Sherwood A 965 dimensions

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