Discovery 18
Copyright 2014 © Troels Gravesen

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January 2014: Normally the speakers presented here have been finished a couple of months before publishing and this speaker is no exception. The final crossover fine-tuning takes time and it's good thing to get over the honeymoon before publishing.
That said, I've rarely enjoyed a "6+1" more than this one - with the stepped baffle and the V3 crossover. These Discovery drivers take some burn-in and after that the bass performance is what first comes to mind. Due to the exceptionally high mechanical Q (= low damping) these drivers deliver a remarkable bass - even at low levels. The fairly rigid cones helps delivering a transparent midrange and I keep wondering what else I can use this driver for. The ring-radiator is my favourite tweeter with this speakers and seriously challenge the higher prices tweeters. Happy reading.

If you do not care for the long explanations, then click here to go to a short presentation of the V3 version.

Nov-Dec 2013.
This speaker really started with a client bringing in a pair of Dali 310 speakers for upgrade. The Dalis had the venerable Vifa P17WJ-00-08 (maybe -00-06) polyprop midbass driver and a fabric dome tweeter protected by a grill. The crossover was tuned for near-wall placement, thus a somewhat forward upper midrange to compensate for the inevitable room-gain. The Dali people knew that despite being presented on nice stands, people would set up the speakers against the wall - or even on a bookshelf near a corner. For speaker designers the impossible task is to predict where people will actually place their speakers, and tune the frequency response accordingly. No need to tune for ideal set-up if people put them on the shelf anyway.
Most likely the Dalis would be driven by the legendary NAD 3020 amplifier and we were in for a lot of good sound for the money. And the 3020 had tone-controls to manage the bass, something most people badly miss these days, not least from the dreadful designer homes with bath room acoustics. Look at recent Dynaudio brochure on the Xeo speakers. These are presented in an acoustic environment that couldn't be worse.
The good sound of the Dali was/is due to the Vifa P17 driver delivering excellent midrange and smooth roll-off, making crossover design easy. The P17 in all its variants must have been made in the tenths of thousands, and they could be found in numerous speakers in various forms, depending on manufacturers' preference for magnet size and voice coil impedance. Usually they were made with an aluminum voice former and and a not too ventilated magnet/chassis by modern standards, delivering a not too impressive mechanical Q, but all in all easy on crossovers due to inherent damping and smooth roll-off. Same goes for the Vifa P13WH-00-08, also used in countless constructions and very popular among DIY'ers, e.g. Lynn Olson's Ariels.


Click images to view large

What if we could revitalise the "6+1"/P17 concept from modern ScanSpeak Discovery drivers? Can the 18W/8434G00 make up to the Vifa P17WG driver? On paper it delivers a fairly promising frequency response and an impressive mechanical Q of 7.58 due to its glass-fiber voice coil former, vented chassis and low-loss suspensions. The magnet system has an aluminium ring - somewhere - a low-cost semi-Symmetric Drive feature. Fs is in the range of 55 Hz and we won't get kick-butt bass from such driver, but neither did the P17WJ. If we want something in the 30-45 Hz range add a subwoofer. And today we might substitute the NAD 3020 amplifier with the NAD D3020, the digital revival of the 3020 legend, and use almost any kind of source material.

Thus, pairs of 18W/8434G00, D2604/830000 and R2604/832000 tweeters were acquired from ScanSpeak. All drivers will be 208 EUR from ScanSpeak webshop (D2604 option) or delivered with the crossover kit from Jantzen Audio.
Any other tweeter will require a different crossover and I cannot help you with this, so please do not ask.

 

DRIVERS
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Click images to view large

 

Phase issues

The aim of this construction is also for the first-time DIY'er to go through four different versions with regard to cabinet design and crossover construction. The problem of having the tweeter in front of the midbas driver is described in the Jenzen intro. With both drivers mounted on the same flat panel, most 6" midbass driver are acoustically some 20-30 mm behind the tweeter. Well explained here at Wikipedia.

