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Testing Amplifier Capability

Please Help Me! I have a few amplifiers that i am having problems with and i need to determine if the amplifiers are bad or if the speaker circuits are bad. each has upwards of 100 speakers on them (70.7 volt system) and they keep shutting down (internal circuit breaker tripping). they only seem to do it when all/most speakers are hooked up, but i can't yet figure out why. I've checked the AC current with a Fluke DMM wired in series, but it only shows what i calc to be around

70 watts (.7 amps to .98 amps) but i should be able to pull close to 120 watts. everyone i talk to tells me this is a bad way to test this system, but nobody can tell me how to. any help will be greatly appreciated.

120 Watt Amps

70 Volt System Lots of Speakers...
Reply to
david
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** Groper Alert !

** Have any idea how to calculate the load presented by all the speakers together ?

Was your amperage test done with a sine wave signal or speech / music ?

The minimum load impedance for a 120 watt 70 volt amp is 41 ohms.

If there are 100 similar speakers, each must be have 4100 ohms load impedance.

Also - beware the existence of low audio frequencies, a big NO-NO with

70volt systems.

....... Phil

Reply to
Phil Allison

What are the 70 volt transformer taps set at?

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

For _any_ kind of "testing" controlled and known conditions are a first step. Try three 60W/115VAC globes in parallel and observe if the thermal protection kicks in---> meaning your load is at fault for whatever reason. (the ammount of connections and load adjustments can give a chance to error).

Have fun

Stanislaw.

Reply to
Stanislaw Flatto

"Stanislaw Flatto"

** Now, watch while this Flat Head does the EXACT OPPOSITE !!

** My Gawd, that is a 8 ohm load with cold filaments !!!

Only rises to 48 ohms when full sine wave power is applied.

Guaranteed to TRIP the thermal breaker !!

Only complete MORONS use filament lamps to test amplifiers.

....... Phil

Reply to
Phil Allison

"Stanislaw Flatto"

are a first

** Now, watch while this Flat Head does the EXACT OPPOSITE !!

** My Gawd, that is a 8 ohm load with cold filaments !!!

Only rises to 48 ohms when full sine wave power is applied.

Guaranteed to TRIP the thermal breaker !!

Only complete MORONS use filament lamps to test amplifiers.

...... Phil

.

---------------- Speaking of a quicky test load... I just measured the small ring on my electric stove..43ohms.. The dynamic resistance compared to filament...dunno. But if it's stable enough then 70VRMS^2/43ohms=114Wattsrms... close to 120W rating of the amplifier. Put element in bucket of water for more stability. D

Reply to
D from BC

Thanks for everyones responses. I'm not sure what a "Groper" is, but if it means i need help, then ya got me :)

together ? No. i'm guessing if i can apply steady power at a steady frequency, then check voltage & current i might be off to a good start?

my last test was done with a speech & tone message. my current readings were peak readings from when the tone was sounding, and if i'm testing properly, it was around 1Khz.

after that i get fuzzy about the impedance measurements, and what would be considered a low frequency that is a big no-no

1/2 Watt for all of them

step. Try three 60W/115VAC globes in parallel and observe if the thermal protection kicks in---> meaning your load is at fault for whatever reason. (the ammount of connections and load adjustments can give a chance to error).

by globes you mean like... light bulbs? if so please explain, i would like to learn more about how these systems really work, not just know how to hook them up, so any insight is appreciated.

Reply to
PDI Dave

** Groper = clueless Google Groups poster.
** Yep.

** No use at all.

You must have a steady, sine wave test signal.

** People who service live voltage PA systems MUST have an impedance meter.

Something like this will tests individual speakers or a whole 70 volt system.

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Low frequencies = bass content of music.

The tiny transformers on each speaker are not able to cope with much bass, their iron cores magnetically saturate and impedance values plummets if driven with frequencies below about 80 Hz.

Also beware of occasional sub sonic signals (ie thumps) that may be generated by switching a mic on - especially a radio mic.

** Forget it - never even dream of using such bulbs.

.......... Phil

Reply to
Phil Allison

Thanks for all the help Phil. is there any reason i couldn't check impedance with a meter or two if i can prove that the amps are good, or is it just not worth the trouble... get the meter? also im thinking of hooking up a known good amp to the circuit to compare, but it is a different typeof amp. should i be able to use any pre-amp input with any amp. or are there different types of input signals which could work with one amp and be bad for another?

Reply to
PDI Dave

Merry Christmas

Reply to
PDI Dave

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