At Gearslutz, the mega-popular recording technology/gear lust site, a member asked…

Originally Posted by deadby100cuts
Ok so I got a pair of Roland ds-90’s from a buddy for 100 bucks. Ive been mixing through my pa system so its GREAT having monitors lol.Anyway, I just got them hooked up and they sound GREAT.I used some instrument cables that I had been using to plug my interface up to my pa. They sounded great, no problems at all. Fast forward an house after I get back from buying some short speaker cables. I hook it up, and I’m now getting a REALLY annoying high pitched noise from the speakers. I use to get this from my Pa system as well but It could be managed by keeping the pa itself on a lower volume. However with the new monitors I don’t know how to get rid of it. Its not super noticeable when a song is playing but the second the song gets softer or ends, its right there in your face. Its SUPER annoying and it didn’t do this when I used the instrument cable. Its not a solid pitch, it changes slightly. it doesn’t matter what settings I change on the interface it still has that annoying sound. Its driving me crazy.I recorded the sound ( I hope) here is a clip Vocaroo | Voice message[1]


Ok, so I grabbed my long like, 20ft speaker cable from my pa system and dragged the speaker into several other rooms so I would know its on a different circuit, thats where it gets weird. If I plug the speaker into the wall in another room BOTH speakers will emit a lower sustained kind of tone. Even though one is in the other room, and is still plugged into the other outlet, so the only way they are “connected” is the fact they are both plugged into the same interface. Keep in mind they are powered separately.

So I don’t THINK its a ground issue, but I’m not sure whats going on. My whole computer setup is a little strange, wires everywhere. If it interference from something, like the power cables, then I’m not sure how to get around it. Because I mean, the only place I can put my tower is right in between the speakers and the monitor is in front of the tower.

[I answered…]
Could that ‘REALLY annoying high pitched noise’ you’re getting from your powered (amplified/active) speakers (and also got from your PA when turned up loud) be the broad spectrum noise often referred to as ‘hiss’? That’s the term often used to refer to such self-noise from amplifiers.

In conventional standalone amplifiers, there is typically a master volume control that controls the output level of the amplifier as it goes to passive speakers. There will always be some noise floor, even with that master volume all the way down — but on most conventional amplifiers, as you turn the output level up, the self-noise rises with the volume. If you have music going, you don’t notice it because the music has ALSO gotten louder — but if there’s no music, the self-noise (hiss) from the amp will likely be very noticeable at the top of its loudness range.

Now, imagine you had to run that amp all the way up (for some arbitrary reason) but then control the volume from an external source like a CD player with a built in volume control. The amp is always running at full bore, so the self-noise is always high, but when you adjust the volume from the CD player down to a comfortable level, the level of the hiss from the amp stays just as loud as it was.

This is analogous to how (almost all) powered speakers work. (Some do have individual volume controls and some variable or switched input trims, which, if designed right, should allow some lowering of the gain the amp is supplying, lowering self noise.)

When you turn down the output level from your computer you control the level of signal going into the input of the amplified speaker — but not the amplifier’s gain. Lower the volume from your computer/DAW and the overall volume goes down — but not the self-noise from the amplifier/speaker.

These powered speakers look like pretty powerful amp/speaker combos judging from the Roland writeup (DS-90A :: Products :: Roland) 60/30 biamped — the ‘equivalent’ in some broad sense, of a conventional 180 watt stereo amp. (And pretty good frequency specs, too, at 48Hz to 20kHz (+/-3dB), at least on paper.)

And that means they’re probably louder than you’re used to, and have higher self-noise than you’re used to.

If the speakers have no volume control or trim (there’d likely be knobs, switches, or small, inset nylon screw shafts on the back — but check the manual, which I didn’t download [not familiar with these units, myself]) then I’m afraid it’s more or less something you’re going to have to get used to.

Now, cables. You should NOT run dedicated speaker cables between your source (DAW/computer/whatever) and the LINE in put of the powered monitors — such cables are for running between the output of an amplifier and passive speakers! You won’t damage anything (but your signal) but they are designed for the impedance and power handling necessary for carrying powered signal between the output of the amp and a speaker’s load.

Signal leads (balanced mic cables or unbalanced ‘instrument’ or patch cables) should be used between the LINE source and the amplifier input of the powered speakers.

Now, finally the low pitched noise you got from running the long cable into the other room where the speakers were plugged in?

That is almost certainly a so-called ground loop. You created a near-perfect setup to introduce ground problems by interconnecting two audio devices that (I’d bet a cyber-dollar) are connected into power outlets that are running to different circuit breaker or fuse panels. That introduces two paths to ground. Electricity has a voodoo like attraction to ground (we won’t get into the physics) so electrical power can, in effect, be drawn through the circuit in unintended ways, causing 50 or 60 Hz modulation of the signal from ‘power contamination.’

The solution: it’s always best to have interconnected video and audio devices supplied from the same panel circuit if at all possible. To be on the safe side, many smaller installations (as in houses that may have funky wiring — and many do) will supply power to all interconnected units from a single point.

Hope that helps.