Does anyone care to ask an amplifier designer a technical question? My door is open.

Roger,

You posted the following

"As I tell everyone. Light loading can be done on any tube amplifier. If you dont play above 80 dB go to the lowest tap. If you play 90 go one higher, if you play more than 90 you may have to use the high tap to get the volume you desire. This change should be far more apparent than a cable change. 7 ohms on the 8 ohm tap is no problem for the amp but lower taps always perform better technically though how it sounds to you is more important.

"4 ohms on the 4 ohm tap produces 100 watts. 8 ohms on the 4 ohm tap around 60 watts. As you go down in taps you spend more time in the class A region. The distortion and damping are much improved.

"Would be nice if you had a way to measure the peak voltage you require, then much better advice can be given. You can also get at that with a SPL meter at one meter at listening level. Then with speaker sensitivity we can compute it

"Everyone needs an SPL meter. For $50 its an execellent investment. People spend more than that on a poorly designed high end fuse. Please do not buy premium fuses, TuningFuses are the worst and the others I have not dissected but the people who sell them should be dissected."

*******

Just an anecdotal experience.  I own an ARC Ref 150SE.  It has 3 output taps: 4, 8 and 16.  I settled on using the 4 ohm tap for many of the reasons you mentioned above.  John Atkinson reviewed an earlier version of the a few years ago and made the following comments:

"As expected, the Ref150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm. All three taps offer quite a low source impedance for a transformer-coupled design; as a result, the modulation of the amplifier's frequency response, due to the Ohm's Law action between that impedance and that of our standard simulated loudspeaker, was relatively mild." See Atkinson report here:

 https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements

So, in my experience, the Ref 150 SE, on balance, sounds best when light loaded off the 4 ohm tap. At times, I thought the 8 ohm taps sounded better, but after a while, I found the sound bright, probably because there is a 20++ ohm peak at the midrange cross over point.  And as you predicted, bass is tighter when played off the 4 ohm taps, although I think speaker impedance in the bass region bounces around quite a bit but has a 3.8 ohm saddle in the 50 to 200 Hz range.

As far as loudness is concerned, my speakers are rated at a 92db sensitivity and the amp has plenty of power to drive the music to uncomfortable loudness levels, albeit with little perceive distortion or signal scrambling.

So the bottom line is that there seems to be something to be said for light loading.

Bruce

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@ramtubes

You wrote the following regarding ARC:

Im sorry, but you asked. I'm sorry to be so negative about ARC, but I was a dealer for them from 1975 to 1980. If you have not heard about the "pink papers" thats a story in itself. ARC treats dealers with a heavy hammer, large opening order and threats if you do not follow the party line. Bill called us up one day because he heard we were doing A/B comparisons for our customers. He said "stop immediately or I will pull the line"

Of course I respect your technical experience and ARC anecdotes, but let me just weigh in on ARC.  Perhaps I have been crazy lucky, but I have owned ARC gear for quite awhile and my anecdotal experiences are quite different than yours.  

In particular, as far as linestages go, I've owned the Ref 3, Ref 5, Ref 5SE and now currently, the Ref 6.  As far as amps go, I've  owned the VS110, the VS 115, the Ref 150 and now the Ref 150SE.  My CDP is the Ref CD-8 and my phone pre is the PH-8.    

To date, I have had only one problem; namely:  blown bias resisters in my older amps.  Other than that, no other problems whatsoever.  In the case of my Ref 150SE, I am running KT-150s. So far, … fingers crossed, ... no problems with arc'ing or blown bias resisters.

As far as bench tests are concerned JA tested the Ref 150 amp, an earlier version of the Ref 150SE.  Here is his report:    https://www.stereophile.com/content/audio-research-reference-150-power-amplifier

Notably, JA reported that "Fig.1 indicates that the Ref150 has a wide bandwidth, particularly into loads higher than the nominal tap value, which correlates with a well-defined 10kHz squarewave (fig.2). While this graph reveals a small but critically damped overshoot on the leading edges of the waveform, presumably due to an ultrasonic transformer resonance, no ringing is visible."  By comparison, the Ref 75 did not test as well.  And you noted the same in your prior post.  

I can't speak to whether the circuits in my ARC gear are elegantly simplistic or labyrinthically complex .  I am not a EE.  But so far, my gear is holding up pretty well.

As far as SQ is concerned, … no comment.  I have not had an opportunity to A/B ARC gear against any other brands.  But to my untrained ears, my gear seems to sound OK.

Perhaps your views about ARC might change if you road tested some of their current models.  I understand that ARC opted to simplify the circuits in its latest offering, the Ref 160 mono blocks.  Haven't heard them, but it doesn't matter.  I am not going to drop $30K on mono blocks.

Btw, I owe you two quads of KT-150s for matching.  The 3rd party vendor from whom I purchase the tubes didn't match the tubes to ARC spec.

Regards,

Bruce

@ramtubes

Roger, you wrote:

"The difference between Bill Johnson amps and the current products is night and day. I appreciate the simplicity but not so much the performance specs. However specs arent everything. We are finding out some very interesting things on some listening tests. Very hard to get a really good handle on an amp without an A/B and a reference."

Not sure what you feel is deficient with the ARC Ref 150SE specs.  If you care to elaborate, please do.

Also, have you ever bench tested the Ref 150 SE or Ref 6 linestage?  Further, have you ever A/B'd any current ARC models against other brands that you think highly about?

My speakers are pretty sensitive.  Rated at 92db.  I can play music (rock or classical) at ear-splitting levels without any apparent sonic congestion. Of course, I can't speak to distortion because I don't have the means or ability to measure those specs.

That said, maybe it really does not matter what I think.  There are a million threads where folks tout the attributes of their gear and so I guess everyone has their own favorites.  

In any case, I would like to know your reactions if you bench tested or critically listened to any *current" ARC gear.

Keep an eye peeled for my KT-150s.  I am also sending some old ARC winged C 6550s that I used on some older ARC amps (roughly 2000 hours on the tubes).  I would like to know if they have juice left in them.  Also, the 6550s might give you a clue about the tubes specs ARC used to match their tubes for purposes of matching my KT-150s.  The KT-150s are da*n expensive tubes.

Regards

BIF