Power amplifiers should be boring. They have a single, well-defined function: Make the input signal large enough to run a loudspeaker so that it makes sound at levels suitable for listening to music. Generally, controls and features are few or none. Peter Walker of Quad famously defined the ideal amplifier as a “straight wire with gain.” That’s just one feature: gain.
That ideal is not easy to achieve, for many reasons. Even a straight wire of any practical length and structure has properties (resistance, capacitance, inductance) that can affect how the signal is transferred to it on one end and how the signal is transferred from it on the other. These effects can be within the audible range depending on what device is at each end. Insert a complex device like a real-world amplifier (with different wires in and out) and Walker’s ideal starts to seem unrealistic, although it should be possible to get close.
Walker further stated that an “audio power amplifier is required to produce an output signal that differs from the input signal in magnitude only.” (footnote 1) In attempting to realize that goal, he tried both tubed and transistor designs (although he averred that the latter were superior). Today, with the availability of tube amps and an expanding range of solid state designs, we have a broad range of options but no better standard than Walker’s.
The NAD C 298 stereo power amplifier is based on the Purifi Eigentakt class-D amplifier module, the most recent brainchild of Bruno Putzeys and Lars Risbo. The C 298’s design incorporates feedback to achieve a linear and accurate transfer function, an approach espoused by Peter Walker, although the Purifi guys are using an approach to amplification—class-D—that even advanced thinkers in Walker’s time may not have been aware of (footnote 2). So, let us not obsess about “how” the amplifier does its job and focus on how well it does it, technically (Cue JA!) and how well it plays music.
The amp arrives
Even as I was reviewing the NAD M33 Streaming Integrated Amplifier last year, it seemed obvious that NAD would soon release a stereo power amplifier using the same Purifi technology. I wrote that the M33 has “a switch to bridge the two channels into a >700W mono monster, and the line outputs allow you to add an external matching amp or two. (Do you think that NAD has something like that in the works?)” But no, the next amp NAD introduced with the Purifi technology was the M27’s successor, the M28, a seven-channel power amp. (See Sidebar 1,) The M33 and the M28 are, as indicated by their common prefix, members of NAD’s generous and stylish Masters Series.
When it arrived, NAD’s two-channel Purifi-based power amplifier was observably not in the Masters Series but rather their more affordable, less luxurious “C” (for “Classic”) series. The C 298 employs the same Eigentakt modules used in the M33 and M28, “but the power supply and input circuitry is specific to the C 298.” The C 298 is heavy for a class-D amp, at about 25lb. Its appearance is plain but clean, and the amp seems well-constructed.
The front panel bears only a Standby button and two small LEDs. When you connect the AC cable and rock the main power rocker switch on the rear—nothing seems to happen. Press the front panel button, though, and after a few seconds’ delay, a relay clicks as the LED above the Standby button blinks in amber. In a few more seconds, the LED turns blue and the amp is ready to make music. The slightly larger LED to the right of the Standby button lights blue if the amp is in bridged mode.
There is more going on in the back: both XLR and RCA inputs for each channel with toggle switches to select them; a pair of RCA line outputs to permit daisy-chaining the input to additional amps; a toggle to select fixed or variable gain; a gain control; and a control for the sensitivity of the Auto-Sense function. Above these connectors is a grounding lug to help remove ground-loop hum. Next is a mini-USB port for servicing, a 12V trigger input and output, and a bridged-mode switch that’s intentionally hard to get at. Finally, there are two pairs of sturdy, multiway speaker terminals, that power rocker, an AC fuse holder, and an IEC receptacle.
Standard bridging strategies are verboten with Purifi amp modules since neither output terminal is at ground potential and should never be connected to an external ground. Unlike regular bridged amplifiers, where the output is taken from the left and right positive terminals, NAD uses the left positive and right negative terminals and cautions against connecting subwoofers, switches, or headphone adapters with common ground connections. Safety is a good reason for the bridging switch to be inconvenient.
If one uses the C 298 in a multichannel system or to power active multiway speakers or if one wants to mix bridged and stereo C 298s with other amps, it’s necessary to adjust the gain—hence the C 298’s gain-control knob, which allows you to select gain between 8.5dB and 28.5dB in stereo and between 14.5dB and 34.5dB bridged. NAD could improve this very useful feature by providing a visible index mark on the tiny knob.
Context and listening
Although my system is multichannel, there is no processor or preamplifier. The output of the DACs drives the power amps directly via balanced connections. I used the NAD’s XLR inputs in both stereo and bridged modes. When playing two-channel music, I used fixed gain. Fixed gain in stereo mode (28.6dB) also worked when playing multichannel with one C 298 powering the front L/R speakers. However, to use a pair of bridged C 298 amps, my options were to use fixed gain and adjust the channel balance in JRiver or the DAC or to use variable gain to match the other channels. The character of the amp remained constant across all these permutations.
Footnote 1: The quote is from Wireless World, December 1975.—Editor
Footnote 2: Class-D amplification was invented in the 1950s, and by the mid-’60s there was even a commercial product from Sinclair Radionics, which put out a whopping 2.5W. It’s likely true, though, that most designers in the hi-fi space weren’t aware of it.—Editor
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COMPANY INFO
NAD Electronics International
633 Granite Ct.
Pickering, Ontario L1W 3K1
Canada
(905) 831-6555
nadelectronics.com
ARTICLE CONTENTS
Page 1
Page 2
NAD M28 seven channel power amplifier
Specifications
Associated Equipment
Measurements
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