Re: Hifi landmarks

"François Yves Le Gal" <flegal@xxxxxxxxxxx> wrote in
message news:ecadf290iqoo2s4cnbmbovpqlqa7qvph2g@xxxxxxx
On Wed, 30 Aug 2006 19:01:25 -0400, "Harry Lavo"
<hlavo@xxxxxxxxxxx> wrote:

The Outlaw Monoblock 200 (now the 2200 with addition of
balanced outputs and $25 in price). It is conventional
A/B up to about 80wpc, then switches operations to a
Class G switching mode and delivers 200-300wpc, sweetly
and transparently with *none* of the switching nasties
I've heard in every other switching power amp I've
listened to.

Huh?

The Outlaw 200 and 2200 don't work in "switching mode".

Agreed, not as far as the signal path is concerned.

They are so-called class G, with dual output stages
(bipolar for AB + Mos-Fet for G) as well as dual power
supplies (standard B for AB, high B for G, drawn from the
same 400 VA xformer).

This would be an unusual implementation of Class G. Typically, Class G uses
the same linear output stage for all power levels, but varies the power
supply voltage as required to minimize device dissipation, and avoid
clipping. This requires a second set of output-level devices, but they are
operating as switches and don't dissipate much heat. By switching the power
supply voltage rapidly, power dissipation in the linear output stage is
minimzed. The MosFETs may be part of the power supply voltage switching
facility, not actually inside the linear portion of the signal path.

Crown, QSC and others have refined this technology to a fairly high degree.
But, they mostly only use it only on their high end, ultra-high power amps.
AFAIK, the lowest-cost pro audio amp using this technology is the QSC RMX
1850 amp at about $500 for a stereo amp delivering about twice the power as
the mono Outlaw 2200.

The RMX 1850 is 2RU like the Outlaw 2200, but puts out 350 wpc @ 8 ohms
under hifi operating conditions. It has a variable-speed fan. I haven't
actually auditioned a RMX 1850HD, but typically QSC amp fans can be
tolerated at their low speed if the amp is not too close to the listener.

Modern Class-G technology is typified in the schematic of the RMX 1850
output module at http://www.qscaudio.com/support/library/schems/rmx1850.pdf
.. Especially note the use of power FETs in the circuitry labelled "POS STEP
DRIVER" and "NEG STEP DRIVER". These are the stepped power supply
controllers. It appears that the two power supply voltages are 44 and 88
volts, nominal. The big brother RMX 2450 appears to use 55 and 110 volts.
http://www.qscaudio.com/support/library/schems/rmx2450.pdf

Class G amps are basically standard amps with a "turbo"
kicking in on peaks, nothing more, nothing new : the
Hitachi Dynaharmony HMA 8300, the first consumer class G
amplifier, was launched in 1977!

Agreed. This technology has been around for decades. The big SQ problem
with Class G relates to power supply rejection by the linear output stage.
With the power supply changing voltage in steps of 30-50% in time with the
signal (there are both 2-step and 3-step implementations), the opportunity
for switching transients to make their way to the speaker terminals is
always there.

http://www.dself.dsl.pipex.com/ampins/classg/g.htm

"Class-G is now out and about. So what has changed? This would be easier to
answer if there had been more published on the technology; as it is I have
to rely pretty much on my own research. I found that Schottky power diodes
have eliminated the commutation-diode glitching. Having peeled this skin
from the distortion onion, linearity was still signficantly poorer than a
Blameless Class-B design. This proved to be due to Early effect in the
output stage devices, stemming from the sudden changes in collector voltage
they experience in Class-G operation. I had my own ideas about how to deal
with this, and you can read all in Electronics World for Dec 2001 and Jan,
Feb 2002. There is a fully worked-out design, and even a PCB available to
ease building it."

The economic problem with class G is that unless the power amp is really big
( today: ca. kilowatt), the costs related to the switching hardware and
multiple power supplies exceeds the benefits of reducing the size of the
heat sinks. In some larger QSC and competive amps, the power supplies are
themselves high frequency switchmode designs, for a double dip at the
efficiency tub.


.

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