REL Subwoofers, Class D Amps and Neutrik Cables.

I don't know what "caps on the black wire" may refer to in this case. But an approach that should work fine is to obtain a simple RCA plug, perhaps at Radio Shack, solder the black wire to the ground sleeve connection point of the plug, while leaving the center pin unconnected, and inserting that plug into an unused RCA connector on the integrated amp. That would establish a circuit ground connection between the amp and the sub. Don't connect to one of the galvanically isolated digital inputs on the amp, though. And connecting to an analog output jack might be slightly preferable to connecting to an analog input jack.

Another approach that might work ok, depending on the internal grounding configuration of the amp, would be to simply connect the black wire to a chassis screw on the amp.

Good luck. Regards,
-- Al

Hey Puff,

Al nailed it - I have one of those cables you speak of, and that's exactly how it works. Music Direct used to sell them, I'm not sure why but they don't anymore. The cable has three wires on the amp end - two positives that connect to the two positive speaker posts on the Grand, and the black cable has an RCA connector that connects to an analog input or output jack for grounding.

I did get one of those cables made with the RCA ground from my stereo dealer, however I'm not sure if they modified the plug in such a way that the center of the plug is not affected per Almarg.

I spoke to Sumiko and they would not commit on a RCA ground as a safe way to go. I also spoke to a gentleman who used to manufacture cables specifically for this purpose and he is now convinced that any high level connection will eventually harm a digital amp and only reccomends using the low level connection.

In regards to the "Caps Thing' I was referring to how the hot leads are modified with 10 UF film caps. Which I know nothing about. l will reference an old post I found on www.computerAudiophile.com from 09/08/2011 : (not from me)
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I know a little about the problems with some Class D amps (as well as class A/B balanced output amps) and using high level inputs to subs, as recommended by Rel.
It is important to understand that all class D designs are somewhat different in this regard, so what applies to one amp will not neccessarily apply to another. My experience is with IcePower based amps, specifically ones using the ASP series Ice modules. These modules have 40 VDC on their speaker output terminals in relation to amplifier ground, and the amps are balanced output, so the black terminal is not ground, the ground is usually found at the chassis. The solution for a Rel sub with this amp is to put a filter on the speaker outputs to block the DC: so the two hot leads get a 10 uF film cap in series and are hooked up to the appropriate speaker terminals. Then the ground lead is hooked up to the chassis, making sure it makes good contact (one may need to scrape of paint or anodising to expose bare metal.
I believe the newer Ice modules are different in this regard.
All balanced output amps (not just class d) need special consideration to hook up to Rel style sub input. For example, my Pass x150.5 is balanced output, and has a separate ground terminal on the back panel: the ground wire from the Rel goes to the ground terminal, and not to any speaker posts, and the two hot leads go to the appropriate speaker posts.
If you are at all unsure how your amp operates, be sure to contact tech support of the manufacturer of your amplifier, and explain to them what you are doing, and ask them to provide an appropriate solution.
BTW: I am a big believer in hooking up subs this way (even subs which do not have a high level input stock (like JL) can be adapted to use the amplifier connection with the appropriate additional circuitry to match impedance/levels) as this connection method tends to make it easier to get a seamless transition between the main speakers and sub(s).

My experience is with IcePower based amps, specifically ones using the ASP series Ice modules. These modules have 40 VDC on their speaker output terminals in relation to amplifier ground....

I was not aware of that. If that is the case with your Peachtree Grand then, yes, as the person you quoted stated you would need to have capacitors in series with the red and yellow wires. Assuming, that is, that the sub itself does not have capacitors in series with its high level inputs. (When you said that "Rel's new subs do not have the issue," I'm not sure how they could be immune to 40 VDC at their inputs unless their inputs were AC-coupled, presumably with capacitors).

Have you asked Peachtree if the Grand Integrated has a significant DC voltage such as 40V on its outputs? FWIW, its manual does not have any cautionary statements regarding the presence of DC on its outputs. Also, they would be the ones, not Sumiko or REL, who presumably could best advise as to the suitability of using an RCA shell for the ground connection.

Also, you could use a multimeter to measure whatever DC may be present. With no signal going into the amp, you would measure between each of the output terminals and the shell of an analog RCA connector.

And if you have or can obtain a multimeter, by measuring for continuity or resistance you could use it to determine if on the cable your dealer supplied the ground connection of the Neutrik connector is connected to the ground sleeve of the RCA plug, and that the center pin of the RCA plug is connected to nothing.

Good luck. Regards,
-- Al