This speaker will sing with high powered amps, like the McIntosh amps, MC402. The myth is these speakers can work fine with low power amps, even with my subs I use the MC402.
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As has been said, a low-power amp is not the best mate for the 63. If you want an Atma-Sphere amp, you'll need at least a pair of M60's. Also look into the Music Reference RM-200 tube amp (Michael Fremer "Class A" listing in Stereophile, if you care). Unlike other tube amps, it's power increases into lower impedance loads like the 63.
I have heard that the designers of this speaker did not assume lots of watts and amperage abundance. I think you may have to find specialty forums for the info you want or even a brief conversation with Quad themselves.
I am sure the McIntosh amps do a good job and the Quicksilver as well but there may well be other alternatives.
I use the 63s in my second system and Atmaspere is absolutely right to my ears. Below 60 watts you won't hear what they can do. I use Gradient subs with them together with their dedicated x-over driven by a pair of old Manley 250, bought for very little money. The Quads are driven be the Atmaspere 60 watters. There is a pair here on A. for sale at a fair price.
This system ( front end is also Atmaspere ) is really heaven to my ears.
We recently sold a customer who ownes these speakers a pr of Atma M60's then a few months later he upgraded to the new Atma Novacrons. When we went to set-up the Novacrons he still didn't have enough vol to get the speakers to sing. It wasn't the amps it was he was using a passive pre. Once we set him up with an Atma pre (about 12-14 dB of gain) everything came together so be careful of the gain of the pre is the lesson learned here.
Thanks to everyone for the responses. Sorry for my delay. It sounds unanimous about low powered amps even without any load below 50hz. My CJ MV-60 sounds pretty amazing so unless I can taste the magic of another circuit like single ended, or maybe a great tube like the 300b, maybe I should stick with the CJ. I.e. would an RM 200 really sound much better?
Also, as I look at amps, how can I tell from the circuit design, parts or measurements whether a particular amp can handle the impedance changes in the esl-63?
Also, I didn't want to lead with this, but my sonic priorities lean more toward midrange magic and musicality. I am not one who seeks ultimate detail, "neutrality" in the audiophile sense, etc. I play at pretty moderate volumes, and never turn it up to extreme volume. So I am very curious about 300b and/or SET sound. But I am keeping my quads, so does this leave out SET entirely?
Another possible amp is an assemblage 300b push pull that might be fun to try. At 18 watts (I think) it should have enough oomph since it doesn't need to do much below 50hz?
Montaldo, you might want to look at this post:
and this link:
Unless you have a very small room, you will not have enough power with a 300b-based SET. They only have 7 watts. You need more like 50-60 as a minimum to really make the speakers sing in the average sized American room (17' by 20' or so).
Quads go lower than 50Hz, unless you are cutting them off with an electronic crossover. Actually they can make quite convincing bass!
I don't think you can go wrong with CJ and Quads. It's more a question of whether you like the older CJ sound that you're getting with the MV-60 or the newer, more neutral and detailed CJ sound. I ran my 63's with several different amps including Music Reference RM-10's both in stereo and then mono blocks, and CJ Premier 12's. CJ combined with the Quads was always a step above everything else. Also, don't necessarily agree that you need at least 50-60 watts with the 63's. My Rm-10's in stereo at about 25 watts did just fine in a 16'x 24' room at all but very loud volumes with all types of music but some Classical.