February 2009

XTZ Sub Amp 1 DSP
Upgrade your room and get to know your PEQ from your GEQ.
Review By Clive Meakins
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  In November 2008 I checked out the XTZ Room Analyzer, this proved to be a tremendously useful tool that that enables optimal subwoofer setup. The Room Analyzer helps you work around room problems that impact bass quality but it cannot totally fix these room problems. To fix your room you could remodel the shape and size of your room – somewhat impractical for most people; you could apply various room treatments; these are not always practical in a domestic environment either. Going down the DSP path to tailor the sound in a specific room is not uncommon nowadays. This can be done to great effect across the entire audio spectrum. When using digital sources I have no issue with this approach but I am not keen to digitize my vinyl, which in turn is played via the purity of a single-ended tube amplifier. As it happens my bass from around 100 Hz downwards is solid-state driven. Running DSP in parallel with a Single-Ended tube amplifier may at first seem incongruous but what is happening here is that I am running what I feel are optimal approaches for bass and mid/treble.

The XTZ Sub Amp 1 DSP is derived from the XTZ 99 W12 DSP subwoofer. The complete XTZ subwoofer uses a 12 inch bass driver in a reflex box along with the Sub Amp 1 DSP electronics. For this review I am using the Sub Amp 1 DSP with my Bastanis 18 inch dipole and 15 inch sealed box subwoofers. The Bastanis subwoofers are married with Bastanis Atlas Open Baffle loudspeakers click for review, which are 100dB/W/m efficient. These are driven via a 300B tube amplifier – the WE91 LadyDay from Diyhifisupply.

Some Of The Technical Details
The inclusion of DSP requires a gain setting stage between input switching and the DSP module itself.

Here is how the Sub Amp 1 DSP is configured: The DSP section includes analogue-to-digital conversion, DSP and digital-to-analog conversion. Input gain can be set in 0.5dB increments from 0dB to 18dB. There is a LED VU meter to help you set the gain correctly. The target is to have the loudest bass played at your maximum volume set to only very occasionally light up the red "clip" LED.

The DSP functions are where this all gets more interesting. There are low-cut and high-cut filters. These have slope options of 12dB and 24dB. The low-cut filter is variable between 10 Hz and 40 Hz, it releases the amplifier from uselessly amplifying very low bass that would only contain amplifier power-sapping noise. The high-cut filter is for the crossover to the main loudspeakers, again slope options are 12dB and 24dB, the frequency can be varied between 40 Hz and 250 Hz. There is also the option to add a time delay to the bass frequencies; this is useful for when you have positioned the subwoofers closer to the listening position than the main loudspeakers. Phase can be inverted and varied from 0 to 180 degrees; therefore there is full 360 degree phase control.

12dB slope (green) versus 24 dB slope (blue) – integration with my open baffles

Now For The Most Important DSP Functions
Graphic And Parametric Equalizers (GEQ and PEQ)
The GEQ provides control over 10 frequency bands from 16 to 125 Hz with a boost or cut of up to 12dB. Using the GEQ is the easiest way to tweak the character of your bass to your taste. The PEQ provides a more sophisticated way to alter frequencies that you need specify by entering the parameters of what you want to do. It is via the PEQ that you can accurately reverse the unwanted effects of Room Modes. Room Modes are resonant frequencies dictated by the dimensions of your room. Room Modes amplify certain frequencies making them overblown. Some music will sound too heavy in the bass; this effect will mask much of the music. Normally all you can do to reduce these effects is to improve speaker and listening positioning, in a room with severe problems you often need to reduce the overall amount of bass you are producing. This is "throwing the baby out with the bathwater", sure you reduce the bass problem but you end up with little bass. Other than using DSP you could tweak the bass characteristics of your room using objects placed in the room corners. DSP is far more accurate and does not require strange objects to be placed around the room.

