subwoofer connection quandry

an acquaintance wishes to connect his nad d7050 integrated (which has dual rca sub outs) to his hus stf-1 sub (which has a single rca input) using a generic y-connector. someone at hsu told him that such connection method would short out the amp. i'm perplexed by this advise (granted, i'm easily perplexed). can someone at or near almarg's level of wisdom enlighten us here?
Hi Loomis,

Thanks for the implicit compliment. See my post here, which explains the concerns, and the rest of the discussion in that thread.

The bottom line: It might work ok, and some people do just that, but I don't consider it to be good practice and I would recommend against it. There might be adverse effects to both sonic quality and the long term reliability of the D7050, depending on the specific design of the circuit in the D7050 which drives the sub outputs. I would recommend either obtaining a separate mixer to combine the two channels, as discussed in the thread, or else connecting at speaker level.

Best regards,
-- Al


As usual, your comments are a pleasure to read and very informative. Would like your opinion on this. Years ago when I worked at an audio dealer, I would make a "y" adapter using 2 resistors. Using a simple stereo RCA cable, just connect both channel outside shields to the outside of the single RCA. Then use a 1/4 watt 1K ohm resistor in line with the L & R center wires before tying them together to the center pin of the single RCA. I had found this published somewhere as, "How to make a stereo to mono adapter." I made quite a few of them to use in different applications, and never had a problem. How does this compare to using the typical y-connector?
(i posted my followup query on the wrong post) but, almarg, in your expert view, would there be an issue with simply connecting one of the two sub outputs of the amp to the sub input as others have opined?
Thanks, Loomis & TLS. There's no technical issue connecting just one output channel, but obviously it won't be sonically optimal with recordings having deep bass content that is not centered (i.e., that is not recorded equally in the two channels, to a close approximation). As Elizabeth indicated in the other thread that probably won't be an issue with many or most recordings, but IME it would definitely be an issue with some.

The resistor-based approach TLS describes amounts to implementing a passive mixer, and can work well. In fact in the other thread I linked to there is a link describing exactly that, although with resistors in the 5K to 10K range. What value is optimal depends on the impedances and other characteristics of the particular components, and also perhaps on the length and characteristics of the cable (although that probably doesn't matter much if at all in an application such as this where effects on high frequencies aren't a concern).

In this case the output impedance of the NAD's sub output and the Hsu's line-level input impedance do not appear to be specified. But my suspicion is that the resistor value that is chosen is probably not very critical. If I were to hazard a guess I would say go with something in the area of 1K to 3K for these particular components, if that approach is pursued.

Best regards,
-- Al
al et. al.: thanks for the input. i should have found the earlier thread, which was expressly in point, but i drank too much cough syrup last night, which is interacting very badly with my pysch meds.

Thanks Al, I just looked at the link in the other post, and that is definitely the adapter I made. I found that using a larger RCA connector like this one, it was fairly easy to secure and conceal the resistors inside the connector, making the adapter cable sturdy and having a very clean appearance.
Al and others ... the OP's Q reminds me of what I had to do to solve my sub-woofer issues. Rather than repeat everything I said before, I'll just put out there that the solution to my sub-woofer problem was picking up a custom made impedance buffer that summed the L/R channels without shorting the linestage's main outputs. In addition, I needed to raise the impedance presented to my linestage. The custom buffer handled all of these issues and one or two other issues as well. Read the link Al provided for more information if interested.