Insane ground loop; anybody wanna try?
@edwyun Thanks for the input. I'm pretty sure it's a ground loop, since lifting grounds does have some clear effect. There is the slightest trace of RFI if I put my ear an inch or two from the speaker. I'm not sure I can slay that, but I can tame it a bit with cable choice. If you think my analysis is off, by all means say so — I'm not an expert here. |
Man, what a brain trust! When I mentioned loose or dirty connection, I was thinking of the connections inside the panel. So the connection where neutral connects to the neutral bus, hot to the hot bus, ground to the ground bus. If you open the panel and shut off the main service breaker, you’ll be able to loosen then re-tighten the connections. It does sound like a ground loop, though one that is related to the branch circuit it’s on. Which is weird. |
@edwyun Acknowledged, and thanks. What I should have said is that there's a little bit of RFI that I can hear, and a LOT of hum that sounds like a ground loop. I'm trying to attack one thing at a time. (I'm not that bright!). @toddverrone Got it. But wouldn't the acceptable performance in locations 1 and 2 seem to indicate it's not the entire electrical service? Doesn't seem to be any harm in pulling the panel, shutting off the main breaker, and having a look tomorrow. I'll disconnect and reconnect the relevant breaker (#3) while I'm in there. If I don't reply further — well, you'll know I skipped the second step. |
Edwyun 2-22-2017+1. Specifically, it’s possible that there is nothing "wrong" with the AC wiring to the listening room, but it may have different characteristics than the more recent wiring to location 1, which in turn are bringing out an inherent susceptibility of the power amp/preamp combo to ground loop issues. More specifically, see pages 31 to 35 of this paper, written by a renowned expert on such matters, in which he describes "what drives 99% of all ground loops." As you’ll see, Romex (which perhaps was used in the relatively recent wiring of location 1) is much less likely to bring out the susceptibility to ground loop issues that is inherent in some designs than wires randomly positioned in conduit (which perhaps was used in the older wiring to the listening room). If that is what is occurring, and it seems like a reasonable bet at this point, a solution may be to have an electrician install new wiring to the listening room, using Romex. Alternatively, inserting a suitably chosen Jensen transformer between the preamp and each of the amps would probably resolve the problem, while also perhaps reducing other forms of noise, at a cost of around $300 plus some additional cabling. Reports that have been provided here regarding the transparency of Jensen transformers have generally been very positive, although I can recall a couple of people who have provided comments that are a little bit mixed. A suitable Jensen model would be the PI-RR, which would accept an RCA cable at its input and provide an RCA output, or the PI-XR, which would accept an XLR cable at its input and provide an RCA output. You would need two of them, one connected via a short RCA cable to the input of each amp. The cable from the preamp to the Jensen transformer could be whatever length is necessary. Here is a good supplier, although they can also be ordered directly from Jensen. Also, although it is apparently unrelated to the problem I would still strongly advise that at some point soon you eliminate the XLR-to-RCA adapters, that are shorting the signal on pin 3 of the preamp’s XLR outputs to ground. If the sub does not provide XLR inputs you could either special order adapters from Cardas which leave pin 3 open, or have a cable supplier custom make an XLR to RCA cable which leaves pin 3 open, or purchase a Jensen PI2-XR transformer which would provide two channels of balanced to unbalanced conversion. Or you could use the preamp’s XLR outputs for the main signal path, connecting them to the amps via the Jensen PI-XR transformers, and connect RCA cables from the preamp to the sub. Good luck. Regards, -- Al |
