When it comes to the operation and sound of this duo I could find absolutely nothing to criticise.
It is my personal view that the world has too many hi-fibrand-names already, so I am a bit peeved that a company with the street-cred of Parasound seems hell-bent on increasing that number. After more than 25 years in business as plain old ‘Parasound’, this Californian-based company seems to have decided to use different ‘branding’ to differentiate between its different ranges, so the top-line Parasound equipment will be known as ‘Halo’ and the second-string line as ‘NewClassic.’ And if that wasn’t enough, Parasound’s rack-mount models are known as its ‘ZCustom’ line.
But if you look closely at the Halo logo, you’ll see that Parasound is playing it safe by incorporating (in smaller type) the words ‘by Parasound’. Likewise, although the NewClassic line is advertised as ‘ParasoundNewClassic’, when you get to look at a product in the flesh, the front panel bears only the word ‘Parasound.’
What has not changed is that the famous John Curl is still a part of the design team at Parasound. If your memory needs a little jogging, you may recall that Curl became famous around the world as the designer of the original Mark Levinson JC-2 preamplifier, which at the time—and for a good number of years afterwards—was widely acclaimed as being the best pre-amplifier in the world. Curiously enough, Curl’s most recent preamplifi er design for Parasound recycles the famous JC-2 model number (the Parasound JC-2). Curl also designed the Halo by Parasound A21 power amplifier that is the subject of this review. (It’s my understanding that although he certainly played a significant part in the design of the P3, this preamplifier is ‘not all his own work.’ )
The Halo by Parasound A21 is rated at 250- watts per channel into 8-ohms and 400-watts into 4-ohms in two-channel mode. If you want even more power, you can bridge the two channels of the A21 and operate it as a mono amplifier, in which case it’s rated to deliver 750-watts into a single 8-ohm load. Although the circuitry is entirely solid-state, Curl has managed to avoid the ‘hard’ sound quality usually associated with high-power solid-state amplifiers by using metal oxide field effect transistors (MOSFETs) to drive four pairs of 60MHz/15-amp Sanken bipolar output transistors. This odd coupling completely removes odd-order harmonic distortion components while at the same time extending the amplifier’s frequency response to beyond 100kHz. Ahead of these, Curl uses matched pairs of discrete JFETS in a dual differential cascade configuration. This means very low noise and that circuit operation is in no way reliant on the output impedance of the source component. Rather than use blocking capacitors or trim controls to prevent unwanted direct current (d.c.) from appearing at the speaker terminals, Curl instead uses a fast d.c. servo. This enables speakers to move around their exact midpoint, rather than an arbitrary offset position. It also effectively immunises the circuit against the deleterious effects of age, so that unlike conventional designs that uses trim controls, which allow d.c. drift over periods of time, the A21’s d.c. offset will always be exactly 0V. There are three other layers of protection built in: relays, fuses, and current sensors. As with all high-end designs, Curl uses a linear power supply, at the heart of which is a massive 1kVA toroidal power transformer and four 22,000μF Elna electrolytics, each of which is bypassed by small-value polypropylene capacitors.