This method is extremely efficient, but significant crossover distortion occurs at the point where the transistors ‘hand off’ to one another. In a class A amplifier, the output transistors are always conducting, regardless of the output signal waveform. Therefore there is no crossover distortion, but the amplifier is wildly inefficient, wastes a lot of power and produces a lot of heat. Pure class A designs are typically reserved for low level driver stages or low power headphone amplifiers. They are almost never used in the output stage of a commercially available power amplifier, as they’re costly to build and costly to run, particularly when high output designs are introduced.
The class A/B amplifier is an attempt to combine the merits of both classes. In a class A/B amplifier both output devices handle just over half of the signal (200 degrees or so). This results in less of a gap in the cycle where crossover distortion can occur and the amplifier is extremely efficient as only half of the output devices are conducting at any given time. However crossover distortion still occurs, and for decades many designs have emerged that attempt to bridge the gap between the sound of class A and the efficiency of class A/B. Examples include Technics’ ‘New Class A’, Doug Self’s Class XD crossover displacement circuit (now patented and used exclusively by Cambridge Audio), class D (often incorrectly referred to as the ‘digital’ amplifier) and classes G and H.
A class G amplifier provides multiple power rails at different voltages, and switches between them as the signal output approaches each level thus increasing efficiency by reducing wasted power at the output transistors. Class H amplifiers take the idea of a class G amplifier a step further by implementing an infinitely variable supply rail. The supply rails are modulated such that they are only ever a few volts above the output signal at any given time, allowing the output stage to operate at its maximum efficiency. The drawback however is increased complexity of the power supply design and an increase in total harmonic distortion.
Arcam have been designing class G amplifiers for some time, the typology first seen in their AV receiver lineup. More recently the technology has found its way into three of the companies amplifiers (the A29, A39 and A49), which now form the entirety of their amplifier lineup since the base model in the range, the A19, was discontinued.
The A29 produces 80 watts per channel into an 8 ohm load, rising to 175W into 4 ohms. Distortion is measured at less than 1 part per million, with frequency response flat to within 0.2dB from 20Hz to 20kHz. Seven line level inputs are provided, along with a moving magnetphono stage with fixed loading at 47K ohms, 100PF. Volume is handled by a digitally controlled analogue ladder chip from Texas Instruments – the PGA2311. The same chip as found in the flagship A49, the PGA2311 offers 120dB of dynamic range, vanishingly low distortion and maintains near perfect channel balance at all levels.
A massive toroidal power supply provides the juice, with Arcam’s class G implementation using 2 sets of supply rails at approx +/-25 and +/-48V for the low and high rails respectively. Under normal load the output stages run from the lower rails, with the first half watt or so delivered in pure class A before the amplifier moves into class AB operation. When the signal output reaches approximately 20 volts peak (or 14 volts RMS, equating to roughly 25W into an 8 ohm load), class G operation kicks in, raising the rail voltage and leaving enough headroom (5V or so) for linear operation of the output transistors.
Input switching is solid state, with what John Dawson, Arcam co-founder and senior electronics designer described to me as “a novel form of distortion cancellation to keep distortion at the parts per million level”. I must thank John for taking the time to share with me an informative explanation of the A29s class G circuit and other technical details mentioned in this review. The amplifier is DC coupled throughout, with a DC servo keeping offsets below 5MV. Metal film and thin film resistors are used exclusively in the signal path as these “don’t suffer from voltage modulation of the resistor value”.
Out of the box the A29 very much resembles the A19. It’s a slimline unit weighing 9.2 kg and nicely finished with a small row of ventilation slots on its top panel. The aluminium front has a slightly curved profile and features a neat array of controls including a control knob for volume, balance and changing various settings. Rows of push buttons provide mute, balance and display dimming controls and access to the seven line level inputs and moving magnet phono stage. I wish the inputs were simply numbered, rather than being pre-named (SAT, CD, BD etc). A 3.5MM input is positioned on the front panel along-side a 3.5MM headphone output. Finally a hard power button cuts power to the unit entirely, while a backlit alphanumeric display shows the volume status, chosen input, and various messages such as channel balance, menu settings and firmware update prompts.