PEQ frequencies can be selected to operate at a frequency of your choice between 16 Hz and 250 Hz. There are five PEQs so you can choose five different frequencies to work with. Along with frequency you must also choose a boost of up to 12dB or a cut of up to 24dB. Apart from the flexibility to pick your five PEQ frequencies and boost/cut amplitude you also pick your desired Q-factor.  The mystical Q-factor defines the breadth of frequencies the PEQ operates over either side of your chosen frequency. A high Q-factor number indicates a narrow frequency band; a low number sets a broad frequency band.

Setting Up
All the functions are selected and set via four buttons below the LCD panel. This is easy enough although there is an even better way. Hook up your Windows computer via an RS-232 cable to the Sub Amp 1 DSP. Having downloaded PC-Remote from the XTZ website you can now do all your setup via your computer, you can save your settings on your computer as well as saving presets on the Sub Amp 1 DSP itself. PC-Remote makes setup really easy but we've not finished yet! XTZ's party piece is to integrate their Room Analyzer with PC-Remote. You measure your room characteristics with the Room Analyzer and then simply click "Load"; room mode correctional settings are loaded into PC-Remote at the click of this button. Within the Room Analyzer you can even drag the measurement curves around to modify them to suit your taste. Note that the Room Analyzer has to be purchased separately from the Sub Amp 1 DSP.

If all this sounds quite a lot to get to grips with, well it possibly is but take it a step at a time and everything drops into place easily. The first area to work on is the traditional integration of the subwoofer with your main loudspeakers, getting the crossover frequency, slope and phase correct. When doing this you should also fine-tune the positioning of the subwoofers. You should get the setup working as well as you can before you use DSP. DSP is a tremendously powerful tool to really make your bass sing but is it best applied once you have done your best via conventional methods first.

Amplifier Impressions
For my first listening impressions I wanted to find out how the Sub Amp 1 DSP amplifier performed, could it drive my subwoofers cleanly and cope easily with a single amplifier driving a pair of subwoofers. Using the amplifier with both types Bastanis of subwoofers demonstrated to me that the XTZ amplifiers were at the very a least a match for the performance of the Bastanis Titan amplifiers. The toughest test was to have one amplifier driving two of the 18-inch dipoles, this the XTZs managed with ease. Using one XTZ amplifier per subwoofer produces a slightly tighter bass but the XTZs had no problem with the 4 Ohm load of two dipole subwoofers. The dipoles sap a lot of power so this was a tough test. If I were not using DSP then running one amplifier to power a pair of subwoofers would be worth considering. With DSP the results will be better when using a pair of amplifiers as left and right channels can then be individually optimized.

DSP Impressions
Time now to move onto fixing my room modes. My room is somewhat problematic in the bass region, the Room Analyzer did a great job detecting room modes and setting the Sub Amp 1 DSP to virtually eliminate these. Following this auto setup I tweaked the settings a little more to get the low bass really very flat. With a pair Bastanis Dipole subwoofers powered by individually setup Sub Amp 1 DSPs I managed to get bass from 30 Hz to 250 Hz flat to within +/-3db. This is a truly fantastic result for in-room bass frequency response. I could work more on the settings to do even better but for now I'm enjoying the music too much to invest the time.

Room Modes (green) versus DSP equalized result (blue)

Room Modes (green) versuss DSP equalized result (blue) – an alternative setting

Whilst I am describing the XTZ amplifiers I should put into context the two types of Bastanis subwoofers I partnered with the XTZ's for my review. I used a pair each of sealed box subwoofers with 15 inch drivers and non-symmetrical W-frame dipole subwoofers with 18 inch drivers. The sealed box subwoofers are a little more efficient and can more easily be tuned to produce a very powerful kick in the bass. The dipoles produce a serious thump too but are a little more refined sounding in that they are a little more detailed than the sealed box subwoofers. Integration is excellent with both bass approaches but is simply superb with the dipoles, the matched dispersion characteristics between bass and main baffle makes the transition from bass to mid totally seamless. With the sealed box bass units there is greater flexibility of room placement, whereas the dipoles need some room to breathe and are really best positioned with the main baffles. Some will prefer the slightly more impressive bass of the sealed boxes; the more detailed and sophisticated bass from the dipoles is my preference.

