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70v PA question

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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FMROB
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Joined: Sun Jan 12, 2003 2:28 pm

70v PA question

Postby FMROB » Sat Sep 16, 2017 1:25 pm

Stupid question. Lets say I have a 70v PA system in a building utilizing a 100 watt 70v amplifier and 4 30 watt speakers (transformers mounted on speakers, tap for 30 watts)

In reference to the wall mounted volume controls, if one were to use a 20 watt control, would that simply limit the wattage to 20 watts to the speaker, or will it damage the volume control? Does one need to use a 100 watt volume control in order to keep everything happy?

Thank you, Rob

RFguy
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Re: 70v PA question

Postby RFguy » Sat Sep 16, 2017 1:34 pm

70-volt systems are sort of like the utility power systems. High voltage lines with huge potential for power with the voltage stepped down by a transformer to a lower voltage to your house. In the case of a utility the transformer at you house just has to be big enough to handle the power in watts that's consumed by your house.

The rule of thumb on 70-volt systems is that if you add up all the power tap settings for each speaker, it shouldn't exceed the power of your amplifier.

Just because you have a 30-watt speaker, doesn't mean it's running 30-watts. You need to see what the tap on the speaker transformer is set at.

As for your volume control, what type is it? It's it on the primary side (typically a 70-volt, multi-tap control), or on the speaker side (typicaly a 20-ohm control)?

Either way, the power rating of the control only needs to handle the power of the speaker(s) connected to it.

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FMROB
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Re: 70v PA question

Postby FMROB » Sat Sep 16, 2017 3:26 pm

So if the amp is 100 watts, and the speaker tap is 20 watts, it would be fine to use a 35 watt 70v volume control?

Also, does the same hold true if two speakers tapped at 20w are run from a 35w volume control?

Rob

RFguy
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Re: 70v PA question

Postby RFguy » Sat Sep 16, 2017 4:16 pm

FMROB wrote:So if the amp is 100 watts, and the speaker tap is 20 watts, it would be fine to use a 35 watt 70v volume control?

Yes

FMROB wrote:Also, does the same hold true if two speakers tapped at 20w are run from a 35w volume control?

If you are controlling 2 speakers both tapped at 20-watts with a 70-volt volume control, I think your 35-watt control is a bit undersized.


A lot would depend on the system levels. How hard is the amp being driven. Just cause it's a 100 watt amp, doesn't mean it's putting out 100-watts. I suspect duty cycle comes into play as well. Is the amp playing continuous music, or short announcements.

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FMROB
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Re: 70v PA question

Postby FMROB » Sun Sep 17, 2017 6:25 am

Ok, great. Thank you. This is a call alert system within a fire house, so the announcements are relatively short in length. I almost always use 100 watt attenuators no matter the tap or speakers connected to it. Its just easier to order and stock.

Thank you, Rob

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motorola_otaku
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What radios do you own?: 2 turntables and a microphone

Re: 70v PA question

Postby motorola_otaku » Mon Sep 18, 2017 7:40 am

I would strongly suggest ordering ceiling speakers with built-in volume controls, as opposed to putting L-pads in the walls. I can't tell you how many "no tones at the station" service calls I've been on where some jackwagon had just turned the volume all the way down. We've also had problems with guys turning the knobs on the L-pads full 360 and damaging them.
When exposed to Rapid Fire Growth or Thermal Emergency, two things are lost:
1. Ability to use fine motor skills
2. Presence of mind

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kb4mdz
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What radios do you own?: Too many for the time I have.

Re: 70v PA question

Postby kb4mdz » Mon Sep 18, 2017 11:35 am

I do a lot of these; been using Bogen CSD2X2VRU; 2x2 square, up to 4 watts via rotary switch on back-can; recessed volume control on face. Connect to proper transformer wires, set wattage switch & drop it in place of a tile. Tho I often set front panel @ max, and put a Brother label over it with wattage & date, so FF's have to work hard to tamper with it.

Damn, it's tough to make things Marine- & FireFighter proof. But then again, that's why the radio & amp are in a locked cabinet now.


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