hello i have a problem, the thing is that i have numark dm905 in which and i wanna have my pc and my tv connnected to it. but when i have the tv and pc connected at the same time it makes a hum sound. i think that the ground lift would help but i don't know how to do it my amplifier is a optimus mpa-250 and also a Gemini EQ300 Graphic Equalizer
Place something like "solving ground loops" in a search engine. There are several ways to solve the problem depending on where the problem originates. You can also read some of the archived threads here to find solutions.
Dave, one amp could become more succepible to hums than the other, based on age or anything else. If the grounds are all equal potential, then if one hums, the problm is just the one unit. With age, the caps can dry out, fasteners could loosen(especially after handling and moving) and things free-up due to expansion and contraction.
If you are able to open the amp and do a minor service, then all may be well again.
I had a Rotel amp with a hum, so I serviced it with compressed air only. I blew a POS away from a FET leg and solved it.
David- My 3b would HUM when I had it in the theater. The NAD did not. However, the Bryston was picking up a ground loop that was coming through the satellite line. It does not hum in the two channel system.
Every piece of equipment has its own ground potential. There are multiple reasons why this is so. If you're been reading the threads regarding ground loops, you'll see some of the possible reasons listed. The addition of surge protectors has only managed to increase the potential problems in any one system. Any cable TV based video system can have ground problems since the real earth ground for the incoming video from a cable feed is not taken at the same location as the ground for your home's electrical system. Once a ground potential varies too much in one component it will usually hum or make noise in some fashion such as physical noise from the transformer. Though electrical hum and transformer hum are two distinct noises there is the possibility both can be caused by grounding problems.
If you introduce a component and noise shows up, then it's a good guess the new component has contributed to the problem - but might not be the problem.
If the noise shows up without changes in the system, then you should consider the offending unit to have potential problems.
The most likely culprit in this case would be power supply or coupling capacitors. If this were the case, the hum would be present when no other components were plugged into the same AC source as the amp and no incoming interconnect was connected to the amplifier.
Tracing noises can be extremely frustrating since a groud loop can show up when two components are linked by a (not quite) common ground point or an AC outlet is wired with reversed conductors. In a pull-your-hair-out scenario an amplifier with border-line capacitors can make noise when another component is on line and not when the component is removed. However, this would be an usual circumstance and one that would quickly abate as the caps continued to fail and the noise became constant.
As with most ground problems the way to start is by stripping the system down to bare essentials and making notes on paper that will allow you to replicate connections that worked and didn't work as you put the system back together. Connect one component and check for noise, disconnect that component and try another working from the amplifier input backwards toward the sources. Take notes at all points and only connect a component when the amplifier is either powered down or the volume control is set to minimum levels.
Tracing ground porblems is 90% logic and 10% guess work. So be consistent and follow a logical path. If you have substitute components to replace a noise inducing unit, this can sometimes help resolve the issue though, as you've probably noted, one amplifier can hum in a situation that doesn't affect another amplifier.
Enter "solving ground loops" into a search engine and start with some simple suggestions.
"If you introduce a component and noise shows up, then it's a good guess the new component has contributed to the problem - but might not be the problem."
I think this statement hits it pretty much right on head. The potential for a ground loop has always been present in my HT system, introducing another component was a catalyst.