Back to DSP, this in not the first time I have used DSP on my subwoofers, the XTZs were taking the place of a very respectable studio DSP unit. The XTZs proved much easier to setup, at no point did I loose my way with the settings. With the studio unit I achieved good results but the lack of a decent automatic setup resulted in my settings being unnecessarily complex for the result I achieved. The XTZs on the other hand got my settings close to perfect out-of-the-box, this gave me a secure foundation on which I could tweak the settings to taste.

When you first hear DSP manipulated bass in a listening room that you know well, you have to recalibrate yourself. You will find that bass lines appear that in the past had only been hinted at. Some albums that you always thought were boomily recorded suddenly become clear and tight in the bass. You can hear the mid-range much more clearly as there is no bass overhang anymore. A couple of albums may not sound as good as they used to, as no more benefit do they from fortuitously juiced-up bass caused by room modes. Overall you will find yourself revelling in authoritative, detailed and textured bass. Integration of bass with mid-range should also be seamless. By so effectively controlling room modes it is likely that you will now be running the overall bass level higher than in the past. To keep room modes in check before-DSP I had to turn the bass down to the point where boom did not intrude. This is fine for the room mode frequencies but at other frequencies the level of bass was too low. Now that I have bass in-room to within +/-3dB I can hear the bass frequencies that I had previously been missing. Hence my earlier mention of the appearance of some previously missing bass lines.

Fundamental Impact
The effect is fundamental; if you want good deep bass you have to go the DSP route. If you are lucky enough to have a very large room with no bass issues then you may not need DSP but many listening rooms are like mine, which is 18 x 15 feet. I have room modes at 33, 58 and 77 Hz at levels of up to 12dB. As you can imagine a 12dB lift if one heck of a big room mode. I could run through a whole series of albums where I have noticed great changes but the same changes may not be reproduced in your room because you will have different room modes. You would likely find similar changes but not the same changes.

On rock I have found bass to be faster and more detailed as well as not clouding the mid-range. On Jazz the effect is to make the sound more natural and believable. I use Open Baffle loudspeakers with single-ended tube amplifiers; naturalness and believability are the raison d'etre for my setup. String and percussive bass is simply deeply impressive. Classical takes on an effortless scale. Vocals take on a believable richness that it is not possible for me to do without. The general effect is to very significantly improve bass impact, scale, detail, texture and timing. Integration with the main loudspeakers and the consequential mid-range improvement is also significant. This is not the sound of DSP but the sound of bass working properly in a typical listening room. Loudspeakers and subwoofers that go deep make a huge difference to a system. Using DSP to make the bass work as it should delivers at least as big an improvement again. Bass is the foundation to music, get it right and you will be hugely rewarded.

Blue Note Score
The scoring is a very tough to do as I am reviewing purely the electronics and software that form part of a subwoofer, I also have my room to take into account. It is difficult to imagine any other electronics and software doing a significantly better job. Maybe the future will see more than five PEQ functions to enable an in-room response of nearer +/-1dB. Whether in practice that would improve the sound is open to debate.

Specifications
Type: Subwoofer amplifier with DSP room correction
Features:
   10-Band Graphic Equalizer (16-125Hz)
   5-Band Parametric Equalizer
   Delay - 0-100 ms
   0 - 180 Phase and Phase Invert
   User Presets 9 presets for saving your favorite settings
   High Cut - (40-250Hz)
   Low Cut - Subsonic filter (10-40Hz)
PC-remote to control your settings on SUB AMP 1 DSP from your computer (RS-232)
Price: 550€ including VAT in the EU (approximately $565)

Company Information
XTZ
Sweden
E-mail: info@xtz.se 
Website: www.xtz.se