Harrison Flex 10

With the Flex 10, Harrison’s designers have delivered features not usually found in a desktop interface, including transformer saturation and real VU meters.

Music tech can be novel and exciting, but it can also be utilitarian. Audio interfaces usually fall under the latter umbrella, and that’s fine. We don’t want our audio interface to spring surprises or fuel creativity. We want it to funnel sound to and from our computer, with a minimum of fuss.

However, audio interfaces are often paired with other gear that does colour the sound in musical ways. In particular, engineers like to feed them from mic preamps with transformers, valves and other sources of non‑linearity. So if transformer‑balanced preamps are desirable, why not build them into the audio interface?

The obvious answer is because fancy preamps are expensive to make, and this is borne out by the sprinkling of existing interfaces that do feature interesting preamps. The likes of Heritage Audio’s i73 series, Focusrite’s ISA C8X and Neve’s 88M and 1073SPX‑D are, inevitably, more costly than typical ‘vanilla’ interfaces with the same number of inputs and outputs. For this reason, manufacturers such as SSL and Focusrite have equipped their lower‑cost interfaces with analogue circuits that mimic transformer saturation using more affordable components.

SSL, Neve and Focusrite all have a rich heritage of analogue preamp and console design, which they’ve successfully translated to the world of audio interfaces. Now, another renowned manufacturer of analogue consoles is attempting the same feat.

Going It Alone

For their first attempt to build an audio interface, Harrison partnered with Icon Pro Audio rather than develop everything themselves. The result was a desktop interface that featured two preamp channels derived from Harrison’s 32Classic console. The 32Ci was a relatively affordable product, in part because it used a transformerless version of Harrison’s preamp circuit. It was also a curious one. Icon’s quirky approach to the digital side of things delivered some unusual features, but presented its share of frustrations.

Two years on, Harrison have had another go at making their preamp tech available in an audio interface, and this time they’re not sharing the limelight. Superficially, the new Flex 10 embodies many characteristics that are common to desktop interfaces. It is a USB interface, housed in a wedge‑shaped case. At heart, it’s a stereo‑in, four‑out device, but adds ADAT Lightpipe I/O for digital expansion. And it provides a simple but effective direct monitoring arrangement that does not require the installation of a software control panel. But the Flex 10 is a little more expensive than your average desktop interface, and the main reasons for this are to do with its analogue circuitry. Specifically, the Flex 10’s two main input channels are anything but average.

In fact, the Flex 10 goes all‑in on providing an authentic Harrison analogue experience in an audio interface. The main input channels use a novel implementation of Harrison’s preamp circuit, which not only allows you to switch a transformer in and out of the signal path, but also to vary the degree of saturation independently of signal level. What’s more, both channels have a balanced, switchable insert point, and instead of offering a single combi socket, each provides an XLR input for mic signals plus separate quarter‑inch jacks for line‑level and instrument sources.

The Flex 10’s two analogue inputs provide separate XLR and jack sockets for mic and line‑level signals.

A continuous gain control on each channel is augmented by buttons for ‑20dB pad, polarity reversal, phantom power and line/mic input selection. The last of these is overridden automatically when anything is plugged into the front‑panel instrument socket.

Also present and correct is perhaps the most characteristic feature of the classic Harrison preamp design: the unique filter configuration, which allows the high‑ and low‑pass filters to be swept over a much wider range than is usual. The low‑pass can be screwed all the...