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I am currently using a pair of Chimera Labs IC's. They are great and musical, but recently I have had some concerns over the fact that they are unshielded. I have noticed an element of hum and hiss/noise on my amp when playing headphones that is present, it is not intrusive when music plays as the noise and hum is well below the noise floor of the recording, but its there, especially when I turn the volume up. However, when I plug in with no source attached, the amp is virtually dead silent, much quieter than with the source plugged in with the IC's. I know the noise could be coming from my source (Meridian G08), but I believe it may be more RFI and electromagnetic interference in general.What are some audio cues that I should be listening for, that could help me ascertain my current situation with shielded vs. unshielded ic's?
Follow Ups:
I do not think RF is an issue, but I believe EMI to be an issue in my rig, how does shielding help out in that regard specifically?
The shield is a conductive jacket that terminates stray electric fields. Most shields are made of foil or copper wire, and do not stop magnetic fields. Magnetic fields from power transformers are troublesome, since most shielded cables are no more immune to them than they would have been if there had been no shield.The twist or braid scheme in a cable, whether shielded or not, is what helps keep magnetic fields from inducing hum into the cable. Each spiral presents two loops to an external field. One loop is in the opposite direction from the other, so the voltages induced in them cancel out as long as the field is uniform across both of them. In the real world, the fields are not uniform over space, so there will be some residual hum voltage induced if the cable gets close to the power transformer. If this cannot be fixed by re-routing the cable, look for a cable with a tighter twist geometry, or put some magnetic shielding material between the cable and the transformer.
Thank you so much for taking the time to thoroughly discuss the issues at hand, I really appreciate it, and you have been really helpful. Your advice will definitely come in handy.
RF noise is not audible by itself. An RF signal may be so strong that it breaks through: you would hear a radio program, for example, if the transmitter were next door, even though your audio amplifier is not a radio receiver. This is properly called RFI.RF noise below the level of breakthrough still causes audio problems, though, because there is some intermodulation within the audio components and at dirty contacts. The noise mixes with the audio signal to produce spurious tones. The symptoms of RF noise pollution are a harsh treble, a dry and disconnected midrange, and a vague bass. Cymbals turn to mush and vocal sibilants become prominent. There is a loss of warmth and relaxed flow to the music. The degree of these symptoms depends on the level and spectrum of the RF noise, so it helps to hear familiar music recordings on a quiet system to judge whether yours has these symptoms.
Hum may be caused by a ground-loop, as Ted mentioned, or by magnetic induction from leakage magnetic flux in a nearby power transformer. Most shielded cables are not shielded against magnetic fields, so the only protection for this is the twisting or braiding scheme. Try moving the cables away from the components as much as possible to see if the hum reduces.
Hiss is thermal noise from electronics, most likely within your source.
i
Regards,
Geoff
I know that the way my amp is setup, the power transformer is somewhat nearby the output transformers, being in the same chassis, so the headphones pick up a slight residual hum, the speakers you dont hear it. My IC's are litz braided which are supposed to be really good in rejecting EMI and RF and stuff, but it appears by your post a shielded IC wouldn't help. The weird part is when nothing is plugged into the amp, with the inputs shorted, there is virtually no noise.
This is a good thing.If you want to check the noise being picked up by your cables by themselves, make some dummy terminations for the ends that would be connected to the source, and load those ends with 50 ohms. Listen to the amp output with the cables plugged in to the amp. Any hum you hear is being picked up by the cables, most likely from the power transformer.
You are right that most shielded cables would not reject this hum. Perhaps some magnetic shielding around the power transformer would help if that is the source of the hum. If not, then perhaps there is a ground loop, or the hum is induced electrostatically. Shielded cables would help in the latter case.
Many folks do not like the effects of shielding on the audio performance, however, even if it reduces hum. The only way to tell is to audition the cables in your system.
Hi Al, this is something new to me so forgive the newbee question. I have used shorting plugs to test my amps noise level and I'm happy to say it is dead silent. However, I'd like to try this experiment to test the cables. Could you tell me an easy way to make the dummy terminations for the source ends of the cables? Thanks
we were out of town for a few days.Yes, just get some RCA jacks, either in-line or chassis-mount, and solder resistors from the center to the outer terminals. 50 to 100 ohm resistors should work fine. These don't need to be expensive components. The point is to simulate the loading that a good source would provide to the cable at the input end.
nt
Thank you very much for your comprehensive replies, I think you have offered every possible angle that I could possibly see. I believe the hiss to be source induced, and the hum mostly is a result from the power transformer being in close proximity to the output transformers. The only thing left would be for me to follow your advice and attach a dummy load to the source end, to see how much the cables introduce into the system. I will try to research some different grounding schemes, I do have a BPT power strip that has a star grounding scheme for all its outlets, that might help determine the problem. But the amp itself is practically dead quiet, which I am happy about.
...with your terminated cable.After you finish listening to your terminated cable in various orientations as Al suggested, try just touching the shield of the terminated end to your source ground and see if that makes a difference. This can help reveal ground loop problems.
I happily use unshielded cables myself.
Rick
HowdyIt could also be a ground loop, especially the hum part. Searching for groundloops (or ground loop(s)) here, in tweakers' and general may bring up some posts of interest. After treating some ground loop issues my system still meets the description in your post but since that hiss only shows up when I turn the gain up about 30dB louder than I normally listen I don't worry about it too much...
-Ted
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