 

My guess is that 99 of out 100 commercial speakers do not address the issue of time-alignment and there are several ways to "overcome" the problem:

#1. Use an asymmetrical crossover usually consisting of 2nd order to the bass and 3rd order to the tweeter to make "proper" phase integration.
#2. Use 4th order filters to both drivers, e.g. LR4, and due to the large phase shift, the lack of alignment is negligible - although I really feel it isn't.
#3. Tilt the front panel so that with a listening height somewhere between the bass and tweeter we more or less compensate for the time difference.
#4. Place the tweeter below the midbass. Having the midbass at ear height, the tweeter will - to some extent - be better aligned with the midbass driver.
#5. Use an all-pass filter to delay tweeter response.
#6. Ignore the problem and implement a simple 2nd order filter as long as we can produce a decent linear frequency response.

The 6th option is what is found in countless commercial speakers. 2nd order filters to reduce cost and to provide a decent protection for the tweeter. And I'm not pointing fingers, because I've found this sometimes sounding better compared to a text-book asymmetrical 2nd/3rd order topology, which on paper looks perfect, but in reality sometimes sounds worse. I've used the combination of 2nd/3rd order filters (option #1) many times and must conclude in all depends on drivers and actual implementation.
The 5th option I used in my ScanSpeak Illuminator monitor and it worked well indeed despite a more complex crossover that takes good quality components.
The Jenzen speakers were made to do something radical about this and it all started with the study of stepped baffles. Stepped baffles can cause diffraction and reflection and it takes a lot of test-baffles to minimise the impact on frequency response. Commercial brands like Vandersteen, Thiel and Wilson Audio are companies that have taken the challenge of doing something about time-alignment. Vandersteen to the extreme, using 1st order filters and all-pass filters to have all drivers connected with same polarity. Please keep in mind there are trade offs in any approch.
I strongly urge you to read the magnificent paper from John Atkinson (Stereophile) on Testing Loudspeakers. Mr. Atkinson has had the opportunity to test and audition numerous loudspeakers, also those addressing the problem of time and phase distortion. Part of his conclusion goes like this:

 Please (!) read all of the articles, they're gold! Click text image above to download file (pdf).

 

The present Discovery W18 will be presented in several versions depending on what is feasible. Some solutions may be technically possible but too complicated.
#1: Flat front panel and simple 2nd order filter regardless of lack of time-alignment.
#2: Flat front panel and "standard" 2nd/3rd order crossover.
#3: Stepped front panel and LR2 filter. My preferred and recommended option.
#4; Stepped front panel and LR4 filter. The LR4 filter does not sound as good as the LR2 filter - to my ears - but provides a better power-response compared to all the other options, i.e. less sensitive to listening position. I usually call the LR4 filter the engineers' filter as it is so good on paper and does so many things right. Studio monitors often feature LR4 filters as they must provide en even power response and allow a wider audience to hear the same thing regardless of where you sit in the studio. Surely we can run an LR4 filter speaker more aggressively due to better protection of mid and tweeter drivers, and almost all electronic crossovers feature LR4 topology. To my ears LR4 filters provide a soundstage with a more dry presence and lack the 3-dimensionality and spaciousness of LR2 filters. Read about Linkwitz-Riley filters here on Wikipedia.
I not only think, but know, that some people will highly disagree on my preferences, but there is no final truth in loudspeaker design. There are more or less clever choices to be made and our ears are the final judge of the outcome of these choices.
The Discovery W18 will also provide you a platform for trying out different crossovers and gain experiences in crossover design and the sound we get from different topologies.

Here's one that will not be tried:


Click image to view large.

