Bottlehead Forum
General Category => Technical topics => Topic started by: Dr. Toobz on July 08, 2010, 05:07:33 PM
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I want to run a line between my iMac/USB I/O (DAC) and S.E.X. amp in my music room, which would require about 20 feet of wire. Problem is, regular RCA cables aren't sheilded, and when I tried one, I picked up a lot of hum and noise. What would be the best way to rig up something on my own? I don't have a DAC right now at the amp (but ironically, I have a cheap 48/24 ADC that goes from the Seduction to the iMac via a long TOSLINK cable to digitize LP's. So I'm digital only for recording, not playback).
For example, would coax work, and could I protect the signal wires from interference by running the foil shields to the chassis ground of the S.E.X. amp? I'm not so concerned with line loss as I am interference, which does seem to be an issue with the cheapo cable I tried.
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Most RCA interconnects are coax, which is "shielded" - the leakage through the shield/outer conductor depends on the actual construction. The question is, what noise are you picking up and why? What I mean is, it may be ground current in the interconnect which has a different solution from radio frequency interference.
Measure the AC current between the cable outer conductor and the RCA ground, at either end, by unplugging the RCA plug. That will give you a hint.
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Use some Belden 89259, shielded RG 59U. It has a stranded center conductor which makes it more flexible than most RG 59U. A good low capacitance shielded cable.
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I use Mogami 2549 balanced microphone cable for all of my IC's. Redco has it for pretty cheap. It was recommended to me years ago and I've since sold off all of my other IC's. I've used it in 15' runs without hum or issue though it is designed for much longer runs.
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You'll want to run the cable through an audio isolation transformer. Audio equipment operating off of two different outlets often develops hum.
Another choice would be to put the DAC closer to audio equipment and try running a longer digital connection.
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. . . Another choice would be to put the DAC closer to audio equipment and try running a longer digital connection.
Insert the slap head smiley face here! That is the best answer. Digital travels much better than analog.
Good suggestion!
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Actually, I can't - the USB soundcard would be limited by the length of USB cables available, which is less than 20 feet. The DAC is a soundcard. I also tried a ten foot USB cable and a shorter audio cable and had the same noise - hum, buzz and periodic clicking and static. I'm almost 100% positive it is ground loop noise, and what I'm hearing is interference from motors and the like.
What I actually may do is just get one of those cheap $70 Geffen mini-DACs to put at the S.E.X. amp and run a toslink cable to the computer some twenty feet away, like I did for the ADC. I can't justify spending $400+ on a "real" DAC right now, and the last one I had sounded like nails on a chalkboard anyway (Cambridge Audio DacMagic). I should have kept my Marantz CDP!
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What you are describing comes out of my on-board computer sound card all the time, and it only has to go about two feet as actual unbalanced audio to some powered speakers.
As an alternative to wire, I've read of people sending S/PDIF around their homes with little local TV-bandwidth transmitters. Then they just set up a receiver wherever they want to feed a DAC. Of course, you could most likely go the 20 feet on RG-59 coax if you were sending S/PDIF, but you do have to have a DAC at the destination.
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Hi Angelo,
Wireworld (and others) make 7 meter usb cables -- the UltraViolet an the very nice StarLight -- both of which perform far above their price points. Also, the HRT MusicStreamer II for $149 is a very nice dac, small, sounds very nice, usb powered, etc..
I had a lot of the noise you are describing until I got a better power supply for my Asus HTPC, which is dedicated to running audio via my Tranquility dac.
BTW, couldn't agree more about the sound of the DacMagic -- that's one I just can't take.
Good luck,
Jim