What s the best Class A integrated tube amp over 80W

@chngjh,

I would never contradict Ralph.  However, I think you may have somewhat reversed what Ralph may have said about amps that operate under the so-called voltage paradigm (i.e., constant voltage at varying speaker impedance levels -- most SS amps which have near zero output impedances) versus amps that operate under the so-called power paradigm (i.e., constant power at varying speaker impedance levels - generally, tubes amps that have high'ish output impedances).  Ralph wrote a white paper on this topic.

Just an fyi, the reason the ARC Ref 150 SE operates as though it has "low'ish" (but not zero) output impedance off the 4 ohm taps (.55 ohms in the lower frequency region) is because the amp uses 14 db of negative feedback (NF).  Some folks and manufacturers prefer amps that use zero NF.

In the case of a SS amp, voltage generally remains constant at all frequency levels, but power output (wattage) will increase as speaker impedance decreases and amp power will decrease as speaker impedance increases.  So, for dynamic speakers that present low impedance levels in the bass region, a SS amp will produce more power than in the frequency regions where the speaker impedance is higher (often at driver crossover points). 

The reason the SPL of a good speaker remains relatively flat at most frequency levels if driven by a SS amp, despite the varying speaker impedance levels, is because the speaker was designed and voiced to be driven by a SS amp. 

You posted that: 

SS output doubles with every doubling of impedance. Therefore my Pass Labs X250.8 SS amp produced about 60W at 30HZ with 30 ohm impedance. On top of that bass frequency takes more power to make the sound than treble.

That seems somewhat odd and contradictory if you were using a SS amp with near zero output impedance.  This is because your ESLs present 30 ohms of impedance in the low frequency region and just 3 ohms of impedance in the high frequency region.  If you used a SS amp, I surmise that bass would be attenuated and treble bright, ... which you kinda indicate in the quoted text below.   

That said, you also posted that:

Soundlabs sounds too bright at treble with most SS and sounds better with tubes which don't double output with halving of impedance.

On the other hand, it may be that your ESLs were voiced to be driven by a tube amp that has a "high'ish" output impedance.  For example, I think many Atmasphere amps have an output impedances in the 4 ohm space. 

Rather than me being disagreeable for the sake of being disagreeable, I think what is important in the end is whether a particular tube amp will mate well with your ESLs.  Aside from electronic stability issues of mating a particular tube amp (or any amp) with your ESLs, I would expect, if it was my setup, that the amp/ESL combo would produce a relatively flat SPL over the audible sound spectrum.  

So returning to your original question, your OP asked for Class A tube amp suggestions to mate with your ESLs.  I think there have been several suggestions already offered.  I do not think ARC amps are Class A, but some type of hybrid Class A/Class A-B variant.  Further, if your ESLs mate better with high'ish output impedance amps, ARC amps may not be a good fit because many of the amps in the ARC line have low'ish output impedances.

In any case, I'll add to the list Atmasphere tube amps, especially if Ralph thinks they would mate well with your ESLs.  I would also have a chat with Sound Labs.  The manufacturer may have some good suggestions too.

Good luck with your amp search and happy listening.
         

@chungjh ... do a search in the Forum for old posts about the ARC VT 100.  I recall that some years back, a couple of techie members explained why biasing the VT100 is so hard, ... and a tad dangerous for non-experienced, non-techies. 

I don't recall if you mentioned your budget, but someone mentioned the ARC Ref 150SE as a possibility.  I own that amp and it is a stunner.

I have question for you that should be answered by a techie.  If your ESLs have an impedance of 30 ohms in the low end of the spectrum and 3 ohms in the treble, is a tube amp a good choice for you?  Your ESLs operate almost like a giant capacitor.

I ask because many tube amps have high'ish output impedances.  Even the Ref 150SE has a "low'ish" (for a tube amp) .55 output impedance off the 4 ohm taps in the bass region; the amp's output impedance creeps up with frequency. 

I mention this because a tube amp's output impedance may color the sound based on Ohm's Law principles.  See, e.g., John Atkinson's bench test report on Ref 150 at: 
Audio Research Reference 150 power amplifier Measurements | Stereophile.com   

Based on the impedance figures you mentioned, I calculate that if you used the REF 150SE as your amp, the treble could be shelved down by approximately 2 db as compared to the bass.  Check with a techie.  You could even call ARC customer service. 

Or, if Ralph Karsten (Atmasphere) catches this thread, perhaps he could weigh in.  Ralph manufactures high quality OTL tube amps.  I recall that Ralph posted a while back that he uses his amp to drive ESLs.  Maybe Ralph could explain the whole impedance matching business. 

Good luck with your search.

Bruce
       

@chungjh, ... if that Q is directed to me, the only brand I know about is Atmasphere OTLs.  I suggest that you make a call to Sound Labs.  They know what electronics are the best fit for their products.

Ironically, most dynamic speakers on the market have fluctuating impedance and phase angle curves, ... but are voiced to be driven by SS amps having near zero output impedances.  Fortunately, most ARC amps have "low'ish" (but not zero) output impedances and mate, for the most part, pretty well with most speakers that have moderate to high sensitivity levels. 

The impedance curve of my speakers fluctuates between a low of 4 ohms in the bass region and have an impedance peak in the high-teens/low 20s in the 2200 kHz mid/tweeter cross-over point.   Because I use my amp's 4 ohm taps, I calculate (as echoed by John Atkinson in his review of the Ref 150) that the SPL delta between the high and low impedance points of my speaker is roughly between .8 to .9 db.  Not too bad for a tube amp.

I would call Sound Labs. 

@chungjh,

I am surprised by the anecdotal advice Sound Labs gave you.  The part about lots of power is ok, but the part about Atmasphere versus JC-1 amps is curious.

As I explained above, Atmasphere amps have "high-ish" output impedances.  See for example  Atma-Sphere M-60 (atma-sphere.com).  Per the Atmasphere website, the M-60 has an output impedance of 4.1 ohms.  By contrast, most SS amps have near ZERO output impedance.  

I surmise that of you drove your Sound Labs ESLs with the M-60, the amp would attenuate the high end of the ESLs.  By contrast, I would expect that the JC-1 would augment the high-end harshness. 

In the end, YOU have to be happy with the match.  In an optimal world, you should plus each amp into your rig, ... in your house,... and take a serious listen.  

Given the advice given by Sound Labs,  maybe you should have another chat with Ralph. He will steer you in the right direction.

Good luck.

Bruce