How do you measure the impedance of speakers connected to a p.a. amplifier? 3

[bdn2004]

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?

 

[ScottyUK]

One thing which isn't clear from your post is whether this is a 100V line system with transformer-tapped speaker connections.

 

[VEBill]

A correctly wired L-pad attenuator is intended to provide a (roughly!) constant impedance - it's kinda the whole point. Here's a wiki link on the subject (but it lacks much detail).

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.

 

[VEBill]

Further to previous (as per ScottyUK's post), "70 volt" (USA/Canada) or "100 volt" (Europe) constant voltage PA systems are typical. Each speaker should have a transformer with taps at various power (loudness) settings. The "70 volt" PA transformer should not be accessible to the occupants. They sometimes also include an L-Pad as per my previous post.

Please explain what exactly the vandals are fiddling with.

 

[bdn2004]

Attached are the spec sheets on the amplifier and speakers, the pdf is 4 sheets.
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!

 

[bdn2004]

And we think the vandals are messing with the impedance on the speakers.

 

[VEBill]

How are the seven impedance choices selected (physically)? I understand the five wattage selections and the switch. But there's no info about the seven "available" impedance settings or selections.

Your post indicates it's by means of the internal wiring.

 

[bdn2004]

The technician said that he has soldered a number of the connections at the speakers to make it harder to change the impedance settings. This implies it is a physical connection.

 

[VEBill]

So the solutions are soldering (good one!), tamper proof screws, epoxy, alarm systems, security cameras, criminal charges.

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. ;-)

 

[ScottyUK]

Hypothesis: the speaker datasheet indicates it has a switch selector between 70V line and 25V line. The miscreants are changing the switch position in an attempt to get more output power. At low levels the output will indeed be louder for a given amp gain. Above about 1/3 of full output voltage - i.e. about 1/9th of rated power - the transformer core will start to saturate if the switch is in the 25V position and connected to a 70V line. Sound quality will distort quite badly (although with some the crap produced today it would be hard to tell ;-) ), and if the power is cranked up a bit further the saturated core will start to look like a short circuit to your amplifier. At that point the magic smoke escapes.

 

[waross]

Measure the resistance.
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

 

[VEBill]

"...when 60 Hz is applied..."

Beware the safety implications if it's assumed that the nearest AC outlet is a convenient source of 60 Hz.

 

[waross]

The speaker impedance at 60 Hz may be less than an Ohm. You probably want just a couple of volts at 60 Hz to avoid a lot of noise. It won't take much voltage at 60 Hz to push the speaker into saturation and a little more may let the smoke out.
Thanks for the heads up VE1BLL.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[geekEE]

Bill, the impedance of a speaker is more complex than a resistance and inductance. The mechanical characteristics of the speaker and the enclosure change the impedance quite a bit. You can see one sample of a typical speaker impedance curve on Wikipedia.

http://en.wikipedia.org/wiki/Electrical_characteristics_of_dynamic_loudspeakers

Glenn

 

[IRstuff]

The amplifier claims to be protected against out of bounds conditions, and ought to blow its fuse first anyway. What does "blow out the circuit boards" mean?

TTFN
faq731-376

 

[awstiver]

The impedance of the speakers is measured by using an impedance bridge, which injects a signal at some known frequency (typically 1000Hz) and measures the impedance. There is really no other way to do it. You can measure individual speakers or the entire speaker circuit. If you believe it is an impedance mismatch that is causing the damage, it would be best to start by measuring the entire load, and then hunting down the culprit(s), if the load impedance is too low. The rated load impedance for the amplifier is calculated as the square of the full-power output voltage divided by the rated output power. 70.7 volt line means that the output voltage at rated power is 70.7v (similar for 25v line). Rated load impedance for 70.7v line at 125w is 40 Ohms; for 25v line it's ~5 Ohms. Anything less will overload the amp, especially if it's regularly run at full output. Several speakers set to 30w, or 25v instead of 70.7v, could overload the amp.

 

[bdn2004]

Awstiver,

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.

 

[biff44]

there are a few ways to respond to this problem:
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.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[biff44]

BTW, you can measure the actual impedance using an "audio impedance meter", which apparently are available new for a few hundred bux.

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.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[zappedagain]

Do you want the complex impedance over the bandwidth of the speaker? If you are interested in the impedance at a a single frequency you might use an AC multimeter. This one is inexpensive and works well for component measurements:

http://www.excelta.com/product_categories/meter

Z