Subject:Cause of HUM from DVD player RCA outputs to active monitors?

Posted by: prairiedogpics

Date:4/21/2005 1:00:44 PM

I normally just post in the Vegas forum, but I have a home A/V question and I wanted to tap some knowledgable people in this forum, hoping for an answer. (hopefully my longevity on the Sony forums entitles me to that...)

I have a set of M-Audio SP-5B active monitors (power supply built in to each one) that I'm trying to use for DVD playback along with my TV (normally use them for video editing).
So: DVD player RCA outputs are connected to the TRS (1/4" Tip/ring/Sleeve) inputs of the monitors. Video components of DVD player are plugged in to my TV. The TV also has CATV input hooked up.

I get a significant HUM with this setup.

I've tried putting one of these

http://www.musiciansfriend.com/srs7/g=live/search/detail/base_pid/180613/

in the path between DVD player and monitors, but that results in a LOUDER hum. (this device changes unbalanced to balanced)

I'm considering using one of these:

http://www.musiciansfriend.com/srs7/g=live/search/detail/base_pid/180634/

since I think it may be a ground loop problem.

Originally, I bought RCA to TS (1/4" mono) cables at Rat Shack. (Couldn't find RCA to TRS -1/4" stereo - cables.) That's when I first noticed the HUM. Then I moved on to the CLEAN BOX I (unbalanced to balanced converter, RCA to XLR), with no change (actually it got worse). See the first link above.

The speakers (active monitors) do, in fact, have balanced XLR and TRS input jacks (using only one or the other, of course), and I am using balanced cables.

Now I'm wondering if it's a ground loop problem (I'm not an electronics wiz, more of a hobbyist video editor), and that's why I'm considering the CLEAN BOX II, which looks like a ground loop isolator.

Am I on the right track?

Thanks,
Newbie Dan

Subject:RE: Cause of HUM from DVD player RCA outputs to active monitors?

Reply by: Chienworks

Date:4/21/2005 2:00:31 PM

Strange that you should be getting that much hum in that situation. I'm guessing it's an impedance mismatch more than anything else, though i would have thought your first box would have taken care of that. What i would suggest trying is making your own cables. Buy a long RCA to RCA cord and cut it in half. Also get a pair of 1/4" stereo plugs. Solder the plugs so that the inner tip goes to the inside conductor of the RCA cable and the ring goes to the shield. This connection will break the ground between the speakers and the DVD player. If this doesn't work, short the ring and the sleeve together. One of these combinations should work. The 1/4" inputs on the speakers should be more forgiving of impedance than the XLR connectors.

Subject:RE: Cause of HUM from DVD player RCA outputs to active monitors?

Reply by: LarryP

Date:4/21/2005 7:01:13 PM

I've built a number of cables similar to what Kelly mentions and they work well. If you look at this note: http://www.rane.com/note110.html and scroll down to Figure 4, configuration 18 you will find a picture of what you want. This is, however, a completely custom built cable.

Another suggestion is to power the units from the same outlet even if it means running a long extension cord to the powered monitors. Just tell your friends that the orange color of the extension cord is important and let them think about that for a bit.

BTW lots of great information on the Rane site.

Larry

Subject:RE: Cause of HUM from DVD player RCA outputs to active monitors?

Reply by: prairiedogpics

Date:4/21/2005 7:23:30 PM

thanks for the replies.

I did wire/solder an RCA to TRS cable as Kelly suggested (trying both options), and still got the HUM.

Today, tired and frustrated, I bought a ground loop isolator at Radio Shack.

What do you know: It works like a charm:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-054

How safe it is, I don't know. A $17.00 solution in the end..

I did bookmark that site, LarryP, to further educate myself.

Let me know if you think I'll fry something using this!!

Thanks,

Dan

Subject:RE: Cause of HUM from DVD player RCA outputs to active monitors?

Reply by: Rednroll

Date:4/21/2005 7:38:57 PM

You definately have a grounding problem. The suggestion to plug all the devices into the same outlet could be a solution. The easiest solution. I'm not familiar with those monitors, but if you have balanced inputs, then the amp for the monitors most likely has a three prong plug that plugs into the AC outlet. Get one of those ground lift adapters. You know one of those 3 pin to 2 pin adapters that plugs onto the end of the AC power cord. This most likely will solve your problem and will cost you a buck. Or if you want, just take the power cord and rip the round ground peg out of it.

Another solution is to use a different cord connected between the devices. If you're using an RCA(male) to 1/4" (male) cord, then most likely that 1/4" end is a mono plug, although you say you're using a balanced cord. This causes the ground to be shorted to one of the balanced sides of the audio inputs for the monitors. Make a RCA to 1/4" (stereo) cord. Don't buy one, because they may wire it so that it acts the same way as the current cord you're using, and probably is what you might have now. Basically the Tip of the RCA end connects to the tip of the 1/4" end. Then the shield of the RCA end connects to the shield of the 1/4" end. This means that nothing is connected to the RING of the 1/4" stereo plug, and therefore elliminates the shorting of the shield to one side of the balanced input.

Kelly's suggestion of connecting the shield of the RCA to the RING of the 1/4" probably won't work, and actually could be a good cause to give you the results you're having now, if the electronic devices are plugged into 2 different outlets.

If I was you, I would first do the cord change that I mentioned and if that doesn't elliminate the hum then use the AC ground lift adapter. The quickest thing to do is the adapter if you've got one hanging around.

{Edit) I guess I was typing this at the sametime you where replying. I would try my ground lift adapter, and go get your $17 back from Radio Shack.