AB Model 200 Power Amp
At the bottom end of the power amplifier market, there are several models offering similar power ratings at similar prices, the model under review adding further to the available choice. This amp is rated at 75 Watts per channel into 8 ohms, 100 Watts per channel into 4 ohms or 180 mono into 8 ohms if used in bridge mode. Unlike many of the budget models which have gone the MOSFET route, the Model 200 is based on bipolar circuitry using two complementary, high-power Motorola output devices per channel. These are biased for class AB/2 operation and thermal compensation is employed to prevent thermal runaway under arduous operating conditions.
The semiconductors in the complementary drive stages have been chosen to maintain circuit stability, while the amplifier uses only 20dB of negative feedback to minimise forward transfer distortion. The design is short-circuit protected, and according to the manufacturers, the output stage is so generously specified that the protection limiting won't come in unless severely reactive loads of under 2 ohms are being driven at full power.
Physically, the amplifier is built into a utilitarian, black 1U rack case. Finned heatsinks protrude from the rear panel, which also houses the speaker binding posts, XLR balanced inputs and phono unbalanced inputs — no fan is fitted. Switches are provided for ground lift and for mono operation; in mono mode, the left hand level control functions for both channels. The mains lead is captive and the binding posts are rather close together, making it a trifle fiddly to fit the speaker cables. However, the binding posts also accept banana plugs, which makes life easier when frequent connection and disconnection is envisaged.
The front panel sports the two level controls, each of which has associated 'signal present' and clip LEDS, while a pair of stereo jacks allow the amplified signal to be monitored using conventional stereo headphones. This is an unexpected feature on a power amplifier but could be useful for silent, midnight monitoring sessions in a home studio. The two sockets appear to be wired in opposite ways so that choosing one socket or the other reverses the left and right hand channels.
Powering up the amplifier reveals that it has some form of 'soft start' system to prevent loudspeaker thumps, which is very welcome in the home studio, where one master switch often turns on the whole shooting match. I must admit being pleasantly surprised by the sound of this amplifier, which was subjectively more detailed and more transparent than some of the amplifiers I've tested. Of course it would be dangerous to assume that this is purely the result of accurate circuit design, as certain types of distortion create the same illusion — witness the audiophile's proclivity for valve amps which produce exactly this effect. Whatever the reason, the sound is open and bright while the bass seems tight and well-controlled, though some bright sounds occasionally tend towards sibilance. Again, it is hard to apportion blame for this effect, as many current CDs are unforgivably sibilant anyway and most of my listening tests were done using CDs as a sound source.
Even though the amplifier is only rated at 75 Watts into eight ohms, I managed to achieve listening levels far in excess of comfort from a pair of Dawn Patrol monitors that I happened to have here on review, and even at those high levels, the sound remained clear. It's hard to get too excited about 'workhorse' power amplifiers, but I feel this one offers a sensible mix of features, quality and power given its price.
AB Model 200 power amp £299 including VAT.
Key Audio Systems, (Contact Details).
Review by Paul White
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