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looking for the UL label anymore. The Occupational
Safety & Health Administration now certifies
multiple companies to test consumer equipment for
safety. These are called Nationally Recognized
Testing Laboratories (NRTL). In most cases the
testing standard used is the one developed by UL
in the past. If the NRTL is not UL it will use its own
Registered Certification Mark. The web site http:/
/www.osha.gov/dts/otpca/nrtl/nrtlmrk.html
shows marks for each NRTL that would be placed
on equipment for each NRTL that could test the unit.
One wishes that OSHA would have established a
single mark to indicate that an NTRL had certified
a product was safe to OSHA standards, but instead
we have to deal with many different marks that
indicated a product has been tested to OSHA
standards of safety.)
Class AB amplifiers running full out are
approximately 70% efficient at full power so the
power coming out of the wall is 13 amps, which is
just under the rating
for a standard wall
outlet (leaving 2 amps
to supply everything
else on the same AC
loop). Amplifier
manufacturers selling
300-watt channels
times 7 will not pass
UL safety tests and an
NSTL label is absent on
these amplifiers. I looked high and low at CES and
not one of the manufacturers sporting an amplifier
rated at 300 watts times 7 had an NSTL label on the
back. In reality, you would need to hang a 15-amp
220-volt power cord on these amplifiers to prevent
them from popping a fuse. These giant amplifiers
do not pop fuses in real life because they rarely
approach full power, at least not simultaneously on
all channels.
Why has no 300 watt multi-channel power amp
been offered with the correct power cable? You
guessed it—the high-end dealers would go crazy
with thoughts of reduced sales. Imagine them
having to advise clients to first call their electrician
before proceeding with the sale. These same high-
end dealers will, of course, gladly sell you $1,000
power cords for your 15-amp circuit.
I do not know about you, but I would worry
about a power amp sitting in my living room that
was not tested to UL standards by an OSHA-
certified laboratory. Apparently, products without
NTRL labels are available for sale in all states but
Oregon. Go Beavers!
OK, back to the Sherwood A-965 and R-965,
which proudly display their UL labels. From a
practical perspective, the R-965 is about as big and
heavy as an AV receiver can get. The single
transformer of the R-965 AV receiver distinguishes
it from the A-965. The R-965 also has less room for
heat sinks since all the front-end electronics fills half
the box. Otherwise, the differences are minor. With
all the metal taken out of the R-965, it can only pass
FTC into 8 ohms with two channels driven at a
slightly higher power rating of 120 watts per
channel. Making that compromise saves $1,000 over
the A-965/P 965 combo.
Both Sherwood units share a nicely designed
circuit topology and some high quality parts.
Attached to the big heat sinks are expensive Sanken
2SA1216/2SC2922 complementary-pair power
transistors. These transistors mean business
packaged in the MT 200 plastic package that
measures 1.5 inches wide. For a single 10 msec burst,
they can source 35mA.
While you can achieve the
same thing more
economically by putting
multiple transistors in
parallel, you would have a
hard time achieving as low
of junction capacitance. The
devices have a unity current
gain frequency (the
frequency that a piece of
wire would do as well) of 40MHz. That’s twice as
fast as most power transistors in this class, which is
why the 2SA1216/2SC 2922 are more than twice the
normal price. Faster is better, because the amplifier
can be made more stable when driving complex
loads at audio frequencies. The A-965 has two pairs
of paralleled pairs of 2SA1216/2SC 2922 per channel
while the R-965 has one pair. In all other respects,
the electronics on the power supply board are the
same.
To generate enough current at the base of the
2SA1216/2SC 2922, a two-transistor compound
common-collector common-emitter pair is
employed instead of a single transistor pre-driver
stage. For the voltage gain stages, the amplifier starts
with a differential pair with an active current-mirror
load to ensure excellent common-mode rejection.
By biasing the differential pair with a Wilson
current-mirror, the rejection ratio is further
improved. A high common-mode rejection ratio
improves the distortion performance of an amplifier
when used in a non-inverting feedback
configuration, which is the standard topology for