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VAF’s new DC-X Gene 12  Silicon Chip siliconchip.com.au VAF Research released their original DC-X loudspeaker in 1997. It offered high sensitivity, smooth frequency and phase responses, and exceptional time domain performance. This was all delivered by a speaker with a very simple crossover network – just a capacitor to feed the tweeters. In this edited article Philip Vafiadis and Simon Wilde explain the design philosophy behind the latest version of the much improved DC-X Generation IV and present construction details for the new speaker kit. For more detail, the full unedited article can be found at www.vaf.com.au. W hen a conventional speaker drivers was directed into a long different to that measured at another is fed with a signal, its tapered lossy transmission line to point. The content of all music and movies drivers radiate sound into dissipate it, eliminating any need for is very dynamic. Even a poor recording a room to be heard by a listener. compensation. will have a dynamic range of at least But much more is happening in this Emphasis on accuracy 50dB. That’s a power ratio of 100,000 transaction. VAF believes the purpose of a loud- from the quietest to the loudest pasEnergy is continually being stored and released by various resonances speaker is to reproduce the signal it sages. The output of all loudspeakers within the speaker system or between is supplied and not to add its own character in the process. This seems will compress to some extent as their its key elements. input power is increased. If the high Consider a conventional speaker. simple but what does it mean? Even frequency response measure- frequencies compress at a different rate Sound radiates from the front of the driver and is heard by the listener. ment is fraught with irrelevance or to the low frequencies then frequency responses measured at different input Sound also radiates from the rear of error. It seems almost self-evident that a powers will also reflect this change. the driver into the cabinet. flat frequency response is ideal but Even just considering this on-axis This rear energy is reflected off the internal surfaces of the cabinet again, what does this mean? Many frequency response we can see that back to the driver and some of it will speakers are measured only at one me- it can be delivered in any number of ways to yield any number of be transmitted through the outcomes. driver’s cone to be heard by SPECIFICATIONS Equally, there are many the listener a moment after Nominal impedance: ....... 8Ω (4.9Ω min; 14.2Ω max) other measurements that can the original sound. Further Sensitivity: ...................... 95dB/watt (2.83V) be used to characterise the reflections inside the cabinet Power rating: .................. 10W to 200W performance of a speaker and will occur until all the energy equally these are subject to is dissipated. Frequency response: ....... 35Hz-18kHz +/-2dB (-3db <at> 32Hz) great interpretation. This stored energy is reDimensions: .................... 1195h x 250w x 350d (mm) The following factors, leased at many different Weight: ........................... 32Kg each together with the internal moments in time after the THD: ................................ 0.31% cabinet reflections described original (direct) sound has projected from the drivers’ diaphragms. So in reality a conventional speaker system’s output is a blend of direct and delayed signals. A conventional crossover network can tailor the system’s output but cannot significantly compensate for Stored Energy. In the original VAF DC-X loudspeaker, the rear energy from the tre and on the tweeter’s axis, whereas most listeners sit at around three to four metres from their speakers and may or may not be aligned to their tweeters’ axes. Furthermore, most speakers have two or more drivers spaced some distance apart on the front baffle. As a consequence, the response yielded at one measurement distance must be above, are the fundamental set of issues that contribute to Stored Energy in loudspeakers. Even speaker types like electrostatic, ribbon, piezo electric and wide-range horns are all subject to the following factors. 1: Diffraction Sound that radiates across the enclosure’s front baffle diffracts off the driver edges, cabinet edges, nearby furniture etc. Conse- eration IV loudspeaker siliconchip.com.au August 2004  13 Here’s the full kit as you would receive it from VAF, including enclosure, drivers, crossovers and the all important foam pieces. (OK, it’s half the kit – for one enclosure. Sorry!). Each of the items is detailed in the parts list. quently, the diffracted sound will be delivered to the listener’s ears at a succession of latter times than the direct sound. 2: Propagation through a driver’s diaphragm At low frequencies, where the loudspeaker’s cone is small with respect to the wavelength of sound being reproduced, it tends to act as a piston and behave as a point sound source. At higher frequencies where the dimensions of the driver’s diaphragm is large with respect to the wavelength of sound being reproduced, it will not act as a piston . While some energy will radiate forward into the air, some will propagate radially outward through the diaphragm to the roll surround which should effectively damp the energy. 