I have just read some piece of information I found interesting. It is taken from http://www.rcm.com.pl/cec/cec_main.htm , and it concerns CEC's new integrated amp, AMP71. The original text is in Polish, here is my (poor :() translation. I may have messed it a bit, I believe HPotter would do a better job. Anyway, here it is:
AMP71 works in Class-A, giving 200wpc max, without generating huge amount of heat. Enough to say, that heatsink temperature of a working amplifiers does not exceed 40 deg. C. What's more interesting, all A class amps, with power exceeding humble 30wpc, have large cases with enormous power supplies. AMP71 has traditional ps with 160000uF caps and two highest (:)) quality toroid transformers (650W each). Using two toroid is caused by fully symmetric design, including speaker terminals, not connected to the ground. Basic idea is very simple from theoretical point of view, yet its implementation is not that simple. To make long story short, signal is amplified by single bi-polar transistor, working in pure class A. It amplifies signal from input, which basically means that AMP71 does not contain pre-amp section, and the whole amplification is done by single gain stage without feedback. (...) The most interesting part is how to gain 200W in class A using only one gain stage. As we were said by designer, Mr. Carlos Candeias, gain is set between 40 and 50, and with given 100mV input sensitivity results in 5V max on the output. So where is that 200W coming from? The gain transistor is supported by current source made of 8 transistors per channel, that can easily generate 200W while working in A class (its very high impedance does not influences the signal). Roughly speaking, the power transistor works as switch controlling the
current transistors. The whole process is controlled by detecting output signal and informing the current source about changes, that are apropriately amplified. (...)
Does it sound familiar? Any comments, guys?
it sounds like a zen with aleph current source, doesn't it?
(I mean I haven't listened to it)
Actually sounds more like a single transistor driving a
complementary version of the Aleph current source. In
this design the N channel current source of say, the Penultimate
Zen, is complemented by a P channel current source and
together they form a push pull version of the current source.
If you set their current gain very high, they don't load the
single transistor very much, and so it works.
My experience with this circuit is that it can be made quite
good in performance, but as a practical matter the current
sources must be run pure Class A to avoid high distortion.
In the case of the CEC, this implies about a 900 watt idle.
Given the description I would be skeptical that you would
want to refer to the amp as Class A or single stage.
Actually, at first glance it looked to me exactly like Dieringe has said... but The One And Only must be right ;) ! I wonder what does the amp sound like?
If you want to see yet another variation of this concept,
check out the Stasis patent #4,107,619. In this approach
a small amplifier stage connected directly to the load is
"current bootstrapped" by an external circuit.
Same basic idea.
Certainly you can operate this small amplifier stage, perhaps
a single transistor, in Class A, but when 95% of the current
to the load comes from an AB power circuit, you are seriously
stretching the idea of single-stage Class A.
The AMP71 looks great! But the heatsinks don't look any bigger than the ones on my old class AB design 100W Hafler DH200. I only see what looks like four pair of IRFP type devices.
in this case I don't find anywhere service manuals and schematics, but I have found a PDF URL in German language
"Leonardo daVinci and the LEF topology"
I think, translation about google's language tools is possible.
There are a detailled circuit describtion and I will try to create (in the next time) by simulation a circuit example (pre- or power amplifier) in order to this describtion. But I guess already now, that is close to the concepts of "Stasis" so as the MJR-Amp
In the new year (i guess in February) this headphone amp I will get for repair and modification service from an user in my aera. If nobody post a schematic diagram to this time, I must perform a reverse engineering for schematic creating.
The input stage consists of a single stage (INSTEAD A DIFFERENTIAL STAGE) amplifier in push-pull followed from a push-pull VAS-Stage (2SJ76/2SK213) with integrated Vbe multiplier for adjust idle current of 120 mA through the output buffer (true complentary push pull, 2SC2921/2SA1215). The NFB goes like by the STASIS from the VAS-Stage to the EMITTERS of input stage (in the same manner like Musical fIDELITY A1/A100/A200). Thus the output stage isn't in the feedback loop.
According the schematic the supply voltage is 2x18VAC (aproximately +/- 24-26VDC). The power amp section exist four times for bridge mode.
I miss emitter resistors in the output stage and the fast fuses in the collector line means two fuses in the signal pad (because the absense of caps behind the fuses resp. at the collectors).
There must be several misprints arround the VAS stage (PP current source), thus i am not able to determine the appropriate current value - but I assume approximately 100 mA (necessary because the output buffer isn't triplet/darlington - this fact is very special for me).
The describtion by the URL
isn't suited to the schematic - maybe there are a wide range of others or "LEF" is only a name for marketing as Mr. Bob Cordell mentioned by one of the LEF threads here on diyaudio.
The different to Musical Fidelities A1 is actually very low. In the A1 is the output stage the VAS stage at the same time. If one replace the both emitter follower stages behind the input stage of the A1 and the output stage by one pair medium power lateral mosfets for the VAS stage include Vbe multiplier and add a pair of BjT power devices from SANKEN in a MT200 outline (not darlington or triplet) one will get the mentioned CEC topology.
I am still looking for the LEF topology.
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