Loud hum from DIY subwoofer for home theatre

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I have a Dayton 315 HO single voice coil hooked up with a car amp and PSU all connected to an Onkyo Tx Nr646, but the woofer produces a loud hum, the input level on the car amp is turned down close to zero because the Hum is so loud, and Audio can only be heard from the sub when he Onkyos volume is up around 60, when comfortable listening level for other speakers is around 15. Everything is hooked up to one surge protector, although I have tried changing the outlet for the PSU with no improved results. When I connect my phone directly to the car amp, and have the car amp volume set to max, there is little Hum, and Audio can be heard out the sub at low volume levels for the phone. Any ideas on what is causing this Hum? Maybe a Ground loop insulator would help?
 
The car amp power supply output is likely not AC ground referenced ie 12 negative in anyway connected to your house electrical systems safety ground or even the neutral.

If there is a voltage differential between the chassis of the Onkyo and the car amp, the shield/ground conductor of the signal cable will be conducting whatever current at what ever frequency of the voltage differential. The RCA shell conductors are likely not directly connected to the chassis and are not realy designed to sink substantial current.

I would try adding a wire connected to the chasis of the car amp and the Onkyo and see if that drops the hum.

Your phone if not connected to a charger is totally floating and should not be affected if the chassis if the amp is also floating.

Another unlikely possibility is the signal/ground connection is flipped between the Onkyo and car amp? It would sounds like you are amplifying the Onkyo chassis voltage ripple and knock the drive signal way way down.

Barry.
 
So, I solved the humming issue by grounding the chassis of the Onkyo to my house, and then grounding the car amp to the Onkyo and to the house. this solved the humming issue, however 1 issue still persists. When the Onkyo is in Dolby Surround mode, or DTS X, the subwoofer volume is very low, and when I turn the gain up on the car amp and the Onkyo, it can be heard, but it is very distorted. On the other hand, when the Onkyo is in All channel stereo mode, and I have the gains on the car amp and the Onkyo turned down to about half of its full level, the bass is very clear and loud and audible. Any ideas how to make it louder when in Surround mode?, I have the same issue with my two height speakers
 
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