[saint.panda]

On a German Audio Forum, there's a rave on a headphone amp "HD53" by Cec (Audiocubes) which utilizes this new LEF (Load Effect Free) technology. This (slightly self-advertising) link explains roughly what LEF does:

http://www.candeias.com/english/lef.htm



A few excerpts:

Amps based on the LEF circuit do not require negative feedback

Quote:

The breakthrough of LEF circuit is: The signal transistor does neither pass through its voltage characteristic Vce nor the current characteristic Ic.

Quote:

As the signal transistor in LEF circuit does not handle the current requested by the speaker, loss in efficiency is negligible.

Quote:

LEF’s most significant progress is splitting the handling of the loudspeaker’s current request from the music signal voltage output stage!

Quote:

LEF means “Load Effect Free". This means: The signal transistor is not loaded by the speaker's current request, because he has strong current handling "assistants" - thus no sound degradation effects due to the load happens.

Has anybody heard of LEF and is it the free lunch it seems to be or simply a real improvement over conventional circuit designs?

 

[drewd]

Maybe something was lost in translation, but the analogy seems pretty tortured. It sure sounds a lot like the Quad 405 and 405 Mk.2 power amplifiers from about 20 years ago, though. They used an opamp for voltage gain which then drove a set of output transistors biased into Class A mode. A resistor and diode network would activate the current dumping transistors, based upon the current demands of the output transistors. Interestingly, the current dumping transistors operated in Class B mode, but near the switching points, it appears that the signal output transistors, in Class A mode, could provide the current that was absent as the current dumping transistors switched off and on, so that crossover distortion was effectively eliminated.

There is an interesting paper here that goes into quite a bit of agonizing detail regarding current dumping amplifiers.

The whole idea is that the signal transistors stay in their linear range, no negative feedback is used (although a DC servo is used) and loads of current is available to the load.

If that's the way that the LEF circuit works, then it's definitely nothing new or revolutionary. But since the article has no technical details, it's pretty tough to say.

-Drew