Review

Seismix 3 mk3 Complete Review - Australian Hi-Fi - Australia

01 Mar 2007

Krix subwoofers have become smarter. The added ‘intelligence’ has been built in thanks to a Krix innovation it calls ‘In Circuit Serial Programming’ (ICSP) that now allows Krix to easily upgrade the Seismix 3 mk3 (and all other Krix subs with ICSP) to incorporate new features as they become available. This is possible because all crucial circuit operations are now controlled using digital circuitry that is entirely designed and manufactured in South Australia.

The Equipment

And the Seismix 3 mk3 has a lot of intelligence built in. The automatic on/off circuitry now has a secondary standby mode that reduces power consumption to just three watts, so even if you left the subwoofer switched on permanently, it would add less than ten cents a week to your household’s electricity bill. More important, this circuit activates lightning fast but waits for a full 15 minutes before deciding to switch itself to stand-by. Switch-off delay is very important if you’re using the LFE output on your receiver because this output can be switched ‘on’ and ‘off’ without your knowledge by whatever DVD is playing! Power status is shown by a small blue LED on the amplifier plate on the rear panel.

There is also a lot of protection circuitry built in for the bass driver and the amplifier, all of which is provided by an Atmel Mega16 microcontroller. Soft limiting and a fast attack opto-compressor guard against amplifier output stage clipping and cone over-drive. The heatsink is monitored by a very clever circuit that doesn’t just switch the amplifier off if the plate gets too hot, but instead gradually reduces the output volume at 3dB per minute until the temperature falls below 65ºC. It will then attempt to return the volume to its original level. Under normal usage conditions, the heatsink is large enough to dissipate the full 400 watt output of the amplifier, so the thermal sensor won’t trigger. It’s mainly intended to guard against situations where the subwoofer might be built into a wall or enclosure without the builder providing adequate ventilation.

Another circuit protects against d.c., whether it’s the result of an internal malfunction or a malfunction of a component connected to the subwoofer. The final layer of protection comes in the shape of a fixed second-order high-pass filter that rolls off the subwoofer’s frequency response below 30Hz. This limits driver excursion at sub-audible (infrasonic) frequencies (that is, below 20Hz) which keeps the driver’s magnet cooler, and stops such frequencies from intermodulating with higher frequencies and muddying the low frequency sound quality.

Using microprocessors has allowed Krix to incorporate some neat features. Input gain is now switchable (between ‘low’ and ‘high’), phase can be switched using a simple push button, and there is now an ‘Earth Lift’ switch. ‘Lifting’ the earth from the signal path is the most effective way to prevent mains hum (to which most subwoofers are, unfortunately, prone) because the most audible mains hum frequencies (50Hz and 100Hz) fall right in the middle of the typical subwoofer pass band. Home theatre systems are more vulnerable to mains hum loops than ordinary home hi-fi systems because of the number and diversity of the components in a home theatre system.

The Seismix 3 mk3 uses a single front firing paper-coned driver in a bass-reflex cabinet in which the vent also fires forward. This is excellent design, firstly because paper is such a wonderful cone material, as it’s very light (much lighter than polypropylene, which is often used) and very rigid. It also has more internal damping than almost any other material used to form speaker cones. Rated with a diameter of 255mm the cone’s Thiele/Small diameter is 211mm, for a piston area (SD) of 350cm². The cone has a soft rubber surround and a rather large (90mm diameter) dust-cap. The cone is driven by an amplifier Krix says delivers 200 watts to the voice coil.

The amplifier’s power supply uses multiple small capacitors for smoothing and storage rather than the two or four ones that are often used in subwoofer amplifier power supplies. This ensures a far lower supply impedance, enabling a greater dynamic capability from the amplifier output stages. The bass reflex vent, incidentally, is huge! It’s 90mm in diameter for most of its 280mm length, but flares to 120mm at both ends. Such a large volume of air is unlikely to chuff!

Control over volume and crossover frequency is provided by standard rotary controls. These are marked with ‘dots’ rather than numeric calibrations, so in the case of the crossover control, one dot indicates a 60Hz turnover (minimum), and three dots a 150Hz turnover (maximum). Somewhat confusingly, the ‘stop’ and ‘start’ points of the rotary controls are neither identical nor conventional. The volume control’s minimum position is at 11 o’clock, and its maximum is at 7 o’clock, while the crossover control’s minimum setting is at 5 o’clock, and its maximum at 2 o’clock. This means you’ll need to pay close attention when setting the controls initially, because if you look only at the pointer on the knob, and not at the dot calibrations, you might think the volume is set at minimum, when in actuality, it’s at maximum! (To its credit, Krix itself warns about this on the very first page of its excellent instruction manual, even providing an illustration – Figure 1 – showing the proper initial setting of both the volume and crossover controls.) The red-coloured LEDs that indicate the gain setting (high/low) are also illogical, because setting the circuit for lowest gain results in the higher of the two LEDs glowing, and setting the circuit for high gain activates the lower of the two LEDs. Illogical thought it is, it’s harmless enough.

