These days it’s not uncommon for an integrated amplifier to include some digital connectivity along-side the analogue inputs of yesteryear. Designed to partner the CD-NT670 or CD-NT670D network CD players and the NS-BP401 bookshelf speakers to form the MusicCast MCR-N870 system, the Yamaha A-U670 takes things a step further. It may look like a simple integrated amplifier, but there’s some clever modern technology packed into that tiny chassis including a DSD-capable 384KHZ asynchronous USB DAC accompanying the single analogue input.
That DAC is powered by an XMOS controller and the ESS ES9010K2M 32-bit DAC chip, fully supporting both DSD64 and DSD128 content as well as PCM sampling rates of up to 384kHz. Class D amplification produces 70W per channel, with a low-pass filter coil made of oxygen free, OFC copper wire. Class D amplifiers are extremely efficient, meaning the A-U670 draws just 30W at full power and an eco friendly 0.5W in standby. The A-U670 features an optimised circuit layout with short signal paths and Yamaha’s pure direct circuit is fitted to optionally bypass the tone and balance control circuits as well as the back buffer amp for the purist sound quality.
Two models are available, the A-670 lacking the USB DAC function and paired with either a CD-NT670 or CD-NT670D network CD player and a pair of NS-BP301 speakers to form a MCR-N670 or MCR-N670D mini hi-fi system. The A-U670 is paired with either of the aforementioned CD players and a pair of the larger NS-BP401 speakers to form the MCR-N870 and MCR-N870D systems. The difference between those systems with an appending D to their model identifier is the inclusion of a DAB+ radio tuner. Both amplifiers are also available to purchase as stand-alone units.
The 2 amplifiers are largely identical besides the omission of the USB DAC on the lesser model, and that model offering less output power and therefore less power consumption (28W as opposed to 30W). The A-U670 is rated to deliver a maximum of 70W per channel at 1kHz into 6 ohm load at 10% THD, with a frequency response of 10 Hz – 40 kHz (+/-3dB). At 30W per channel, that distortion figure drops to 0.05% which is a far more acceptable value. It is for this reason that i prefer to think of the A-U670 as a 30W per channel amplifier, and it is that value I’d advise you take into account when choosing a matching pair of speakers.
In the box, you’ll find the amplifier itself along with some documentation and a system control cable. That cable connects to the matching CD player to allow the power and volume of the amp to be controlled by the player’s remote and of course the MusicCast control app. Weighing 3.3KG, the amp feels heavy for its size, and measuring 314 x 70 x 342 mm (W x H x D) it’s slightly larger than your average half-sized separate. It’s well made though, with contoured dials, a thick aluminium front panel and piano-effect side panels which lend it a sleek appearance when it’s sitting on the rack.
On the front, you’ll find a row of vertical dials for bass, treble and balance and a rotary dial to control the volume. There are buttons for power and pure direct, the latter with an LED which lights when the function is enabled. The amp doesn’t remember the status of the pure direct function, so you’ll need to re-enable it each time you switch on if you wish to use it.
Speaking of LED indicators, the amp doesn’t feature a sample rate display, instead a row of indicators to show the status of the DAC. It also lacks an input selector, instead switching to DAC mode when it detects a PC is connected. If you connect a compatible Yamaha network CD player, you can switch to the USB DAC mode using the input control on the front of the player, the player’s remote or the MusicCast app. Rounding out the front panel functionality is a full-size quarter inch headphone jack, specified for use with 32 ohm headphones. Connecting headphones mutes the speakers automatically as you would expect.
Around back, you’ll find a permanently attached power cable along with a set of speaker terminals which will accommodate bear wire or banana plugs. There’s a single mono sub woofer terminal, along with a pair of RCA analogue inputs, the system control jack and the type B USB computer input.
I’m a huge fan of the A-U670s design. I love the vertical dials, a hallmark of many a classic amplifier over the years, not to mention the solid feel of not only those dials but the casework itself. The parts are beautifully manufactured and the result is a product finished to a standard that is not always the norm when it comes to smaller hi-fi components. Typically such components are so feature laden that their build quality becomes something of an afterthought. That’s not the case here, Yamaha’s decision to opt for a simple design approach meaning that the attention to detail and the quality of the components in use hasn’t suffered.
Sound wise the A-U670 is a very neutral amplifier. The highs are clean with plenty of top end detail, as are the mids. The bass is tightly controlled, though realistically with 30W of power at acceptable distortion levels you can’t expect the A-U670 to offer earth-shaking sound unless you pair it with some particularly efficient speakers. It’s certainly capable of providing more than enough volume in an average domestic setting however, running my reference Tannoys to very high levels with admirable control. After all, a good hi-fi is designed to faithfully reproduce a piece of music as the artist intended, not to reproduce it as loudly as possible.
Noise levels are reasonable, though the amp does generate some hiss that will be evident if you connect high sensitivity speakers with particularly revealing tweeters. It handles sensitive headphones very well though, easily on a par with any integrated amp at this price and beyond, and better than some dedicated headphone amplifiers. The sound through headphones is much like that delivered through the speaker outputs – detailed, involving and true to the source material.
The A-U670 is a worthy choice if you’re looking for a simple, compact and relatively affordable integrated amplifier. It’s equally at home as part of a compact hi-fi setup where space is at a premium as it is on a desktop where its USB DAC makes it the ideal companion for high-resolution computer audio. Pair it with a CD-NT670 for the ultimate mini hi-fi system. Highly recommended.
