The Best Stereo Power Amplifiersby Candace Horgan
Stereo power amplifiers can have a significant effect on the overall sound of a stereo system. Matching the power amplifier to the power demands of the speaker is critical to achieving system synergy. Power amplifiers can be made with a tube circuit or a solid state circuit. Audiophiles debate the pros and cons of each often. Tubes have a more "liquid" midrange that makes music sparkle, while solid state power amplifiers don't put out as much heat and have more control over the bottom end. Tube amps need to be biased to match the current draw to the power circuit; some amps are self-biasing, while others are biased by the user.
Weighing an astonishing 240 pounds, the Boulder 2060 ($44,000) is a behemoth of an amplifier. It is 18 inches high, 10.75 inches wide, and 26.75 inches deep. The amp is constructed to minimize any external vibrations, having vibration dampening feat and heavily built heat sinks. The amp outputs 600 watts per channel into an 8 ohm speaker load (the amount of resistance the current from the amp encounters in the speaker.) Like all high end power amps, the power output doubles with each halving of the impedance load, so the 2060 outputs 1200 watts per channel into a 4 ohm load and 2400 watts per channel into a 2 ohm load. The 2060 only has balanced XLR connections, so if your preamp has RCA unbalanced outs, you will need to get RCA to XLR cables.
The Halcro DM38 ($22,900) uses the award-winning circuit topology of the Halcro DM58 monoblock but puts it in a single chassis with a stereo output. The DM38 outputs 180 watts per channel into an 8 ohm load, and 350 watts per channel into a 4 ohm load. The amp has both balanced XLR and unbalanced RCA inputs, as well as an unbalanced RCA current mode input with a 60 ohm impedance to help control cable distortion problems. The amp weighs 120 pounds and stands 31 inches high, 16 inches wide, and 16 inches deep.
Conrad Johnson LP70S
Conrad Johnson has been one of the leaders in the renaissance of tube circuit audio gear. The LP70S ($7500) is the company's latest stereo tube power amplifier. It uses three 6922 tubes for the triode and two 6550 power tubes, and measures 6.38 inches high, 16.75 inches deep, and 19 inches wide. It weighs 50 pounds. Conrad Johnson wires the amp to output 70 watts per channel into a 4 ohm speaker load. It is also capable of being wired for 8 ohm and 16 ohm loads. The amp has a simple user bias setting that is adjusted with a screwdriver. The amp has unbalanced RCA inputs.
VTL S-400 Reference
Vacuum Tube Logic (VTL) created their flagship S-400 Reference ($25,000) with an RS-232 input to make controlling the amp from a computer easy. The amp outputs 400 watts per channel into a 4 ohm load in tetrode mode, and 200 watts per channel into a 4 ohm load in triode circuit mode; with an 8 ohm load, it outputs 300 watts per channel in tetrode mode and 150 watts per channel in triode mode. Triode mode is best used with efficient loudspeakers like horns, to achieve more musicality, while tetrode is good with standard loudspeakers for its power output. The amp has both balanced XLR and unbalanced RCA inputs. It has an autobias circuit, as well as a display panel on the front panel that tells the owner when the power tubes need to replaced. The amp measures 11.5 inches wide, 24 inches tall, and 24 inches wide, and weighs 220 pounds.
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