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Old 11-08-2009, 03:14   #1
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Hum when New External Speaker Switched On

I installed a little external speaker in the cockpit and wired it using the correct external speaker accessory wires on a radio mounted down below. I put a toggle switch in the speaker's DC(+) wire to turn on and off the speaker. When I toggle the external speaker on and turn up the VHF volume past about half, a hum develops in the Solara's speaker that increases as I turn up the volume. Nothing else in the boat is on (engine, battery charger and all other electronics off). What are the mostly likely causes? Mismatch between the external speaker and the radio's electronics? Speaker feedback (but they are separated about 8 feet and through a bulkhead, and facing almost away from each other)? Something else? Thanks.
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Old 12-08-2009, 06:43   #2
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Try unplugging shore power completely. Sound like 60 cycle hum to me.

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Old 12-08-2009, 19:08   #3
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Sunspot Baby may be on the right track, especially if the newly-installed speaker wire runs parallel to AC wiring for any appreciable length. If so, separate the wires by at least six inches, more is better.

However, if the cause of the hum is indeed 60Hz AC inductive coupling, I'd think that the hum would be heard at all volumes, not just halfway and up. Granted, with a tiny external speaker, the acoustic response at 60Hz is way down in the noise, but what you may be hearing is higher harmonics of that 60Hz waveform.

It could still be acoustic feedback between the two speakers; just because they're aimed away from each other doesn't mean that sound isn't being reflected of a hard surface. Or lower-frequencies (which aren't nearly as directional as higher ones) might be causing it...

Finally, what kind of external speaker? The Solara (according to the user manual) has an 8 ohm internal speaker. If your external speaker is 4 ohms, then I wouldn't be surprised if the radio's audio amplifier is being over-driven when cranked to half-volume, since it's driving a nominal 2.67 ohm load (not the 6 ohms you might be thinking)... Remember that parallel resistances are 1/R = 1/R1 + 1/R2, not (R1 + R2)/2...

(yeah - I know for the nitpickers out there that we should be talking impedences and not DC resistance, but in this case it's conceptually easier to think DC)...
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Old 14-08-2009, 08:09   #4
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Originally Posted by Beausoleil View Post
Finally, what kind of external speaker? The Solara (according to the user manual) has an 8 ohm internal speaker. If your external speaker is 4 ohms, then I wouldn't be surprised if the radio's audio amplifier is being over-driven when cranked to half-volume, since it's driving a nominal 2.67 ohm load (not the 6 ohms you might be thinking)...

Poly Planar Mb21 2.5" Vhf Surface Mount Extension Speakers



MB-21 Poly Planar 2.5" speaker. Apparently 8 ohms, according to a listing on one distributor's website, but neither other distributors nor Poly Planar list ohms specs for this speaker on their websites, from what I can tell.


Poly Planar Mb21 2.5" Vhf Surface Mount Extension Speakers




Poly Planar Mb21 2.5" Vhf Surface Mount Extension Speakers
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Old 14-08-2009, 08:33   #5
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Check your grounds. I had that problem with a turntable once, and it turned out to be a bad ground.
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Old 14-08-2009, 08:40   #6
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If disconnecting your shorepower or checking your grounds does not work then try twisting your wire. Twisting will reduce the inductance from AC noise sources into the wire. The easiest way of doing this is to pull the wire back out, tie off one of the ends, put a drill motor on the other end, stretch it out tight, run the drill and there ya go!
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Old 14-08-2009, 10:00   #7
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Try connecting the speaker close to the radio with a short (6") wire--if the hum is still there, talk to Uniden about their radio.
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Old 14-08-2009, 18:27   #8
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Went for a sail and noticed no hum at all. Plugged in shore power and still no hum. No idea what could have changed, but seems to be working fine now.

(By the way, sorry about the odd extra lines and bolding in my last post -- must have hit ctrl-paste too often and then left without realizing it needed editing in the 30-minute window)
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