Question about Dynamo MartinLogan Subwoofer Speaker

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Bought a used Martin Logan Dynamo subwoofer a couple days ago and have a very loud hum when the ac cord is plugged in, even if the speaker is not connected to an audio source. The "hum" can be heard in any room in the house, even with doors closed. Also, setting the standby/auto/on switch to auto or standby does not turn the power or the hum off. Now the strange part. Attach speaker cables and sens a signal to the Dynamo and it sounds great. Terrific bass and no hum at all, even during change between CD tracks. So the systems works perfectly if I power up the amp, then the sub, but hums loaudly if the sub comes on before the amp. Even with the Dynamo switch in auto. It doesn't seem it can be a ground problem, since the speaker doesn't have a grounded connector in the cord socket. What is wrong with this thing?

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  • lightimages Jul 03, 2009

    Do they share the same power source, surge, outlet?

    Yes. Neither the amp or the sub have grounded power cords.

    I have discovered the ML Dynamo takes about 30 minutes to switch to standby in auto mode.

    With speaker level inputs connected to the Onkyo A807 amp, both units powered from the same wall outlet, Dynamo sub on Auto, the sub hums for about half an hour after the Onkyo is switched off, then the sub goes quiet. Not so good.

    I connected a bonding wire between the two units on 3_7_09. If I smoked some old audio gear I could just look at that as an opportunity, right?

    Works fine. Now I have the ML powered from the wall and working normally.

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Neither the amp or the sub have grounded power cords.

I have discovered the ML Dynamo takes about 30 minutes to switch to standby in auto mode.

With speaker level inputs connected to the Onkyo A807 amp, both units powered from the same wall outlet, Dynamo sub on Auto, the sub hums for about half an hour after the Onkyo is switched off, then the sub goes quiet. Not so good.

I connected a bonding wire between the two units on 3_7_09. If I smoked some old audio gear I could just look at that as an opportunity, right?

Works fine. Now I have the ML powered from the wall and working normally.

Posted on Jul 03, 2009

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Are the sub and receiver in seperate locations in the room? Do they share the same power source, surge, outlet? If plugged in to different outlets, move the sub to the same power source as the receiver.
The ground leg of the outlet you are useing could be bad or circuits phased together.
The hum is a ground issue as the problem is resolved when it is connected to the receiver, thus being grounded.

Posted on Jul 03, 2009

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