Copyright 2010-2012 Troels Gravesen

mkII crossover     Damping rear radiation     Audax mods      Damping of magnet    
Planar tweeter     Fountek JP2     ScanSpeak 710003 tweeter    SEAS T35C002 dome

It's been more than 3 years since the work on the DTQWT began. Already in 2006 when the driver parameters were decided in collaboration with SEAS/Norway, the speaker following the TQWT was in preparation and it's been a great satisfaction seeing it in the hands of many diy'ers. That the JA8008 driver also has gained interest outside the diy community was no less pleasing. As pointed out in the TQWT file, there are numerous applications for the JA8008 driver and it's great fun having reports back on what people have created.

I need to address some sincere thanks to my friend Michael, who got me in touch with Jantzen Audio taking the financial risk of the whole project of launching an extended range 8" driver. Had it not been for Michael and our discussions on how we could have a new "Vifa C17" produced somehow, the JA8008 driver would never have been made. Yes, it all started with the Vifa C17 driver, but taking this driver into production would require significant investments, where most parts for the JA8008 driver were already at hand at SEAS. Generally I find an 8" driver more versatile, where a 6" driver soon calls for a companion (d'Appolito or 2-way).

The DTQWT construction has been been my main speaker for the last two years and I don't think this will change for a foreseeable future. There are numerous projects I'd like to do with the JA8008 as the main driver, but time is a limiting factor and making big cabinets takes time. Even the QUATTRO construction took longer than anticipated despite an uncomplicated 40 liter vented cabinet.
For those - hopefully - enjoying the sound of their TQWT/DTQWTs, this article to show that their speakers still have my full attention in the persuit of better sound. Any speaker is a compromise and I've had reports back asking for more bass, less bass, more forward midrange, less forward midrange, more treble - but not less treble, so I guess treble level suits most customers. Many commercial speakers have an elevated treble range - because treble sells, but it's basically linear distortion and adds an unnatural sound to acoustic instruments. For purely electronic music we can't know what it's supposed to sound like anyway.
The treble question is the easy one: If you want more treble change R1011 from 2R2 to 1R8 or 1R5.
Tweaking the midrange is more troublesome as changing any of the components here requires adjusting all others. Fortunately I've only had this question once.
Tweaking bass performance is the most troublesome question as it's not only a question of what the TQWT/DTQWTs can deliver - but how they interact with your listening environment - and it's impossible for me to have any idea of the acoustic performance of your listening room. Only thing I can suggest here is trying out different rooms for the speakers, different placement and different amps. This just to get an idea of what is happening. Then we might start discussing what can be done in the actual listening room. Generally this is an area where I can do very little - for obvious reasons.


The search for better sound is a never ending story and many experiments have been carried out over the last two years to possibly improve the performance of the DTQWT, but nothing came up that would produce a DTQWT-mkII until recently. Below comments on some of the tweaks tried, some reported briefly, some in more detail.

Tweaking the crossover was where the most significant change for the better appeared. Read about the mkII crossover below, tweak #1. The simpler the crossover, the more options available for final voicing of the speaker it appears. With an LR2 crossover even minor changes to values of the cap, the coil (including its series resistor) can make radical changes to the voicing of the speaker.
The TQWT mkII and DTQWT mkII will be implemented in the standard kits from Jantzen Audio and the only changes to the website will be schematics and headings. As this change only has minor impact on measuring performance the measurement files will remain the same.

SWANS RT2H-A planar tweeters unit were in my test cabs for 3 months last summer - and it took a good friend to tell me "this isn't really as good as the old one, or...?" And he was right, it wasn't. Treble as such was great, but overall integration just didn't work out. I render this more important than any specific tweeter quality we may fancy. I don't like listening to tweeters, I like listening to treble, good treble that blends well with the midbass driver.

Fountek JP2 tweeter was tried for a long time but driver integration was even worse than the SWANS planar. Ribbons usually require steep crossovers unless crossed very high and this wasn't a good match for the JA8008, at least not with my set-up and with my crossover. Others may have found differently.

