Review

Seismix 1 mk1 Complete Review - Australian Hi-Fi - Australia

14 Sep 2007

'Bigger is better' is a motto that has stood the test of time. Although it's not always true, it's correct often enough that it remains the rule while the exceptions, remain...umm,...the exceptions.

It's certainly still true of subwoofers. Although it is now possible to build a subwoofer with small physical dimensions, you have to pull some pretty clever tricks with DSP equalisation and Class-D digital amplifiers to get it to work as it should. The rub is that if you then use exactly the same DSP and Class-D tricks in a larger subwoofer, with a larger bass driver, you will get better performance.

So I was a little baffled as to why Krix has pitted its tiny Seismix 1 mk1 up against the Seismix 3 mk3, with both models selling at exactly the same price, which ranges from $850 (in vinyl), through $1,000 (black and Atlantic Jarrah veneer) up to $1,100.00 for beech. Both models have exactly the same amplifier and the same driver. Both even sport Krix's new 'In Circuit Serial Programming' (ICSP) circuitry.

The Equipment

The Seismix 1 mk1's single front-firing paper coned driver fires forward as does its vent. (Yep, it's a bass reflex design.) Paper is a great cone material, as it's very light (much lighter than polypropylene, which is often used in smallish cone sizes) 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 has Thiele/Small diameter of 211mm, for a piston area (SD) of 350cm2. Krix says the Seismix 1 mk1's power amplifier will deliver 200-watts to the voice-coil. To ensure low power supply impedance - and thus greater dynamic capability - Krix has fitted multiple small capacitors for smoothing and storage rather than the two of four larger ones that are more often used in subwoofer amplifier power supplies.

Control over volume and crossover frequency is provided by standard rotary controls. As with the Seismix 3 mk3, they're marked with small white 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 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 will 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 for both volume and crossover controls.) The red-coloured LEDs that indicate the gain setting (high/low) positions 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. Despite my personal opinion that the control labeling is illogical, it won't affect the subwoofer's operation.

Like its larger stable mate, the Krix Seismix 1 mk1 has automatic on/off circuitry with a secondary standby mode that reduces power consumption to just three watts, so if you leave the subwoofer switched on permanently, it will add less than ten cents a week to your electricity bill. More important, this circuit activates lightning-fast but waits for a full 15 minutes before deciding to switch itself to standby. (Switch-off delay is very important if you connect a subwoofer to the LFE output on your receiver because this output can be switched 'on' and 'off' without your knowledge by whatever DVD is playing!). The subwoofer's power status is shown by a small blue LED on the amplifier plate on the rear panel.

Krix incorporates protection circuitry for both the bass driver and the internal power amplifier, all of which is provided by an Amtel Mega16 microcontroller. Soft-limiting and a fast-attack opto-compressor guard against amplifier output stage clipping and the cone being overdriven. The heatsink's temperature is monitored by a very clever circuit that doesn't switch the amplifier off if the plate gets too hot, but instead gradually reduces the output volume until the plate temperature reaches a safe level. 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 output of the amplifier, so the thermal sensor should never trigger. (It may, however, trigger if you build the subwoofer into an enclosure or put it somewhere without adequate ventilation.) Yet another circuit protects against d.c. at the speaker terminals, whether it's the result of an internal malfunction or a malfunction in 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 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 to muddy the sound quality.

Using microprocessors has allowed Krix to incorporate some neat features that are rarely found at this price point. For example, the input gain is switchable , phase can be switched using a pushbutton (rather than a switch) and there is an 'Earth Lift' switch. 'Lifting' the earth from the signal path is an 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 a typical subwoofer's 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 and the increased likelihood of one or more of the components having a switch-mode power supply. Krix has been able to incorporate all these neat features because rather than using an amplifier imported from England or China, the amplifiers used in Krix Seismix subwoofers are entirely designed and manufactured in South Australia.

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.

