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School PA system speaker impedance problem.
School PA system speaker impedance problem.
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Old 12th October 2018, 12:43 AM   #21
JMFahey is offline JMFahey  Argentina
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Location: Buenos Aires - Argentina
Just read that Pyle selector specs: you "always measure 2 ohms, even without speakers" because the Pyle **already has transformers inside *****
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reading more, itīs absolutely unsuitable for your needs, so disconnect it now.

Itīs a mess:

* it can handle only 50W RMS per channel amplifiers,

* it only feeds up to 18W RMS to each speaker (so it has 18W RMS "line transformers", period)

* it can handle only 8 ohm each speakers

* it puts all speakers in parallel so when all speakers are selected, amplifier sees 1.7 ohm load

* so you need a "1 ohm stable" amplifier to drive it.
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In a nutshell: drop it in the nearest junk bin, specially to avoid the temptati0on of ever using it again ... serious.

All this comes from reading the Pyle manual ... which obviously nobody did.
PyleHome - PSPVC6 - Home and Office - Digital Tuners - Speaker Selectors - Sound and Recording - Digital Tuners - Speaker Selectors

Almost forgot: it is VERY inefficient, because the line/adapter transformers are inside the selector box, near the amplifier, and not at the far end, near the speakers.

Maximum suggested speaker distance: 80 feet ... and that with 14 gauge wire.
Any longer, they suggest heavier gauge (of course) ... which the terminals can NOT handle (14 to 22 ga only!!!)
Did I say itīs unsuitable for your task?

I repeat earlier suggestion, backed by others Forum Members:

1) put each room speakers in series to achive a sensible *room* impedance, 16 ohm or higher.
Donīt be afraid of reaching up to 32 ohm in a single room, thatīs what using 70V transformers do anyway : present a way higher impedance than what speakers show by themselves.

2) youīll need to add an extra 8 ohm speaker in series in room 5, to raise room impedance to 16 ohm, and so pair puts out roughly same power as other rooms, otherwise either it will be way too loud or others way too low.

3) distribute load (rooms) between available power amps, I understand you have 4 of them.
Think Mono signal feed, forget Stereo, this is not a Living Room Hi Fi system but an improved PA/distributed Music one.

If one amp channel feeds 2 rooms and you want to cut sound in one of them, add a simple switch in series with each speaker line.
Similar to what the Pyle unit does, but in a simple functional way.

*OR*
go the full way, get a large 70V amplifier, and add individual 70V transformers at each speaker, your choice of system and cost.
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Old 12th October 2018, 02:21 AM   #22
wg_ski is offline wg_ski  United States
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The problem of being able to switch any room between either of two sources (or off) does add additional challenges on the budget. Requiring *two* 70 volt channels. Yeah, you can buy an inexpensive low impedance P.A. amp that will drive 70 volts bridged, but two would cost $400 and be way overkill on power. So distribute it the other way. Find two more amplifier channels (ie, another receiver) and drive each room with its own amplifier. Then you just wire each room to a sensible impedance (6 to 16 ohms) which will never change. Handle all the source switching at the input with a bank of switches and you can even put a pot on each channel to individually adjust levels. The little black box won’t cost much to make and won’t require power.
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Old 12th October 2018, 05:47 PM   #23
ndcraig is offline ndcraig
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One other way if you want to stay with home-stereo low-impedance equipment is to look into those standard in-wall volume controls that are sold for whole-house audio systems.

They have a jumper or switch on the back labelled 2X, 4X, 8X (or similar).

All that does is multiply the impedance of the connected speaker so that when all combined on an amp the overall impedance is within 4 to 16 ohms.

I have used them set to full volume and hidden behind each speaker, or stacked behind the amp at the distribution point. Also handy if you have different speakers you want to adjust the relative volume between them.

They generally are rated for home stereo use so the low frequency transformer saturation issue is handled better.


Ideally though you are trying to re-invent an already established art. Used 70V amps can be had fairly cheap and are designed for this purpose. Transformers can be added to existing 8-ohm speakers and are easy to find and re-purpose.
I was digging around in one of our town's school storeroom and found stacked in a pile just about every piece of vintage electronics they have ever had -including 2 generations of DuKane intercom systems and boxes of old 8" 70V & 25V speakers -some NOS still in box.
So ask around to other schools in your area, you may be able to get this type of stuff for free.
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Old 12th October 2018, 07:03 PM   #24
turk 182 is offline turk 182  Canada
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i would look at something like Atlas Sound TSD-DA28 2 x 8 Distribution Amplifier
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