Damping upper pole piece of the JA8008 was tried and didn't improve performance. Reported below. The discussion on reflections from the magnet system onto the thin cones of dynamic drivers is still going strong and some have even taken out patents to protect their non-reflection designs (Vandersteen) and I had hopes there was some gain here, but to the best of my auditive ability this simply didn't make any difference. Recent years have seen a lot of designs trying to minimise reflections from the magnet - and I'm not sure this is as important as it is claimed to be. It takes more to make a good sounding driver than the size of the magnet behind the cone.

Smoothing impedance peaks in the 20-100 Hz region by LCR circuits was pretty much a disaster. I wanted to hear what would happen from getting a flatter impedance but as the 8008 and the bass drivers need their own individually tuned circuits, this proved extremely difficult and when I finally got it right ( - or did I?) it just killed the intentional collaboration between the front driver and rear bass drivers loading the same center horn. Not a good idea.

Modifying TW034 dome by venting the center pole piece and cavity below voice coil has been tried and yes, it makes a slightly better tweeter suitable for very low points of crossover. For the DTQWT this tweak didn't pay off, having a point of crossover at ~3 kHz. Partly reported below with link to page describing the full modification.

Damping rear radiation from the bass driver has been tried and yes, some experiments are useful here. Read below.

Tweak #1
The DTQWT mkII crossover

The QUATTRO speaker was where new ideas to a simplified crossover emerged based on work done on the PRELUDE crossover in combination with the article on Siri'S Killer note. The QUATTRO having a different working point to the crossover, due to the baffle step compensation circuit, allowed a very simple LR2 crossover to the tweeter and quickly this topology was tested in modelling and practice for the TQWT/DTQWT speakers as well. The revised crossover was tested for another long period of time and found to improve the sound enough to justify an "mkII" addition to the TQWT/DTQWT names although the changes on paper seem very modest.
Due to shallower crossover slope, the optimal driver integration moves a bit up-wards compared to the original crossover. The optimal listening point is now at tweeter height where before it was right between JA8008 and TW034 axis.

The impact on sound can be described very simple: Enhanced transparency. I could also put it this way: Easier to listen to, more natural timbre, a slightly "darker" sound although I'm hesitant to write this as people may think treble level has gone down, where in fact it is slightly increased in amplitude, but within experimental error. You will more easily be able to identify the location of events in the musical landscape, e.g. I have found it easier to hear Keith Jarrett's "aahhh-aahhh..." or "beep-beep-beep...." singing, which may not be to everyone's taste, but nevertheless more fidelity!

I'm sure I'll have the question if it is worth while and my answer is yes, but some may have gotten used to the "mkI" sound and prefer this. For those with a box of coil/cap leftovers, it may be beneficial to try it out before investing in the best of components. Whether your ears will hear the same as mine I certainly cannot tell.

The crossover changes have been implemented in the TQWT and DTQWT constructions and file headings are TQWT mkII and DTQWT mkII. Obviously the changes can be implemented in OB9 too, but I haven't yet heard from anyone building these large panels.

Check up-dated files on TQWT mkII and DTQWT mkII

Above the simplified crossover to the tweeter. We now have a series resistor to L1031.
The resistance of L1031 itself can be anything from 0.1-0.3 ohm.
Everything else to the TQWT/DTQWT crossovers remain unchanged.

The complete DTQWT mkII crossover.

Should you want to upgrade your existing DTQWT crossover, you need one of the following options:

Option 1:
2 x 0.18 mH 0.6-0.8 mm wire coils (0.1-0.3 ohm)
2 x 1 uF Superior Z-caps
2 x 1.0 ohm 5-10 watt MOX resistors.
The 1 uF capacitor is added parallel to the existing 4.7 uF and will make 5.7 uF rather than 5.6 uF. No big deal! This is within the tolerance of the caps anyway. If you have the STANDARD Z-cap version, you obviously need 1.0 uF STANDARD Z-caps.

Option 2: 2 x 16 AWG 0.18 mH wax coils, 2 x 5.6 uF Superior Z-caps, 2 x 1.0 ohm 5-10 watt MOX resistors.

Option 3: 2 x 16 AWG 0.18 mH wax coils, 2 x 5.6 uF Silver Z-caps, 2 x 1.0 ohm 5-10 watt MOX resistors.

Option 4: 2 x 16 AWG 0.18 mH wax coils, 2 x 2.2 + 3.3 uF Silver Gold Z-caps, 2 x 1.0 ohm 5-10 watt MOX resistors.

