TAPERED QUARTER WAVE TUBE
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.
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). This doesn't necessarily mean we must use a
Fz = 40 Hz driver to 40 Hz horn. We can load a horn with whatever we
want except that drivers with a very high Qt may perform better in a
traditionally stuffed transmission line. The DTQWTs were tried with
10" drivers having a Qt = 1.2 (brought in by DTQWT customer) and
the bass simply became too boomy (too low electrical damping).
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.
DTQWT is in principle a 2-way with
integrated subwoofers, thus no high-pass filter on the main front
driver. This has the indisputable advantage of not needing any series
capacitors, which would be excessively expensive if of good quality
and sonically inferior to no caps, no matter how good the caps are.
the description above, many people have trouble understanding why
we don't need bass drivers with huge Xmax and low Fs. Please compare
the combined membrane area to an e.g. 10" bass driver in a vented
cabinet with the DTQWT. Here we have some 2 x 330 cm^2 from the bass
drivers and 220 cm^2 from the front driver, in total 880 cm^2. This is
2.6 times the membrane area of a single 10" driver. If a single
10" bass-reflex loaded driver has to move +/- 5 mm, we only need
5/2.6= +/- 1.9 mm. Next, all drivers in the DTQWT loads a horn, an
acoustic transformer. Wikipedia:
A horn loudspeaker is a complete loudspeaker or loudspeaker element
which uses a horn to increase the overall efficiency of the driving
element, typically a diaphragm driven by an electromagnet. The horn
itself is a passive component and does not amplify the sound from the
driving element as such, but rather improves the coupling efficiency
between the speaker driver and the air. The horn can be thought of as
an "acoustic transformer" that provides impedance matching
between the relatively dense diaphragm material and the air of low
density. The result is greater acoustic output from a given driver.
Initially the DTQWT was built with a single 10" 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 a bit 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.
The story behind the front drivers can be read here: TQWT
Changing your mkI crossover to mkII, download image here.
DTQWT drivers and crossover Parts
If STANDARD Z-CAP, wire coils or Cross Coils are preferred, please ask for price. Numerous options available.
You may also replace all MOX resistors with wire-wound SUPERES types.
If you also want to bi-wire your speakers, you have to order 4 pairs of terminals.
Download kit sale presentations:
DTQWT mkII wax-coils/Superior Z-Cap/Silver-Z-Cap (1035 EUR incl. VAT)
mkII, wire coils/Standard-Z-Cap
(890 EUR incl. VAT)
notice that the orientation of an RC circuit (1.0 uF + 15R) in the MT
section can be
either way. RC or CR, doesn't matter, does the same thing.
Crossover for the JA8008/TW034 front drivers is the same as the for TQWT construction.
Crossover for bass drivers
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.
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.
Left: 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.
Right: Distortion (THD) measured at 0.25 meter distance to 8008/TW034 drivers, 1/48 octave intervals, 2R7 to tweeter, 2.8, 5.6 and 11.2 volt input equivalent to 95, 101 and 107 dB at 1 meter. At normal listening level, e.g. 90 dB, we're below 0.5% THD from 500-20000 Hz. Doesn't get much better than this for home audio.
Builders' response on TQWT and DTQWT