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100v line amp for P.A.
100v line amp for P.A.
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Old 10th January 2021, 03:12 PM   #1
srcveiga is offline srcveiga
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Join Date: Jan 2021
Default 100v line amp for P.A.

Hello, I work for a school and in 1 of its pavilions the P.A. system is old and unused for more then 10 years.

In the other pavilions the PA system used is much more recent and used but not the same speakers and amps.(mostly where events take place and gyms, these are working and fresh and clean)

I don't know anything about sound but I was interested anyways in trying to figure out what happened and if it can work again.

I took a peak at the speakers. XMR 512PL.
I fished for the cable (black one) and it went into 3 other speakers (4 in total in parallels).
All connected to the brown tap (2.5w) of the speaker for a total of 10w.

To test them I tapped them to black tap of the speaker transformer and pugged it to a mobile phone via AUX set up.

They work but very low volume. Understandable. I reverted them back to the brown tap and checked the old stuff they had stored.

Like I said I know nothing about sound, and this 100v line is a first to me. but I looked for what could just be a myth to me in the storage sound room and I actually found this...thing...

Crest audio FCV440 power amp.

Now this thing is a beast and it obviously drives these 4 speakers at 2.5w but its school property and its not something I want to risk malfunction when someone comes asking for it.

Plus there are other speakers id like to have hooked up in the future for emergency situations where we would need to evacuate the building but I would need at least control over 4 different channels.

So now I'm looking for an amp with 50w to 120w with this mythical 100v line. but I'm not finding much on stuff like Aliexpress.

The amps they sell run for at least 100euros plus shipping fees but I'm thinking I'm just being stupid on the search.
Even searching on this forum there isn't much about 100v line amps.

Am I confusing anything? are these normal stereo 4 to 8 ohm amps with some sort of converter? are they just called power amps? because i seem to find mostly 4 to 8 ohms power amps.

Could someone point me out somewhere I could find info on maybe a diy board ? or just a plain simple cheap 100v line amp ?
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Old 10th January 2021, 04:40 PM   #2
conanski is offline conanski  Canada
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Join Date: Mar 2013
These distributed sound systems use transformers on the speakers and the amp to spread the output power of the amp evenly among all the speakers. In this case for example the amp produces 220w/ch so in theory it could power 11 of those speakers to 20w or 15 to about 15w. This system is by far the most economical way to get sound to that quantity of speakers, the wiring necessary is very modest compared to the low impedance option where cable losses have to be factored in which makes covering long distances impractical.

So it seems like this gear still works, you made a mistake with the wiring change and should have moved the speaker connection to one of the other transformer taps, you got low sound when you moved it to the black tap because there isn't a direct connection to that back at the amplifier when it is wired for 100v operation.

Can you post a picture of the wiring connections on the back of the amplifier?
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Old 10th January 2021, 04:55 PM   #3
srcveiga is offline srcveiga
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Join Date: Jan 2021
yes, this is how i tested the sound with the 100v line amp.

For input its an AUX cable connected to phone. Blue positive, White negative and ground is the shielding.

For output i wired it directly to speakers as it was originally set up to brown tap 2.5w each.
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Old 10th January 2021, 07:03 PM   #4
conanski is offline conanski  Canada
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Join Date: Mar 2013
A couple things.

You should not connect a stereo signal to a balanced mono input because everything common on the stereo signal(usually all low frequencies and vocals) gets canceled. So you either need something between the source and amp to convert to mono or just use a single channel.. ie disconnect the - wire. For testing purposes this should work fine.

The output transformer is isolated so a small jumper has to be installed to get signal to it, so get a short piece of wire and install it between the 3rd and 4th screws on the output barrier strip labelled "4ohm Direct" and "XMFR IN"

Also note that the user manual for this amp is available online, just do a search for the model number(Crest Audio FCV440) and you will find it.

Last edited by conanski; 10th January 2021 at 07:13 PM.
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Old 10th January 2021, 07:42 PM   #5
srcveiga is offline srcveiga
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like this ?
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Old 10th January 2021, 08:18 PM   #6
conanski is offline conanski  Canada
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Yes that is correct.

If you want the speakers to be louder move the connection at each speaker to a higher wattage transformer tap.

How many speakers are connected to this amp anyway?
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Old 12th January 2021, 10:52 PM   #7
MAAC0 is offline MAAC0  Portugal
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Join Date: May 2010
Quote:
ie disconnect the - wire
or twist both L & R and connect to + and ground the - terminal.
Don't be afraid of frying Your phone or mp3 player by shorting the channels, since the headphone output is protected by resistors anyway.

Quote:
and this 100v line is a first to me.
Wanna learn someting about ?

High Voltage Audio
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Old 12th January 2021, 11:19 PM   #8
srcveiga is offline srcveiga
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Join Date: Jan 2021
oh i see, i will try conecting both.

I tried disconnecting the - signal, but the sound went dead. So i connected + to the + of channel A and - to the + of channel B. set it to stereo instead of parallel and it worked but it was humming so I disconnected it again.

I will try tomorrow with both lines on + channel A.

I do I put a jumper between - and ground ? or just leave ground disconnected ?

Thank you for the link, that is very helpful.
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Old 13th January 2021, 01:16 AM   #9
conanski is offline conanski  Canada
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Join Date: Mar 2013
Did you download the user manual yet?

FYI... With the input selector switch set to parallel the CH A input signal is sent to both output channels.
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Old 13th January 2021, 01:26 AM   #10
srcveiga is offline srcveiga
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Join Date: Jan 2021
yes i did.

the manual says inputs are actively balanced.

for use with unbalanced source, tie the inverting (I assume the - ) input to ground, if left floating a 6db loss in gain will result.

With the input selector switch set to parallel the CH A input signal is sent to both output channels.
I don't mind this at all. Don't want to use bridge (no need) and I don't want stereo output.
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