Technical Paper >> INTRODUCING THE 5/1500A 'BALANCED' FIVE CHANNEL FIVE MONO BLOCK HOME THEATER AMPLIFIER AND ITS COMPANION 2/600A 'BALANCED' TWO CHANNEL VERSION.   Design Background Sherbourn established very specific goals when designing its original 5/1500 'Five' mono block amplifier and these same goals are applicable to the new 5/1500A and 2600A which both include XLR Balanced and RCA unbalanced inputs. The goals were to engineer a mono block amplifier that would perform accurately in a listening room and on a test bench; to engineer an amplifier that would drive all kinds of loads - resistive, inductive or capacitive with a consistent performance over the entire audio bandwidth; to design an amplifier that would excel for safety in its thermal performance and efficiently in its use of power without generating excessive heat and to design a mono block amplifier that could be grouped onto a single chassis in combinations of 'five' and 'two ' for home theater and stereo applications.   Engineering Design Execution For difficult loads, both voltage feedback and current feedback are employed. Triple stage current amplification is used in the output circuitry for very high current gain so as to effectively isolate the load from the voltage amplifier, which drives the output circuitry. The use of a large number of robust high current output devices ensures that ample current is always available to a load. The design allows for a fairly constant amplifier input and output impedance over a wider range of frequencies and means that the 5/1500A is better able to preserve the incoming signal and to drive 'difficult' loads than is often the case with traditionally designed amplifiers. Other multiple 200 watt amplifiers may use only four or six power transistors per channel. Because Sherbourn uses eight 250V Bi-Polar transistors per channel, each transistor dissipates 25% less heat than each of (say) six devices used in another amplifier. The overall approach of using more power devices, large heatsinks and rugged power supplies means that the 5/1500A's power silicon runs cooler in most conditions as compared to other amplifiers. Unlike amplifiers that use one or two transformers to power all five channels the 5/1500A uses five toroidal transformers each of which has a very tight load regulation. The power supply voltage for each channel does not drop appreciably from a light load to the full 200 watt output and even then each channel only draws 7 amperes peak current from its own transformer versus 35 amperes drawn totally by all five channels in the case of multi channel amplifiers that use one transformer. To minimize sonic differences resulting from environmental temperature change, amplifier warm up time or aging effect, the 5/1500A's biasing circuit uses a multiple thermal tracking technique which senses the temperature from the heatsinks and from an output 'current dependent' heating element. The result is a fast thermal response to stabilize the bias current of the output circuit. The amp is capable of maintaining a constant performance from a cold start or after a lengthy warm up. This feature also permits the product to automatically shut down a few minutes after the discontinuation of a music source. In the 'standby' mode the 5/1500A shuts off all current flows in the main power amplifier circuits leaving 'on' only a signal monitoring circuit thus consuming less energy. Sherbourn's input stage uses a high quality Burr Brown J-FET operational amplifier to provide input amplification. This device is also a 'summing' amplifier for voltage and current feedback from the amplifier output and it is this 'current feedback' that allows the amplifier to have an extended open loop power bandwidth. A 'current feedback' amplifier responds faster to the current demanded by its load and this wider power bandwidth allows for a flatter input and output impedance over a wider frequency range. This allows for a more constant loading to a preamplifier and a more uniform damping factor to drive the presented load.   Protection Circuitry Sherbourn protects each of its five channels from the AC input side to the speaker output. Each transformer has two fuses, dual voltage windings and its own internal thermal protection. A voltage switch is incorporated for 120V-230V switching and automatically configures so that only one fuse is used under a 230V setting and two in parallel to double the current passing capacity under the 120V setting. The input stage is protected from being overdriven via any extraneous surge signal such as a static surcharge or some 'floating' AC voltage during system set up. The power supply to the input stage is regulated and a 'dynamic clipping' circuit is employed to limit the current drawn by the input stage during any deep clipping of amplifier output. This ensures that the first stage of the 5/1500A will never enter saturation or be biased to an 'off' condition under such clipping conditions. Such a problem may be found in amplifiers with traditional differential or double-differential transistor or FET designs in which one side of the differential device is driven by a continuously rising signal and the other side by the feedback clipped signal. A large differential signal could develop between the inverting and non inverting inputs of the differential amplifiers thus biasing one side into saturation and the other into 'off' completely. Such amplifiers could also exhibit a high degree of distortion because they no longer follow the original input signal...not so with the 5/1500A.   Installation Flexibility The 5/1500A's chassis is also used for the 2/600A BALANCED two channel amplifier. Both models use either a 19" rack mount or a standard sized front panel. Having five, two and one channel (1/300MB mono block) versions with identical appearance and performance design allows for integrated styling and awesome BALANCED power in any configuration.   Summary In summary, what does all of the above mean? Simply that Sherbourn's 5/1500A and 2/600A are more accurate and more musical than conventionally designed amplifiers and represents outstanding value for money.