Scott,
Not knowing how your system is "exactly" built makes it difficult to second
guess the lightning.
However, here is one way to protect your amplifiers: Move the protection
interface out to the edge of your building. Use rod located there with all
referenced to ground. At this same location use a 10A AC mains line filter
between the amplifier and the speakers (cheap filtering which should be able
to pass the audio) with AC line towards amp and load towards speakers.
Between filter and speakers place fusing in series (won't do much for truly
high voltage which will jump, but will take care of a lot of nuisance
discharges) place gas discharge tubes there. [ If you can get a surplus
telephone entry block, the type with the carbon "shorts", they work great
here, too. ] Back side of filter place tranzorbs, then back at amp place
more tranzorbs.
You prevent lightning damage by designing a filtering system which limits
the maximum amplitude that can get into your electronics. And the most
effective rejection filter is always high impedance in series, low impedance
to ground, high impedance in series and low impedance to ground, ad nauseum.
Just make multiple paths that do this and you can even sustain a direct hit.
Speakers and wires will probably fry though.
- Robert -
Robert A. Macy, PE [email protected]
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: Scott Lacey <[email protected]>
To: [email protected] <[email protected]>
List-Post: [email protected]
Date: Saturday, June 15, 2002 9:19 AM
Subject: Lightning Protection for PA System
To the group,
I am seeking advice as to the best methods of protecting a Public Address
system
against recurring lightning damage. The system uses several commercial PA
amplifiers, each driving several speakers at indoor and outdoor locations.
There are
also several locations where microphones can be plugged in. The longest
speaker
wires may be up to 250 yards long. It is believed that the charge is being
coupled to
the speaker wires where it then returns to ground at the amplifier location
within the
building, destroying the solid state devices within the amplifier. A
technician has
added fuses to all external microphone inputs and speaker outputs. While
these
have blown several times during storms without obvious damage to the
amplifiers it
is my belief that fuses are generally too slow to protect semiconductor
devices. I am
seeking advice as to surge suppression devices.
System particulars are as follows:
1) The PA amplifiers have 70 volt outputs. All speakers are transformer
coupled.
2) All microphones use standard XLR connectors. They plug into metallic
conduit
mounted jacks at locations inside and outside the building. The outside
microphones
are unplugged during storms.
3) The amplifiers are located on the second floor of the building. Each
amplifier is
dedicated to a set of speakers at one location. The amplifier driving the
longest wires
is the one which most often has to be replaced.
4) The building is in a location known to be susceptible to lightning
activity. Electrical
appliances have been destroyed on at least two occasions.
5) All protective grounding efforts to date have been made to the conduit. I
am
recommending that this be supplemented by driven rods.
It is pretty easy to buy commercial surge suppression devices for the ac
lines. I feel I
need advice as to the best methods to protect the audio inputs and outputs.
For the 70 volt outputs I am thinking of using gas tubes to earth where the
wires
enter the building supplemented by MOV, Tranzorb, or other devices near the
amplifier location. I welcome suggestions as to device types.
For the microphone inputs I am thinking of using semiconductor transient
voltage
suppressors near the amplifier. Again, any suggestions are welcome.
Thanks in advance for any advice and guidance.
Scott Lacey
[email protected]
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