Download Kit Sale, Option 1 here.
Download Kit Sale, Option 2
Download Kit Sale, Option 3
Download Kit Sale, Option 4

I'm deliberately calling these suggestions "options" and not "levels", although there may be sonic differences. Going from standard low-cost polyprops (STANDARD Z-Cap) to the super caps - to my ears - deliver significant enhanced level of detail and transparency. There may be sonic differences between Superior Z-Caps and Silver Z-caps, but in the end it may be a matter of taste. I haven't tried the Silver/Gold option, but some people just want the best regardless of price.

The future default DTQWT kit will include option 3 as the omission of the two 8.2 uF Superior Z-caps more than pays for now the single Silver Z-cap. The majority of kits sold are with Superior Z-caps and WAX coils according to Jantzen Audio.
If you want to fine-tune tweeter level to your room/amp/taste, order 2 x 1R8 and 2 x 2R7 too.

Download image here to upgrade your existing crossover.

Tweak #2
Damping rear radiation from the Eminence bass drivers

The two Eminence bass drivers facing your front wall are driven from a simple 1st order filter and do radiate some energy in upper bass/lower mid into the room. Being in phase with the front driver we have a bi-pole situation in contrast to a di-pole speaker, where rear energy is out of phase with front radiation. A bi-pole speaker approaches a 360 deg. soundfield, where a di-pole have a "null" (dead) zone that may require extra precautions with regard to speaker placement and sitting position. However, for the DTQWT only working in lower registers, this is of less importance as bass notes are omnidirectional anyway.

I have to point out immediately that we cannot wire the DTQWT bass driver with inverted polarity as these and the front driver load the same center horn and the result would be no bass - or very little.
Partly damping rear radiation of the bass drivers does have an impact on sound, which - in particular the midrange - becomes a little dryer and appear to be more focused from the JA8008 driver. It's not a major tweak, but my panels are going to stay. As my speakers' rear panels face a window, the panels help preventing the sun light bleaching the cones of my bass drivers ;-)

My mockup panels really don't deserve photos. I'll add pics once the final panels are finished. The panels are 280 x 600 mm and made from 12 mm Baltic birch with 10 mm slots as shown on drawing. This actually does take some time with the router I can tell. Find your own solution if this seems too laborous. Initially it was my plan to make a rear grille with standard grille cloth, but I thought the Baltic birch solution nicer. The panel is attached with 6 screws with 15 mm spacers made from aluminium tube. Two 22 cm circular sheets of 10 mm felt material is placed concentric to the bass drivers. That's all. The slots and the quality of the felt are open for experimentation. With no slots, I'd leave 20 mm to the rear panel. Experiment with felt material and thickness as this is not overly critical. For initial experiments, just hang a towel from the top with a 10-20 mm spacer and hear what happens.

Finally, 4 years after doing the DTQWT I had these panels ready. 21 cm felt (10 mm thickness) discs placed right at center of bass drivers.
Better late than never. Panels attached with M5 screws and 20 mm spacers (alu tube).

Tweak #3
Modifying the Audax TW034 tweeter

First of all: With the chosen point of crossover and the 2nd order filter topology for the DTQWT tweeter, there is no advantage in modifying the TW034 dome. The modification is only justified when we think of using the TW034 dome down to around 1.5 kHz and examples will be shown at a later moment.

TW034 tweeters come with foam damping domes on the solid center pole piece and as a result we have a minor wrinkle in response at 2 kHz.

Left: TW034 (no waveguide) before and after modification. Middle: CSD/40dB un-modded. Right: CSD/40 dB modded.

A full report on TW034 modification can be found here.

Tweak #4
Damping of JA8008 magnet system

Damping the upper pole plate with felt as seen on photos wasn't as easy as anticipated.
If it has to be done it should take place before the cone surrounds are glued to the basket.
However, the story here is short: No improvement in sound could be detected from this - cheap - tweak.

Tweak #5

SWANS RT2H-A planar tweeter was tried out extensively last summer and just didn't integrate well with the JA8008 driver. I badly wanted this to work out right but had to give it up. Too many visitors had thumbs down and they were right. The crossover below was one of the solutions tried, but no matter what I tried the upper mid and treble didn't gel properly.

