Designed and built in New Zealand, these three boxes (one pre-amplifier and two of the power amplifiers) look like nothing else out there, yet share the basic three box layout and feature set that’s preferred by the audiophile community. Stephen Dawson reports.

As you move from mid-range hi-fi to the top end in the high fidelity world, there are two clear trends. First, the feature set and basic layouts of the options tend to converge into near uniformity.  Second, the styling tends to diverge into many interesting and often attractive forms.

That’s certainly the case with Pure Audio control pre-amplifier and Reference Class A monoblock power amplifiers.

WHAT IT IS
A control pre-amplifier is the control centre for a system such as this. You can plug a number of different sources into it, select between them and adjust their volume. A power amplifier takes the output from a pre-amplifier and boosts the signal in such a way as to be able to drive a loudspeaker. That involves boosting the output voltage, typically from 1-2V to the tens of volts, and allowing plenty of current to be drawn, typically two or three amperes.

As you go into the high end, some of the features provided on lesser preamplifiers – tone controls and such – disappear and a more pure signal path is provided. The Pure Audio control pre-amplifier certainly does that.

At first glance it has only one control: a large volume knob on the right hand side of the front panel. This doubles as two controls: rotate it fully counter-clockwise and it switches the unit into standby/bypass mode. This is an unusual control. It isn’t a digital control, nor is it a standard potentiometer (aka continuously variable resistor). It is a 32 position rotating switch to select different levels of attenuation using high quality fixed-value resistors. As such it ought to be immune to the degradation – operation noise, usually, introduced by dust leaking into it – to which normal potentiometers are subject.

There are also three switches on the back: a main hard-wired power switch, a small one to switch the front panel light on and off (in one position the light is on all the time, in the other the light is only on when the unit is in standby mode), and a bypass switch, which we’ll return to later.

Notice anything missing here? There is no input selector. There are four line level inputs on the back – high quality RCA sockets are provided – but no obvious way to choose between them. In fact, switching is automatic. By default ‘Input 1’ is always used. But if a signal is detected at one of the other three inputs, then the unit instantly switches to that.

I will confess that I did not like this arrangement. The reason is when it comes to switching back to Input 1. This happens after the current input has ceased to provide a signal for thirty seconds. That can be quite a wait if you’ve an impatient temperament.

If you want to use a turntable you will need an external phono pre-amplifier (Pure Audio has a model of its own).

The bypass switch is to do with an additional input labelled ‘Processor Input’. Switch it to enable and plug in the stereo line-level ouputs from your home theatre receiver, and – this is important – turn the volume knob to the standby/bypass position, and the control pre-amp will pass the signal directly through to its outputs (from where it is fed to the power amplifiers) without any attenuation.

This is very clever indeed. It means that you can use this high end system to conveniently support your stereo speakers in home theatre mode without having to re-wire things on an ad hoc basis, and without surrendering the high end stereo performance of the sources plugged directly into the control unit.

Internally this uses a ‘dual mono’ design, which means that the left and right channel circuits are entirely separate, including even using separate power amplifiers.

The Reference Class A monoblock power amplifiers take this a further step by having the left and right channels in two entirely separate units. If you like you can plug them into different power outlets.

Stylistically they are similar to the control unit, lacking the volume knob. They are constructed of a thick (5mm) aluminium sheet folded into shape, but with much of its surface cut away. This is powder coated with a light silver grey. The cutaways are filled in with a fine stainless steel mesh allowing you to peer into the interior. This is closely enough woven to act as a faraday cage, protecting the interior from stray electrical fields.

The power amplifiers add full length, angled heatsinks with fins of the same thickness aluminium.

At the back of each are trigger connections, a single RCA line level input and two sets of output terminals. These are Cardas binding posts, in which a decent sized knob screws a clamp in to tightly grip both of leads at the same time. The documentation provided with the unit is minimal and there is no way to switch between one and the other output, so I can only assume that the two outputs are provided to facilitate bi-wiring. In any case, these are very easy to use and grip very tightly. They are best used with bare or tinned speaker cable or spade lugs, not banana plugs.

IN USE

The power amplifiers switch on an off automatically, according to the presence of a signal, or they switch on under the control a 12V signal.

The switch-off period is 30 minutes without a signal. The switch-on is very nearly instantaneous.

Aside from my complaint about the inability to switch quickly back to Input 1, the system worked extremely effectively and remarkably well in a practical sense. The control pre-amplifier comes with a kind of remote control. This is a largish wooden unit with just two buttons, volume up and volume down. And this is used only to ‘trim’ the volume. It has a total range of about ten decibels from maximum to minimum.

These units sounded absolutely magnificent. I approached this review with some trepidation, suspecting that I would be unable to hear any difference between this system and my own Marantz monoblock power amplifiers, but it turned out that these amps were sweeter with my speakers (VAF Research I-93 floorstanders), which present a very difficult load (they go below 3Ω at some frequencies). The stereo image was placed lower – height wise – with the Pure Audio system, while guitar plucking was panned across the sound stage in a more delicate and precise manner than my usual system.

The VAF speakers with their twin 200mm drivers go solidly down to 20Hz, so they can present a challenge to an amplifier’s bottom end control, but the Pure Audio amps exercised excellent control, delivering kick drums with clean punch and no overhang.

Only at the extremes of output did the Pure Audio amps fall behind my Marantz ones, which are rated at three times the 65W specification of the Pure Audio units. Even then the incipient stress was very subtle, while the volume level being produced was immense.

Class A is an amplifier design which avoids so-called ‘crossover distortion’, where Class B designs can introduce a small discontinuity in the signal as their output transistors switch from a negative to a positive state and vice versa. Class A/B designs use a DC bias to shift the zero point on the input waveform into either the positive or negative realm. Only at high volumes is there any switching between negative and positive.

Pure Class A, like these amplifiers, has the bias so high that they operate entirely within the positive or negative realm, and never cross from one to the other. There is no crossover distortion. The downside of this is that they use a lot of power. One of these, according to my power meter, used around 180W all the time that it was switched on, regardless of whether music was actually playing, and regardless of its volume level. That’s actually about thirty watts more than Pure Audio specifies, and about 30W more than the other one.

When they switch automatically to standby – half an hour after receiving no signal – this drops fairly rapidly to around 35W (both of them, this time), again about 30W more than specification.

CONCLUSION
The price of these units is high. If you’re after a bargain, then don’t bother looking into these units. But if money is no problem for you and you want fine amplifiers to drive very high quality loudspeakers, you’d do well to audition the Pure Audio control pre-amplifier and Pure Class A monoblock power amplifiers.