Tube Amp for Martin Logan Speakers

"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.

Well stated Mapman! :)

I think the EPDR link I provided supports Ralph's comments that low impedance speakers may have an even lower EPDR rating than what is nominally reported or measured. Even more important is that speakers presenting low EPDR factors significantly contract an amp's so-called SOA.

I am not saying that 4 ohm speakers are not good performers. Just that a nominal 4 ohm load in the bass region may really squeeze the amp and possibly result in a referral to the SPCA. At a minimum, a less than optimal amp/speaker match may, as the article explains, result in distortion. So, I happen to think Ralph's comments make sense for the reasons set forth in the attached article.

Bruce

P.S. The SPCA is the Society for the Prevention of Cruelty to Amplifers.

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.

Hi Unsound, yes it is, and one should consider that I do refrain from that sort of thing as much as I can. Recommending the ZERO is something I have done a lot, and not just for our amps and I will continue to do so despite George's remonstrations. This is for no other reason than the ZERO really does work.

One other thing- to clarify, I am not stating that 4 ohm speakers are bad speakers. I am stating that any amplifier driving them will sound harsher and less detailed as opposed to the same amplifier driving the same speaker that was 8 ohms, were all other matters to be equal. Put another way, a simple method of making a speaker seem more transparent and easier to listen to is to make it be higher impedance.

This might be all about how important it is to an audiophile to have increased transparency/detail, coupled with a smoother presentation. I like those things myself.

Something I have not mentioned yet is the effect of the speaker cable on 4 ohm speakers- but it should be obvious that they are far more critical for a 4 ohm speaker than the cables are for 8 or 16 ohms. Damping factor of amplifiers is normally stated with respect to an 8 ohm load; that number is initially cut in half with a 4 ohm speaker. Add to that the seemingly low DC resistance of the speaker cable and the damping factor of the amplifier is reduced considerably more that one might intuit. RCA published a nomograph that showed this relationship several decades ago.