Tweak #6
Fountek JP2 ribbon tweeter

Trying the Fountek JP2 ribbon tweeter went no better than the SWANS planar.
Point of crossover was taken a bit higher to fully integrate - technically - but sonically poor integration.


Tweak #7
ScanSpeak 710003 tweeter with waveguide

ScanSpeak D2904/710003 tweeters

I have a box of Monacor waveguides cut to various depths for a quick test on tweeters to pick those suitable for applying a waveguide. Not all tweeters perform well with a waveguide and I've found it pretty much unpredictable. It has to be tried.
The ScanSpeak tweeter is a high-sensitivity tweeter displaying an impressive 95+ dB/2.8V sensitivity due to the low-ohmic voice coil and strong neo magnet, thus a potential candidate for the DTQWT. Initially I picked a shallow waveguide taking advantage of the 710003 faceplate's initial rounding towards the dome. This turned out bad and produced a serious dip around 10-12 kHz. Thus, a deep waveguide was tried and produced the results seen below. Really nice! My DTQWT prototype cabs had new front panels for I don't know many times and surprisingly the crossover didn't need any modifications at all. Quite a coincident.

The "DTQWT-71" has been evaluated extensively against my reference DTQWT and there are minor differences although hard to pin-point. Due to the deeper waveguide - I guess - I find the best sound from less toeing in compared to the Audax, which can cope with on-axis listening without problems. I really can't tell which is the better performer, but for sure the 7100 gets my full recommendation. Basically I like having alternatives to provide choices for the builders, although I'm constantly asked which is the better. In that sense it's better having a single product, which is put together from specific drivers, period!
Post honeymoon (3 months): I inserted the TW034 again - and kind of felt being back to where I initially wanted to be. Maybe it's my several years with this dome that has become my reference standard. The TW034 makes a more coherent soundstage with the 8008 midbass, where it seems that whenever we optimise on certain parametres it's at the expense of others.  

Left: 7100 on DTQWT cabinet at 0, 10 and 20o, no crossover attached
. Right: Same including 30 and 40o.

Go to website on how to implement the Monacor waveguide here.

How to make an adaptor for your existing waveguide to accommodate the 7100 tweeter here.


Tweak #8 May 2013
SEAS T35C002 tweeter with waveguide

When the SEAS T35C002 dome was released in Europe I ordered a pair to see if they would fit the TW034 waveguide because due to the rear chamber it might do the same thing as the modified Audax TW034 dome can do: Go lower and allowing the point of crossover between 8008 and dome tweeter to be taken down to around 2 kHz providing a bit more presence and better dispersion in upper mid and lower treble.
The T35 does not sound any better than the TW034 dome, in that respect they're too alike. The SEAS has a voice coil diameter of 35 mm, the Audax 34 mm, fabric weave slightly different, coatings very much alike, dome shape very much the same, etc. Only major difference to the standard TW034 is the rear chamber.
Any difference in sound is based on minor differences in frequency response and this can be corrected in the crossover. I've used my modded TW034 the same way as I now use the T35 but never released a new version of TQWT, DTQWT and OBL-11 as very few people have the tools to do the TW034 modifications. No point in making a speaker only a very few people can copy.
I suggest taking a look at performance of the T35C002 and modified TW034 here. Study the waterfall plots and you'll find no significant differences.

The T35 dome including surround is actually slightly smaller than the TW034 dome, probably about 0.8 mm difference in diameter despite having a 1 mm larger voice coil, but nevertheless fits perfectly into the TW034 waveguide. Only problem is making four threads that will take the four screws holding the standard T35 front plate. I never succeeded doing this properly by hand and eventually mounted the T35 dome with a flange, the same used for the ScanSpeak 7100 dome. When the next batch of waveguides were ordered I went to the factory and had them make a small batch of waveguides suitable for the T35 dome. Thus, only thing to do is loosen the four screws, remove the face plate and fasten the T35 to the waveguide. The threads in the waveguide are obviously in the POM plastic and DO NOT overdo tightening of the screws. Firm - but no more.

The crossover for the SEAS T35C002 versions of TQWT, DTQWT and DTQWT-12 goes like this:

Go to TQWT, DTQWT and DTQWT-12 page for full details on T35 implementation.