PSW505 - Yet another ground loop problem - I found a solution, but need a better one.

tojo_m

Q. Why does a subwoofer hum?
A. Coz it does not know the words.

Now, when I had read the above in the PSW505 manual, I had found it very funny. But after spending a few hours troubleshooting this problem, I'm bit past the point of funny!

I just setup my brand new home theater system:
1) Front L/R -- Monitor 70 II
2) Surrounds - RM 7
3) Center - CS2 II
4) Sub - PSW505
Receiver - Onkyo SR508

I have the sub at the rear corner of the room and am running a 25' RCA cable from the receiver to the sub. Onkyo's Preout --> LFE input of the sub. Crossover is set at 120hz at the receiver level (Don't know why Audessey chose 120Hz instead of something like 80, but that question belongs to a different thread). The cable is tightly tucked together with the two 16 AWG surround speaker wires that run along with it (inside cable raceways).


I hear a hum from the sub when I switch off my home theater system (The sub is set to auto, so remains on).
**If I switch off the sub manually, the hum disappears.
**If I plug out the RCA cable from the sub, the hum disappears
**If I change the volume on the sub, the hum's volume will change accordingly (slightly audible at 50% volume and very loud at 100%)

From what I have read on the internet, it appears to be a ground loop issue. Right?

Now this is where it gets interesting. I have figured out a sequence of steps that will get rid of the hum:
1) Turn off the home theater system (with my harmony remote). I will now hear the loud hum.
2) Unplug the receiver. The hum will disappear for 2 seconds but will reappear with the same intensity.
3) Plug back the receiver. The hum will instantly go off, then in about a second it will come back for about half a second, and will finally disappear for ever (till I turn on the system again and turn it off).

I have tried different power points in the same room, but no difference in behavior.

So all I need to do is to unplug my receiver and plug it back again, every time I turn off my home theater system.

You guys think there is a better solution?

Thanks.

tonyb

Ground loops are tricky to locate the culprit with lots of possabilities. Usually it occurs in a cable box connection or a bad cable from the sub to the receiver. Try a cheater plug on the power cord for the sub. Costs 2 bucks at any hardware store. It converts a 3 prong plug to a 2 prong,lifting the ground. If that doesn't work,move the cheater plug to the receiver and see if it works. There are many other options to deal with ground loops but this is the cheapest way and easiest way first.

kawizx9r

^ Good suggestion.

Just some food for thought, one of my RT1KP towers and one of my subs (either my Velodyne or the one I built) had a nasty 60hz hum. Solved it when I hooked it up to a power conditioner, however it's not where I wanted my power conditioner. I checked the outlet and sure enough it was wired incorrectly. I lived in an old apartment though during that time so it wasn't uncommon to have s@#$$y wiring.

tojo_m

Just a few more inputs:
1) I don't have a cable box (nor OTA antenna) connected.
2) I noticed that my receiver's power cord has two pins (no ground) -- so is the cheater plug solution even applicable in this case?

tonyb

tojo_m wrote: »

Just a few more inputs:
1) I don't have a cable box (nor OTA antenna) connected.
2) I noticed that my receiver's power cord has two pins (no ground) -- so is the cheater plug solution even applicable in this case?

Only on the sub's power cord. How do you watch TV ? No antenna or cable box ? Do you have a power conditioner ? What kind of cables are you using to hook all this up ?

Also, never run power cords along side speaker cables. Or your sub cable. Change your crossover setting to 80 to start with on the receiver,and turn the one on the sub all the way up to the highest point.

tojo_m

tonyb wrote: »

Only on the sub's power cord. How do you watch TV ? No antenna or cable box ? Do you have a power conditioner ? What kind of cables are you using to hook all this up ?

Also, never run power cords along side speaker cables. Or your sub cable. Change your crossover setting to 80 to start with on the receiver,and turn the one on the sub all the way up to the highest point.

I watch Bluray and Roku (wireless streaming from another room). Also have an XBox.

Bluray/Roku/Xbox are all connected via HDMI to receiver. The receiver's HDMI out is connected to TV's HDMI in.
Also, the receiver's pre out is connected to the sub's LFE via a RCA cable.

I don't have a power conditioner, but do use a spike buster.

tonyb

A good power conditioner won't hurt ya bro. Why is the sub so far away ? Just askin' is all.

tojo_m

tonyb wrote: »

A good power conditioner won't hurt ya bro. Why is the sub so far away ? Just askin' is all.

alright... I'll get going with research on some good affordable power conditioners.

Thanks!!

EDIT: The sub is far away since that is closer to the main listening position and it sounds much louder than if placed in the front wall. Can't afford to turn the sub volume too high since I'm in an apartment;)

nguyendot

Run a wire from the receiver ground to a screw on the amp plate of the sub. Watch the hum disappear.........lol

tonyb

nguyendot wrote: »

Run a wire from the receiver ground to a screw on the amp plate of the sub. Watch the hum disappear.........lol

That may work too. Lots of ways to get the hum gone but more so a trial and error thing. I always look for the simplest and cheapest solutions first.

Erik Tracy

Some sub cables have built in grounding wires.

My Audioquest Sub-x cable does. You can ground it at either the avr or the sub to see what works best.

Hawkeye

Are you using a real sub cable? A friend of mine picked up a rat shack rca for his sub and had the loop. I took over a sub cable I had and it eliminated the hum.

Gordon