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Old 5th September 2008, 05:25 AM   #1
Ute is offline Ute  United States
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Join Date: Sep 2008
Default Newbie question about an MP3 speaker amp

I am not an experienced DIYer, but I am looking for direction.

The following is a description of an old military speaker from WW2. I would like to find or build a battery-powered amp that would allow me to drive the speaker and allow me to connect an MP3 player to the speaker. BTW, this is an old Signal Corps speaker, not the large outdoor speaker used by the BBC which has the same designation.

Quote:
The input impedance to the LS-3 is 8000 ohms.Inside the box is a 6" permanent magnet loudspeaker with 3.2 ohm voice coil. Mounted on the speaker frame is an 8000 ohm to voice coil matching transformer rated at 10 watts. The jack on the front panel, a JK-33, is a 7/32" ID 3-circuit jack that mates with Plug PL-68 (Postwar PJ-068) (and does not look anything like an RCA phono plug). Contacts on JK-33 and PL-68 are called Tip, Ring and Sleeve. Sleeve is grounded to the LS-3 panel and connected to the low side of the transformer primary. Ring is not used. Tip goes to the high side of the transformer primary.

Background trivia - the reason for using JK-33 and PL-68 was for backwards compatibility. The loudspeaker that was replaced by LS-3 was a 30's vintage field coil unit (required an external bias supply for the coil supplying the magnetic field to make the loudspeaker work). The ring terminal brought in this bias supply. The BC-189 and BC-312 receivers originally used this earlier loudspeaker. When it was replaced by the permanent magnet LS-3, the LS-3 was designed to be a plug-n-play replacement.

PJ-068's available, $10.00 plus shipping.

Before you attempt to connect an MP3 player to an LS-3, you need to find someone who knows what impedance means to tell you how to do it. I've given enough information above for them to figure out what needs to be done. If they change the internal matching transformer, be sure that a note to that effect is affixed to the back of the case. So that the next person to try to use it doesn't throw it away in disgust when it doesn't work properly.

Thanks for your help.
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Old 5th September 2008, 08:09 AM   #2
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Join Date: May 2008
You have two options.

1) Use a tube amp. That's what that speaker was designed for, and that's why it has an audio transformer built-in.

2) Disconnect the audio transformer and use any amplifier that is good for 4 Ohm loads directly on the speaker. The 3,2 Ohms will be the DC resistance, which makes the speaker a 4 Ohm nominal type.

At 10 Watts you might try a chipamp like LM1875 or TDA 2003, TDA2030, TDA2040, TDA2050.

For battery powered use, you may also find a class-T (aka class-D) amp appealing.
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