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[Sean Lehman]

Hi, What is the practice to diagnose a 70v system to make sure it is not shorting out or damaging the amplifier?  70v amplifier is feeding about 15 to 20 rooms and each room has two speakers that are transformer tapped. Is there a way to test the load at the amplifier end or will a continuity test suffice if I know everything is within spec? what's the best way to go about this?

thanks for your help,
Sean

[John Roberts {JR}]

For serious debugging they make AC impedance meters that can measure the actual load, while in general an actual short should be easy enough to see with an VOM.

JR

[Ivan Beaver]

Hi, What is the practice to diagnose a 70v system to make sure it is not shorting out or damaging the amplifier?  70v amplifier is feeding about 15 to 20 rooms and each room has two speakers that are transformer tapped. Is there a way to test the load at the amplifier end or will a continuity test suffice if I know everything is within spec? what's the best way to go about this?

thanks for your help,
Sean

You really need to do an impedance test-across the entire bandwidth-not just at 1Khz.  Then convert that impedance to watts and see how it lines up with the amp power rating.

An Ohm meter will be seeing the DC resistance of the wire (speaker wire and transformer windings) only.  If there is a short on the other side-you will not be able to see it with an ohm meter.

[John Roberts {JR}]

Yup,, To measure loading wrt 70V amp you need AC Z and then do the math. I built that measurement capability (20-20k) into my old Loftech TS-1 , but it seems a few spot (midrange) audio frequencies will tell you enough. 



JR

JR

[Tom Young]

Yup,, To measure loading wrt 70V amp you need AC Z and then do the math. I built that measurement capability (20-20k) into my old Loftech TS-1 , but it seems a few spot (midrange) audio frequencies will tell you enough. 

JR

The Dayton Audio WT3 impedance measurement software should do the trick.

http://www.daytonaudio.com/index.php/wt3-woofer-tester.html

and:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390-804

[Sean Lehman]

thanks for the replies and info. appreciated.

[Steven Barnes]

The Dayton Audio WT3 impedance measurement software should do the trick.

http://www.daytonaudio.com/index.php/wt3-woofer-tester.html

and:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390-804

This is what I have been using with great success

[David Sturzenbecher]

This guy does impedance measurements across multiple frequencies...and is a great signal generator.

http://www.nti-audio.com/Home/Products/Minstruments/MiniratorMRPRO/tabid/119/Default.aspx

[Billy Wood]

I have a Fluke and it has a amperage clamp that I use on all kinds of equipment.  You can push a tone down the line (120hz if it is a cheap meter) then get your amp reading.  Then multiply that by the voltage which you can also measure with it.  I know 70v would be logical but I like to measure rather than assume.  That has helped me diagnose a busted Yorkville amp before.  You can get your system wattage from that and know if you are over loading your amp.  Easiest way to fix a overload problem is to change the volume knobs in rooms to bring your total wattage down.  Alot of people buy 100w volume knobs assuming that it is related to its durability and max load but it limits that zone to said wattage.  You can put 1000w worth of speakers on the other side and it will only get 100w.  It is just a multi-tap transformer with a knob on the front to pick the multiple taps.

Ok with that said testing at the amp will ussually show you nothing because you have volume knobs at every zone.  Everything past the volume knob will not show at the AMP.  That is usually where most of your problems lie due to them being above ceiling tiles that everyone and their mom gets into!

Impedance will not tell you much also. 70v is not a impedance based system. That is why it is used in distributed audio.  You need to turn every zone off at the zone controls and then run signal through your system.  If you are getting amperage draw over the hot line at the amp you have a leak to building ground somewhere (ie. - a short). 

The impedance remark is only applicable if you do not have a step up transformer at your amp and it is a transformerless amp system.  This is saying you don't have a regular 2/4/8 ohm amp at the start and a step up transformer feeding your 70v.  I have not ran into this in any recent installations. 

Billy Wood
www.avprogroup.com
The Woodlands, TX

[John Roberts {JR}]

Impedance will not tell you much also. 70v is not a impedance based system. That is why it is used in distributed audio.  You need to turn every zone off at the zone controls and then run signal through your system.  If you are getting amperage draw over the hot line at the amp you have a leak to building ground somewhere (ie. - a short). 

The impedance remark is only applicable if you do not have a step up transformer at your amp and it is a transformerless amp system.  This is saying you don't have a regular 2/4/8 ohm amp at the start and a step up transformer feeding your 70v.  I have not ran into this in any recent installations. 

Billy Wood
www.avprogroup.com
The Woodlands, TX

"Huh"?    70/100V systems are called "constant voltage" systems, and impedance is indeed a useful measure of how heavily that amp is loaded for that nominal (not really constant) system output voltage..

It is fairly simple math to convert 70V at X watts, into a minimum Y ohms, and unlike your clamp on ammeter you can debug the system when it's turned off or broken.

Of course do what works for you...


JR

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