Review

Seismix 7 Complete Review - Australian Hi-Fi - Australia

01 Apr 1998

Designing and building a very high-quality subwoofer requires a leap of faith on the part of the manufacturer. The reason is that all speaker manufacturers know the only way to make a truly great subwoofer – one which offers extended bass response and very low distortion at high sound pressure levels – is to use a very large driver in an even larger cabinet. However, these self-same manufacturers also know, from their consumer research, that the very last thing the average consumer wants in the lounge room is a very large cabinet. So if the manufacturer goes ahead and builds a large subwoofer, it can take it for granted that it won’t sell too many of them.

Confronted with this marketing reality, most manufacturers put commercial practicalities ahead of design commonsense (and good engineering practice), and proceed to compromise the performance of their subwoofers by making both the cabinets and the drivers too small. However, a few manufacturers, (too few, unfortunately) place their trust and confidence in their customers, believing they will understand that in order to obtain superior performance at low frequencies, they are going to have to put up with a large cabinet in their lounge room. That trust and confidence is where that ‘leap of faith’ comes in.

With its Seismix 7, Krix has taken one of the biggest leaps of faith we’ve ever seen, because this subwoofer (attractive though it is!), is around half a metre wide, half a metre high and very nearly a metre long (768 x 495 x 520mm, to be precise). It weighs a not-inconsiderable 52kg. Even at the design stage, Krix knew this project was going to be a big ask on the part of its customers. A spokesperson for the Krix design team said: “Enclosures of a smaller size produced far too much distortion and (were) unacceptable. Multiple smaller drivers were tried, and a number of enclosure types, but a bass reflex enclosure with a single 15 inch driver was eventually used for its god distortion characteristics and the obvious low frequency enhancement.”

An advantage of using a very large bass reflex environment (around 198 litres!) is that it allows the use of a short, large-area vent, so the ‘impedance’ of the vent is very low, and there’s no chance of vent noise (either low frequency ‘chuffing’, or higher frequency ‘whistles’).

The bass driver, which has a diameter of 380mm, has a paper cone and uses not one, but two spiders (flexible circular suspension elements) to ensure that the cone excursion is linear, and that the very large-diameter (100mm) voice coil always stays properly centred in the gap. The bass driver’s magnet alone weighs 9kg. The driver’s basket adds only 1kg to this weight, because it's cast from aluminium. Krix says that this driver, which is rated for professional use, is able to handle 400 watts RMS. (Krix’s theory about bass response is: “To produce bass, you need to move lots of air, and to move lots of air, you need a large radiating area.” We’d agree, but note that we could hardly disagree with such a basic law of physics!)

Needless to say, the Seismix is an active design, and in this case the driving amplifier is a MOSFET design rated at 300 watts RMS ‘music power’. When questioned about the music power rating, Krix replied that if the Seismix 7’s power amplifier is tested in the manner one would a conventional amplifier, into a purely resistive 8-ohm load, it is able to produce 230 watts continuously. However Krix says such a measurement would be “misleading and inappropriate in this case, because the amplifier will only be working below 100 cycles and hardly into a resistive load. This results in less continuous drain on the power supply (the mains transformer to be more specific) allowing for greater transient performance.” To ensure continued protection for the bass driver’s voice-coil, and indeed for the amplifier itself, Krix has elected to incorporate into the Seismix 7 a number of protection circuits, amongst them clipping protection, input limiting, short-circuit protection and that final fail-safe, the humble fusible link.

Providing protection on a subwoofer is always a good idea (almost every manufacturer does), because most people tend to drive subwoofers extremely hard. The problem for the designer is that if you provide too much protection, or protection that acts too quickly, it’s easy to ruin the sound quality. Krix has provided the best of both worlds with its limiter, which is a very slow-acting RMS type. This type of limiter allows transients to slip through, but prevents continuous high-level signals of any type (including clipping). The result is that very loud signals, that will drive the amplifier into clipping, are permitted through the limiter, so long as the overall power level is low.

As the overall power level creeps upwards, the limiter starts operating, but will still permit transients to pass through. Krix has provided a ‘Limiter LED’ so users can monitor the status of the circuit. If the LED glows constantly, you’re overdriving the subwoofer. (But might we say that if you are overdriving a Seismix 7, your neighbours will probably warn you well before the LED does, either by calling the police or yelling at you to “turn that bloody thing down.” The Seismix 7 can produce around 100dB anywhere between 30Hz and 90Hz, which is seriously LOUD and will be more audible outside your home than it is in.)

