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24.13.112.255
In Reply to: What you hear in the absence of signal is audio band EMI or noise. posted by Al Sekela on March 24, 2007 at 15:27:35:
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.
Follow Ups:
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
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