Around back a 2-pin IEC socket provides power, and a row of RCA jacks cater for input and output, including both fixed and variable level outputs for recording devices and external power amplifiers. A chunky pair of speaker terminals for a single pair of speakers are provided in a somewhat unusual layout. There’s a jack for powering an Arcam rSeries component (with a cable provided in the box), and a USB port which can be used as both a power supply and a software update port.
The build quality is outstanding. The chassis is acoustically damped and offers up very little ringing when tapped. It sits on 4 large rubber dome feet which do an exceptional job of damping vibrations. The terminals are solid as are the controls, particularly the control knob which has none of the flexing and wobbling commonly associated with cheap digital encoders. It’s clear that the components used in the A29 are of high quality, and as a result it it feels like an amplifier that’s built to last.
The A29 is supplied with Arcam’s CR902 system remote, the same as that supplied with the A39, A49, C49 and CDS27. The remote is light in the hand and feels cheaper than rival units, though its buttons do offer a nice ‘click’ when pressed which is a nice touch. It takes a pair of AAA batteries which slot into a compartment at the back and follows the RC5 standard so it may well control other components in your system too. I would like to see larger volume buttons as those on offer are identically sized to most of the other controls, but the CR902 is certainly a major step up from the remote supplied with the A19.
In use, the A29 is as basic as it gets. Pressing the power button results in a couple of relay clicks as the circuits stabilise and the speaker protection relay is released. Pressing the display button toggles between 3 brightness levels (bright, dim and off), while pressing the balance button and then rotating the volume control gives you +/-12dB of adjustment in either direction. There is no provision for speaker switching, nor are their tone controls not that you need either. Connecting headphones mutes the speakers automatically.
Rather than offering a menu system, the A29 incorporates a clever control system whereby multiple controls are pressed simultaneously to alter various settings. For example, pressing the phono and balance buttons and then operating the encoder knob allows the onboard phono stage to be disabled, at which point the input can be used with a standard line level source such as an external phono stage. Pressing ‘balance’ and ‘aux’ allows the automatic standby time to be configured, while pressing ‘Mute’ + ’Sat’ + ‘CD’ allows you to switch between RC5 system code 16 and 19 to avoid remote control conflicts. A full list of menu options and the default values is provided in the FAQ document on Arcam’s website.
Arcam’s ‘house sound’ is not one intended to instantly impress, instead becoming more involving the longer you listen. That’s certainly the case with the A29, though it sounded very good straight out of the box. It spent a couple of weeks providing background music in the system before any serious listening took place, allowing it ample run in time but also allowing me to become acquainted with its sound.
When I did sit down for some quality listening time, I was immediately struck by the A29’s effortless power and ability to drive difficult loads. This is an amp with plenty of power on tap, and it’s only too happy to deliver it when required. It’s loud, bold and explosive, but it’s not without delicacy and refinement. It’s also extremely neutral and transparent, allowing the character of the source components, source material and speakers to define the overall tonal quality of the system.
When I span the recent Bon Jovi remasters via the A29’s internal phono stage, I was presented with a warm tonal character as you might expect from a good vinyl source. The same was true for Queen’s ‘Studio Collection’ remasters and most other LPs. The phono stage is unusually quiet and hugely detailed, on a par with a very good external unit. It makes the best of poor pressings too, something I hadn’t expected given the neutral revealing nature of the amp. The fixed loading was perfect for an Audio-Technica AT150SA which likes to see a lower capacitance than most internal stages (and cheap external ones) provide.
The headphone stage is great too. It had no problem driving anything I had on hand with ease. While it can’t be expected to keep up with Arcam’s dedicated rHead amp, it’s more than the typical headphone socket and is a great inclusion.
Arcam’s A29 packs some clever tech, useful features and bags of power into a sleek, slimline chassis. There are no digital bells and whistles here – only pure analogue goodness with cracking headphone and phono stages onboard to boot. With the ability to seamlessly interface with Arcam’s rSeries components and more analogue inputs than many will ever need, the A29 can form the basis of a system that is as complex as you want it to be. It’s a lot of amp for what is – in hi-fi terms at least – little money. Highly recommended.
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