Subwoofer for Ayre AX-7e?

@taww, thanks for the good info. Between the commonality of circuit ground and chassis ground and your finding that the RCA inputs are apparently treated as if they were balanced the design is certainly different than what I was envisioning. So, yes, my suggestion of connecting a sub’s negative speaker-level input terminal to the ground shell of an RCA input connector would not be suitable, for either you or the OP.

Just as a matter of curiosity, though, I wonder if the tape out RCA connectors are being provided with a balanced pair of signals, or if (as I would expect to be more likely) the ground shells of those particular connectors are connected to circuit ground. Although even if those ground shells are connected to circuit ground it would seem unlikely that connecting the negative sub wire to one of them would provide any benefit compared to a chassis connection, given that you’ve found chassis and circuit ground to be common.

Best regards,
-- Al

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

I also am using the AX-7e in my office with a pair of Zu Omen Bookshelf speakers and HSU VTF-1 subs.
The Ayre handles them at line level with no problem.

Hi Bob,

Doesn’t that mean you have to adjust the volume control on the sub whenever you change the volume of the signals provided to the main speakers, to keep them at the proper level relative to each other?

Best regards,
-- Al

In addition to the fact that the AX-7e doesn’t provide volume-controlled outputs from its preamp section, there are significant potential issues if its speaker-level outputs are connected to a sub. Note the following statement in its manual:

The Ayre AX-7e drives the loudspeakers with balanced outputs. Since none of the output terminals are grounded, connecting any of them to ground may result in damage to the amplifier. Do not connect the loudspeaker outputs to any speaker switch-box, accessory, or test equipment that has a common ground connection.

The negative speaker-level input terminal of many and probably most powered subs which provide speaker-level inputs is connected to AC safety ground through a low impedance, or in some cases perhaps even directly. In those cases connecting the negative speaker-level input terminal of the sub to the negative output terminal of the amp risks damage to both the amp and the sub, depending on how circuit ground and AC safety ground are interconnected within both the sub and the amp.

And what would probably be a much worse possibility is that if you were to use just one sub, and if that sub provides inputs for two channels, if you were to connect the sub’s two negative input terminals to the two negative output terminals of the amp the result would most likely be a direct short between the amp’s two negative output terminals.

An approach that would probably work ok, which REL recommends for use in such cases, would be to connect the sub’s negative input terminal(s) to a chassis screw on the amp. Although it is possible that hum could result with that approach, depending on the specific designs that are involved. 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.

Regards,
-- Al