The above is what I call the cost-efficient Dali solution - and again, not pointing fingers because I know marketing people rule and have a target price point. Has to be said it's not that bad, providing a decent frequency response and very few crossover components. What lack is some weight in the 300-600 Hz range due to baffle step loss, tweeter roll-off could be better and provide better power handling, midbass upper roll-off has too much energy in the 4-6 kHz range impacting treble performance. What Dali also would have done - at least back then - is a smaller midbass series coil elevating upper mid some 2-3 dB due to the inevitable book-shelf placement. Midbass driver is set back some 30 mm in the simulation software favouring a listening height of the tweeter. What we have is a filter consisting of (close to -) LR2 to the both midbass and tweeter but due to the time difference we have both drivers connected with positive polarity. Phase integration is so-and-so, but seen worse.

 


Click images to view large

 

 

CABINET
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I've had the question on vented vs. closed box designs many times. Modern drivers generally feature low Qt (high mechanical and electrical damping) and if we put them in closed boxes we just don't get any bass. Simple as that, regardless of the closed box' superiority in transient response. With the current Discovery W18/8434G00 in a 18 litre closed box we get F3 = 80 Hz (!). Adding a vent we have F3 = 52 Hz. Now, almost 30 Hz makes a difference I can tell. In a closed box it sounds utterly lean and anemic. When you have made the cabinets you can try stuffing the vents and hear for yourself. Thus, vented it must be.


Click image

Modeling the 18W/8434G00 in LspCAD proved bit a challenge because SS TS data doesn't exactly fit in well when doing cross-calculation of parametres. I measure significantly higher Fs compared to ScanSpeak, rather 55-60 Hz depending on input voltage. However, my 18 litre volume doesn't appear too far away from what may suit this driver well and provide an acceptable low-end response down to ~50 Hz. Do not expect gut-shaking bass response from such driver. If you want deeper, add a sub. And do not expect a sub will all of a sudden make your small two-way into a true 3-way system. The small 6" driver still has to manage everything to down below 100 Hz and and it won't be more capable of handling the important 100-200 Hz range. Read ATS-4 to get my point.

If you have a router - and you need a router to make loudspeaker cabinets - then you may also have a 45 deg. router bit to chamfer driver holes. Whether you chamfer the sides of the front panel or not will have little impact on performance - but it looks nice. From the images below you can see how to make the outer bass front panel. I rounded the outer bass panel towards the tweeter a little for the stepped baffle. If you do not have a rounding bit, leave it sharp. This is as easy as it gets for making non-stepped baffles.

 


Use 19-22 mm MDF or 21 mm Baltic birch. For bracing I used 15 mm Baltic birch. You do not have to adjust volume regardless of panel thickness, just stick to outer dimensions. All internal panels are covered with 4 mm bitumen pad, which reduce volume, but the felt/acoustilux will increase virtual volume and +/- 1-2 Hz variance in Fb (port tuning) does not matter. Port is Ø50 x 120 mm length for Fb = 40 Hz. You can boost bass a little by reducing port length to 8 cm (Fb = 45Hz).
You can make the front midbass panel with a straight cut-off at 127 mm from top, angled at some 20 deg or just rounded like I did here with the curved panel.

Cabinet construction pics


What most first-time diy'ers want: Rectangular box, flat front panel.
Cabinets here made from 21 mm Baltic birch. Braces 15 mm Baltic birch.
Make 4 x 80 mm holes in braces and a smaller one in the middle.


Left: Ready for gluing. Right: Test front panel, here the flat, non-stepped baffle.


Left: DO NOT forget to chamfer bass driver holes, 45 deg. down to some 5 mm from edge of rebate.
Right: Test baffle with D2604 tweeter.


Blocks added at corners to fasten test front panels. Do not add these unless you want to experiment with front panels.


Left: DO NOT forget to chamfer driver holes for bass driver! Right: Stepped front panel ready for mounting.


Damping of cabinets:
Damping of panel resonances with bitumen pads is optional.
1: 8 mm felt material on all internal panels, except front panel, rear panel behind midbass driver (crossover will be here) -
and rear panel at bottom where terminals will be.
2: One layer of 30 mm acoustilux at top and bottom. 3 layers of 30 mm acoustilux on rear panel behind 18W driver (covering crossover).
If you use bitumen pads, glue with elastic vinyl glue. The same can be used to fix 8 mm felt damping.
DO NOT obstruct air passage between 18W driver and port!
(Do not add the blocks seen above at top and bottom of cabinet. These were used to fasten test front panels.)

