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Author Topic: Low Power FM at Racetrack  (Read 5570 times)

William Power

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Low Power FM at Racetrack
« on: February 12, 2013, 01:22:01 am »

I'm working on a racetrack that has a low power fm installed on the wall beside the pa system rack. installed in a metal utility box w/off on switch and pilot light on side of box to indicate "on" state. FM antenna outside and system broadcasts on 100.3 . A 2 wire lead was connected from the transmitter box to the 70 volt and com leads on a 120watt pa amp from University Sound. One of our guys tuned a service trucks radio to 100.3 and heard strong clear cd music that was playing thru the University. Is that the way the transmitter receives its signal? 70volt? Who makes this thing? No name anywhere that we can find. I'm tired and hope I explained everything right. 70volts?
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Hal Bissinger/COMSYSTEC

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Re: Low Power FM at Racetrack
« Reply #1 on: February 12, 2013, 09:02:40 pm »

You have to understand how a 70 volt constant voltage system works. No reason you couldn't feed the input on something like a transmitter off of it. Use a transformer, pad or just turn the input level down. I don't like to do things like that because of noise and distortion considerations but this is only PA after all.
 
-Hal
« Last Edit: February 12, 2013, 09:04:52 pm by Hal Bissinger/COMSYSTEC »
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William Power

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Re: Low Power FM at Racetrack
« Reply #2 on: February 12, 2013, 11:46:45 pm »

You have to understand how a 70 volt constant voltage system works. No reason you couldn't feed the input on something like a transmitter off of it. Use a transformer, pad or just turn the input level down. I don't like to do things like that because of noise and distortion considerations but this is only PA after all.
 
-Hal
Yea, I know that there's no actual "70 Volts", but I should have made my consternation clearer, why not use a baseband signal?
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Tim Perry

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Re: Low Power FM at Racetrack
« Reply #3 on: February 13, 2013, 12:25:50 am »

Yea, I know that there's no actual "70 Volts", but I should have made my consternation clearer, why not use a baseband signal?

You are going to have to clarify what you mean by baseband.

If you mean composite, there is no need for stereo in this application.

If you mean a low level audio out from the amp, I could speculate that the transmitter was looking for +4 and the output is -10 so a convenient higher level point was used.



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William Power

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Re: Low Power FM at Racetrack
« Reply #4 on: February 13, 2013, 12:47:52 am »

You are going to have to clarify what you mean by baseband.

If you mean composite, there is no need for stereo in this application.

If you mean a low level audio out from the amp, I could speculate that the transmitter was looking for +4 and the output is -10 so a convenient higher level point was used.
Thanks. I got it now.
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William Power

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Re: Low Power FM at Racetrack
« Reply #5 on: February 13, 2013, 12:52:21 am »

You are going to have to clarify what you mean by baseband.

If you mean composite, there is no need for stereo in this application.

If you mean a low level audio out from the amp, I could speculate that the transmitter was looking for +4 and the output is -10 so a convenient higher level point was used.
One other issue. this track has 12 horns all tapped at 25watts for a total of 300watts. the amp they have purchased is a 600w per channel. will this amp burn those horns if it's set at about half power? 300watts of horns vs 600watts of amp?
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Brad Weber

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Re: Low Power FM at Racetrack
« Reply #6 on: February 13, 2013, 05:53:45 am »

One other issue. this track has 12 horns all tapped at 25watts for a total of 300watts. the amp they have purchased is a 600w per channel. will this amp burn those horns if it's set at about half power? 300watts of horns vs 600watts of amp?
If they have 12 horns and a 2 channel amp then are there are 6 horns per channel?  And that amp is rated 600W into what load and at what frequencies?

If the horns are tapped for 25W than that is what they draw at 70.7V, however if the amp is 600W into 8 Ohms then that is actually a 69.3V system (70.7V**2/8 Ohms is 624.8W).

Chances are setting the amp at 'half power' is not reducing the potential amp output, just educing the input signal required to get that output.

Unless you know they are a licensed operator for the FM transmitter then I would not go anywhere near it.
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Hal Bissinger/COMSYSTEC

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Re: Low Power FM at Racetrack
« Reply #7 on: February 13, 2013, 07:05:46 pm »

One other issue. this track has 12 horns all tapped at 25watts for a total of 300watts. the amp they have purchased is a 600w per channel. will this amp burn those horns if it's set at about half power? 300watts of horns vs 600watts of amp?

No more than if you connected twelve 25 watt light bulbs to a 600 watt generator through a dimmer. Works kinda the same way.
 
-Hal
 
 
 
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Tim Perry

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Re: Low Power FM at Racetrack
« Reply #8 on: February 13, 2013, 11:43:41 pm »

One other issue. this track has 12 horns all tapped at 25watts for a total of 300watts. the amp they have purchased is a 600w per channel. will this amp burn those horns if it's set at about half power? 300watts of horns vs 600watts of amp?

70V is kinda hard to to understand all the ramifications without quite a bit of thought and study... and to take a stab at answering we must make a lot of assumptions. (make and model numbers would help a lot)

Firstly you cannot set an amp to half power, you can only reduce input gain.

Secondly, power will be divided equally among as long as they are the same load and tapped the same in the stepdown transformer.  (long lengths of undersized wire between speakers may affect this statement but lets assume it negligible for now)

Next, PA horns are generally very loud with very little power. Yes everything has it limits, but these things generally get loud enough to an announcer to be heard over screaming fans and tractor pulls.

I'm guessing what you really want to know is 'how do I idiot proof my system?' The answer is you can't really. Too small and amp and it will distort Under normal use. An amp large enough to do the job under normal condition will also be large enough to harm the system when operators abuse it.

Someone may say "hey just throw a limiter in it and properly set it up". I'd say even a "properly" set up limiter would not insure damage prevention due to user abuse.

And finally a lot of sound guys advocate using 2x rated power of load for the amplifier (to maximize the SPL to investment ratio.)

My recommendation: the larger amp should be just fine.  Think of it as headroom.       
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Geoff Doane

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Re: Low Power FM at Racetrack
« Reply #9 on: February 14, 2013, 08:38:52 am »

70V is kinda hard to to understand all the ramifications without quite a bit of thought and study... and to take a stab at answering we must make a lot of assumptions. (make and model numbers would help a lot)

(Snip all that stuff in between)

My recommendation: the larger amp should be just fine.  Think of it as headroom.     

Go back and re-read what Hal wrote.  You apparently don't understand 70V systems either.

The power rating of the 70V amplifier is irrelevant, as long as it's higher than the total load of the speakers.  Your conclusion is correct, but the path that lead you there appears to be rather meandering.  :)

GTD
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Re: Low Power FM at Racetrack
« Reply #9 on: February 14, 2013, 08:38:52 am »


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