|Manley Labs MAHI Mono Power Amplifier Review|
|Home Theater Power Amplifiers Mono Amplifiers|
|Written by Andre Marc|
|Wednesday, 18 November 2009|
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The first time I got to hear a piece of Manley Labs tube gear was when I reviewed their superb Neo Classic 300B Preamplifier a few months ago. To say I was impressed would be an understatement. Albert Schippits, Manley Labs National Sales Manager, was also sure I would also be interested in reviewing their MAHI® mono block tube amplifiers.
The EL84 tube has quite an interesting history. It was originally developed by Philips in Europe for audio amplification. It soon found its way around the world, being designated as the 6BQ5 in North America. I won't go into an eye glazing, micro history of the tube, but suffice it to say it has proven quite versatile for more than 50 years. It was also put to use by British Invasion bands during in the mid 60's inside their guitar amps, especially in the prized Vox AC30.
Construction and Features:
Coming back to what I mentioned as unique features to the MAHI's, there are several clever design elements. First, there is a toggle switch on the top right side of each chassis that give the user the option of switching between Ultra Linear mode and Triode mode. According to Manley, Ultra Linear mode offers more power, and a slightly more aggressive sound. Triode mode offers the sweeter, more textured presentation, but is a less efficient way of using a tube; hence less output power.
Next, the user has the option of toggling a three position switch that controls Negative Feedback settings from MIN (3dB's), STD (6dB's), and MAX (10dB's). Feedback is often employed in amplifier designs for greater stability and to lower Total Harmonic Distortion (THD). Basically, a portion of the output signal is fed into the input signal. There have been age old arguments on how much, or if any, to include in the input signal. There is a lot of information about it on the Manley website. I generally preferred Triode Mode with the Feedback setting at MIN, but will describe what I heard with each setting later in the review. The MAHI's are fittied with high quality German made WBT speaker binding posts and Manley's design of gold-plated, teflon insulated RCA jacks. The Manley’s are specified to out put 46 W max in Ultra linear Mode, and 25 W max in Triode mode.
I set up the MAHI's on the floor, supported by two blocks of solid Myrtle wood and connected my preamp / speakers. I turned the bias pots to set the amps at 250 millivolts as specified in the manual, a simple adjustment to achieve 25mA current draw for the output tubes. I did experience a ground loop hum when I first powered up the MAHI's. I solved the issue by plugging each mono block into a different side of the room, on separate AC lines. I didn’t encounter any other set up issues during the review period.