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rooster250f 7th January 2010 05:29 PM

MP3 player to PA system
Hello, I am looking for some help and it looks like there are many knowledgable people on this forum.

Current situation:
In our manufacturing facility we play alarm message WAV files over the PA system. The messages are triggered from a DCS (distributed control system). Currently the source of the alarm messages is the sound card output from a PC. The output from the sound card goes to an AT&T PagePal and from there it goes into the PA system. This setup works OK but the interface between the DCS and the PC is kind of a nightmare. We would like to change this.

We have a 'box' that has a much cleaner interface to the DCS that we would like to use. The box is basically an MP3 player with an earphone output. We tried connecting the earphone output to the same input that the PC sound card output goes to but the signal appears to be much too weak. With all volume controls at max the output on the PA is barely audible. So obviously some type of amplifier is required.

The input on the AT&T PagePal is listed as '100K at 2.5V max'.
There is also an input available listed as '600 ohm +10dbm max', we haven't tried using that input.
The current wiring uses phone cable and is about 100 feet in length, we will have to use this the way it is.

I would like to stay with a single ended supply of either 5 or 12 vdc (wall wart) for powering the amplifier circuit.
The messages are just voice so no hi-fi is required.

Can someone recommend an amplifier setup that might work?

Thanks, any and all suggestions will be greatly appreciated!

jaycee 8th January 2010 06:31 AM

1 Attachment(s)
You need one opamp configured with some gain. You could easily run this off of a single 12V supply.

Something like the attached should work. You should be able to make this on stripboard pretty easily. I'd make sure your 12v wall wart is regulated. If it isnt though, you could always put a simple 78L12 regulator.

I've shown NE5532 but any opamp should be OK, such as a 4558. I'm assuming you need stereo - if not then simply use a single opamp, or tie the unused opamps inputs to ground.

rooster250f 8th January 2010 12:00 PM

Thanks jaycee!

Is R6 part of the new circuit or does it represent the input impedance of the PagePal?

jaycee 8th January 2010 12:09 PM

R6 is there to drain the capacitor C3 but you could leave it out if the circuit will never be disconnected, i guess.

If you're driving a long cable, I would recommend the NE5532 as it has superior drive capability. You may even want to use the second half of it configured as a buffer to drive the cable.

It'd be helpful if you can find out or measure the MP3's players peak output. My guess though is that you dont actually need much if any voltage gain, but you do need more current than the player will source. The opamp stage here should take care of both of those. The circuit I posted has a gain of 2.

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