Hum from scope outputs

minipax · Post by **minipax** » Thu Jan 31, 2008 6:52 am

Hi i've got the pulsar 2 board with the classic outputs, i connect my monitors directly to the 2 analogue phono outputs but i am noticing a low frequency hum in the woofers and a little interference in the tweeters. if i disconnect the cables from scope the hum stops so i was thinking it's no general interferance from my other equipment...

anyone else get a little hum from the scope outputs or else know what could cause this.. is there anything i can do to prevent it?

thanks.

zangsta · Post by **zangsta** » Thu Jan 31, 2008 7:09 am

It´s probably a lack of earth-connection, or an earth-loop due to two earth-connections. Check your electric cables and see where you have, or don´t have, earth. What country are you in?
You seem to have self-powered monitors, and may have just to get rid of the earth there, I usually just put a bit of tape over the earth-connector to see if that does it.
Ohh..sorry, the proper word is "ground" where I did a direct translation from Swedish, and wrote "earth", hope this makes sense to you.

Cheers
Jörgen

minipax · Post by **minipax** » Thu Jan 31, 2008 7:44 am

Hi Jörgen, yes that makes sense i guess, but i'm in the UK and my pc and speakers both have a 3rd plug pin that i thought would earth them? i don't really understand the "or an earth-loop due to two earth-connections"..this could happen if both the pc and powered monitors were earthed? if so, how would i fix it?

i'll unplug things to see if that makes a difference and then get back to you...

zangsta · Post by **zangsta** » Thu Jan 31, 2008 7:53 am

[quote="minipax"]this could happen if both the pc and powered monitors were earthed? if so, how would i fix it?
[quote]
That´s where I isolate of one of the earth-plugs in whatever way is possible, in the swedish plugs, it´s quite easy to just tape over the earth-plate, thats on the outer side of the plug, and when shure that the earth is not needed, just cut that third cable.

Post by **garyb** » Thu Jan 31, 2008 9:49 am

first, both speakers and computer must be on the same ac outlet. the speakers may have a ground lift switch. you can also get a an isolation transformer, "hum eliminator" is a popular brand and hosa makes this and this. it's not really a good idea to cut off ground lugs, they are for your safety.

before i bought anything, i'd be sure that i had good cables as a bad ground on a cable will do this, and i'd be sure both speaker and computer were on the same circuit, even better, on the same plug. i might also try a different power supply in the computer.....

borg · Post by **borg** » Thu Jan 31, 2008 1:17 pm

If your handy with a soldering iron, you can make a little box that safely removes ground from your powercord. I have the schematics somewhere in a box. Should someone be interested, i could scan it.
I haven't tried it, and I'm no expert, but it is a very simple device.

I got it from a very experienced sound engineer and he said it has proven its use on many occasions (especially guitar amps).

minipax · Post by **minipax** » Thu Jan 31, 2008 2:21 pm

great, thanks for all you help guys!

the19thbear · Post by **the19thbear** » Fri Feb 01, 2008 12:12 am

hi borg.. could you send me a schem? and what does it really do?
i made a post called grounding problems, cause i have problems too!
thanks

t_tangent · Post by **t_tangent** » Fri Feb 01, 2008 12:24 am

Borg, yes that would be an interesting schematic to look at. Would you mind posting that here somewhere in the forums. Thanks

minipax · Post by **minipax** » Fri Feb 01, 2008 4:49 am

so, in my search for hum eliminators i find these 2 products on ebay:

http://cgi.ebay.co.uk/Hum-Buzz-Eliminat ... dZViewItem

http://cgi.ebay.co.uk/ART-CLEANBOX-II-H ... dZViewItem

obviously there's a big price difference and one is aimed at laptop/dj user, but in principle (i know component quality will be better) is there any difference between these two types of hum eliminator? would the cheap one suffice??

borg · Post by **borg** » Fri Feb 01, 2008 1:48 pm

I'll post a drawing early next week.
basically it removes 'dirt' from the ground in a power cord. In case of severe hum, you may still hear it, but significantly less.

Mr Arkadin · Post by **Mr Arkadin** » Fri Feb 01, 2008 2:22 pm

Contrary to what zangsta said, DO NOT UNDER ANY CIRCUMSTANCES CUT ANY EARTH CABLES ON ANY EQUIPMENT. It is dangerous, you will invalidate any equipment warranties and i doubt any insurance company would be to pleased if they found out either.

As stated by others use some form of isolating/earth lift box that works on the audio earth loop. Electrical earths must be left in place.

That concludes this health and safety message.

Post by **garyb** » Fri Feb 01, 2008 11:02 pm

those "hum eliminators" work because they isolate both devices with a transformer. obviously, the better the transformer, the better you can hear what's going on. better than buying a premade box would be to buy high quality jensen or other top notch transformers and make your own box, but if if you buy a storebought product, get the best iso box you can afford, the difference in price is usually the transformers. when the box is under $100-$125, however, it's likely that there'll be no better transformers, so if that's your budget, get one that looks well made and has the connectors you need......

bassdude · Post by **bassdude** » Sun Feb 03, 2008 1:34 pm

Why do people always have to go for the power cord earth???? As Mr Arkadin says, do it at the audio interconnects. It does the same thing and is much better for your health.
Read this page (and print it out for reference) for all the information you will ever need.
http://www.rane.com/note110.html

And also it helps to keep your equipment running off the one household power circuit where possible (most households have more than one).

valis · Post by **valis** » Sun Feb 03, 2008 5:13 pm

My recommendation would be to get a small monitor mixer and use that. In addition to being able to select one to fit the inputs (rca style) and outputs (match your speakers, either XLR or phono balanced) you have you'll also have something that can do gain changes in the ANALOG realm. Turning things up & down in scope is basically bit reduction.

Lastly, when something goes wrong with the PC (Crashes, unexpected 3khz tones at full volume, feedback etc) you will have a handy way to mute things instantly.

minipax · Post by **minipax** » Fri Feb 08, 2008 4:03 am

Hi all, just for your information I got the cheaper of the 'hum eliminators' i was talking about and it works a treat!