
narrow, and yet we still need both Near and Far Field coverage
without increasing the horizontal coverage angle. We can fly
the two loudspeakers one above the other and configure the
top (Far Field) speaker in the above bridge mode application,
and then the bottom (Near Field) speaker can be connected across
the A channels' red and black banana posts. This will work
the same way as the first three-speaker array, i.e., the Near
Field speaker will have the proper gain reduction (-6 dB).
I believe we covered a lot of ground in this article. If you
thoroughly understand the operation of a power amplifier in
bridge mode, you may want to undertake some of the outlined
applications. If you are still a little cloudy on how bridge
mode works, I would advise you not to try the above applications.
After I thought up this approach several months ago, I explained
my proposed application to Jack Sondermeyer, Peavey Electronics'
director of analog engineering. Since Jack is the engineer
that actually designed the Peavey CS-X series of amplifiers,
I knew that he would be able to spot any flaws in my thinking,
regarding this application. After I explained this method to
Jack he laughed a little, and I said, "On no, don't tell me
there is a fly in the ointment"? Jack laughed once again, and
said that he was laughing because it was a great idea, he couldn't
believe that he hadn't thought of it.
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