help with hum when connecting second sub

[Adam321]

Hello,

I built a new sub which uses a pro amplifier. I decided I wanted to reconnect my old Velodyne sub back up as well to improve headroom. I used a y-adapter at the input of the proamp to daisy chain over to the old sub.

When I first turned everything back on I was getting a loud hum from the Velodyne. This sub always seemed very susceptible to hum. When I first got it, it hummed and Velodyne told me to use a cheater plug, and it worked. I reconnected the cheater plug to the Velodyne which helped a little. I then connected both the pro amp and the Velodyne amp through cheater plugs, which completely eliminated the hum at first. But, when the pro amps fan kicked in all hell broke loose. I do not like to use the cheater plugs anyway. I disconnected the old sub for now. The proamp by itself does not hum, but has a very slight hum in its transformer, if you put your ear up near the case. I am not sure what to do next. I tried connecting the two sub amps to different outlets but this did not work either.

Any suggestions?

Thanks in advance.

[---k---]

Not cheap, but will work:

http://www.jensen-transformers.com/index.html

see also:

http://www.audioxpress.com/reviews/media/AE600HA.pdf

http://www.audioxpress.com/magsdirx/ax/addenda/media/hansen-galo2706.pdf

[Adam321]

Yeah, I looked at those. they are really expensive. I thought since the old sub was just sitting around right now, why not put it into service and add a little extra umph to my system. But, for that much money I might as well just built another passive sub and add it to the system.

Thanks for the reply!

[ThomasW]

The low level humming coming from the equipment is simply the transformers. Loud humming means ground loops.

The worst thing to do is have equipment on separate circuits, doing that instantly creates a new ground loop.

I've tried the Jensen transformers and they don't work for me. Since the offer a money-back-guarantee no harm in trying them

One must be careful with either transformers or baluns because some of them filter out frequencies we want to hear.

I run tri and quad amped systems, systems like this are ground loop magnets. My solution is a combination of balanced power transformers and the judicious use of cheater plugs.

I realize the paranoia pundits have the world thinking cheaters equal instant death. The chain of events necessary for serious personal injury to occur with the use of cheaters is lower than that of being hit by lightning.

[Adam321]

Thomas,


"My solution is a combination of balanced power transformers and the judicious use of cheater plugs."

Does balanced power transformers mean using balanced connections, i.e. amps with XLR inputs. Or, is the balanced transformer something you actually add into the circuit. If so, can you give an example?

My sub output from my preamp is not balanced. I am using a RCA to 1/4" adapter.

[ThomasW]

Balanced power transformers are large devices use on AC lines themselves.

Here's one of several threads we have on the devices...

http://www.htguide.com/forum/showthread.php4?t=11045&highlight=balanced+power

[Dave Bullet]

Is your pro audio amp earthed? If not - earth from the chassis.

My sub was earthed but my old Pioneer receiver wasn't. Earthed it and hum "nearly" eliminated.

David.

[JonW]

Is your pro audio amp earthed? If not - earth from the chassis.

My sub was earthed but my old Pioneer receiver wasn't. Earthed it and hum "nearly" eliminated.

David.

By "earthed" do you mean you run a wire from the case of the amp to something? And where might that go? Thanks.

[Hdale85]

You run it to a piece of equipment that is earthed so they are all pulling from the same ground thus eliminating the hum. Thats what the cheater plugs help with.

[JonW]

So it's that simple, eh? Just run a wire between the cases of one thing that's grounded and another that's not? I'll have to try that out.

[Hdale85]

Well you have to make sure its bare metal And make sure the second isn't grounded through something else and what not. But other then that yeah its pretty simple. Doesn't always work but hey? Cheap to try

[JonW]

Yup, I got the metal-to-metal part. :P I'll give it a shot. Thanks.

[Dennis H]

Just be aware that connecting the chassis can make things worse not better. The key to avoiding ground loops is having one and only one path to ground and it's complicated by the fact that signal ground is often isolated from chassis ground. Experimentation is good.

[Dave Bullet]

Yeah - I should have been clearer.

You run an earth wire from bare metal on your chassis of the pro amp into the earth pin on a power plug (without any other live pins connected) and preferably plug that into the same socket that your subwoofer is using. I am assuming the socket in question is earthed (not sure what the standards are where you live).

As Dennis was saying - the goal is to make the earthing potential the same between all devices. By earthing to the same socket as the subwoofer - you are minimising the potential differences (ie. the same circuit to earth will be used - since it is the same socket). Where the potential is different (which could be the case when different sockets are used - wired differently in a house, diffrerent resistance due to cable lengths, interference etc...), "residual" current will find the path of least resistance. This means a little bit of current may leak through the low / high level cable from the amp to the subwoofer to make it an easier path out to the earth.

David.