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View Full Version : Larsen effect noise in speakers.



zetoune
02-25-2005, 11:18 AM
(I don't know if it's the good forum).

I have a noise in my speakers. I've been told it should be a ground issue.(the noise seems to be the 60 Hz larsen)
I only hear it when capturing video and audio through firewire. I don't hear it when I listen to music (mp3, CD audio) or I watch DVDs.


How can I stop that ?

M.Brane
02-26-2005, 12:54 AM
If it is a true 60hz tone you likely have what is commonly known as a ground loop.

The hum is produced by small currents flowing through the shields of the unbalanced analog cable toward the input of the recording device. The best (and safe) solution is to make sure all your equipment grounds terminate at the same point. Easier said than done.

Other sources of hum/buzz are anything that creates strong magnetic fields:
CRTs, PSUs, electric motors, certain types of lighting, etc. Always keep low-level cables away from these sources. If you have to cross low-level signal cables, and power cables make sure you cross at right angles. Don't run parallel.

More info here. (http://www.rane.com/note151.html)

biggles
02-26-2005, 07:58 PM
M.Brane,

a damned fine link, thank you. I'm not sure why zetoune has the problem only when using FireWire, but at the risk of hijacking this thread, my only noise problem is some kind of noise which my monitor speakers are picking up from my display. Doubtless it's RF noise from the screen, not a ground-loop problem, and I haven't figured out yet how far I have to remove the speakers from the display to lose the noise. You appear to be au fait with this kind of problem and I'd be grateful for any advice. It's not an insurmountable problem, I'm not in the business of mixing audio, and I can simply de-power the speakers when I'm tracking to lose the noise, but it sure would be nice to lose the noise altogether if I could. The speakers are Alesis M1's and sit close to my flat-screen display. I like the idea and convenience of powered monitor speakers, but I'm guessing that it's the amplifiers which are picking up the noise. Can you suggest powered monitor speakers which would be less susceptible than the ones I have, or should I be looking at a separate amplifier for the monitors?

Nicolas
02-27-2005, 05:45 AM
Hello,

I solved a ground loop by using this ground loop isolator (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F007%5F013%5F000%5F000&product%5Fid=270%2D054&MSCSProfile=745D84CBF04D14A48AA6FF9C89D722C0BA68C1 B04FE384678A5285FCD6E056B17AF21627FDABE316B90B3C03 8D68EBD6B7F9F3BD1712EAA9951ACB2590A05C6517EFE46941 FEFDD1985D4EFD6321F5E70B4DE9B6C1D45512DCD9FB3DBCAC B947BCD25C158FB0CB82FF3733F8904CF204668A07C3F078FE 5CFB2C8A260A9BF5CCC7B8167EEB75D2F0) I have tried nearly everything on my MDD before I bought this.

Some suggestions:
A defective cable can produce a hum.
Cheap cables (non shielded) could be the culprit.
Using symetric connections helps (XLR).
A bad connection (use raw spirit to clean the phone jack')

Regards

Nicolas

M.Brane
02-27-2005, 10:26 AM
biggles;

When you say flat-screen do you mean LCD? Shouldn't be getting any EMI from one of those unless it has a poorly shielded PSU.

If all your cabling is good, and routed correctly then the only things to try would be some kind of transformer isolation on the input to the speakers like Nicolas suggested (be aware that transformers will always have some effect on the sound), and/or physically shielding the speakers from the display with sheet steel. Some dual CRT users have used cookie sheets to keep the displays from interfering with each other. Just don't get yourself in trouble with the boss by stealing her gear. :D

Powered speakers are convenient, but the lower-priced ones are usually lacking in the amp dept. Always better to use passives, and a good dedicated power amp. I run a Carver PM 1.5 (450PC@8ohms) into my JBL 8" 2-ways. It sounds great, but I recently fried both tweeters with about 1sec of feedback. :eek: That'll teach me to not put fuses on the inputs. :rolleyes:

One thing I should have mentioned for you people on the other side of the pond is your ground-loop hum will be at 50hz, not 60 like us Yanks. ;)

rwm
02-27-2005, 10:39 AM
M. Brane nice to see you again. Always appreciate your audio expertise with the threads.

Again thanks, how you and guitar doing? When you going to test X. :D

Take Care
Randy

zetoune
02-27-2005, 03:28 PM
Thank you guys for you help.
I ran to Radio Shack today just before it closed to look for the ground loop isolator Nicolas talked about. I bought it. I replaced the audio cable (plugged into my firewire AV/DV converter) by the ground loop isolator and the ground noise disappeared.
Nicolas you are the king :-).

I'm so happy! Thanks again.

Nicolas
02-28-2005, 03:14 PM
Zetoune,

glad you solved the problem.

Your welcome.

Regards

Nicolas

M.Brane
02-28-2005, 10:43 PM
M. Brane nice to see you again. Always appreciate your audio expertise with the threads.

Again thanks, how you and guitar doing? When you going to test X. :D

Take Care
Randy

I'm always lurking.
:D

I've been working a lot of OT due to the rain. Not much time for music lately between that, and vehicle repairs. I may be able to afford a G4 soon though. I'm sure I'll have plenty of questions when it comes time to buy.

Thanks for asking. :)

zetoune:Good to see the transformer did the trick. Ground loops can be a real nightmare sometimes.

:jollyrog: