HELP! 1075/3805 speakers humming

[tomm]

hi
just hooked up my new 3805 and new 1075 today and my speakers are humming, including my subwoofer(which is not connected to the 1075). All other speakers are run through the 1075. does someone know what this could be?
tom

[Taito]

Hi, did the subwoofer ever hum before? I'm assuming it's an active sub.
-Ben

[bigburner]

The Cause of Hum

Hums are caused by ground loops. Ground loops can be a difficult task to track down and remove because there are numerous reasons for them to exist. The loop is caused by a difference in the ground potentials in your system.

When you plug a power amp into one receptacle and a preamp into another receptacle, the metal cases of these two units can have a small potential difference in their safety grounds which means that the metal cases of each piece of equipment are at a slightly different potential. When you connect a single ended (RCA) cable between these two devices, a small AC current can flow in the shield because of the potential difference. This signal is in the signal loop circuit and can cause a hum. An interconnect circuit has a loop path (completed circuit) that flows through the centre conductor of the interconnect cable and back on the shield. If there is an AC signal on the shield flowing because of the ground difference potential, you'll hear a hum.

Solution 1 – Use Cheater Plugs

A cheater plug is a mains AC plug where the third prong has been eliminated. The third prong is the electrical safety ground and is there to protect you and anyone who uses your equipment from electrocution. This safety ground is a cold conductor designed to provide a path to ground for safety protection against internal shorts inside your equipment when equipped with the three prong plug.

The third prong connects back to your service panel (where it is bonded to the neutral wire), and from there is connected either or both to your house plumbing or external ground rod.

On equipment with a third prong, the metal case and external metal parts are all connected to this safety ground when you plug the electrical cord into the wall. If a component fails inside a piece of equipment and the mains power shorts to the equipment’s case, then that case is now live and can electrocute you. If the safety ground was attached, then a breaker would trip to indicate you had a fault.

On equipment with two prong plugs, they are internally insulated from the external case to provide this protection without the third wire.

If you have a hum from an amplifier caused by a ground loop, then removing the safety ground connection to the amplifier (and perhaps other devices as well), can clear the resultant effect of the ground loop, and that is why the hum goes away with the use of cheater plugs.

This easy fix is obviously tempting, but it creates the safety issue discussed above. Unfortunately sometimes it's the only thing in the end that works on some troublesome equipment, so you should ensure that any “cheated” piece of equipment is hidden away from anyone touching it.

Solution 2 – Eliminate the Cause of the Problem

It is more advisable to try and clear the condition causing the ground loop than to mask its effect by applying unsafe practices.

Generally a ground loop can be tracked down to its source. Removing the problem can sometimes be a little more difficult.

Firstly, disconnect everything from your amp except its speakers and see if it still hums. If it does – you don't have a ground loop – you have a problem with your amp.

If you have no hum with the single amp and its speakers, then hook your preamp up to the amp with a single set of interconnects, ensuring the preamp has no source components connected to it and that it is plugged into the same receptacle as the power amp. If it doesn't hum, then add a single source component to the preamp with the same conditions. If this minimum system has no hum, and you usually plug the devices into different receptacles, then you can try and plug the preamp or source component into a different receptacle and check for hum. Using this approach you must rebuild your entire system, until the hum reappears. You should find the source of the problem this way.

One of the major culprits in creating ground loops is the use of more than one circuit in a system. Residential houses in the USA use 240 volt "single phase" three wire power. The two "hot" legs are 120 volts and are 180 degrees out of phase. The loads in your house are balanced between these two legs at the service panel. If possible, try to use just one circuit for all your equipment. If that’s not possible, for example because your system requires a lot of power, ensure that you use a common leg.

Another major cause of ground loops, except usually worse, is when you introduce a new ground into the system from cable TV or a satellite. The ground on the shield may possess a different potential than the ground in your system and current will flow in all the interconnects. Centralizing and bonding all external grounds to the common house ground gives you the best chance of reducing this problem.

Acknowledgement

I am not an expert in removing hum. I had a hum problem and I sought advice within this forum. I received advice from a number of members, in particular from brucek who provided most of the information provided above. If you can’t solve your problem using the above information then I’m afraid you’ll have to get further advice elsewhere!

The Solution to My Problem

I solved my hum problem by getting my electrician to install a new dedicated 20 amp power cable from my switchboard to my listening room. All my hi-fi devices are plugged into this circuit, and I don’t use this circuit for anything else.

[tomm]

well after much searching i have found the problem. power cords. problem now fixed. thanks for the help.