Line-level input is via RCA sockets, with the left input doubling as the LFE input (if only a single connection is available). You also have the option of using speaker level inputs. Speaker outputs are also provided, but this is a ‘thru’ function: there is no filter between the speaker inputs and outputs. The speaker terminals are large, multi-way, gold plated and banana capable.

Krix provides a standard IEC mains power socket (with integral power switch and fuse) rather than a permanently fixed mains lead. This is my preferred arrangement, since it means you can choose a mains lead of your preferred length and colour. (If you like the standard black 1.5 metre cable, you’re in luck, because Krix supplies one of these for free!)

Recognising that many people hide their subwoofers, so the exterior finish is of no importance to them, Krix provides a basic vinyl-clad (though still very presentable!) version of the Seismix 3 mk3 for just $850. Krix’s classic wood finishes (Black Ash and Atlantic Jarrah) work out a tad more expensive ($1,000), while there’s a further $100 premium to pay if you want to step up to the classiest Light Beech finish. The Seismix stands 415mm high, and is 360mm wide. Krix’s specification says the subwoofer is 390mm deep, but this is the dimension of the cabinet itself. If you allow for the grille on the front and the amplifier heatsink fins protruding from the rear, the depth increases to 450mm.

Listening Sessions

I was a little surprised to find the bottom of the cabinet had no feet, nor even threaded sockets for spikes. Krix instead supplies small stick-on rubber feet for those who want to protect the veneer (or the vinyl!). The rationale is, apparently, that because most people build their subwoofers into recesses, or hide them under large pieces of furniture, the lower the cabinet profile, the better. If you’re tempted to place the slick bottom surface on a slippery carpet, keep an eye out that the Seismix 3 mk3 doesn’t ‘creep’ along the carpet while it’s playing: that forward mounted cone generates quite a bit of thrust!

As luck would have it, I got to try out the Earth Lift button right away, because an earth loop in my home theatre system resulted in a small buzz coming from the subwoofer. Pressing the earth lift button eliminated the buzz instantly and completely: so much so that I couldn’t hear it even with the subwoofer’s volume control at maximum. In my book the Earth Lift is a great idea: every powered sub should have one. If you’d like to try this in a store to see how effectively it works, ask the salesperson to disconnect all the wires from the Seismix (except the mains, of course!) and then turn the volume and crossover controls to maximum. Now push your finger up against the LFE input. You’ll hear a buzzing sound from the subwoofer. Press the Earth Lift button and the buzzing will disappear. You can’t fail to be impressed. Non-technical readers should note that you should never attempt to eliminate an earth loop by cutting off the earth pin of the 240V plug. This is an unsafe and dangerous practice. Despite its name, Krix’s Earth Lift button does not actually disconnect the earth from the subwoofer, it breaks the link between signal ground and earth, which is not the same thing at all.

Before I set up the system, I connected a CD player directly to the inputs of the Seismix 3 mk3 and played back a recording of a synthesiser playing back a repeated ‘E’, and slowly increased the volume on the subwoofer. The Krix handled it perfectly: the volume just increased linearly until the protection cut in. There were no untoward noises, no voice-coil poling, and no distortion audible from the sub. That said, I noticed small changes in tonal quality of the note as the volume neared maximum, but it wasn’t until the volume was incredibly loud: you would never play music at such high levels.

Hooked into a home theatre system configured as a two-channel sub/sat system, I played a DVD-A of organ music and was amazed not only by the high sound pressure levels Krix’s Seismix 3 mk3 was able to generate, but also by the quality and depth of the bass. Few people will ever require the lowest notes the Krix can happily reproduce, but if they do, you can be sure the Krix will deliver them. More importantly, the Krix is very fast and nimble where it will do most of its work, which is delivering music between B0 and E3. Here the Sesimix 3 mk3 was outstanding: I could hear the pitch of every note precisely and the cone was so ‘fast’ that machine-gun-like drum sounds were reproduced with uncanny accuracy. The attack was instantaneous and so was the recovery, so the sub was always instantly ready for the next sound.

Conclusion

The usual rule is that a subwoofer has either to be large or expensive – or both – in order to deliver great-sounding bass. Krix’s Seismix 3 mk3 is an exception, because it’s neither large nor expensive, yet still manages to deliver great-sounding, powerful bass.

- Australian HiFi - Australia , Greg Borrowman

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