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In Reply to: RE: Isolation Transformer For Digital Sources? posted by knewton on June 24, 2016 at 14:23:07
Appreciate your interest, Ken.
1. The noise is always the same and always steady. It is a very high-frequency, narrow band sort of "whine" or low volume "screeching" raspy type noise. When I am troubleshooting, the volume pot is turned down, a digital device is selected, and the noise is audible. Turning on MUTE will defeat the noise. (Atma-Sphere SE preamp, which replaced a Rogue).
(I have tinnitus which comes and goes, so for a while I thought the noise was my medical issue. I have been tested and have excellent hearing for a mid-50 year old; some low frequency loss, but I can still hear 19K just like when I was a teenager).
I'm guessing that the noise is in the 5-10kHz range.
2. My components have dedicated pairs of interconnect cables, mostly Purist Audio. For the sake of testing, right now Belden IC's are connected, and I have tried each input.
The reason I am asking about the use of an isolation transformer for the digital components is because I recently tried using a Tripp-Lite Isobar for the ARC and the DAC. It's selling point is that it has isolated filter banks for receptacles and toroidal chokes.
Anyway, there were times when the digital noise was eliminated, but overall the unit was not effective. Sometimes it would suppress the noise, then the filtering would fail...about a 25% success rate.
Follow Ups:
Just wanted to say that I'm no EE, but I know that some digital circuits can feed noise from the device back down the power cord into the AC line that is shared by analogue components.
What I don't know is if the gain of my CDP and amp plays a part in the level of this return signal. The ARC CDP and my amp both have very high gain.
Okay, this is as I had suspected. A high pitched noise is likely not due to a mains ground loop. You may be experiencing either RF intrusion, or some gain stage oscillation (oscillation is more my suspicion, since the noise only appears after warm up). So, we appear to have reduced the possibility of the digital source components, the interconnects or the power amp being the problem. I say the power amp too, because the noise stops once you mute the preamp output feeding the amp. Signs appear to be pointing to the preamp.Next, I suggest bypassing (completely removing) your Atma-Sphere preamp from your system chain. If you don't have a second preamp unit laying around in your basement then beg, borrow or steal one from a friend. Any cheap unit will do, the quality doesn't matter for your noise isolation test purposes. If you cannot access another preamp from somewhere, then perhaps your Chord DAC features a built in volume control, enabling it to be directly connected to your power amp.
If the preamp turns out to be the source of the noise, it may have a bad tube or a tube with pins poorly contacting it's socket. If it's not due to something easy to fix yourself, like that, then the preamp will probably need professional servicing. If the problem turns out NOT to be the preamp then, unfortunately, it would be hard to determine where else to point you via remote troubleshooting. So, keep your fingers crossed. :)
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Ken Newton
Edits: 06/25/16 06/25/16 06/25/16 06/25/16 06/25/16 06/25/16
Ken, I appreciate your time and you may be right about having a preamp issue. I bought the pre new in January and it has been modded. Gain has been lowered to match the high gain of my SS amp and a 6dB pad added for the gain of the ARC CD. Higher quality pots replaced the stock. I have to believe that Ralph would not let a unit leave his shop with any tech problems. He even setup the preamp and tested it using a SS amp.
I have rolled the tubes, but I can check again, and I do own a small line stage.
I'm certainly not dismissing your diagnosis, since it was done very thoughtfully and methodically.
The fact that when I ran the digital from the separate AC line for a few days and the offending noise was eliminated still makes me wonder if isolation would work. Also, the use of the Tripp-Lite power strip did provide some periods of noise-free use of my system.
This may not be relevant but I just eliminated a 7.5 kHz intermittent pulse of about 10 db by changing the balanced XLR cables to a model that may be better shielded. I had this same problem with different sets of equipment and i think in both instances it was due to poorly shielded interconnects. The ones i needed to replace were not cheap and are generally well reviewed. The interconnects were between a tube preamp and SS amp.
motberg,
That's an interesting thought regarding my issue. I've substituted many ICs from the various digital, but never tried swapping out the preamp to amp IC. (It's unbalanced).
Lowrider, you're welcome.The fact that the noise went away when using different power outlets may not be as significant as the fact that it later returned. Which may mean that the temporary absence of the noise was simply coincidental, possibly being more correlated with the simple fact that the components had to be powered off in order to switch them over to other outlets, however, it's really difficult to say anything conclusive yet.
Definitely insert that back up line stage in place of the Atma-Sphere unit to see whether the noise is affected. Good luck.
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Ken Newton
Edits: 06/26/16 06/26/16 06/26/16 06/26/16 06/26/16
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