Audient ASP800

Less than a year after refreshing their eight-channel preamp design, Audient have released another. What makes this one different?

If you read last year’s review of the Audient ASP880 (http://sosm.ag/audientasp880) you’re probably wondering why the company are releasing another eight-channel 1U rackmount preamp. Actually, though, the ASP880 and the new ASP800 provide usefully dissimilar feature sets at different prices. Although the ASP880 was designed as a replacement for the company’s popular ASP008 (http://sosm.ag/apr06-asp008), it was also conceived to serve as an input expander for the iD22 audio interface/monitor controller (http://sosm.ag/audientid22). In much the same way, the ASP800 can be used stand-alone or as a cost-effective expander for the newer iD14 compact USB bus-powered interface.

The ASP800 costs around 20 percent less than the ASP880. The production cost has been lowered not by corner-cutting — it shares the same highly regarded class-A preamp circuit design as its elder sibling — but by employing a different power supply and by simplifying the facilities provided for most input channels. The first two ‘Retro’ channels, though, also offer something more, notably the HMX tube saturation emulator (used in some previous products) and a brand new ‘Iron’ processor, which introduces transformer saturation.

Design & Construction

The 1U rackmounting chassis extends 250mm behind the rack ears and weighs 4.5kg, which is a little shallower than its sibling but about 0.5kg heavier. That extra weight comes mostly from that linear power supply, which is less expensive to manufacture than its sibling’s elaborate SMPS design and employs a custom-made toroidal mains transformer, which is surprisingly compact and fully shielded. This last point is critical because a radiating magnetic field from the transformer can induce hum into nearby circuitry. In many designs (not this one!) it’s not unusual to find more hum in channel 8 than channel 1. The only downsides of the linear PSU are a little extra weight and the requirement for a mains AC voltage selector (100, 120, 220, or 230 Volts). As the PSU is passively cooled, there’s no fan noise.

Examination of the rear panel reveals some of the ASP800’s reduced facilities compared with the ASP880. The analogue outputs are provided on an AES59 (Tascam-standard) 25-pin D-sub socket again, but with ground-sensing (rather than fully symmetrical) balanced outputs. This ground-sensing design is borrowed from the ASP8024 console’s insert sends and works equally well with balanced or unbalanced destinations, without suffering from ground-loop hum. The ASP800 lacks the ASP880’s line-level insert points, which allowed external processing equipment to be patched in after the preamp but before the A-D converter.

Digital outputs from the converters are via dual ADAT ports. At base sample rates both ports provide all eight channels but S/MUX4 formatting is applied automatically when operating at double sample rates, with channels 1-4 transmitted by one port and 5-8 via the other. The ASP880’s AES and S/PDIF outputs are also absent, but an external word-clock sync reference can still be connected via a BNC socket (with switchable 75Ω termination). The A-D converter’s operating level can be switched between the EBU standard of 0dBFS = +18dBu and a lower +12dBu peak level. The latter matches the converter sensitivity to that of the iD14 audio interface (that device’s converters have a lower operating level due to the USB bus-powered design), while the +18dBu mode aligns it with the iD22.On the rear panel, you’ll find a switch to optimise the ASP800 for use alongside the company’s iD14 or iD22 audio interface — but it will, of course, work as an expander for any ADAT-equipped audio interface.

Preamp Performance

The combi-XLR input connectors cater for mic or line signals. The electronically balanced line inputs are padded down by 10dB and routed to the mic preamps. The ‘Retro’ channels also offer separate front-panel instrument inputs, which are routed through a high-impedance JFET buffer into the corresponding mic preamp circuitry. Each of the six basic preamps (channels 3-8) is provided only with illuminated buttons for phantom power and a 15dB pad, plus a rotary gain control. The adjustable high-pass filter, polarity invert and switchable impedance options of the ASP880 are all omitted to reduce costs. A side benefit of this is that, on each channel, Audient have been able to redeploy the half of the dual op amp which previously provided the high-pass filter gain stage as an additional gain stage for the preamp. By sharing the amplification load in this way an additional 10dB of gain has been realised (up to 70dB) and, more importantly, the gain-control has become more linear in its gain/rotation, with noticeably less ‘bunching’ at the loud end. This is welcome, and particularly so when matching channels for stereo use.

The Retro channels (1-2) also lack the high-pass filter and switchable input impedance of the ASP880, but otherwise enjoy a more comprehensive range of facilities. In addition to the push-buttons that activate phantom power and the 15dB pad, others provide polarity inversion and activate the HMX and Iron coloration circuits (of which more later). Rotary controls are provided to set the gain as well as to adjust the HMX and Iron drive levels. As mentioned earlier, the front-panel TS instrument input sockets are selected automatically when something’s plugged in and feature the same musical-sounding n-channel JFET class-A input buffer circuitry as the ASP880, presenting around a 1MΩ input impedance.