 

CROSSOVERS
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Flat front panel: Crossover for D2604 version. For R2604, use 1.0 ohm for R1011.

 


Flat front panel: Crossover for D2604 version. For R2604, use 0 ohm (short) for R1011.

 


Stepped front panel: R1011 for R2604 = 1.5 ohm default (parallel 2R7 and 3R3 following the kit). Tune to personal taste.
R1011 for D2604 = 2.7 ohm. Tune to personal taste.

Full V3 schematics come with crossover kit from Jantzen Audio




C1021 = 2 x 6.8 uF in parallel

Stepped front panel: Crossover for R2604 version. For D2604, use 2.2 ohm for R1011.


 

THE SOUND
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First of all, please read these comments on describing sound. OK, agree?
These Discovery drivers takes some time breaking in, which is not unusual for high-Qm drivers. The intrinsic flat response in all of the important midrange is paying off. These fiber glass cones deliver excellent midrange transparency and a responsive bass performance as far down as it goes. I favour the ring-radiator over the dome, but others may choose differently preferring the slightly more vivid performance of the dome. Good thing here is that they are interchangeable by only adjusting attenuation resistor.
Setting up V1 crossover was easy and overall tonal balance was good, but the transition from upper mid to treble appeared rather fizzy and blurred. Something just wasn't right despite the ability to play all kinds of music without any distress to the ear.
The V2 crossover solved this problem and kind of made the sound well known from many other speakers I've made, but not up to the level of stepped baffle/LR2 speakers I've made ever since the launch of the first Jenzen speaker - or the ScanSpeak Illuminator with its all-pass filter to the tweeter. The quality of the W18 driver strongly suggested there were more goodies in store.
V3 crossover: Making the stepped baffle and implementing a true LR2 filter made a world of a difference. Suddenly music started flowing and sense of depth and perspective improved vastly. I never heard the difference between a flat baffle/2nd-3rd order crossover and a stepped baffle/true LR2 filter so clearly. It's night and day. It also made me think I have to do the Ellam XT once more with an easy stepped baffle because people continue building the Ellam XT speaker rather than the much better Ellam FLEX with its more complex front panel layout. There are more to be gained from these Ellam XT. And I would love to do the 9800 tweeter again in such set-up... well, well, back to the Discovery here.
V4 crossover (stepped baffle): This has the well known 4th order characteristics. Even power response and a balanced presentation with a noticeable presence character. Good for monitoring! I know some people like this a lot but to my ears it sounds kind of flat and without the sense of depth and 3-dimensionality of the LR2 filter. Matter of taste in the end.

General for all versions is the dynamic bass. These high-Qm Discovery midbass drivers has the ability to not only let your hear the bass, but also feel it. OK, not like a 15" on an open baffle, but for the size, quite remarkable. And this driven by my GlowMaster KT88 PP tube amplifier. A good 40-60 watt PP valve amp is all these speakers need.

 

COMPLETE KIT incl DRIVERS
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For price quotation incl. shipping, please contact Jantzen Audio at: contact@jantzen-audio.com
Remember to state where you live for calculating shipping cost!

Kits available w/wo drivers.

All kit and component prices may be subject to change and are always to be confirmed by Jantzen Audio Denmark.

DOWNLOAD ALL KIT SALES PRESENTATIONS HERE (pdf file):

 

All technical questions at: troels.gravesen@hotmail.com

 

 

CROSSOVER LAYOUT for version #3.
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CROSSOVER WIRING, version #3
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Value of components comes with the kit.


Fully mounted crossover.

 


Left: Crossover mounted on rear panel in cabinet. Right: Two layers of 30 mm acoustilux covering crossover.