3: Driver & Cabinet resonances All loudspeakers have resonances and this will cause them to deliver sound for some time after they are excited. Simi14  Silicon Chip larly, no speaker cabinet can be made infinitely rigid or perfectly damped. 4: Electrical damping Complex electrical crossovers can compromise the ability of the amplifier to provide electrical damping particularly if the DISTORTION Fundamental Frequency 60 120 240 480 960 1920 3840 7680 THD (%) 0.48 0.34 0.31 0.31 0.25 0.40 0.25 0.19 These very impressive THD specs show the linearity of the drivers and the design. Figures of less than 1% across the range are very unusual in speakers. inductors have significant resistance. The higher the electrical Q of the filter, the worse the problem will be. 5: Time alignment If the distance from a listening position to each of the drivers differs, the listener will receive the sound from each driver at different moments in time. This problem is compounded by the fact that most drivers do not radiate from just one fixed point at all the frequencies that they reproduce. It is compounded again as some frequencies are the combined output of two or more drivers. The result of poor time alignment is poor overall impulse, transient and phase responses. Solutions VAF’s Generation IV DC-X loudspeaker addresses these issues with the following aims: • Flat Frequency Response target siliconchip.com.au Fig 1: unsmoothed Frequency response 30Hz-20KHz. Although not shown here it is worthy of note that the flatness of this response is maintained over almost all of the frequency range at sound pressure levels as high as 110dB! of ±2dB over most of the frequency range. • Flat Phase Response of better than ±50° over most frequencies above bass resonance, and almost flat phase through the critical mid range (Fig 3). Speakers with excellent phase performance create a more tangible three dimensional sonic representation, ie, it sounds more real. • Deep Bass: To avoid the need for a subwoofer in Surround Sound Theatre systems, low distortion bass down to 30Hz and audible output to around 25Hz was the aim. The new DC-X produces cleaner and more powerful bass than many dedicated subwoofers. • High efficiency and 110dB dynamic range with low compression. Many Surround Sound receivers will not drive all channels to rated capacity simultaneously. So target efficiency was 95dB SPL at 1 one meter • Low Distortion: Less than 1% at most frequencies at up to 100dB. As well as using low distortion drivers each woofer is in its own separate enclosure which is proportioned differently to yield differing impedance responses. This reduces cabinet related distortion levels significantly. • Wide Power Response (a measure of the total output of a speaker in all directions). In the new DC-X two tweeters are aimed in slightly different directions to broaden the high frequency dispersion. Both tweeters are time aligned with the woofers so there are minimal offaxis response lobes and almost double the off-axis energy of the original DC-X above 5kHz. • Minimum stored energy. Characterised by the cumulative spectral decay measurement. The New DC-X uses many mechanisms to minimise stored energy including the extreme efforts made to control diffraction around the tweeters. Meeting the challenge You can see from the accompany- Fig 3: impedance response lower bass driver and tweeters. siliconchip.com.au Fig 2: here is an extreme close up of the unsmoothed Frequency response. This type of unsmoothed raw data is almost never published. While a remarkable +/-2dB is achieved, under the more common 1/3 octave smoothing this plot would resemble a straight line! ing Step Function (Fig.7), Impulse Response (Fig.8), Phase Response (Fig.9), Cumulative Spectral Decay (Fig.6) and Distortion Measurements that the new DC-X loudspeakers are very accurate indeed. The new model DC-X is basically a product of the following building blocks. Woofers: Each enclosure uses two 210mm woofers with Composite Black Soft Kevlar cones. They feature T-shaped pole pieces and large, high temperature voice coils. The voice coil and spider are both is vented to provide dramatically improved voice coil cooling which significantly reduces thermally related compression. This venting facilitates exceptional dynamic linearity as does the linear excursion of 9.5mm and a total excursion of over 22mm. The Kevlar cones are fitted with a number of proprietary pads which provide further damping of propagation of sound through the cone itself which cone is terminated in Fig 4: impedance response upper bass driver and tweeters. August 2004  15 Fig 5: combined impedance of bass drivers and tweeters. Note that the impedance peaks around bass resonance are now effectively controlled ensuring greater power delivery from almost any amplifier that drives them. Also note how the minor variations in the individual woofer responses have been damped Fig 6. Cumulative Spectral Decay, unsmoothed, unfiltered. Note the very rapid early decay being around 15dB down in under 0.2mS! a very