Revelation Two – Monitor MkII BE

Concept:

 

Revelation Two – Monitor MkII BE is a small sized 2-way stand-mount loudspeaker for small to medium sized listening rooms. It offers several tuning options like two different bass-reflex tunings as well as tweeter tuning options.. This is the “Beryllium” tweeter option of the Revelation Two – Monitor MkII loudspeaker.

Loudspeaker driver units:

 

The Revelation Two – Monitor MkII BE uses two driver units from ScanSpeak. The mid-woofer section consists of the ScanSpeak 15W/4531G00 Revelator. The 15W/4531G00 is one of the best mid-woofers available to the DIY market. It has a nice low extension and a very nice mid-range reproduction.

The tweeter section consists of the ScanSpeak D3004/664000 “Beryllium” tweeter. This is a very low distortion tweeter which has a slightly dropping top-end frequency response from 15kHz to 25kHz. 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.

The “Beryllium” tweeter has a neutral effortless sound characteristic and with great high frequency resolution, but without ever sounding exaggerated or harsh.

For further details see:
ScanSpeak 15W/4531G00
ScanSpeak D3004/664000

Cabinet:

 

The Revelation Two – Monitor MkII uses an 11 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.

 

 

Cabinet drawing:Dayton 11 liters Curved Cabinet #302-711

 

 

 

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:

 

There are three different box tuning options in order to satisfy personal bass preferences and room setups.

Option 1 (reference):
11 liter bass-reflex box with Fb = 44Hz (anechoic response)
-6db = 39Hz
-12db = 31Hz

Option 2:
14 liter bass-reflex box with Fb = 39Hz (anechoic response)
-6db = 35Hz
-12db = 28Hz

Option 3:
14 liter closed box with Qt = 0,7 (anechoic response)
-6db = 51Hz
-12db = 34Hz

The closed box version has a surprisingly good bass performance when room gain is included. Option 2 goes remarkable low and sounds almost like a much bigger floor-stander. Option 1 used in this design has more mid-bass at the cost of low end extension compared to option 2.

Any of these box tuning options can be used without compromising the design as long as the baffle width and driver unit lay-out is the same.

Cross-over design:

 

The Revelation Two – Monitor MkII 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 two response shaping filter circuit (C1+L2+R1) and (C2+L3+R2) which shapes the cross-over slopes to a LR2 roll-off with a targeted 3kHz cross-over point. The large inductor (L1) tunes the design with a full “Baffle Step Compensation” (BSC).

The effect of the response shaping circuits can be seen in this picture. Without the response shaping circuit (C2+L3+R2) the loudspeaker have too much energy in the mid-range and is too much forward sounding to sound natural.

 

The tweeter cross-over filter section consists of a single tweeter padding resistor (R3) and a second-order electrical filter (C3+L4) that shapes the cross-over slope to a LR2 roll-off with a targeted 3kHz cross-over point. The “Beryllium tweeter has a slightly rolled off frequency response from 15kHz (-3dB @ 20kHz). The optional cap value of (C8) can be changed to adjust the tweeters top frequency to personal preferences.

 

C8 = 1.5uF -> +2.5dB @ 20kHz
C8 = 2.2uF -> +4dB @ 20kHz

(C6+L7+R4) 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 (C6) 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.

(C4+L5 and C5+L6) 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.

Cross-over simulation:

 

 

Top: Tweeter-axis, 15deg off-axis response.

Left: Tweeter-axis, 15deg off-axis response, reverse polarity.

Right: Individual driver unit phase tracking.

The reverse polarity simulation shows a deep reverse null indicating an excellent phase tracking between the two driver units at the cross-over point.

 

Impedance measurements:

 

 

Blue=C8 = 2.2uF, Green=Without C8

4 Ohm impedance minimum @ 200Hz.

 

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.

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

Frequency response 15deg off-axis:

Blue: Without C8
Red: C8=2.2uF

(click on picture to zoom)

Upper Left: Tweeter-axis @ 2m, 0, 15 and 22.5deg off-axis.
Upper Right: Tweeter-axis @ 2m, 0, 15 and 22.5deg off-axis C8=2.2uF.
Lower Left: Tweeter-axis @ 2m, 30, 45 and 60deg off-axis.
Lower Right: Tweeter-axis @ 2m, 30, 45 and 60deg off-axis C8=2.2uF.

 

Distortion measurement:

 

Measurement setup:

  • Tweeter-axis near-field measurement at 20cm
  • Frequency Range: 200-10000Hz
  • Baffle size WxH: 20,32×35,56cm (8×14″) (baffle edge round-over r=12mm)

The distortion measurements are done in near-field and the amplifier output level was adjusted for each driver so that the fundamental is 85dB, 90dB and 95dB at 1m. This setting simulates normal, medium and high listening levels.

 

(click on picture to zoom)

Upper left: Left loudspeaker, 85dB @ 1m

Upper right: Right loudspeaker, 85dB @ 1m

Mid left: Left loudspeaker, 90dB @ 1m

Mid right: Right loudspeaker, 90dB @ 1m

Lower left: Left loudspeaker, 95dB @ 1m

Lower right: Right loudspeaker, 95dB @ 1m

This is a low distortion loudspeaker with very low odd-order harmonics. At higher listening levels the second-order distortion rises, but the odd-order remains very low.

 

Component list:

 

The suggested components would cost about 2050 US$, but the cost can be reduced by choosing less expensive cross-over components and by building your own enclosures.

 

Summary:

 

Even though the Revelation Two – Monitor MkII BE loudspeaker is an expensive DIY design it can compete with similar far more expensive commercial designs.

It shares many of the same sound qualities as the Revelation Two – Monitor MkII loudspeaker design and it uses the same “Ladder Delay Network” technique to time align the tweeter with the mid-woofer.

When using the ScanSpeak 15W Revelator mid-woofer in the smaller sized 11 liter enclosure you get a faster mid-bass articulation compared to the larger 14 liter option, which in turns digs a bit deeper at the cost of speed. To choose enclosure size is more a matter of personal taste and sound preferences. They are both very good options.

The tweeter is well extended and neutral in its character. It never sounds harsh, exaggerated or tries to draw your attention from the music. Compared to the Revelation Two – Monitor MkII the Beryllium version reveals additional micro-details, is a bit more transparent and is a bit more analytic sounding in its character.

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 soundstage. Less good recordings are also enjoyable, since the loudspeaker is forgiving and non-analytic in its character.


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:

 

Revelation Two – Monitor MkII BE
Type: Two-way small sized stand-mount loudspeaker
Enclosure type: Bass-reflex
Woofer driver unit: 5,5” Sliced paper cone. ScanSpeak 15W/4531G00
Tweeter driver unit: 1.1” Beryllium dome ScanSpeak D3004/664000
Cross-over frequency: 2.9kHz
Cross-over function: LR second-order acoustically, first-order electrically (wo) and second-order electrically (tw)
Frequency response: 40-35000Hz -6/+0 db (on-axis)
Sensitivity: 87db 1m/2.83v
Impedance: 4 Ohm
Enclosure volume: 10 liter internal net volume
Weight: 9.5kg
Enclosure dimensions: (height x width x depth): 356x203x288mm (14″x8″x11,8″)

 

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Thank you!

/Göran

Author of the “AudioExcite Loudspeaker Design” website