Subwoofer for Ayre AX-7e?

Taww 4-2-2018
... This wiring setup works, HOWEVER, if you turn off the Ayre, you will get a fairly loud hum from the REL. This is because the Ayre outputs are floating and not referenced to ground at all. I haven’t figured out a way to get around this short of modifying the Ayre with a dedicated output for the sub with a muting relay.

A solution might be the suggestion I made earlier:

... An even better approach, IMO, would be to solder the wire from the sub’s negative input terminal(s) to the ground shell of an RCA plug, leaving the center pin unconnected, and inserting that plug into an unused RCA connector on the amp.

That would result in a direct (essentially zero ohm) connection between the circuit grounds of the sub and the amp. While I would expect in the case of an Ayre or other well designed amp connecting to chassis would interpose a significant impedance between the two circuit grounds, resulting in the possibility of hum as I had mentioned. Although whether or not my suggestion would be helpful when the amp is turned off is probably not predictable, since as you alluded to the outputs of the amp are not in a controlled state in that situation.

In any event, thanks for your informative and well written posts.

Regards,
-- Al

@almarg .
My office rig is in a relatively small room (10'x10'), so I don't play music loudly and usually within a small range of volume. In any case, after setting things up the way I liked, I don't find the need to adjust the subs' volume. In fact, I hardly have them outputting, as the Zu's have pretty good bass output by themselves.
B

regret and Ayre
...rare as really deep and flat bass in a 10’ x 10’ box

@almarg wrote:

... An even better approach, IMO, would be to solder the wire from the sub’s negative input terminal(s) to the ground shell of an RCA plug, leaving the center pin unconnected, and inserting that plug into an unused RCA connector on the amp. 

That would result in a direct (essentially zero ohm) connection between the circuit grounds of the sub and the amp.

Hi @almarg - I don't believe that would work because the RCA grounds are not referenced to circuit ground. In fact they are totally isolated from one another, I verified this with a multimeter. The Ayre circuit is fully differential from input to output so I believe they treat the RCA ground the same as an inverting signal, which I'm guessing is why Ayre gear doesn't sound great with single-ended sources (a phase splitter or transformer coupling would better drive the balanced circuit, though at the cost of another passive/active stage).

The impedance from chassis to circuit ground is negligible - there's a single star grounding point in the middle of the PCB where the ground plane is exposed to the screw, which goes through a brass standoff to the chassis. I also had this impedance concern but I measured and it was in the 10's of milliohms, and there is no hum whatsoever. However if I were taking my mod further, yes, I would put a ring connector under that grounding screw, making direct contact with the ground plane, and wire that to a proper connector for the subwoofer output. My more elaborate plan was to do this, add a muting relay and hooking it up to a 1/4" TRS connector with 10 ohm series resistors isolate/protect against shorts. I decided it wasn't worth the trouble though. :) 

@audioconnection wrote:

The reason I disagree a single sub by the time he hears the bass for what op has paid for can easily overload that room spoiling or confusing what he already has right in the midrange balance, to begin with, it's still a can of worms. 
2 properly adjusted small subs work better than one but forward firing drivers tend to draw more attention away chuffing and mooing with artifacts. 

Hi Johnny - definitely a potential issue, but I have not had any of these problems at all with the REL in a few different setups. They do work better with a speaker that has reasonable extension (say, -6dB @ 60Hz) and a smooth rolloff, so you can keep the crossover point low. I am not sure if the OP's Epos fits the bill. Also remember that REL does not have any sort of high-pass for the mains, so they are expected to run full range. It is truly "bass augmentation" only.