compliant high loss soft rubber ing to each woofer from the cabinet is the woofers is negligible, to ensure surround. damped through the other’s voice coil. providing maximum ideal electrical Tweeters: The 25mm dome tweeters This more than halves cabinet-related damping from the amplifier. have ferro-fluid damping, rare-earth distortions and is critical to the DC-X magnets and heatsinks for overall cool- system’s operation as an accurate low The speaker kits. . . ing. A shallow horn is fitted in front distortion transducer. DC-X GEN-IV kits are only available of the dome, shaped to provide a flat Crossover: The DC-X electronic com- directly from VAF Research and are overall response. provided with all parts required Cabinet: This has numerous “The sound quality is quite exceptional, with very to complete their construction. internal angled surfaces and smooth and wide treble dispersion and extended bass. A three year guarantee is standthe geometry is such that Combine that with very high efficiency (for a wide range ard on all VAF speakers even the location of the woofers, speaker) and you have a pair of remarkable speakers.” when bought as kits. (Leo Simpson) ports and cabinet boundaThe cabinets supplied are ries minimise transmission finished with a high quality line effects. ponents consist of high quality 5% two-pack satin lacquer over the timber Carefully positioned pieces of VAF tolerance metallised polypropylene veneer. Hypersoft III foam are used to attenu- capacitors, close tolerance high power Various options are available from ate rear radiation from the woofers. non inductive resistors and 1% toler- a build-it-yourself flatpack, through Each woofer in its separate enclosure ance resin bound air cored inductors. pre-built enclosures and, if you don’t is electrically connected in parallel. The combined DC resistance of all have the time nor inclination to do it The small amount of energy return- wire and components in series with yourself, completely finished enclo- The VAF W200FR1 composite Kevlar woofer has unique venting behind the suspension and venting through the top of the voice coil. The two venting techniques provide more air flow around the coil, therefore better heat dissipation. 16  Silicon Chip The VAF T25DTH1 tweeter utilises a Neodymium magnet with a heat sink to make this a very low compression/high power handling driver. siliconchip.com.au Fig 6. Note the unusual horizontal offset of the tweeters to increase dispersion and complex acoustic treatment to control diffraction. Fig 7. The near perfect Step function, again unfiltered. Only time aligned, phase coherent speakers are able to produce a step function like this. sures ready to plug in and enjoy. A kit without any enclosures is also available for those who wish to build their own from scratch. For constructors who intend using the DC-X loudspeakers in a home theatre system, VAF offers a range of centre channel and rear channel speakers with similar features to the DC-X. Contact VAF for details. Parts List – VAF DC-X Generation IV Speakers 2 fully assembled and pre-finished veneered MDF speaker enclosures, (or build your own) 4 VAF W200FR1 200mm woofers 4VAF T25DTH1 dome tweeters 2 bi-wire gold-plated terminal panels with wiring looms and circuitry attached 20 pieces Hypersoft III foam (two parts A,B,E & G and four parts C, D and F) 4 male/femal grille clips 2 pieces 300 x 1200mm grille cloth 1 5m length cloth fastening spline 10- pieces felt (four each of parts A&B; two part C) 1 0.5m length closed-cell foam tape (to airtighten tweeters) 40 pan-head screws (driver mounting) 8 countersunk-head screws (terminal mounting) 4 20mm pan-head screws (electronics mounting) 1 tube glue 2 self-adhesive VAF badges Fig 8. The Impulse response of the DC-X clearly shows the absence of ongoing ‘ringing’. Fig 9. Actual measured phase response in the critical midrange area 500Hz to 7KHz. Where from; how much . . . The VAF DC-X Generation IV speakers are only available from VAF Research (see below) Three different options are available: (1) No enclosures; speaker drivers and all other components only – $1149 pair (2) As described above with pre-built enclosures and all other components – $1999 pair (3) Fully built, assembled and tested enclosures, ready to use – $2499 pair All prices included GST. Freight on any option is $80 per pair. An eight-piece adjustable floor spike set is available for $45.00. Contact VAF Research at 52-54 North Terrace, Kent Town, SA 5067. FreeCall 1800 818 882 or fax (08) 8363 9997; email vaf<at>vaf.com.au siliconchip.com.au August 2004  17 VAF DC-X Generation IV Assembly procedure (1) With the cabinet lying on its front, fit the two Hypersoft III foam part Ds through the rear port hole. Carefully squeeze the foam pieces through this hole so that they will be positioned as in the supplied assembly drawing. Position both pieces to the side of the cabinet. Find part E, with a sharp knife, cut this piece into two sections one 330mm long, the other 250mm long. Put these aside, as you will need them right after the next step. (2) Fit the circuit board and the terminal housing. Place the cabinet on its side, with the front facing away