Sherbourn 5/1500A Power Supply and Features

Perhaps one of the most important aspects of an amplifier is its ability to deliver stable and continuous DC power to the output transistors. The Sherbourn 5/1500A is extremely generous in its ability to deliver power. The unit is divided into 5 equal mono-block amplifiers (see picture below), each able to produce 200-Watts of continuous power at 8-ohms and 300-Watts of continuous power at 4-ohms (20 Hz to 20,000 Hz). The power for each channel is provided by individual 400-VA transformers combined with two 10,000-uf low ESR capacitors (20,000-uf total per channel). Combining the specifications of each mono-block amp totals to a staggering 2,000 VA equivalent transformer with 100,000-uf capacitance, thus the reason for its robust power and weight.

One could argue that a disadvantage of a mono-block design is that the two channel power is limited by the use of small multiple power supplies as opposed to a single and much larger power supply reserve found on other multi channel power amplifiers. Normally there is truth to that fact, assuming there are enough current capable output devices to handle the extra power without running into thermal runaway. However, the Sherbourn is an exception in this case, since it offers more high current output devices (8 per channel), power supply capacitance, and transformer power to each channel with its mono-block topology for under $2k, than any other 5-channel, single power supply designed amplifier in and above its price range that we have seen. As an example, I evaluated an Acurus 5 channel amplifier (A200x5), which also retailed for $2000, that utilized a single 1KVA or so power transformer and combined power supply capacitance of only 46,000uF.

In my opinion, this amp didn't even hold a candle to the Sherbourn with respect to dynamics and raw deliverable power in low impedance loads. At best, the amplifier section from the Acurus was a slight improvement over a typical top of the line home theater receiver.

Transistor Output Devices

A good power supply is useless without an ample output stage to take advantage of it. There are 40 bipolar transistor output devices in operation; 8 on each channel in push pull configuration, all working hard to provide clean power to the speakers. The transistors are kept cool with a whopping 2500-square inches of heat sink, 500-square inches per channel. It's no wonder this unit never breaks a sweat when cranking five 4-ohm speakers. This amplifier does an impeccable job at driving low impedance loads, reminding me of an ideal voltage source. Many lesser amps tend to wimp out when speaker impedance halves. The Sherbourn welcomes the challenge and remains quite musical, with plenty of headroom for the task.

Features & Ergonomics

There are a few extra features in the Sherbourn, which includes balanced input and automatic signal sensing. I don't usually find much merit to the "balanced inputs" of amplifiers unless the entire design from input to output is dual differential. The real benefit of a fully balanced system is the cancellation of 2nd and 3rd order harmonics. However, implementing a fully balanced amplifier involves more component cost and thus reduces profitability margins. Since the Sherbourn amps are conservatively designed to operate in the most linear region of the VI curve, include a push pull topology, and utilize the right amount of feedback, the design is not so limited. I can't fault Sherbourn on the single ended output stage implementation because 99% of the audio industry does this, and reserves dual differential amplifier designs only for the most expensive, cream of the crop models offered.

The intent of balanced inputs in this case is to help minimize RF injection from other sources, which could be beneficial for some applications.

The front panel is made for a standard 19" rack interface at a height of 7". The depth of the unit is 17.7 inches and every bit is used to pack in the electronics, although still managing to leave sufficient room for air flow around the massive heat sinks. The only negative I see with this unit is that it takes two normal people to lift it onto your entertainment center, or one bodybuilder.