JDK Audio 8MX2

Adding a mixer to an eight-channel preamp may not sound revolutionary, but if it all fits in a 1U rack case it could be very useful indeed!

JDK Audio only came into existence fairly recently, but the company are a subsidiary of the more familiar API, who have been making high-end audio gear for about 45 years, and were acquired by ATI (Audio Toys Inc) back in 1999. The JDK product catalogue comprises three 500-series modules and four rackmount products, which are essentially single- and dual-channel versions of the same preamp, compressor, and four-band EQ, plus the eight-channel rack mixer reviewed here. Much of the design and technology comes from ATI's Paragon live sound consoles.

8MX2

The 8MX2 comprises eight preamps, each with an independent limiter, a separate eight-way stereo summing/monitoring mixer, and a dedicated monitoring section. All of this is squeezed into 1U of rack space. If this seems familiar, I'm not surprised; I reviewed the ATI 8MX2 in February 1997, when it had a bright-red front panel and slightly different panel layouts. The JDK version is essentially the same, but with a few updates and a green front panel.

With 18 inputs, 12 outputs, eight preamps, an 8:2 mixer and a monitor section, the front panel is inevitably pretty crowded. The dual-concentric knobs and compact buttons, though, make it surprisingly usable, even for those who take their gloves in size L! Critical functions such as phantom power and cue monitoring have LED indicators and most of the rotary controls have a detented action, to give the illusion of expensive controls! The internal mains power supply can be switched between 100-120 and 220-240 Volts AC on the rear panel. An On-Off miniature toggle switch and power LED are on the extreme right-hand side of the front panel.

The eight electronically balanced mic preamps are all equipped with direct balanced outputs via an AES59 connector (Tascam DB25 format), and this is intended for feeding a multitrack recorder or DAW interface. Another AES59 connector receives eight balanced unity-gain line input returns for monitoring and/or mixing. Each preamp section contains two dual-concentric sets of controls. The grey-capped set in the lower left corner set the preamp gain and the threshold of a protective (infinity-to-one ratio) VCA limiter. The inner/upper knob sets the gain (scaled from 0 to 65dB), while the outer/lower one adjusts the limiter threshold (+4 to +24dBu). Since the mic input can cope with signals up to +24dBu, no input pad is required, but push-buttons in the column to the left apply 48V phantom power and input polarity reversal. Sadly, there's no high-pass filter option.

On the rear panel, a toggle switch above each input XLR activates individual channel ground-lifts, which is a useful feature if you're working with stage splits, for example. However, activating the ground lift will inherently disable that channel's phantom power, because the return side of the supply through the ground connection is broken. Normally, though, in situations where a ground-lift might be helpful, phantom is usually being provided by another console anyway.

A view of the preamp internals. The fan and toroidal transformer are in the bottom left-hand corner, while the chunky, multi-voltage, regulated power supply is on the right at the back.

Each preamp channel can be assessed independently through the monitoring section by pressing Cue at the bottom of the column of buttons (multiple channel selections mix together in mono). The selected channel(s) can be auditioned on headphones or via the monitor outputs, and viewed on a mono bar-graph meter (see below). Unusually, activating the Cue button actually makes three separate audio signals available to the monitor section: both pre- and post-limiter feeds from the preamp, and a feed of the multitrack return signal — the relevant one being selected from the monitoring section. The channel limiter's gain-reduction control signal is sent and displayed as well.

Mixing & Monitoring

Every channel section also includes routing to the stereo-mix bus, controlled by the upper set of black-capped, dual-concentric controls, with the mix send level set by the inner/upper knob and the panning by the lower/outer knob (imposing roughly 4dB attenuation at the centre). A Mix button below the rotary controls routes the output to the mix bus, while the input can be selected between either the preamp send (post-limiter) or the multitrack return via the black Ret button at the top of the column on the left-hand side of each channels section.

The stereo-mix master controls on the right of the front panel are very simple, with another dual control adjusting the stereo balance on the lower/outer knob, and output level on the upper/inner knob. I don't really understand why anyone would want or need a balance control on the output, and the function might have been more useful (and safer!) on the monitoring section. The mix master level control range is from fully off, through unity at two o'clock, and up to +6dB of additional gain when fully open. The electronically balanced mix outputs are presented on a pair of quarter-inch TRS sockets on the rear panel.

Monitoring is via a front-panel headphone socket or rear-panel balanced line outputs on quarter-inch TRS sockets, and the level of both is controlled with a single red-capped knob. The manual carries a stark warning about only using 'professional high-impedance' headphones and not 'consumer low-impedance' phones, but nowhere can I find a definition of what the minimum acceptable impedance might be! The headphone drivers generated a very generous level into a pair of 600Ω AKG K240s quite happily, although when I switched to some 24Ω Sony MDR7509s the level was, as you might expect, significantly greater — but the headphone amps complained audibly, with obvious distortion at higher level settings. However, the level was plentiful before the onset of distortion.

The front and rear panels of the 8MX2.