from you. Place the circuit board through the square cutout in the rear of the cabinet. Secure it in place with the supplied screws. Feed the cables through the cabinet to the driver holes in the front. The cables for the lower bass driver and the tweeters will have to be inserted through the holes provided through the internal bracing and baffles to reach their respective drivers. You will have to tightly wrap the labels around the cable for the tweeter and lower bass driver to fit through the holes in the separating internal baffles. Allow around 100mm of wire to protrude through the baffle to allow easy soldering to the drivers. The internal wire access holes do not need to be sealed and this will not affect the performance of the cabinet in any way. The wires to the drivers should now be positioned in a way that leaves no tangles and with each wire able to reach its appropriate driver. (Note: All wire ends are labelled, but please take particular care not to mix up the wires going to the woofers and tweeter. Now you can use those two pieces of part E that were previously cut. Place the 250mm piece between the terminal housing and the rear port on the rear of the cabinet. Place the 330mm piece above the port hole leading up to the top of the cabinet. (3) Place the cabinet face-down. Watch that the wires coming out of the front of the cabinet are safely inside so they do not damage the front of the cabinet when you roll it Terminal housing ready to be screwed into cabinet. 18  Silicon Chip Cable access through internal baffles. over. Place the terminal housing into the rebated square hole and using the holes in the terminal as a guide, drill four 1.5mm diameter pilot holes for the mounting screws. Use four countersunk screws (supplied in sealed bag) to secure the terminal housing. Do not over-tighten the screws; enough pressure to compress the gasket behind the housing is sufficient. Next, the rear vent can be inserted into the cabinet. Push it in with the palm of your hand to set it flush with the cabinet. (4) Place the cabinet on its back so that the rest of the damping can be fitted. Follow the DC X GEN-IV Foam Placement Guide precisely. The performance will suffer if the internal damping is not fitted exactly to plan. The foam is very resilient and will not be damaged through fitting and removing it several times to get it just right. (5) Fit the second vent tube. (6) Mount the drivers. The bass drivers have a soft sealing gasket but the tweeters require a separate sealing gasket. Apply the foam tape to the tweeter cutout holes as close to the inside edge of the hole as possible. Rear port and terminal housing in place. siliconchip.com.au Woofer with labelled cable soldered onto terminals ready to be fitted into cabinet. Solder the wires to the bass drivers, making sure that the positive wire is connected to the terminal marked with the ‘+’. Double-check to make sure these wires are connected as marked, while no damage will result if the polarity is reversed, the sound of the loudspeaker will suffer. Apply a small amount of solder to the driver terminals, enough to cover one side of the terminal.    Hold the tinned wire end on the terminal and apply heat to the wire as it touches the terminal. The solder on the wire will melt and incorporate into the solder on the terminal, fusing the two together. Do not apply excessive heat to the wire and terminal. The drivers can now be screwed into place using Pan-head screws. Align the pre-drilled holes with the holes in the baskets of the drivers. (7) The DC-X uses specialised felt treatments around the tweeter. This is an integral part of the design and must be placed carefully. Using the DCX Felt Placement Guide, fit felt kits A, B and C. A total of six separate pieces are fitted to each cabinet. A tube of glue is supplied to hold the felt in place. Do not glue the felt parts C that go over the tweeters to the cabinet. Speaker drivers in place (front vent tube not in photo). siliconchip.com.au Hammer in grille clips using the female clip to protect the other clip. (8)The grille cloth frame can now be covered and the clips inserted. Follow the grille covering section for the best way to cover the grilles. After the cloth is attached, the male clips can be inserted into the frames. Using the female part of the clip as a protector, tap the male clip into the holes in the frame. The flange on the clip will finish flush with the frame.   After putting in all eight male clips into the grille frame you can now insert the female clips into the cabinet. Insert the clip into the hole and then gently tap it into the cabinet with a hammer. (9) Fit the self-adhesive VAF badge around the apex at the bottom of the grille. About 10mm above the bottom edge of the grille. (10) Repeat all of the above for the second speaker. Stand them up and connect to your amplifier. As with any new project, we recommend that you use your speakers for the first time at a low volume initially to confirm their correct operation. (11) When you have confirmed that all is well, put on your favourite music (or movie), turn up the volume, sit back and be amazed. SC One down, one to go. . . August 2004  19