Sequence Two – Monitor BE



Sequence Two – Monitor BE is a medium sized 2-way stand-mount loudspeaker for small to medium sized listening rooms.

This is basically the same loudspeaker as the Sequence Two – Monitor, but uses a ScanSpeak “top of line” Beryllium tweeter instead of the ScanSpeak D3004/660000 textile soft dome tweeter.

 Loudspeaker driver units:


Sequence Two – Monitor BE uses a mid-woofer driver unit from AudioTechnology. The 15H520613SDK mid-woofer uses a Kapton voice-coil in order to reproduce a more detailed midrange with higher dynamic and less coloration and distortion. This mid-woofer is one of the very best mid-woofers available to the DIY market. It has not the lowest bass extension, but has a wonderful “snap” and bass articulation together with a remarkable natural mid-range presentation, a class of its own.

The tweeter section consists of the ScanSpeak Illuminator D3004/664000 “Beryllium” tweeter. This is a very low distorting tweeter and it has an extended frequency response in both ends. There is no sign of the odd-order distortion remnants from the cone break-up, which is common for metal domes.

This tweeter is perfect for designs requiring a low cross-over point or as in this design a low-order cross-over with shallow cross-over slopes.

Sonically it matches the AudioTechnology mid-woofer beautifully and it has a neutral effortless sound characteristic and with great high frequency extension, but without ever sounding exaggerated or harsh or for that matter “metallic”.

For further details see:





In this design I’ve used 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:


~13 liter bass-reflex box with port tuned to 45Hz (anechoic response)

-3db = 46Hz

-6db = 40Hz

-12db = 33Hz

A classic QB3  bass-reflex alignment.

Cross-over design:


The Sequence Two – Monitor BE uses a true symmetrical second-order Linkwitz-Riley (LR2) filter topology.

The most common way to address the driver units relative acoustic center off-set is to use asymmetrical cross-over slopes, but in this design I use an electrical way to time delay the tweeter to adjust the acoustical center off-set. This is done by using a “Ladder Delay Network” filter circuit, also often called an “All-pass Network”, “Lattice Network” or “Phase Control Circuit”.

The mid-woofer cross-over filter section is very simple and consists of a large coil (L1) and a response shaping filter circuit (C1+L2+R1) that shapes the cross-over slopes to a LR2 roll-off with a targeted 2.7kHz cross-over point. The large inductor (L1) tunes the design with a full “Baffle Step Compensation” (BSC).

The tweeter cross-over filter section consists of a single tweeter padding resistor (R2) and a second-order electrical filter (C2+L3) that shapes the cross-over slope to a LR2 roll-off with a targeted 2.7kHz cross-over point. The value of (R2) can be changed to tailor the tweeter level to personal preferences.

R2 = 3.3Ohm -> +1dB
R2 = 3.9Ohm -> 0dB (Reference Tweeter Level).
R2 = 4.7Ohm -> -1dB

(C5+L6+R3) flattens out the tweeters impedance peak at the resonance frequency and it’s necessary to shape the filter to a LR2 filter slope, as well as to avoid “ringing” at the resonance frequency that could cause audible distortion. It’s ok to use a 68uF value for (C5) when using a high quality component or as in this design combine a cheaper electrolyte cap (68uF) together with a high quality 1.0uF cap.

(C3+L4 and C4+L5) contains the “Ladder Delay Network” circuit that time delays the tweeter so its acoustic center off-set matches the mid-woofer and makes a perfect phase behavior around the cross-over frequency with minimal phase distortion.

The tweeter is connected with reverse polarity and as can been seen in the schematics, the tweeter isn’t connected directly to ground. It’s of the outmost importance that the tweeter is connected exactly as shown, otherwise the delay network will fail.

(L7+C6+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 7 Ohms +/- 1 Ohm within 200-20000Hz.

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).

Impedance measurements:


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

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

Overall an easy load for the amplifier. Nice consistency between left and right loudspeaker. With the System Impedance Correction circuit the average impedance is 7 Ohms and makes the loudspeaker 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 350Hz.

Frequency response 15deg off-axis, reference tweeter level:

Blue: Left Loudspeaker
Red: Right Loudspeaker

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

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.

Upper left: Left loudspeaker, 85dB @ 1m

Upper right: Right loudspeaker, 85dB @ 1m

Lower left: Left loudspeaker, 90dB @ 1m

Lower right: Right loudspeaker, 90dB @ 1m

Overall a very low distortion loudspeaker. The only thing that smudges the measurements is the cone-edge resonance at 1.25kHz that contributes to an elevated, but sonically harmless second-order harmonics and to some extent a rise in third-order harmonics.

Component list:


At 2150 US$ a pair this DIY loudspeaker is quite expensive, but the cost can be reduced by choosing less expensive cross-over components and by building your own enclosures.

However I strongly advice to use the suggested cross-over component quality because it gives a very noticeable sound quality boost together with these two very fine loudspeaker drivers.



Sound description:

Even though this loudspeaker is a very expensive DIY design it easily outperforms similar commercial loudspeakers costing substantially more. It’s a big investment, but well worth it.

When comparing this loudspeaker with the Revelation Two – Monitor it doesn’t dig as deep in the low bass, but what it lacks in the lowest bass it exceeds in speed and micro articulation.

The midrange is clean, neutral and is effortless in its character. The reproduction of voices is very realistic and natural.

The tweeter is well extended, neutral and transparent in its character.  It never sounds harsh, exaggerated or “metallic”. The tweeter works in perfect harmony with the neutral and effortless character of the mid-woofer and they are a perfect match to each other.

The loudspeaker has a very generous sound stage with a nice width, height and depth.

With a well-made recording you are rewarded with a huge crisp soundstage. Less good recordings are also enjoyable. The Beryllium tweeter has a tremendous ability to present micro-details and has greater transparency than its textile tweeter cousin.

When you close your eyes and listen you can easily hear the location of the individual instruments and musicians in the recording.

It’s the marvelous mid-range reproduction that really makes this loudspeaker shine. AudioTechnology have succeeded to build a mid-woofer that can combine micro detail and speed, but without ever sounding could or analytic, together with a very convincing and natural voice reproduction.

Final words:

Compared to the Sequence Two – Monitor, which uses a textile dome tweeter the Beryllium version is very similar sounding. In a positive way the Beryllium version reveals additional micro-details, is a bit more transparent and is a bit more analytic sounding in its character.

Which one to choose?

If the budget isn’t an issue, go for the Beryllium version. Otherwise the textile version has a better price/performance ratio.

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:


Sequence Two – Monitor BE
Type: Two-way medium sized stand-mount loudspeaker
Enclosure type: Bass-reflex
Woofer driver unit: 5,5” Polypropylene cone. AudioTechnology 15H520613SDK
Tweeter driver unit: 1,1” Beryllium dome ScanSpeak D3004/664000
Cross-over frequency: 2.7kHz.
Cross-over function: LR second-order acoustically, first-order electrically (wo) and second-order electrically (tw)
Frequency response: 40-20000Hz -6/-1db (on-axis)
Sensitivity: 85db 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″)


I’m happy to accept any kind of donations!

Thank you!


Author of the “AudioExcite Loudspeaker Design” website