That's a good question, and I'm hoping some of the amp manufacturers that lurk hereabouts will post a response.
Many SET amp manufacturers and at least one OTL amp manufacturer combine zero feedback and Class A operation in the same package (three of the six amplifier lines I carry have both characteristics, so obviously I voted in favor with my wallet). I think that Nelson Pass's "First Watt" solid state amps do as well.
I'm under the impression that the Aerius has a fairly nasty impedance curve, which can work against such amps. Can you tell us anything about the impedance?
You need some power with those speakers and VAC made some pretty high powered class A 300B push-pull amps that you can run with no feedback. There are probably others.
Ayre claims their SS amps are zero feedback designs but they mean no global feedback. Any SS amp has to use at least local feedback.
First Watt amps are current sourse amps and only work with high efficiency single driver speakers.
My Aerius speakers, are 6 ohms' but can dip down to 1.6 ohms.
My Aerius speakers, (...) can dip down to 1.6 ohms
Given that, you need an extremely well stabilised amplifier circuit to drive those speakers. Some map manufacturers do spec how difficult a load their product is designed to take -- so go with that.
"Class A" simply denotes the circuit biasing. If you like the sound (I for one, do like it) then go for ultra "stabilised" class A amp.
Why don't you ask Ralph from Atma (lurks around here s/times). He should know a thing or two -- he designs tube-based amps.
'Lurk' is hardly the term!
For a speaker like this, a zero-feedback amplifier will need a little help. Fortunately there are two things going for you- the first is that the speaker is an ESL and the second is that there is a way to deal with the impedance.
The ZERO is the way to deal with the impedance and will allow any zero-feedback amplifier to operate the speaker so long as the amp is able to make enough power into a benign load.
The fact that the speaker is an ESL means that the speaker is expecting the amp to make constant power regardless of the load impedance of the speaker (which is why transistors for the most part tend to sound bright on ESLs). The ZERO will present the amplifier with the benign load that we are looking for, and a zero-feedback amplifier will have the proper 'constant power' characteristic.
Class A is important here as a zero-feedback amplifier does not have feedback to reduce distortion. Distortion is reduced by other means- class A is certainly one of those ways as it is the lowest distortion class of operation known to man. Feedback oddly enough causes some types of audibly undesirable distortions and so you are seeing a good number of manufacturers, tube and solid state, that eschew the technique. To do so means that component quality and topology suddenly got a lot more important to the overall success of the amplifier!
So the answer to the thread is: both are almost equally important.
I expect digging around the archives could lead you to a respectable body of opinion, especially if you widen your search to include other ML models which have similar minimum impedance levels.
The argument of zero negative feedback and Class A are also in the archives (with lots of contribution from the aforementioned Ralph, a respected designer who also makes for an interesting read when he gets involved in a thread). Long story short, I expect a great deal of the result of using one, both, or neither attribute successfully is about implementation, not about inherent characteristics.
And with regard to the 5 watts, unless you listen to thrash metal or bass drums constantly, the vast majority (in terms of minutes listened) of what you listen to will probably not exceed a 5 watt pull, it's just that some of it will, and when it does, it will exceed that by a long way, so yes, you need more wattage than that.
Great responce,to the tread,can we talk about specific amps! What about Manley's adjustable feedback? I think it's on the mahi mahi,or any feelings on simaudio's I-5? Then again how important is tube biasing? Some amps bias the tubes as a group others individually,and primluna has a patent on there auto bias that I think Biases constantly and to a much more refined degree hence there great sound,all this said I haven't heard any of these amps yet,I would like some input about them and how they my suit my martin Logan's Aerius speakers,(I need to keep price levels at a maxium of $3000,so some amps like Pathos that has both zero feed back and is class A (Pathos TT) is not in the running for me because of price,thanks Nick
Happynick, Ralph speaks the truth here. I own Logan ascent i speakers and have used more than a few amps with these speakers both solid state and tube. I recently purchased a pair of the original atma-sphere MA-1 mono blocks used. I use them with the Speltz zero autoformers. The ascent i drops to 1.2ohms at 20kHz. These amps sound fantastic with these speakers, the only thing I could see myself changing these amps out for would be newer atma-spheres. You may want to look at the 60 series atma-sphere amps also. Hope that helps.
I hadn't thought about matching OTLs with auto-formers as I would have thought the autoformers negate the OTL-ness but the answer is probably that I was not thinking creatively enough. Sounds like a great idea.
Is the reason why SS amps sound "bright" on ESLs (because of the constant power demand) because ESLs see low impedance at high frequencies, and the SS amps double down in power at the low impedances, making SS amps, on an ESL-specific frequency response curve, relatively "bass-shy"?
T_bone, you got it exactly right. Most SS amps make constant voltage with respect to load; on ESLs that is not the right response.
With respect to the ZERO/OTL thing, we made a device similar to the ZERO about 10-15 years ago. It got to be a bit of a chore explaining why it did not negate the 'OTL character', so eventually we stopped. Paul Speltz picked up the slack.
Here is the explanation: OTLs already have low output impedance and do not make DC, so some of the bigger requirements transformers face have been reduced. To start with, the device does not have to block DC, so its an autoformer with no secondary. This is a lot simpler. 2nd, we're only going from 16 ohms down to 2,3 or 4 ohms, so the turns ratio and distributed capacitance is such that the device has bandwidth out to 1MHz, something that is impossible for a regular transformer. In fact the technology only works if you have a low output impedance and no DC- so its fine for an OTL- particularly in the light of a speaker load that the amp would not work with otherwise! Finally, in the case of our amps, you do not negate the triode-class-A-single-stage-of-gain-fully-differential-zero-feedback aspects. So it is a nice solution. Paul sells a lot to the single-ended crowd as well.
Which is better?
Neither. Comparing apples and oranges. Some may go as far to say that a zero-feedback Class A would be the ideal amp.
(Actually, the further a "zero-feedback" amp is biased into Class A, the better it will sound. To work well in a wide range of applications, amps of this nature are best when they have lots of output devices, drawing a ton of current. I live in Texas, so we changed over to making switching amps. Got tired of turning off the system 4 months of the year.)
I have to chime in here having listened to MA-1 MK II.3 through ML Summits and the ZEROs would be required. The MLs sucked the life out of an otherwise very fine Amp....Steve
Hmmmm, im running the Martin Logan Aerius I's with bi-wire with the SimAudio Moon P-3 preamp,the Moon W-5 Amp, and the Moon Equinox SE CD Player, and the Martins just love this setup, more so then my $10,000 Theils....Go to the moon man. u wont regret it with the Martins....Rick