Signal levels are indicated by a pair of LEDs for each channel, with green showing for signals above –38dBFS, and red at -2dBFS. Although the digital converter is arranged to clip at +18dBu, the maximum analogue output level at the rear-panel DB25 connector is +22dBu (a little lower than the ASP880’s +27.5dBu). The difference in performance is mainly attributable to the symmetrical output format of the ASP880, although the two models also have different power rail arrangements. Whereas the latter’s SMPS provided ±18V rails for the audio circuitry (as well as +48V for phantom power and +5V for the digital electronics), the ASP800’s linear supply uses ±15V rails for the audio stages (hence slightly less headroom), +48V for phantom, 9V for the digital board, and +32V for the HMX and Iron saturation circuitry. The phantom power supply is rated to deliver the specified maximum of 10mA to each and every channel.

The first two channels each feature two sonic coloration options, HMX (a tube-emulation circuit) and Iron, which imparts transformer saturation. You really need to use your ears to judge these effects, but to help you understand what the different ‘flavours’ add to the sound and how they might interact with each other, Audient have developed a ‘tone triangle’ diagram.

The final front-panel control is a push-button to change the converter’s sample rate and clock source, cycling around 44.1, 48, 88.2, 96 kHz, and external. There’s no mains on/off switch on either the front or rear panels.

HMX & Iron

The ‘basic’ preamp is wonderfully clean sounding but the adjustable HMX and Iron facilities allow the user to introduce the kinds of musically beneficial distortions associated with valves (tubes) and transformers on the Retro channels. First introduced in Audient’s modular Black Series (http://sosm.ag/audientblackseries), the HMX facility came to popularity in the two-channel Mico preamp/converter (sadly, the latter has been discontinued). The Iron processor, on the other hand, is new. Although the front-panel layout implies that the HMX circuit precedes the Iron processor, the actual signal flow is the reverse — Audient tell me they made this decision based on listening tests.

The HMX circuitry comprises three cascaded, discrete MOSFET class-A gain stages, with an attenuator between each gain stage to maintain unity gain overall. The audio signal is also equalised with low-frequency pre-emphasis before the gain stages, and a corresponding de-emphasis afterwards — the effect being to encourage more harmonic distortion from the lower end of the source’s spectrum. Running on an elevated 32V single-sided power rail, this asymmetrical HMX ‘saturation’ circuitry generates copious amounts of harmonic distortion (predominately comprising the second, third and fourth harmonics) at up to about 2.5 percent. These harmonics are all strongly musically supportive intervals that enhance the source, rather than introducing any dissonance, and the effect is to make the source sound fatter, thicker and ‘larger than life’. In musical terms, these distortion components are an octave, an octave plus a fifth, and two octaves above the fundamental — the equivalent of pulling the 4’, 2-2/3’ and 2’ drawbars out a bit on a Hammond organ.

The HMX rotary control increases the drive across all three gain stages, simultaneously attenuating the final output level to maintain approximately unity gain overall. However, the effect of the pre-emphasis and de-emphasis equalisation also results in some small but important frequency response variations as the control is adjusted. At the ‘Sweet’ end of the scale, the harmonic distortion is audible but relatively modest, while the low end is lifted gently by around 1.5dB. At the ‘Thick’ end of the range, though, not only is there a great deal more harmonic distortion with some higher components, but there’s also a substantial LF peak in the response above a pronounced bass roll-off. This response combination resembles the characteristic ‘head bump’ associated with high-speed analogue tape machines and brings a noticeable warmth to the sound.

With all of the additional gain and attenuation involved in the HMX processing, it’s not surprising that the noise floor suffers. In this specific incarnation of the circuitry, introducing the HMX mode raises the noise floor by about 20dB. That sounds a lot but the unit’s natural noise floor is very low, and it remains quite acceptable in most circumstances when HMX is engaged. Moreover, the raised noise floor is smooth and benign, and it’s not dissimilar to that of a very good analogue tape machine or vintage valve preamplifier — just think of it as warm analogue dither!

Supplementing the HMX’s 1960s valve-emulation character is the brand-new Iron feature: a naturally complementary partner that invokes the character of typical British consoles from the 1970s. This clever circuit block uses two more high-voltage MOSFET gain stages as the driver and receiver around a miniature 600:600 Ohm transformer. The rotary drive control is a three-gang pot which simultaneously adjusts the driving impedance, the signal level into the transformer, and the secondary winding’s output damping. The minimum setting is labelled ‘Sparkle’ and the effect is predominantly to add a gentle ‘air’ EQ lift, with a modest amount of symmetrical harmonic distortion — mainly second, third and fifth. There’s also a modest rise in the noise floor of around 10dB in this mode. At the maximum setting the transformer saturates beautifully and the distortion levels build, mostly in the odd harmonics, introducing a distinctive rich ‘growly’ quality, just as the control’s labelling suggests! The noise floor appears not to suffer at all in this configuration.