Whereas the Seismix 3 mk3 stands 415mm high, 360mm wide and 390mm deep (actually, 450mm if you allow for the grille on the front and the amplifier heatsink fins protruding from the rear), the Seismix 1 mk1 is just 357mm high, 311mm wide and 397mm deep including grille and heatsink. The other physical difference is the size of the bass reflex port, which of course had to be re-sized to reflect the smaller cabinet volume. The port on the Seismix 1 mk1 is 137mm long, with a 65mm internal diameter for most of its length, flaring to 85mm at the exit.

Listening Sessions

Rather than provide integral feet or threaded sockets for spikes, Krix instead supplies small stick-on rubber feet. The idea is to ensure the subwoofer is as low as possible, to make it as easy as possible to hide.

As a part of the process of evaluating the Seismix 1 mk1 I decided to treat it exactly the same way as I had the Seismix 3 mk3. I connected a CD player directly to its inputs and played a recording of a synthesiser playing back a repeated low 'E', during which time I slowly increased the subwoofer volume. Just as the larger Krix had this perfectly, so too did the Seismix 1 mk1: the volume just increased linearly until the protection cut in without any unwanted noises from the cone, port or voice-coil (i.e poling). Nor could I hear any distortion while I was increasing the volume. Some small changes in tonal quality became evident as the volume neared maximum, but I discounted these, since you'd never play music at such high levels - it was only because it was a single tone I was playing that I'd managed to get the volume so loud in the first place.

As you'd expect, pipe organ music is a very good test of a subwoofer's performance, so that was next up in my evaluation process, this time using an unusual DVD-A organ recording I picked up in Japan but was actually recorded in Germany, at the Basilika St. Johann, in Saarbrucken, where Bernhard Leonardy is Kantor. On this disc Leonardy plays some old favourites (Bach's Toccata and Fugue in D-minor) and some less well-known ones (Leon Boellmann's Suite Gothique). It's extremely handy for evaluating subwoofers because it has both a 48kHz 24-bit PCM five channel mix and a stereo PCM downmix, so you're always getting the full frequency range to all channels, and not a separated band-limited 0.1 channel.

Despite its diminutive size, the Krix Seismix 1 mk1 was able to generate very high sound pressure levels in my room. The quality of sound was very good, with no evident doubling even at high levels, and I could hear that the bass dug very deep. Did the bass dig as deep as the Seismix 3 mk3? I'd say it does, but by comparison is a bit shy on level at these low frequencies, but the differences are so slight that it's unlikely you'd ever notice in anything other than a direct A-B comparison...and even then you'd have to have your wits about you.

In common with the Seismix 3 mk3, the Seismix 1 mk1 is fast and nimble between around 40Hz and 150Hz where it will do most of its work, as this is where most of the bass energy is concentrated, irrespective of whether you're listening to music (via CDs or DVD soundtracks) or movie sound effects. When comparing different subwoofers (even the one Seismix against the other!) you should listen to make sure that (for example) you can easily follow the tune or riff of a bass guitarist is playing. Can you hear each note distinct from the next? If two consecutive notes are of a different pitch, is the difference in pitch clearly audible? If the two are the same pitch, is this instantly obvious, and is the tonal quality exactly the same? Certainly, when listening to the Seismix 1 mk1, I could answer all these questions in the affirmative.

Conclusion

It's inevitable the Seismix 1 mk1 will be compared with the Seismix 3 mk3, and my guess is that the decision as to which you'll buy will depend mostly on whether you're a 'glass half empty' person, or a 'glass half full' type, as well as on the size of your listening room. It's my opinion that the Seismix 3 mk3 would be the better choice in larger rooms, particularly if you're comfortable with a larger (but by no means big!) subwoofer. But in smaller rooms, I think the performance of the Seismix 1 mk1 would be all but indistinguishable from that of the larger model, particularly if you have the opportunity to extract maximum performance by positioning it in a corner. Either way, you can be assured of getting top performance for your dollar.

- Australian HiFi - Australia , Greg Borrowman

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