NAD M23 and a Stereo Pair of REL Subs

Hello folks,

I recently purchased a Class D NAD M23 amp to replace an older Class AB NAD C272. Ive got it hooked up to a pair of bookshelf speakers and everything appears to be working just fine. The trouble happens when I try to integrate a stereo pair of REL subs (via high level connection). There are instructions from REL on the peculiarities of connecting in high level to Class D amps, and the proper way to connect a stereo pair, there was also this recent post from another member:

Well, despite it all I can only seem to get the left channel speaker outs to activate either sub. Try as I might I cannot get the right channel speaker out to do the same. Clearly both speaker outs work since the bookshelf speakers are producing sound just fine. And clearly both subs are working since I can get either to work with the left speaker out. In case you're wondering I've also eliminated the cables to the subs as a factor.

I've contacted both REL and NAD but I probably won't hear from either for a few days. Just curious if anyone else has run into a similar issue? I'm hoping this isn't a curious artifact of this particular amplifier design.



It turns out this is a result of NAD's design (for the M-series of eigentakt amps at least). Someone with an M33 integrated ran into the same issue. I'm posting the response they recieved from REL below in case this helps anyone else:

" is not only a Class D amplifier but it also inverts the polarity for one output channel in order to maximize the efficiency of it's power transformer. As a result, the following instructions should be used when connecting a stereo pair of subwoofers to this amplifier using the High Level input:

Combine the red and yellow wires of the right channel subwoofer's High Level cable, and connect to the right channel negative output terminal on the amplifier.

Combine the red and yellow wires of the left channel subwoofer's High Level cable, and connect to the left channel positive output terminal.

Connect the black High Level wires for both subwoofers to the ground thumb screw, located on the amplifier's back panel near the power cord inlet."

Since one channel output inverts phase, doesn't this mean that with "normal" connections that the main speakers would be out of phase resulting in the main speaker's woofers canceling each other out?