Tube Amp for Martin Logan Speakers

03-26-14: Atmasphere
This can be so significant that the use of an autoformer to raise the load impedance to an otherwise very capable solid state amplifier can result in improved sound. Were this not the case, the insertion of the autoformer would have adverse effects.

OMG, it does, if the listener had ears. I've tried them on a very capable ss amp, and the sound took a bad turn for the worse.

Quotes: from the Dick Olsher review that was convienently left out of the excerpts of the reveiw of the link you provided on the Zero website, read between the lines, we all know that reviewers don't like to rock the boat too much.

"It is not a panacea, and as you can see from my experience, it does not always pan out. However, if you're the proud owner of an OTL or a low-power tube amplifier and presently driving a 4-ohm loudspeaker load, you owe it to yourself to give the ZERO a try."

Cheers George

Atmasphere, on the surface, I don't think anyone would argue that reducing distortion would be a good thing. But we have seen when distortion reduction specifically and unto itself with disregard for the whole can cause more harm than good.
From the above; it seems that Steve McCormack's thinks that the autoformers made his amp "happier". Well, I might believe that, but that unto itself doesn't mean a whole lot when considering the whole lot.
There is apparently more to low impedance than just "sound pressure" (I'm not putting words in your mouth, am I?). For many years in the past, and very much in the present, and I'll hazard a guess, that for many years into the foreseeable future there will be speakers with low impedances with high end aspirations. I would caution those considering acquiring loudspeakers not to dismiss loudspeakers that have low impedances. There are many, many superior loudspeakers that might be missed out on. There are many amplifiers capable of driving them just as they come from the factory.
With all due respect, we've going back and forth on this subject for years now. Unless there is a drastic change in the audio landscape; as long as you keep making the assertions:
"Now its a simple fact, that there is no good argument for lower impedance (4 ohms or less) loudspeakers in high end audio; that is to say if sound **quality** is your goal."
I will probably object. In an effort to save time and bandwidth, perhaps we can come to some sort of gentleman's agreement on how to deal with this in the future?
BTW, as I read it; George didn't call you stupid, he called one of your suggestions stupid, and that was after you called him a "troll", which was after he posted a link where he recommend your product. This line of posting is somewhat out of character for you. You're better than that. But, hey, we're all human.
Best Regards,
Unsound.

"You can see in the specs of any amplifier (solid
state, class D or tube) that the distortion is indeed higher
driving a lower impedance load. "

That is true, but still all relative, and 4 ohms does not
mean many amps stop performing well.

Needless to say, the amp has to be up to the task. Most
good quality SS amps these days are quite capable of
performing well into 4 ohms these days it seems. Lower
than that becomes more of an issue perhaps, but better to
listen and determine for ones self the whole story rather
than focus on any one technical scenario. 4 ohm loads are
quite common these days and for good design reasons, not by
error or ommission. There are many valid ways to achieve
excellent results.

Tube amps are a different beast that work best in different
scenarios and subject to their own unique distortion
characteristics, so comparing SS and tube amp distortions is
not apples/apples and of limite.d value IMHO

Folks, some may find the Stereophile article about "EPDR" quite interesting at this link:

http://www.stereophile.com/reference/707heavy/index.html

I think one important take-a-way is that speaker impedance ratings, by themselves, do not take into account the full picture of speaker's load difficulty. Other factors include a speaker's phase angle characteristics. Take a look at the Final 600i ESL. Talk about an amp ball-breaker.

But perhaps even more relevant to this thread and Ralph's comments is that most amps have a real world "SOA" re power dissipation that is a lot less than one might expect. As defined in the article, the acronym SOA stands for Safe Operating Area. Please note that an amp's SOA power dissipation contracts considerably as impedance decreases and negative phase angles increase in magnitude.

I'll let our techy members comment further, but suffice to say that low impedance loads can cause an amp to operate at or outside its SOA, thereby causing distortion. I surmise that if a particular brand amp's SOA is small when driving a low impedance load, the answer is to buy a more powerful amp that has a less restrictive SOA or try ZEROs.

Final two points. First, the article speaks about SS amps. Nonetheless, I surmise that similar principles apply to tube amps. I understand the SS amps hard clip while tube amps generally soft clip. But I think common to both types of amp is that distortion dramtically increases once the amp exceeds its SOA. The second point is that when Ralph uses the term "happy amp" he no doubt is referring to an amp that is operating within its SOA.

My heads hurts because I have no idea what I just wrote.

Goodnight.

Clipping is always the devil to avoid when chosing an amp for speakers. Effects of clipping start as subtle and increase to major. Whatever the speakers, be sure to chose an amp that is well beyond being up to the task. Most speaker vendors quote minimum or recommended amp power rating that are suitable to get decent results for most, but not suited to get the best results possible, which is what audiophiles seek. Result is many underpowered systems out there over the years and that is valid fuel for the argument towards speakers that present an easy load.

High efficiency/easy load speakers are a viable solution to the problem, but no the only one, especially these days. Amp technology has progressed and offers major improvement in efficiency these days as well. Monster heavy and big power amps ala traditional Krell, etc. are no longer the only robust option. Class D amps are lightweight, small, efficient and offer more power for the $ than ever.

SO do not disregard modern innovations in amplifier technology when assessing options. THere are more good and affordable ones out there than ever, especially when TCO is considered.

Also, use of powered subs to offload the heavy work in the bass is another very practical tool to choose. WHen powered subs are used (most use Class D amps to very good effect here) a lot of the issues with matching speakers to amps that exist otherwise tend to go away in that the main amp is asked to do much less to drive the speakers optimally than otherwise.

Also when playing vinyl, always make sure rumble and subsonic noise in herent in many phono setups is managed properly and under control. Otherwise, most of the amps headroom will be used to produce noise. High efficiency/easy load speakers might be a band aid in this case, but not the solution.