Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[John Lackner]

I'm upgrading a previous install. There are some Bogen paging horns that are about 45 feet up a light pole - inaccessible without a (beyond the budget) scissor lift. What would be the best way to determine if they are set for 8ohms or 70 volt without damaging them? Should I connect a 4ohm amp output first to see if they work and then try 70 volt
or vice versa?

[Ivan Beaver]

I'm upgrading a previous install. There are some Bogen paging horns that are about 45 feet up a light pole - inaccessible without a (beyond the budget) scissor lift. What would be the best way to determine if they are set for 8ohms or 70 volt without damaging them? Should I connect a 4ohm amp output first to see if they work and then try 70 volt
or vice versa?

They will not be set for "70V".

They may be set for some wattage at 70V.  BIG difference.

All a 70V transformer does is change the impedance load presented to the amp.

A quick "rule of thumb" is when tapped at 1 watt, the impedance will be 5000 ohms.

10 watts is 500 ohm and 100 watts is 50 ohms.  600 watt is 8 ohm.

you will have to measure THE IMPEDANCE-NOT RESISTANCE to get an idea.

If you hook a DC ohm meter to them-the only thing you are reading is the DC resistance of the wire-NOT the impedance.

A single tone impedance meter may give some kind of an idea-but what if at that 1K tone there is a peak or a dip in the impedance?  You can only guess as to what it is.

By measuring the CURVE of the impedance-you can get an idea of the overall load.

Another way to "get an idea" is to put a series resistor in the line-say 1000 or 100 ohms.  The value is not really important-as long as you KNOW the value.

Now measure the voltage at a couple of freq (100-500-1K-4K etc) across the resistor and across the load.

You can then figure out the difference and calculate the impedance at those freq.

You don't need a lot of voltage-just a few volts will work.  It kinda depends on how accurate your meter is at low voltages.

[John Roberts {JR}]

I kind of like the idea of connecting a low power 4-8 ohm amp and see what happens. If it gets loud enough you know, if it doesn't you know. OTOH connecting a high power 8 ohm amp might not give you a definitive answer.

JR

[Jason Lavoie]

I'm upgrading a previous install. There are some Bogen paging horns that are about 45 feet up a light pole - inaccessible without a (beyond the budget) scissor lift. What would be the best way to determine if they are set for 8ohms or 70 volt without damaging them? Should I connect a 4ohm amp output first to see if they work and then try 70 volt
or vice versa?

Unless they are high wattage horns the DC resistance should give you a good idea, but whatever you find out, how are you going to size the new amplifier properly without a real impedance measurement (if they turn out to be 70V) or knowing the model #s and power handling capability if they turn out to be low Z?

Jason

[Tom Bourke]

They could also be tapped for 25 volts.  An impedance meter and some math math would go a long way towards an answer.

You could also use a Wheatstone bridge and a meter that is pretty flat across the band of interest.

[Ivan Beaver]

They could also be tapped for 25 volts.  An impedance meter and some math math would go a long way towards an answer.

You could also use a Wheatstone bridge and a meter that is pretty flat across the band of interest.

With the key word IMPEDNACE-NOT resistance-which is what most people have.

[Stephen Swaffer]

Does the "watt" rating on a 70 V transformer have anything to do with actual power handling capabilities, or is really just an impedance indicator?  My understanding is that 70 volt systems are not as sensitive to the number of speakers in parallel-but I am not sure the people that taught me that years ago really understood?

[Brad Weber]

Does the "watt" rating on a 70 V transformer have anything to do with actual power handling capabilities, or is really just an impedance indicator?  My understanding is that 70 volt systems are not as sensitive to the number of speakers in parallel-but I am not sure the people that taught me that years ago really understood?

If you are referencing the tap setting that is on the transformer then that relates to the impedance the amplifier sees and has nothing to do with the speaker power rating other than that you probably don't want to use a transformer with a tap value greater than the rated power handling of the associated speaker.  It's not at all unusual to see speaker and transformer combinations where the highest tap on the transformer is well below the rated power handling of the speaker.
 
You can add or subtract speakers from constant voltage (25V, 70V, 100V, etc.) systems as long as the amplifier can drive the total load of speakers and line losses.  It's fairly common to initially oversize amplifiers for constant voltage systems in order to allow for future expansion and/or tap values being changed.
 
There are dedicated impedance meters like the TOA ZM-104A or general use audio meters like the NTI MR-PRO Minirator that can be used to measure impedance, however many people seem to use http://www.daytonaudio.com/index.php/test-measurement/dats-dayton-audio-test-system.html as a lower cost option for measuring and documenting speaker impedance, it's about $100 at Parts Express, http://www.parts-express.com/dayton-audio-dats-dayton-audio-test-system--390-806. 

[Steve M Smith]

Does the "watt" rating on a 70 V transformer have anything to do with actual power handling capabilities, or is really just an impedance indicator?

I'm not sure if you can correlate impedance from it (I'm sure someone can) but as has already been stated, the transformer just changes the speaker impedance as seen by the amplifier (square root of turns ratio).

It's no different to having an amplifier rated to 8 ohms driving up to twenty speakers of 160 ohms impedance (for example).

If it was easy and/or cost effective to make speakers with higher impedances, it would be done like that rather than use transformers.


Steve.

[Sean Chen]

I'm upgrading a previous install. There are some Bogen paging horns that are about 45 feet up a light pole - inaccessible without a (beyond the budget) scissor lift. What would be the best way to determine if they are set for 8ohms or 70 volt without damaging them? Should I connect a 4ohm amp output first to see if they work and then try 70 volt
or vice versa?

Can you measure the DC resistance of one?  If it is close to 8 Ohm, then it doesn't matter. If it is much higher, then take power rating, multiply the resistance, and take square root of that product.  V^2 = P * R.

[Tom Bourke]

Can you measure the DC resistance of one?  If it is close to 8 Ohm, then it doesn't matter. If it is much higher, then take power rating, multiply the resistance, and take square root of that product.  V^2 = P * R.

DC resistance is useless in this context.  It could be well under 8 Ohms even with transformers.

[Sean Chen]

DC resistance is useless in this context.  It could be well under 8 Ohms even with transformers.

Thanks for your correction. I learned something.

[David Sturzenbecher]

Sean
Remember that at DC a transformer will read very close to zero ohms across the primary , add that to the low resistance of the speaker line and you may easily get below an ohm with a standard DMM on a 70V ( or 25v or 100v) system.

[ProSoundWeb Community]