Proper Distributed Audio and Amplification
Crucial factors to keep in mind for high-quality intelligibility
I felt the need to write this article after a recent
cross-country trip, and I’m pretty sure many of you can relate to
my experience. At a newer, modern airport, I was waiting to board a connecting
flight, and everything appeared to be going as planned until the departure
time suddenly changed on the monitor - a one hour delay.
However, I never learned the reason for the delay, because the gate attendant
making the announcement either had the mic down her throat - or, more
likely - the paging system was defective. Speech intelligibility was poor
to non-existent.
This can’t be, I thought. A modern airport certainly must have a
paging system utilizing the latest technology, properly designed and installed
to provide maximum intelligibility. We all need to “hear and understand”
the announcements, right? After all, this is kind of the point, and unfortunately,
it was a point completely missed by this system.
There can be several reasons for poor intelligibility with a paging system.
In this case, the system was most plagued by bad loudspeakers combined
with poor placement, and I’m sure “Mr. Budget” had negative
impact as well, as it often does in systems of this nature. I wonder how
many people miss their flights daily due to a poor paging system? One
would think the amount of money wasted on compensating for missed flights
would justify the cost of insuring every airport paging system sound do
what it’s supposed to do: deliver intelligible audio to everyone
in the facility.
Many audio professionals believe distributed audio cannot sound good.
And in fact, it’s never going to sound as good as a finely tuned
PA, but it also doesn’t need to sound like a badly tuned AM radio.
If a distributed system is designed and budgeted properly, there is absolutely
no reason for poor intelligibility or background music quality.
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Distributed audio products have improved
much over the past 10 years, because many manufacturers realized
that distributed audio is big business. For example, a decade ago,
there were only a few manufacturers offering ceiling loudspeakers,
and now, just about everyone in the loudspeaker business has jumped
on the bandwagon. Keep in mind that a major brand name on a loudspeaker
doesn’t at all mean it provides the appropriate and necessary
performance. Quality ceiling products take time to perfect. |
THE WEAKEST LINK
While choosing proper loudspeakers for a distributed application is essential
for good results, if you’re feeding these drivers limited audio
signal and power, they still will not perform properly. The system will
only be as good as its weakest link, therefore, every component in the
chain - microphone(s), mixer, power amplifiers, step-up power transformers,
wire gauge, step-down transformers and loudspeakers - must deliver good
frequency bandwidth.
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Another significant problem with distributed
systems is the power amplifier side of the equation. Unfortunately,
many installers lack understanding as to how to properly utilize
power amplifiers for distributed applications. As with loudspeakers,
most power amp manufacturers now offer products for distributed
applications. I’m often asked about the differences between
a standard twochannel power amp and a 70.7-volt or 100-volt power
amp. Besides connectors and security features, there’s not
much discrepancy. |
Many distributed systems require only 100-watts at a 70.7-volt system
operating voltage. (A 100-watt amplifier only has a voltage swing of 28
volts RMS at eight ohms.) Thus a step-up transformer needs to be attached
to achieve the required distributed voltage. There are two types of stepup
transformers, isolation and autoformer, and both have pros and cons.
The autoformer is the easier to engineer, and the cheaper option. As a
result, it’s the step-up transformer found in most power amps. Autoformers
also supply good frequency response, but the downside is lack of protection
between the loudspeaker and the power amp. Therefore, odds are pretty
good that a system using an autoformer will have a short circuit on a
line of 10 loudspeakers running on 300 feet of wiring. And if a short
occurs, the power amp will shut down or even fail completely.
A step-up isolation transformer is a more solid approach in terms of power
amp protection. With this approach, the power amp always sees a constant
load impedance. If a short occurs anywhere in the chain, most likely the
power amp will continue to be able to drive the system. However, frequency
response suffers, limited to about 50 Hz to 8 kHz. A good bandwidth for
an isolation step-up transformer would be 40 Hz to 18 kHz (+/- 1 dB).
So prior to purchasing a power amp with a step-up transformer, find out
what type of transformer it is as well as its bandwidth. Be sure to check
(or ask for) specifications of both frequency response and dB.
THE REQUIRED “SWING”
Not all power amps require a step-up transformer to deliver high-output
voltage. For example, Crown offers a line without output transformers,
which are instead outfitted with high enough internal DC rails to produce
the required output “swing” for the load. These power amps
also load protect against lower distributed impedances. It’s a very
good solution for eliminating bandwidth limitations.
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And in fact, many of the latest hybrid
of high-power amps now on the market deliver enough voltage to drive
a 70- volt system without need of step-up transformers. With higher
power amps comes a larger voltage swing. For a 600- watt power amp
running at eight ohms, voltage output is 69 volts. The impedance
of a 600-watt 70.7-volt distributed system is 8.33 ohms. (This can
be calculated using Ohm’s law.) What this means is that you
don’t necessarily need to use all of the power provided by
the amp. If you only need 300 watts, the 600-watt power amp used
in my example will deliver the necessary power along with the required
voltage swing. |
On the other hand, a standard 150-watt power amp at eight ohms will
not deliver the required 70.7 volts unless it is bridged or outfitted
with a step-up transformer.
Here is an example of how to bridge a power amp for 70.7-volt operation.
Let’s say we have a distributed system requirement of 150 watts
at 70.7 volts. This will require a power amp that delivers 150 watts per
channel at eight ohms, resulting in an output voltage of 34.6 volts per
channel. For a 70.7-volt system requiring 150 watts, load impedance is
calculated at 32 ohms.
When this power amp is bridged, it sees only half the impedance load applied
to it. For a 32-ohm load, each channel sees 16 ohms. Most manufacturers
do not publish 16-ohm specifications, so to determine this, divide the
8-ohm rating in half. So in this example, a 150-watt at 8 ohms is 75 watts
at 16 ohms. The voltage swing for 75 watts at 16 ohms is 34.6 volts, and
with the power amp bridged, power and voltage outputs double. Thus after
bridging this power amp, output of 70.7 volts and 150 watts is attained,
close enough to adequately drive the system.
CHECK THAT SATURATION
Overdriving is a common mistake with distributed systems. If more than
70.7 volts (or 100 volts) is applied, step-down transformers may saturate,
causing them to go from high impedance to an almost dead short. Not only
will sound quality suffer immensely, but the power amp may fail. And be
extra careful that this does not happen if you are bridging a power amp.
The channel outputs will be shorted together if saturation occurs, and
this is not good!
Step-up transformers can also be saturated if too much power is applied
to the primary. When selecting a step-up transformer, be sure to match
the power output of the power amp to the primary impedance of the transformer.
(Most primaries are either 4 ohms or 8 ohms.)
Another issue of which to be aware is the combined frequency response
of a loudspeaker and step-down transformer. Most only go flat to 120 Hz.
To conserve power from the power amp while also avoiding low-frequency
saturation, place a high-pass filter on the input of the power amp, which
will limit its low-frequency response. It is recommended to use high-pass
filter rated at 80 Hz to 120 Hz, 12 dB to 24 dB.
Always remember that 70.7 volts or 100 volts is the maximum voltage to
be applied to the step-down transformer. Further, a distributed system
design should operate between 50 and 75 percent of its total capability,
allowing for additional headroom when needed and preventing potential
transformer saturation.
To attain a distributed system offering excellent intelligibility and
quality background music reproduction, it’s essential to have a
good understanding of all the pieces in the chain as well as the potential
problems that may occur.
Jeff Kuells is an audio engineer and audio manufacturing consultant
and was previously director of engineering for a major amplifier manufacturer.
He is Live Sound’s amplifier technical editor. Reach him at proamps@foothill.net