Prestigious Two – Monitor DXT

Concept:

 

Prestigious Two – Monitor DXT is a medium sized 2-way stand-mount bass-reflex loudspeaker for small to medium sized listening rooms.

The loudspeaker uses “Diffracton Expansion Technology” (DXT) for optimum power response, reduced baffle diffraction influence, off-axis dispersion and directivity control.

Loudspeaker driver units:

 

The Prestigious Two – Monitor DXT loudspeaker uses affordable driver units from the SEAS Prestige series.

The SEAS U16RCY/P is a mid-woofer with a woven polypropylene cone. It has an extended frequency response with a smooth frequency roll-off above 5kHz.

Considering its size the mid-woofer has an astonishingly fast, powerful and accurate bass reproduction.

The SEAS 27TBCD/GB-DXT is an aluminium/magnesium alloy dome tweeter with a DXT lens. The DXT technology solves several well-known issues regarding; directivity control, off-axis response, integration with midrange units and baffle diffractions.

For further details see:
SEAS U16RCY/P (H1520)
SEAS 27TBCD/GB-DXT (H1499)

 

Cabinet:

 

The Prestigious Two – Monitor DXT uses a 14 liter “Dayton Audio” enclosure. This is a very nicely built curved cabinet with a 25mm (1″) MDF front-baffle. The enclosure walls are made of 18mm MDF as well as a 15mm brace from the bottom to the top. The enclosure net volume is around 13 liters (enclosure volume – drivers, filter and port).

 

 

Cabinet drawing: Dayton 14 liters Curved Cabinet #302-721

 

 

The internal cabinet walls are taped with strings of bitumen pads to reduce resonances. The enclosure is lightly filled with sheep wool. All drivers are flush mount and in order to let the mid-woofer “breathe” properly, don’t forget to chamfer the baffle.

The internal cabinet walls are taped with strings of bitumen pads to reduce resonances. The enclosure is lightly filled with sheep wool. All drivers are flush mount and in order to let the mid-woofer “breathe” properly, don’t forget to chamfer the baffle.

 

 

Box simulation:

 

The  bass-reflex port tuning frequency can be fine-tuned to fit personal preferences. I think the Fp=44Hz gives the best balance between fast bass articulation and bass depth.

The closed box version isn’t really an option, unless it’s paired and crossed-over with a sub-woofer.

~13 liter bass-reflex box with port tuned to 44Hz (anechoic response)

-3db = 51Hz

-6db = 42Hz

-12db = 33Hz

 

Cross-over design:

 

The Prestigious Two – Monitor DXT uses a true symmetrical second-order Linkwitz-Riley (LR2) filter topology and has a minimalistic design approach with only one cross-over component + tweeter padding resistor in series with the driver units.

The most common way to address the driver units relative acoustic center off-set is to use asymmetrical cross-over slopes, but as in this design when using LR2 filter topologies it’s not feasible.

Other choices for dealing with the driver units relative acoustic center off-set is to use a slanted or stepped baffle as well as the use of an electrical circuit to time delay the tweeter in order to adjust the acoustical center off-set.

However, thanks to the depth of the DXT lens, the acoustical center off-set is perfectly aligned without the need for such techniques.

The mid-woofer cross-over filter section is an electrical first-order and is very simple and consists only of a large coil (L1) that shapes the cross-over slopes to a LR2 roll-off with a targeted 3kHz cross-over point. The inductor (L1) also tunes the “Baffle Step Compensation” (BSC).

The tweeter cross-over filter section consists of a single tweeter padding resistor (R1) and a first-order electrical filter (C1) that shapes the cross-over slope to a LR2 roll-off with a targeted 3kHz cross-over point. The value of (R1) can be changed to tailor the tweeter level to personal preferences.

R1 = 1.8Ohm -> +1dB
R1 = 2.2Ohm -> 0dB (Reference Tweeter Level).
R1 = 2.7Ohm -> -1dB

The tweeter is connected with reverse polarity and (C2+R2+R3) shape the tweeters frequency response and flatten the response at higher frequencies.

(L2+C3+R4) is a “System Impedance Correction Circuit” used to flatten out the impedance when using the loudspeaker with tube amplifiers. It’s optional to use this filter section and it can be omitted when using conventional amplifiers. By using this circuit the impedance is 6 Ohms, within 200-10000Hz.

Update! (20120710)

I recommend everyone who is interesting in this loudspeaker design to look at the additional tweeter level options that’s available here:

Additional tweeter level options

I highly recommend buying a couple of more resistors in order to fine tune the tweeter level to your personal liking.

 

Cross-over simulation:

 

 

Top: Tweeter-axis, 15deg off-axis response (0dB Reference level).

Upper left: Tweeter-axis, 15deg off-axis response (0dB Reference level) reverse polarity.

Upper right: Individual driver unit phase tracking.

Lower left: Tweeter-axis, 15deg off-axis response (+1dB).

Lower right: Tweeter-axis, 15deg off-axis response (-1dB).

The reverse polarity simulation shows a deep reverse null indicating an excellent phase tracking between the two driver units. The reverse polarity response is a little ragged , but as can be seen further down in the frequency measurements section it’s only visible in the cross-over simulation.

I personally prefer the 0dB or the +1dB tweeter level depending on which HiFi equipment I’m currently using or for that matter, which recording I’m listening to. As usual, tweeter level padding must be fine-tuned to personal taste and preferences, hence I always give some level options to try out.

 

Impedance measurements:

 

(click on picture to zoom)

Left: Blue=Left loudspeaker, Green=Right loudspeaker.

Right: Blue=Left loudspeaker, Green=Right loudspeaker + “System Impedance Correction”.