The metering arrangements are somewhat unusual, as the left (orange LED) meter shows the selected channel limiter's gain reduction (0-20dB in 2dB increments), while the right meter shows either the mono channel Cue signal, or a mono sum of the stereo mix output (attenuated by 6dB). The meter scale spans -3 to +24dBu in 3dB steps, with six green LEDs at the bottom, two orange at +15dBu and +18dBu, and two red LEDs at the top.

The default monitoring signal is the stereo mix-bus output, but if a channel's Cue button is pressed, the monitoring switches to that source. Monitor section buttons labelled Pre and Ret select the relevant element of the channel signal path: with both buttons out, the preamp output (post-limiter) is auditioned. Pressing the Pre button switches over to the pre-limiter signal, while the Ret button selects the multitrack return signal instead. Another black button, Mix Ret, enables a dedicated stereo two-track return signal to be monitored instead.

If the two-track monitor return is not required, internal jumper links on the main circuit board can be moved to route the two-track return directly into the stereo mix bus (at a fixed unity gain) instead. This could be used for a reverb effects return, perhaps, or to accept the output of an external mixer, if required.

Talking of external mixers, the 8MX2 incorporates facilities for concatenating additional units to form a larger system. Two nine-pin D-sub sockets, Slave Out and Master In, send and receive the stereo mix bus and all the Cue-channel audio and control signals. Multiple 8MX2s can be linked together simply by connecting the Slave Out of one unit to the Master In of the next, and so on down the chain (appropriate linking cables are available). In this way, a 24- or 32-channel system can be constructed, with convenient monitoring controlled from the final unit in the chain, along with the stereo mix-bus output.

In Use

Operating the 8MX2 is logical enough, although care is needed initially, as labels are placed variously above, below or beside the associated control or button. Despite the small buttons and their close proximity to one another, I didn't have any trouble configuring and adjusting the preamp, mix and monitoring settings, and while the cooling fan generates a gentle whir with the 8MX2 on the bench, it becomes virtually inaudible when the unit is mounted in a rack. It certainly didn't trouble me at all.

Sonically, the preamps are very clean and quiet, with a wide-open bandwidth, and enormous headroom. I don't normally record with limiters, but they handled brief transients very transparently and were easy to set up. Comparing the 8MX2 with my own reference preamps (GML8304, ISA 428, and SSL Alpha VHD), I found the closest similarity of character with my Focusrite ISA 428. That's a little surprising, given the transformer front-end of that unit, but it shows the calibre of this design.

Conclusion

Like ATI's original, this relaunched 8MX2 is a versatile box of tricks that's ideal for high-quality location-recording rigs or adding recording channels in the studio. Being able to record channels individually at the same time as creating a stereo mix is very useful. I also liked the ability to check and audition the recorder returns, or to build a cue mix from the returns while mixing in live mics for latency-free monitoring. The 8MX2 can be used as a unity-gain summing box, of course, but for a simple home recording setup, the eight decent mic preamps and independent eight-channel stereo mix summing section — both of which can be expanded with additional units — combine to provide a very neat and versatile DAW front or back end. The technical performance and build quality are impressive, and the sonics are clean, quiet, effortless and transparent. What more could you want?  

Technicalities

Frequency-response plots showing two adjacent channels at minimum and maximum gains. At maximum gain, the bandwidth is slightly curtailed, but nothing that anyone would notice! At minimum gain, the bandwidth is slightly extended at both ends, but the gain mismatch between channels is obvious.The phase response of one channel, showing excellent linearity (within five degrees) between 25Hz and 10kHz.

A chunky linear power supply occupies the rear half of the chassis, with a large toroidal transformer close to the centre. Cooling air is drawn in through a small grille on the left, and extracted with a powerful fan. The audio circuitry is constructed mainly with surface-mount components, and appears to use multiple discrete transistors for the preamp front-ends. Analog Devices OP275 op-amps are used for return inputs, intermediate stages and mix buses, and SSM2142 balanced line drivers provide the outputs. The limiter is built around THAT 2181 VCAs in partnership with THAT 2252 RMS-level detectors.

The frequency response from mic input to multitrack output is flat within 1dB between 20Hz and 45kHz at full gain, and 8Hz and 80kHz at unity gain! The THD+N distortion figure is below 0.008 percent at a nominal +4dBu operating level, under 0.05 percent at +23dBu, and still below 0.06 percent at full gain with a -60dBu input. Clipping is +24dBu, even at the mic input, thanks to a proprietary, high-voltage, discrete preamp circuit design which runs on ±48V power rails. The preamp input impedance varies between 5.4kΩ and 6.4kΩ depending on gain setting, phantom power is spot-on at 48V (minimal load), and the equivalent input noise is a healthy -129dBu with a 150Ω source impedance (-132dBu with the input shorted).

The maximum preamp gain varied a little between channels, with discrepancies of up to 1.5dB in the worst case, and precise channel matching was hampered a little by the pot detents. The channel pan-pot provides about 4dB centre attenuation, but although the channel pan pots appear to have accurate centre detents, I found there was an imbalance between the stereo mix outputs of 1.5dB. This was correctable with the mix balance control, but suggests poor quality control somewhere in the system. Routing all preamp channels (at unity gain) or multitrack returns to the mix bus produced no significant increase in the ambient noise floor, which was consistently around -88dBu, giving a total dynamic range capability of 112dB.