DTQWT mkII JA8008 + TW034 + Eminence DeltaLite II 2510, 95 dB high-efficiency 3-way speaker system The DTQWT mkII is a revised version of the
DTQWT construction having a new 2nd order high-pass
filter to the tweeter, where DTQWT uses 3rd order. |
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LINKS Cabinet
construction page. HOW, WHERE AND HOW MUCH?
INTRO To my ears, this is the best speaker I've ever made - and the biggest! 150 liter brutto volume is what you have to be prepared for should you decide to follow. From the front it seems fairly modest, 28 cm wide and 105 cm high, but depth is 50 cm and it will require some 20-25 cm minimum to the rear wall to breathe. Overall sensitivity is 95 dB/2.8 volts and having a minimum impedance of 6 ohms, it can be driven by low-wattage SET amps. I do not recommend 2-4 watts SET amps; these amps need 98-100 dB speakers, no matter what manufacturers may claim. I have no trouble driving the DTQWTs from my 8 wpc 300B SET amp. Deep bass - and that very special 300B midrange. With a proper 300B SET amp you can play these speakers for hours and hours without listening fatigue, a sign of low distortion and proper balancing of drivers. 20 watts do even better in terms of bass slam and punch. For a DTQWT amp review, please read here. In many ways life gets easier
when we make big speakers. We have more drivers, bigger
drivers and distortion may all of a sudden be reduced
significantly as none of the drivers have to move much to
produce high sound levels, and we don't have to rely on
drivers being capable of large cone excursions either. The DTQWT construction is based on the TQWT construction supplemented by two 10" Eminence Deltalite II 2510 bass drivers added, providing a total membrane area exceeding a 15" driver. The front drivers and crossovers are the same as for TQWT. The rear bass drivers are fed from a simple 1st order crossover. It is thus possible to up-grade an existing JA8008/TQWT design by building new cabs and buying four bass drivers, 2 coils, 2 caps and 2 resistors, that's all. So, what's a double tapered
quarter wave tube (DTQWT)? Well, nothing new
under sun. The quarter wave tube, transmission line - or
Voigt pipe - came up around 1930 thanks to Paul
Voigt. A tube having a length of 25% of the
driver's resonance frequency, i.e. if we have a driver
with Fs = 40, we would need a pipe of 34400/40/4 = 215 cm
length (34400 = velocity of sound in air in
centimeters/second). Loading a tapered tube or transmission line can be done in numerous ways and nothing prevents us from using multiple drivers loading the horn - even from drivers having different size. Same thing can be seen in some bass reflex designs where we find e.g. 10" and 12" in same cabinet, sharing the same port.
The DTQWT construction
has an 8" front driver and two 10" rear mounted
drivers, all connected with positive polarity. They're
placed opposite to one another and the rear drivers are
fed through a huge coil, thus only adding weight to the
lower octaves. All drivers load a center horn and you may
ask whether the large bass drivers won't have an impact
on the front driver from sharing the same open space and
the answer is yes - and this is intentional. Air goes
where pressure is low, thus when the rear bass
drivers move inwards, they will push air into
the large center horn and by doing so they will suck
air from the front horn and pull the membrane of
the front driver. Obviously the front driver will do the
same thing for the bass drivers. We have a situation
similar to a push-push configuration of two opposite
placed bass drivers in a vented cabinet. Initially the DTQWT was added only a single bass driver, providing a system impedance of ~4 ohms minimum, not something that should make most decent low-wattage amps uncomfortable considering the overall system sensitivity. My 20 wpc PSE mono-blocks (6AS7 triodes) did well, but adding a second unit in series with the first driver left no doubt that the amps felt better about ~6 ohms minimum. Actually they felt much better and I admit I was surprised by the difference. Distortion was significantly reduced at (very) high levels and the grip on the bass drivers significantly improved. Possibly the reduced cone motion from having two bass drivers also reduces back-EMF (back-electro-motive-force) and SET amps have low damping factor with poor grip on bass drivers compared to push-pull valve amps or solid state amps.
DTQWT mkII crossover
DTQWT drivers and crossover Parts
FAQ: If you also want to bi-wire your
speakers, you have to order 4 pairs of terminals.
Crossover pics Crossover for the JA8008/TW034 front drivers is the same as the for TQWT construction.
Crossover for bass drivers
10" bass drivers suitable for DTQWT
- or use your vintage 12"/16 ohms drivers for the DTQWT Two 16 ohms 12" vintage bass drivers have been found suitable for the DTQWT construction: ALTEC 414Z (16 ohms) and Goodmans Axiom 150 mkII (16 ohms). One driver/cabinet will do and 6 ohms minimum impedance is maintained. Click image to go to page:
Initially only one bass driver was used, but there is only one thing better than a 10" bass driver: Two! Total membrane area = ~988 cm^2 from 2 x Beyma @ 768 cm^2 plus 1 x JA8008 @ 220 cm^2. A typical 15" driver is around 830 cm^2. Download additional cabinet drawings here. Measurements Presenting proper images of bass
capability from this construction is difficult due to
front and rear mounted drivers. Normally we would take
nearfield readings of both drivers and "port"
(horn mouth) and splice the readings at e.g. 100 Hz and
at 350 Hz (JA8008). The port output is usually presented
separately. This approach doesn't work because a
significant part of the lower bass comes from the horn
and in order to measure lower bass we need a measuring
window so wide ( > 100 milli-seconds) that we measure
everything; that is 1) what comes from the drivers and
port - and 2) what is reflected from room boundaries.
Left: 320 ms reading of speaker in six
different locations. ½ octave smoothing. Right: SPL @ 1
meter, 2.8 volts, left and right speaker. The left graph
also tells how enormously important room placement is for
the response coming from a loudspeaker, and this goes for
any speaker, not only the DTQWT seen here.
As can been seen from the graph above, we have a minimum 6 ohms impedance (red line inserted) and as stated in the beginning of this article, 8 watts from a SET amp can drive this speaker to significant levels before running into trouble. In fact much louder compared to average home-audio listening level, which is around 85 dB. This does not mean 20 wpc won't do better if you want to play even louder. The DTQWT can play enormously loud before significant distortion occurs. Remember, with 1-2 watt input we may have ~95 dB at 1 metre distance - and this is loud, really loud, but it doesn't mean I recommend 2-4 watts being sufficient. It isn't. Small amps may soon get into clipping no matter what manufacturers may claim. Builders' response on TQWT and DTQWT |
