Owner's Manual

AtlasIED.com – 16 –
Specifications are subject to change without notice.
Owner’s Manual
AA120G & AA240G
Commercial Mixer Amplifier
1601 Jack McKay Blvd. • Ennis, Texas 75119 U.S.A.
Telephone: [phone] • Fax: [phone]
Measuring a Speaker System’s Impedance
Note: It is important to only use an Audio Impedance Meter and not a conventional volt/Ω meter. A true audio frequency impedance
meter is essential for reliable installation of background music and paging systems in residences, offices buildings, and public areas.
Avoid costly service calls and amplifier damage by verifying actual speaker system impedance prior to operation. Unlike conventional
volt/Ω meters, which measure DC resistance, an Audio Impedance Meter unit utilizes an internal frequency oscillator to measure
true impedance. It may also be utilized with 25V/70.7V and 100V speaker line transformers, L-pads and matching impedance volume
controls. There are several Audio Impedance meters on the market, if you need to buy one we suggest going to MCM Electronics or
search the internet for Audio Impedance Meter.
Measuring 25V/70.7V Distributed Speaker Systems
Large distributed systems typically utilize 25.2V or 70.7V transformers (50V and 100V in Europe), to greatly ease the connection of
multiple speakers and facilitate long cable runs. These speakers are connected in parallel, as shown below, with total wattage ratings
added to calculate the overall rating of the system. Connecting this meter to a speaker arrangement such as this will provide the overall
impedance of the system. Using the following formula, you can calculate the wattage. Simply put, when connected to a distributed
system, take the voltage of the system (normally 70.7V or 25.2V), squared, divided by the impedance displayed on the meter. Your
answer will be the total system wattage. This total wattage must not exceed the wattage output rating of the amplifier or damage
may occur. In this example the measurement for a 70.7V design with speakers of 10W, 30W and 60W, the system impedance would
measure close to 50Ω. Formula 70.7V x 70.7V = 4998, 4998 / 100W = 49.9Ω. If this speaker system load is connected to at least a
120W amp @ 70.7V, the system will operate properly. Note: It is always recommended to use a larger amp than needed with output
power headroom of at least 25%.
Max Load Chart
Speaker Tap AA120G AA240G
4Ω 4Ω 4Ω
25V 5.25Ω 2.6Ω
70.7V 41Ω 20Ω
100V 83Ω 41Ω