REVIEW: Pure Audio control pre-amp and Reference Class A monoblock power amplifiers
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-amplifi er 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 amplifi er takes the output from a pre-amplifi er 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-amplifi er (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 amplifi ers) 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.