Low-frequency sources, in particular, really benefit from the Iron stage’s harmonic enhancement, but there are a lot of other intricate things going on too, including more even-harmonic distortion from the MOSFET gain stages, complex phase shifts and odd-order harmonic distortion through the transformer, transient smearing (due to magnetic hysteresis in the transformer’s core), and some substantial frequency response shaping (due to inductive resonances in the transformer). The practical outcome of all this clever technology is that this Iron processing has an ability to add a delightful sense of three-dimensional depth and spaciousness, especially when processing a stereo mix. I’d like to see Audient putting the HMX and Iron circuitry in a high-quality mastering processor — they are that useful and effective!

Used in combination, the Iron and HMX processing can shape the source in myriad interesting ways, and the superbly informative but readable manual includes a very descriptive graph illustrating how the different processing effects change the sound character. Introducing the Iron and HMX tools moves the tonality away from Audient’s default ‘fast and clean’ character, and into areas with softer, slower or harder tonalities, and with levels of coloration ranging from very subtle right through to a really dirty growl! These are impressively creative and precisely controllable effects, and embellish an otherwise fairly ordinary multi-channel preamp with extraordinary capabilities.

Tech Specs & Tests

Dave Dearden’s celebrated mic preamp design has been with us for over 18 years now, and combines a sophisticated, transformerless, discrete eight-transistor class-A front-end driving an op amp gain stage which also provides the differential to single-ended conversion. This topology has a generous headroom margin, wide bandwidth and very low distortion, and in terms of tonal character it sits comfortably in the ‘fast and clean’ camp, faithfully passing on whatever the microphone delivers without imposing any overt character of its own.

As mentioned elsewhere, the circuitry has been slightly revised in the ASP800 to incorporate a second op amp which provides just under half the overall gain requirement, easing the load on the first op amp. This configuration provides 10dB more maximum gain and allows greater linearity in the gain control, but with the minor penalty of 0.5dB of extra input noise, which seems a very reasonable compromise. The published specifications show a very respectable equivalent input noise (EIN) of -127dB (with a 150Ω source and 20Hz to 20kHz measurement bandwidth), and THD around 0.005 percent.

A-D conversion in the ASP800 is provided by two quad-channel Burr-Brown PCM4204 chips, which are from the same family as the ASP880’s four, dual-channel PCM4220s, but deliver a very slightly lower level of performance. I measured an AES17 dynamic range figure of 112dB (A-wtd) which is about 4.5dB less than the ASP880, and consistent with the difference in Burr-Brown’s claims for the 4202 and 4220 converter chips. In the performance stakes, this figure ranks the ASP800 alongside products like SSL’s Alpha MX, Drawmer’s A2D2 and Millennia Media’s AD596 interface module. The THD+N figure was 0.004 percent (nearly all third harmonic), and the noise floor was around -108dBFS.

With the gain control at minimum and the pad engaged, a line input of +24dBu produced a digital output of -20dBFS, while at maximum gain the preamp could raise a line input of -60dBu to -20dBFS. Moving to the mic input, minimum gain with the pad engaged provided an output level of –10dBFS for a +24dBu input. With the gain at maximum the same output level required an input of -62dB — exactly 10dB lower than for the ASP880 under the same conditions. For the instrument inputs, an output level of -10dBFS was achieved with unbalanced signals ranging from +7dBu (min gain) to -62dBu (max gain). The input pad switch does not work on the instrument inputs. I measured an EIN figure for the mic preamp of -127.1dBu, the phantom power voltage remained sturdy under load, and the mic input impedance measured 2.3kΩ.

Lasting Impressions

Building on the strength of the ASP880, this new model maintains the quality expected of an Audient product but scales back the feature set to reduce the cost in a well-balanced way. It sounds exactly the same as the ASP880 in clean mode (clean, quiet, accurate, transparent) and has generous headroom margins, but the two Retro channels can get down and dirty when required, in very musical ways. Their instrument inputs and the Iron and HMX tone-shaping facilities are an impressive highlight. Not only do they allow sources to be enhanced during recording, they also enable complete mixes to be given a very analogue-style polish as a mix-bus process in a range of genres, or in home mastering applications — in effect allowing you to emulate the ‘stacking’ effect of big-console channels during production.

A significant influence on the design was the need to produce a well-priced channel expander for the company’s iD14 interface, and the ASP800 fulfils that role perfectly. While the technical performance doesn’t quite match the ASP880 in a few areas, it’s only a smidgen off and is unnoticeable in practice, while the additional mic gain and better control linearity will be noticed and appreciated.

This is an excellent multi-channel preamp/converter at an excellent price, and the new Iron emulator works perfectly with the HMX processor, promising great things for future Audient products.

Audient ASP800: Audio Examples

The ZIP file hosted on this page contains a number of audio examples of sources processed with the HMX and Iron circuits of the Audient ASP880 eight-channel preamp/ADAT expander. The results are obviously fairly subtle — the idea is that you can either use the colouration subtly when recording/mixing, or that you can ‘paint’ it in several layers, from recording, through mixing, master-bus processing and on to mastering. In this way, it’s possible to mimic some of the so-called ‘stacking’ effect of recording and mixing through an analogue console.