Review: Audient iD22 USB Audio Interface & ASP880 Microphone Preamp ADC

It’s hard to avoid the review clogging stream of specification features and statistics when compiling an overview of any new product. Audient’s Technical Director Tom Waterman takes AT beyond the numbers for a more detailed view of their design philosophy and implementation.

Tom: “The goal for the iD22 interface was simple: make it feel like a ‘mini Audient console experience’ in a small format. Make it affordable, easy to use and most importantly, make it sound great. Steve Flower, myself and David Dearden (the design legend) worked on the backbone for iD22.

“It builds around the Audient mic preamp; a design which has been optimised by David Dearden over the last 15 years, each iteration perfecting the PCB layout, grounding, noise performance and THD optimisation. The mic pre is fundamentally Class A (same as our console), with eight input transistors running as a discrete gain stage with low noise input pair. These are ‘linearised’ by wrapping the discrete stage up with an IC op-amp second stage in the feedback loop so they provide 60dB of clean and quiet gain.

“The mic pres and analogue stages of iD22 run on ±15VDC (similar to an API console for example) and provide plenty of headroom and professional output drive compared to a lot of 5V USB interfaces. To that end, the power supply design of iD22 is critical, with eight internal voltage rails provided regulated audio rails, AD/DA reference voltage sources and various low voltage rails for digital processing, etc — each section of iD22 is separated at the supply. Thus the audio rails are well isolated from dangerous switching digital power rail noise.

“We benchmarked against the usual mid/high-priced desktop interfaces (some German, some American if you can read between the lines) and decided upon a converter set after much testing and evaluation of various AD/DA chips to find the most natural sounding parts with excellent dynamic range specs. We settled on Burr Brown PCM4220 and PCM1798 (used in some very favourable hi-end mastering converters) and have optimised the ADC and DAC circuits to provide performance and sound quality we feel reflects the Audient sonic ethos (clean, open and sweet) and would stand up well against the competition.

“Clocking is performed locally by a high quality crystal oscillator and we use a two-stage Cirrus DPLL to regulate clocking and make sure the unit has stable stereo image. From there, we have a discrete headphone amp driver — very similar to the classic Neve BA640 op-amp idea; an op-amp with a current-boosting Class AB transistor output stage — and user-assignable functions for monitor control, directly on the hardware. 

“We use some very tried and tested analogue building blocks that hark back to David’s heritage in the ’70s and ’80s of the ‘Best of British audio’ era. But never seen in the public eye is our relentless optimisation stage which can last up to six months, where op-amps, capacitor types, resistor types are all auditioned and measured to find the optimum sound quality and (measured) performance for the target price. Ultimately the circuits are quite simple because they are distilled down and simple often sounds better. Design is all about implementation and not just parts choice.

“With the ASP880, we started with the same classic Audient mic pre, except it runs on ±18VDC just like the consoles; offering 2dB more headroom than the iD22. Eight channels of mic preamp in 1RU is definitely a squeeze, but we designed a completely new PSU which is actually lower noise than the old ASP008 supply, has virtually no mains hum component, and allowed us to remove the fan. Even running power from rear to front in a crammed 1RU case can be a bit dangerous from a noise point of view so there is no power switch. Truthfully, there’s no space on the rear or front panel even if we were inclined to compromise on this.

“The key is keeping the PSU noise as far out of the audio bandwidth as possible (in a switch mode type) and from there, critical layout, managing current loops and using appropriate inductive filtering to reduce any output noise is essential. The benefit to regulating current loops and keeping them as tight as possible, in David’s words, is to “separate the dirty current from the clean current” (grounds in particular) so that we can reduce any mains hum and other gremlins to almost nothing compared to a toroidal linear supply. The other benefit is that with no significant magnetic field in the enclosure, Channels 1-8 are pretty even noise-wise, whereas on the original ASP008, you might find that there is more induced hum in Channel 8 nearest the transformer. 

“Although specs matter to some degree, and so do all of these design decisions. When it comes to the crunch, whether a product is a killer unit or not, comes from the benefit it brings to either sound and/or workflow. If your ears say yes and the workflow is transparent so that creativity can happen, then that is most important and a great benchmark for a successful design.”