It doesn’t get better than this, a perfect impedance match between left and right loudspeaker. With the “System Impedance Correction” circuit the average impedance is 6 Ohms, which makes the loudspeaker an easy amplifier load that is tube-amp friendly.

 

Frequency measurements:

 

If nothing else is noted in the comments, the following frequency measurements of the finished loudspeaker are made at a 2m distance at tweeter height and for the 0db tweeter reference level.

All frequency response charts are presented in a 50db scale with 1/24 octave smoothing and the measurements are valid down to 400Hz.

(click on picture to zoom)

Top: Blue = Left Loudspeaker Red = Right Loudspeaker – 15deg off-axis @ 2m

Lower left: System and individual driver unit frequency response

Lower right: Reverse null response

Nothing to complain about here, a nice frequency response match between the two loudspeakers. Above 17kHz some variation in the top-end response.

A very deep reverse null indicating excellent phase behavior in the cross-over region.

(click on picture to zoom)

Left: Tweeter-axis @ 2m, 0, 15 and 22.5deg off-axis.
Right: Tweeter-axis @ 2m, 30, 45 and 60deg off-axis.

“Power response”, showing the average frequency response for the 0, 15, 22.5, 30, 45 and 60deg frequency measurements.

 

Distortion measurement:

 

Measurement setup:

  •         Tweeter-axis near-field measurement at 20cm
  •         Frequency Range: 200-10000Hz

 

The distortion measurements are done in near-field and the amplifier output level was adjusted for the loudspeaker so that the fundamental is 85dB at 1m and 90dB at 1m. This setting simulates normal to medium-high listening levels.

(click on picture to zoom)

Upper left: Left loudspeaker, 85dB @ 1m

Upper right: Right loudspeaker, 85dB @ 1m

Lower left: Left loudspeaker, 90dB @ 1m

Lower right: Right loudspeaker, 90dB @ 1m

This loudspeaker is overall a low distorting loudspeaker. The measurements show the cone-edge resonance at 1.2kHz that contributes to an elevated, but sonically harmless second-order harmonics.

Even though these measurement pictures doesn’t show the typical tweeter hard dome cone break-ups and its associated odd-order harmonics it’s still there, but not visible in these measurements and it contributes to the tweeters sonic character from approximately 9kHz and upwards.

Below 9kHz the odd-order harmonics are a non-issue.

 

Component list:

 

The suggested components would cost about 700 US$, but the cost can be reduced by choosing less expensive cross-over components and by building your own enclosures.

 

Summary:

 

Sound description:

The bass reproduction of this loudspeaker is very good and the SEAS U16RCY/P mid-woofer has the ability to play bass with authority comparable with the 3 times more expensive ScanSpeak Revelator 15W. In fact it has a “faster” bass character compared to the ScanSpeak driver, whereas the Revelator mid-woofer has the ability to play deeper bass.

The mid-range is “light dark” in its character. This could be a good or a bad thing depending on personal taste and preferences, but it doesn’t have the same mid-range resolution and clarity as more expensive driver units from AudioTechnology, ScanSpeak Revelator or for that matter SEAS Excel magnesium drivers.

With its non-analytic warm character the loudspeaker is very easy listened and less good recordings is fully enjoyable without listening fatigue becomes an issue.

The tweeter is very neutral in its character and it doesn’t have the exaggerated top-end shimmer or sparkle that some soft domes have. This could also of course be a good or bad thing depending on personal preferences. The tweeters sonic character works perfectly with the warm character of the mid-woofer and they complement each other nicely.

The loudspeaker has a very generous sound stage with a nice width and height, combined with an average depth sense. Thanks to the tweeters DXT lens the sweet spot exceptionally large.

With a sensitivity of 83-84dB 2.83v/1m, this loudspeaker does need some power to come alive and bloom and I recommend an amplifier of at least 50W or more. However, I’ve tested the loudspeaker with the “System Impedance Correction” circuit together with my 36W tube amp and it worked nicely as long as you don’t play very strong.

 

Final words:

The SEAS DXT tweeter is what makes this design distinguish itself from other two-way loudspeakers of similar size. With its unique smooth power response it has an extraordinary large listening sweet spot and a nice stereophonic image combined with a generous soundstage presentation in both height and width.

The SEAS U16RCY/P is a budget “Revelator” in terms of bass reproduction and the bass quality this loudspeaker can play is very good and outperforms many similar or larger sized mid-woofers and loudspeakers.

This very affordable DIY loudspeaker with its very simple cross-over design is both technologically advanced and conceptually interesting as well as high performing. Recommended!

 

Recommended loudspeaker placement:

  •         No or little toe-in.
  •         >0.5 meters from back wall.
  •         2-3 meters apart.
  •         2.5-3 meters listening distance

 

The loudspeaker is optimized for a listening distance of 2.5m, but everything between 2-3.5m works fine. As always it pays off to test different loudspeaker placements and see what fits the room and personal preferences the best.

Technical Specification:

 

Prestigious Two – Monitor DXT
Type: Two-way medium sized stand-mount loudspeaker
Enclosure type: Bass-reflex
Woofer driver unit: 5” Woven polypropylene cone. SEAS U16RCY/P (H1520)
Tweeter driver unit: 1” Magnesium/alu dome with DXT lens SEAS 27TBCD/GB-DXT (1499)
Cross-over frequency: 2.4-3kHz. (depending on tweeter level option)
Cross-over function: LR second-order acoustically, first-order electrically
Frequency response: 42-20000Hz -6/0db (on-axis)
Sensitivity: 83-84db 1m/2.83v
Impedance: 8 Ohm
Enclosure volume: 13 liter internal net volume
Weight: 11kg (24,25lb)
Enclosure dimensions: (height x width x depth): 356x216x317mm (14″x8.5″x12.5″)

 

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Thank you!

/Göran

Author of the “AudioExcite Loudspeaker Design” website