Controls on the Seismix 7 include the inevitable volume control, a low-pass filter turnover control (35Hz-89Hz), a phase switch (0º / 180º) and a Filter Order switch. This last control is rather unusual, in that it allows users to select between 2nd order (12dB / octave) and 4th order (24dB / octave) filter slopes.

Aligning the Krix Seismix to your main speakers is largely a matter of juggling the volume, phase, low-pass turnover frequency and roll-off slope until everything is perfectly balanced. Best results are usually obtained by using a pink noise generator (or a pink noise test signal from a test CD) and a sound pressure level meter (SLM), but it is possible to adjust solely by ear, although this usually takes much longer. Even if you do set the controls using test signals and instruments, you will probably find you still have to do a little fine-tuning, using ordinary music (or movie) signals and your ears.

Input to the Krix can be via line-level or speaker-level signals. There’s a common auto-on sensor that switches the subwoofer on more-or-less instantly whenever it detects an audio signal coming down the line. When you have finished a session, the Krix will wait for around 18 minutes to see if you want to keep listening before switching itself into stand-by mode in order to conserve power. The subwoofer is fitted with a high-pass output that can be used to drive any power amplifier. It has a turnover frequency of 80Hz and a filter slope of 24dB per octave.

If any of the foregoing sounded like double-Dutch to you, you’ll be pleased to learn that Krix supplies a very useful and informative 12 page Operation Manual that will allow you to connect and tune the Seismix 7 even if you’ve had no experience with subwoofers at all. We would have liked the (black and white) manual to be of the same quality as the (colour) glossy brochure used to promote the Seismix 7, but we guess you can’t always have everything.

Listening Sessions

We first auditioned the Krix on its own. This is an excellent way of detecting port noises, distortion and overall speed. If a subwoofer can’t reproduce a fairly convincing bass sound on its own, it’s never going to be able to provide a convincing foundation for a full-range audio signal. The Krix sailed though this test, providing tuneful and engaging renditions from a variety of instruments that operate in this region, including pipe organ, drums, percussion, electric bass guitar, double-bass, and piano. Piano? To test a subwoofer? Yep, the lowest note on a standard piano is 27.5Hz and a few grand pianos go down to 16Hz. In fact, the lowest two octaves on a standard piano are all within the ambit of a subwoofer. That’s why it helps if the subwoofer is fast. Any quick boogie riff in the left hand will be reproduced solely by the subwoofer, and if it can’t keep up with the pianist’s fingers, well…(A good disc for testing bass is the Japan Audio Society’s ‘Super Bass Sound’ disc, CD-10.)

This out of the way, it was time to trial how well the Krix would integrate with standard hi-fi speakers. In the interests of being as comprehensive as possible, we trialed the Seismix 7s with both floorstanding speakers (both bass reflex and infinite baffle) and bookshelf loudspeakers (again, both bass reflex and infinite baffle). In all four cases, it was fairly easy (well, it is a little time-consuming) to get a good seamless match between the sub and the mains. Krix has obviously made all the variables work in the Seismix 7’s favour. (A really good test disc to use for this purpose is the ProSonus Studio Reference CD. This has 62 bands of test signals, but the one to use is the 88-note piano scale. If the sub is adjusted properly, you shouldn’t be able to hear where the scale changes from being reproduced by the sub to being reproduced by the main speakers.)

Listening to a complete system (using large floorstanding bass reflex speakers coupled to the Krix) revealed how important a subwoofer is, because the extra bass information (delivered effortlessly, we should say) turned what was already an enjoyable musical experience into a true musical event. Whenever we switched the subwoofer off, it was if the sound collapsed. For music purposes, the Krix proved to be exceptional, not because of the bass extension or even the maximum SPLs it was able to generate, but because of the super-low distortion. The bass was crystal clean. We had no doubt that the Seismix 7 is the lowest distortion subwoofer we’ve ever had the pleasure to audition.

Fired up in a full 5.1 home theatre system, using a Dolby Digital soundtrack from DVD as a source, the contribution of the Seismix was even more tangible, with the Seismix able to prove its worth with non-musical signals such as earthquake rumbles, elephant footsteps and other low frequency ‘FX’. (No wonder it picked up a gong for ‘Best Subwoofer’ in the 96-97 Sound & Image Awards!)

Conclusion

It isn’t too surprising that Krix has been able to develop a subwoofer like the Seismix 7. The company has, after all, been building subwoofers for use in city and suburban theatres for more than 20 years. If you want a subwoofer that goes loud, goes deep, will integrate well with almost any satellite speakers and has exceptionally low distortion, Krix’s Seismix 7 is most definitely the one to beat.

- Australian HiFi - Australia , Greg Borrowman

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