DTQWT Go to cabinet
construction page. Get kit price at: contact@jantzen-audio.com, please
include address for shipping cost. Download sales
presentation here. Kit contains all drivers (excl. bass drivers), waveguides, crossover components incl. comp. for bass drivers. Specify wax coils if wanted. All technical questions at: troels.gravesen@stofanet.dk Read response to TQWT and DTQWT. Some thoughts on amps for the DTQWT (and TQWT)
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 watts SET; these amps need 98-100 dB minimum no matter what manufacturers may claim. I have no trouble driving the DTQWTs from my 8 wpc 300B SET amp. Deep powerful 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 clear sign of low distortion and proper balancing of drivers. In
many ways life gets easier when we make big speakers. We have more
and 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 JA8008/TQWT construction with 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 JA8008/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.
The JA8008-DTQWT Crossover
DTQWT Crossover Parts
Crossover pics Crossover for the
JA8008 and TW034 is the same as the for the 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. 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 doesn't mean that 20 wpc can'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. Builders' response on TQWT and DTQWT DTQWT,
built by Peter/Denmark |
