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Old 28th October 2014, 01:21 AM   #1
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Default LM 3886 Hum

Hi Guys
I turned on my amp and got a hum from my speakers with no load
I put a multimeter on the speaker outputs and it measured 3 volts ac
I quickly unplugged the amplifier
Does this hum do damage to your speakers
I was told that the ic had not stabilized and this is the reason for the hum until it reaches operating temperature
does this make sense
I have since added a timing relay to the disconnect switch of my lm 3886 amplifier to prevent future mishaps
Have I done any speaker damage no dc was present

Opinions appreciated
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Old 28th October 2014, 02:27 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by bluemike View Post
Hi Guys
I turned on my amp and got a hum from my speakers with no load
I put a multimeter on the speaker outputs and it measured 3 volts ac
I quickly unplugged the amplifier
Does this hum do damage to your speakers
If this was 60 or 120 Hz it was at about 1W according to your meter, so probably the woofer is ok.
If it was hf oscillation, the tweeters could have been damaged.
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Old 28th October 2014, 03:02 AM   #3
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How can I tell if it's hf oscillation
The volume was at zero
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Old 28th October 2014, 03:24 AM   #4
rayma is offline rayma  United States
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Location: Upper midwest
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Originally Posted by bluemike View Post
How can I tell if it's hf oscillation
The volume was at zero
There's no way to be sure. You can check the tweeters. If you have an FM tuner, turn off the muting and listen to the hiss at a moderate level.
Or, use a web white noise source. You should be able to hear high frequencies easily, especially on the axis of the tweeter.
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Old 28th October 2014, 03:43 PM   #5
AndrewT is offline AndrewT  Scotland
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Have you worked out why you had 3Vac on the output?

BTW,
It had nothing to do with warm up time.
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Old 28th October 2014, 03:58 PM   #6
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Originally Posted by AndrewT View Post
Have you worked out why you had 3Vac on the output? Cold solder joint

BTW,
It had nothing to do with warm up time.
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Old 30th October 2014, 07:57 PM   #7
AndrewT is offline AndrewT  Scotland
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No comment?
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Old 30th October 2014, 08:42 PM   #8
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Quote:
Originally Posted by AndrewT View Post
No comment?
Sorry it was a cold solder joint that was going
My tech said because I have a tube pre the tubes need to warm up if the valves are not stabilized and you turn the amp on you will get a hum that goes through the speakers
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