BillFitzmaurice.info

I was asked if mic cable could be used to make up a cable to go from a laptop to a powered speaker. All the classrooms have their own individual powered speaker for class-reinforcement. Typical distance for the cable might be about 30 feet.

What advice do you have? I was concerned with induced hum with just a regular store-bought 30" cable. Cost is important.

Tom Smit wrote:I was concerned with induced hum with just a regular store-bought 30" cable. Cost is important.

Induced hum isn't an issue with line level signals. Ground loop hum is, and laptops are notorious for that. Transformer isolation is the usual fix.

buy cheap 1/4 TRS cables, add 1/8 adapter to one end.

A couple months ago Parts express had INSANE deals on a brand called Talent. think I paid 4 dollars for a 25 foot TRS cable.

Thanks, I appreciate the replies.

If you choose to make your own cables, Neutrik makes a really robust 1/8" / 3.5mm TRS right angle plug. It has a good strain relief, and the right angle helps prevent it from accidentally being unplugged. It's a little tight with 22/2 shielded cable, so you don't want to go any heavier than that.

Depending on the make of the powered speaker, it may be isolated and not grounded, in which case ground loops aren't possible.

My favorite tool for this type of thing is the Whirlwind PCDI. It's a stereo DI with 1/4", RCA, and Stereo TRS 1/8" inputs. It has pads, ground lift switches and a switchable summing circuit so you can properly sum to mono. It's a tad on the expensive side at about $130, but a very useful "Swiss Army Knife" direct box. Also useful is the Rapco LTIBLOX, which is a simple 1/8" stereo to mono XLR DI with a volume control. These are a bit less, usually about $40 I keep one of each in my box of short cables and DIs, and between them, I can interface just about anything.

I have done that from my desktop 1/8 inch trs to each of the following.......stereo rca, 1/4 unbalanced, 1/4 balanced and to xlr with no hum issues and no devices to prevent hum but cable length never exceeded 16'.

Some of those connections even violated my own rules about multiple adapters to get the right connection.

Is it just laptops that do that Bill? Would plugging your charger in give your laptop a good ground? I have to plead ignorance on this one because I have always used a desktop, but never owned a laptop! Sorry!

bjm362 wrote:Is it just laptops that do that Bill? Would plugging your charger in give your laptop a good ground? I have to plead ignorance on this one because I have always used a desktop, but never owned a laptop! Sorry!

Other way around, a laptop with charger introduces the hum in my experience. Using it on battery solved the issue entirely... Of course if you pick up the above bits it's not an issue.

One of these could work: http://peavey.com/products/index.cfm/it ... SBPlayback but probably too pricey for your application.