Boulder amp from hell...

[fas42]

There is no need to start an arms race with Halcro 'cos Halcro no longer exists

Whoa, what happened??!

Frank

[DonH50]

Just had a quick look at Duncan's book, High Performance Audio Power Amplifiers:

"Class A efficiency is highest at an ideal 50% when the output stage is push-pull.
This could be one of several of the topologies shown earlier in Figures 4.9, 4.27 and 4.28 for example. It is second best at 25% if the output is single ended with an active collector load."

Now any seriously powerful amp is always push pull, otherwise the number of output devices gets totally out of control ...

Frank

OK, that makes sense. The number I recall is 27% max, single-ended, whether common-emitter (source) or common-collector (drain). I should also note the vast majority of my experience is designing GHz ICs, not power amps of any sort (not since my early audiophile and Ham days).

I would not say "any seriously powerful amp is always push-pull" -- high-speed complementary power devices are hard to come by. Of course, I try to avoid speaking in absolutes, as there is always an exception. For that matter, transformer coupling is often used to create a balanced/push-pull design.

[fas42]

I would not say "any seriously powerful amp is always push-pull" -- high-speed complementary power devices are hard to come by. Of course, I try to avoid speaking in absolutes, as there is always an exception. For that matter, transformer coupling is often used to create a balanced/push-pull design.

And of course there are techniques like the Sziklai configuration, so that high power devices of a single polarity can be used. A lot of the earlier high powered audio gear used this "trick", brought in another set of problems of course, but enabled the early "monster" amps to come into being ...

Frank

[Orb]

Hmm how does one truly define efficiency for a Class A amp when the draw is always the maximum the amp can deliver?
As an example into 8ohms; the Musical Fidelity 100watt Class A amp consumes 1000W (230v mains), while the Robert Koda 45watt amp consumes 800W (230v mains), however the current drawn is always a constant so efficiency comes down to how much power is used to drive the speakers.
Pretty inefficient way if just using 5-10watts, and both an example of really inefficient Class A.
I could give the Pass Labs, which is more efficient but the figure is difficult due to it still able to slide into AB even with the pure XA models at their limits, it consumes 150W idle (this is the XA60) but when measured at full output it delivers 133watts into 8ohms and consumes only 193W.

These figures are from measurements done as part of the Hifi news reviews, I do not think power consumption-efficiency is on his Miller Audio Research site.
Cheers
Orb

[DonH50]

Efficiency is defined at max spec'd power output by default. All classes drop down from there, including class D, though different loss mechanisms apply. Also, some classes vary efficiency over their power range, so max n* is not always at peak output power. Like everything else, it's complicated... - Don

* n = eta = efficiency. Hey Mods, can we add the symbol font? I need my Greek...

[Orb]

But I am talking about from a listeners perspective and not engineering efficiency spec of an amp
50% efficiency has no meaning to a listener if they only use 5watts of say that MF Class A, that is consuming 1000W.

The problem is from a listeners perspective is that a true Class A amp has a contant consumption as in this horrendus example of 1000W, it does not matter if it is poor efficiency for a 100W Class A amp when say music is not linear and the amp is spending most of its time only delivering 5W to the speaker, and peaks at 60W - makes it an efficiency disaster.
I appreciate this is not the same as engineering efficiency spec as you say but is more realistic-practical for the amp which is rarely delivering its maximum output to the speaker.


Coming back to engineering spec related efficiency; I need to try and find it, but there was a Class A amp reviewed ages ago that hit the 50% mark at Hifi News.
Anyway the MF and Robert Koda are good examples of poor efficiency ones (which can be seen from their output to power consumed).
Cheers
Orb

[microstrip]

I quot a a post from the audiosylum "Radiotron Designer's Handbook by Langford-Smith, Fourth Edition, 1947, Page 572, Section 5, paragraph B: Using a definition from the 40s a Class A Amplifier is an amplifier in which the grid bias and alternating grid voltages are such that the plate current of the output valve or valves flows at all times. " This is the most often used definition of Class A (we just change the device)

As Don wisely said, using some simple calculus we can derive efficiencies for this mode of operation. However in practical use you have to enter the impedance of the load to design an amplifier, and most people are not aware of it when buying a class A amplifier - most of the time it is specified for 8 ohms and the impedance of your speaker will not be a fixed 8 ohm, some ones will even have very low dips. So the efficiency discussion is of little practical use, and some will say that as a rule of thumb, the lower the efficiency the best it will sound...

[Orb]

Looking at the very good class A amps Micro in the Hifi News reviews, their efficiency is pretty close throughout the difference impedence 8/4/2, it does deviate but for the very good amps it is reasonably close.
But these are the very well engineered ones.
Why I only bothered with the 8ohm for the MF and Koda.

Thanks
Orb

[DonH50]

Thanks microstrip, "wise" is something I rarely hear in association with myself... And, yeah, whether plate, collector, or drain current, definition of class A is the same.

Orb, most amps have very low output impedance relative to the speakers so efficiency stays pretty flat, but microstrip is right that there is load dependence. Never seen a load-pull test on an audio amp, however. Regarding "user efficiency", I don't know how to define that since it varies by user. What can be easily generated is a plot of efficiency vs. output (and vs. load, for that matter). By that measure, all amps are 0% efficient with no output, assuming they are turned on. If they are off, I guess you'd have to measure leakage, or use L'Hopital's Rule, or something...

[Orb]

Don a misunderstanding with my post.
I am stating Paul miller does load-pull test at 8/4/2 ohms looking at the consumption from mains outlett against the dynamic outputt.
So I can say reading PM reviews what the consumption is vs the actual dynamic power of the amps, if you look I even mentioned the example of the Pass Labs XA60 that is meant to be pure Class A but at the limits of its dynamic power it must switch to AB.
This is known because we have the steady state consumption and again with rated-dynamic ouput, in this example the Pass Labs only draws 193watts into its most dynamic output.
This is what my post is about, not output impedance but rating of maximum dynamic output and consumption at 8/4ohms
Hope the clarification helps.
Cheers
Orb