How do you measure the impedance of speakers connected to a p.a. amplifier?
How do you measure the impedance of speakers connected to a p.a. amplifier?
(OP)
We have automated P.A. systems that sporatically blow out the circuit boards on the amplifiers. One of the technicians claims that it is because the speaker systems can be jacked with (against our rules) - as in their impedances are adjustible and can be changed. This changing of the impedence does affect the loud volume...which is why, he claims, the sabotagers desire to do it. This creates a lower impedance than the amplifiers are rated for. How do you measure this impedance? Any other explainations that seem plausible for why this would happen?





RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
That's why PA systems, if they provide a local volume adjustment, are supposed to use L-pads.
Double check that you're using PA type L-Pads, and then double check how they're wired to make sure that they're not installed incorrectly.
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
Please explain what exactly the vandals are fiddling with.
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
You can't buy this unit anymore. From what I can tell the unit is 70V.
Even though the connections aren't supposed to be accessible - if someone has the know how and the will they will find a way to do it.
Again, how do you measure this impedance? Thanks!
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
Your post indicates it's by means of the internal wiring.
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
If you needed to measure a particular speaker, you can use a DVOM. Beware that the DC resistance isn't going to match the speaker's nameplate impedance (the DC resistance will be significantly lower). Because they should be in a monotonic relationship, you should be able to make a chart (based on one sample, if they're all the same model) for future reference.
70-volt PA systems normally have each speaker group wired in parallel. So you'll need to disconnect the speaker-under-test to measure it.
Also, beware where the 70-volt transformer is in the speaker circuit. You can't measure resistance through a transformer.
It might be simpler to seal them with a tamper seal. Then deal with the ones that have been tampered with by inspection and repair.
I'm not going to ask where this fiasco is occurring - it must be a junior high school.
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
Measure the current when 60 Hz is applied. Calculate the impedance at the frequency used.
Use Pythagoras' theorem to calculate the inductive reactance at 60 Hz.
Use 1000Hz/60Hz to calculate the inductive reactance at 1000 Hz.
Use Pythagoras' theorem, the measured resistance, and the calculated inductive reactance at 1000 Hz to calculate the speaker impedance.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
Beware the safety implications if it's assumed that the nearest AC outlet is a convenient source of 60 Hz.
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
Thanks for the heads up VE1BLL.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
http://en.wikipedia.org/wiki/Electrical_characteri...
Glenn
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
I think you answered my question. There are at least 10 speakers connected to each output and long distances apart. We want to look at the impedance the amp is actually seeing.
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
1) lock up the amplifiers and their speaker connections in a metal cabinet
2) go inside of the amplifier itself and install a series resistor so that the maximum output current is not exceeded.
3) go inside of the amplifier itself and install a series fusing device. That way you at least can "repair" the amplifier easily at each occurence.
4) You can add an electronic current limiting pass transistor to the amplifier output. That way if too much current is flowing out of the unit, the peak current will get automatically clipped--saving the actual amplifier stage. The clipping will sound like audio distortion, which might further keep the sabateur from doing it again in the future.
5) Install a loud alarm circuit on the door to the amplifier, and have it go off if the door is opened.
www.MaguffinMicrowave.com
Maguffin Microwave wireless design consulting
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
If you are crafty, you can do the same with an oscilloscope--display the voltage and current sine wave outputs and Zload=V/I (complex math. You can see the current output with either a coil probe, or by putting in a 1 ohm resistor and measuring the volts across it.
www.MaguffinMicrowave.com
Maguffin Microwave wireless design consulting
RE: How do you measure the impedance of speakers connected to a p.a. amplifier?
http://www.excelta.com/product_categories/meter
Z