Läste gammal recension av Stereophile om Parasound HCA-3500. Robert skriver att han hör ett svagt brus pga en jordloop. Han får inte bort den utan får ett nytt exemplar som är modifierat. Någon av er som kan kan gissa vad dom gjort? Jag har själv ingen aning. Robert beskriver det som "modifying the amplifier's internal grounding arrangement"
Grounded
Tracking down noise caused by ground loops in an audio system is often a tricky business. You can have a system that's perfectly quiet, and then you add a new component, and all of a sudden there's buzz/hum.
This was the case when the Parasound HCA-3500 was introduced to my system. The HCA-3500 driven by the CAT SL-1 Ultimate preamplifier resulted in what sounded like AC noise—low enough in level to be masked by music most of the time, but audible from the listening seat between CD tracks. (Mechanical hum from the transformers was exceptionally low.) The system was dead quiet with the CAT driving the Rotel RB-1090 or the Bryston 9B-ST. I tried all the usual remedies—checking connections, lifting/reconnecting AC grounds, plugging components into the same outlet/different outlets, changing interconnects—to no avail. The noise was still present when I plugged the PS Audio Ultralink Two digital processor directly into the HCA-3500, which might be taken to indicate that the problem was with the amplifier.
But when I used the Parasound AVC-2500 surround processor/preamp, the system was quiet. I eventually concluded that the problem was not due to any one component, but to the interaction between components. At this point I thought of getting hold of another high-end preamplifier, but that would have meant spending more time getting accustomed to the sound of the system with the new preamp—assuming that it did not interact negatively with the HCA-3500.
The solution came courtesy of Bryston vice president James Tanner and Bryston design engineer Stuart Taylor (the "ST" of the Bryston 9B-ST, 7B-ST, etc.). I gave James a call because I remembered that I had had a similar problem in my home-theater system, involving an early sample of the 9B-ST and a surround processor. That time, the problem was solved by Bryston modifying the amplifier's internal grounding arrangement. Would they help a rival amplifier manufacturer deal with a grounding problem?
Yes, indeed! With a spirit of cooperation that should be a model for the industry, Stuart Taylor told Parasound technical ace Andy Murray about the exact changes made to the Bryston 9B-ST that eliminated the AC noise they had encountered in some applications. Andy sent me another sample of the HCA-3500, modified as per Stuart's suggestions, and, like magic, the AC noise was gone. The amplifier also sounded a bit smoother. With this problem out of the way, I could start listening again in earnest. All comments in the review about the HCA-3500's sound refer to the second, modified sample. I'm told that all current and future production of the HCA-3500 incorporates the modified grounding scheme.
https://www.stereophile.com/solidpoweramps/100parasound/index.html
