MAC Autoformers?

@ramtubes 

Thanks Roger.  Thought you might find the attached websites of interest.  Atkinson tested an earlier version of my amp, the Ref 150.  The test results should be comparable to my amp, the Ref 150SE, because the Ref 150and Ref 150SE share almost identical electrical characteristics.  

Atkinson uses a simulated speaker load, which is actually kind.  See below.  Impedance doesn't drop much below 8 ohms.  Atkinson measured Ref 150 tap impedance as follows:  "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."  Running the Ref 150 into the simulated speaker load, he measured FR as follows:  "From the 8 ohm tap (fig.1, gray trace), it was ±0.8dB; the 4 ohm tap offered ±0.4dB, the 16 ohm tap ±1dB." 

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

https://www.stereophile.com/reference/60/index.html

So in summary, I gather that even if speaker impedance swings between 3.9 and 20++ ohms at its peak, it is still better to use the 4 ohm taps because it will draw less current off the output tubes and thereby cause less tube wear and less distortion.  Also, the 4 ohm tap is close tothe bass dip impedance, which is good.  Max power may be compromised at higher impedance, but if I am not pushing the amp, that shouldn't be a problem.

Btw, based on Atkinson's measurements, I gather that if my speakers do dip to approximately 4 ohms in the bass, the damping factor should be roughly 7.27 (4/.55), which ain't too bad.

Am I getting it now??

Btw, ARC uses 14db of negative feedback in ultralinear mode to achieve these electrical characteristics.  The amp is also runs in modified AB mode.

Thanks again,

BIF 

BIF,

Thanks for the info. Let me clairfy this part which many people make the same mistake. 

Max power may be compromised at higher impedance, but if I am not pushing the amp, that shouldn't be a problem.

I've heard this from ESL owners where the impedance rises in the bass to very high values. They believe that OTL amplifiers are better for their speakers because they can supply the extra voltage to drive the high impedance in the bass. They indeed get more bass if the damping factor is low, but not the bass the speaker was designed to give. They get more and perhaps like more but it is one note bass.

Speaker makers are doing a better and better job making speakers have flat response, which is a good thing. To to this they generally let the impedance do what it will. They could add a lot of parts to the crossover to try to flatten the impedance curve but they assume you are using an amplifier with reasonable damping of 10 and above. Ten is not a hard limit but 5 is better than 1.

This is where we get in trouble with the power paradigm which had led people to believe that the speaker wants constant power. It does not. I cannot think of or find a modern speaker that wants constant power, the varying impedance and flat response insure that the designer uses a constant voltage amplifier, ie one with high damping. 

The reason that John Atkinson puts the response with the simulated speaker first is that he and I feel it is the first thing you are going to hear when you audition that amplifier. 

What the power paradigm is saying is that flat response is not as important as ills of feedback. Sorry, but I am not in that camp.  Proper feedback has no ills. The Futterman amplifiers have over 60 dB of feedback and a good reputation. Thats a lot of feedback, more than most transistor amplifiers. I plan to start a thread on the Futterman design when I get done with this topic.

Since everyone hears what they hear and like what they like I would rather not promote one paradigm over the other, I just want to point out what is going on based on my experience and my rules of amplifier and speaker design.

I would rather see all speakers have flat impedance curves but this is not how the world is currently set up. Even the DeVore (which has been recommended) has wide impedance variations.

While I agree with JA and Ralph that it is easy to drive (stays above 8 ohms) its peaks go off the chart above 20 ohms. A damping factor of 1 or 2 or 3 will modify the nicely flat frequency response several dB.

http://https//www.stereophile.com/content/devore-fidelity-orangutan-o96-loudspeaker-measurements

Thanks for your question, I hope this clears it up for you and others. Feel free to ask for any further clairication.

C1Ferrari 9-6-2018
Very cool thread ;-)

 +1.  Great thread.  And thanks to Ralph and Roger for providing their informative if somewhat different perspectives.

Roger, regarding...

The Futterman amplifiers have over 60 dB of feedback and a good reputation.

I've seen that stated before, but I'm wondering how an amp can be designed with 60 db of feedback.  Wouldn't its open loop gain have to be enormous to support a closed loop gain that is reasonable?

Best regards,
-- Al

P.S:  Bruce (Bifwynne), great to see you posting again recently. 

@ramtubes 

So Roger, …. bottom line, do you concur that using the 4 ohms taps on my Ref 150SE should yield a flatter FR, less distortion and longer tube life?? I gather you are saying yes. 

P.S. the ARC engineers obviously were trying to balance a lot of design ideas and techniques when putting the Ref 150SE together.  Just enough negative feedback to get the output impedance low enough to handle a host of speakers designed to be driven my low impedance SS amps and a robust power supply (1040 joules) to handle the tough spots that require a lot of current (i.e., low impedance and highly reactive phase angles in the bass region).

P.S.S. -  I have about 1800 hours on my KT-150s.  Can you help me source some matched tubes that will bias well my amp?  I have 8 fresh KT-150s sourced from another vendor sitting in a box.  They don't bias well.  Could you match them for me?  

@almarg - been distracted.  Glad to be back.  Al, if you get a chance, check out Roger's amp design presentation here:

https://www.youtube.com/watch?v=aO1Q281FQxo

The guy knows his stuff.

BIF