{"id":3261,"date":"2013-01-25T16:40:15","date_gmt":"2013-01-25T16:40:15","guid":{"rendered":"http:\/\/www.audioexcite.com\/?page_id=3261"},"modified":"2020-11-28T14:43:53","modified_gmt":"2020-11-28T14:43:53","slug":"revelation-two-monitor-mkii-be","status":"publish","type":"page","link":"https:\/\/www.audioexcite.com\/?page_id=3261","title":{"rendered":"Revelation Two \u2013 Monitor MkII BE"},"content":{"rendered":"

Concept:<\/span><\/h3>\n

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Revelation Two \u2013 Monitor MkII BE<\/strong><\/span> 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 \u2013 Monitor MkII<\/a> loudspeaker.<\/p>\n

Loudspeaker driver units:<\/span><\/h3>\n

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The Revelation Two \u2013 Monitor MkII<\/strong><\/span> 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.<\/p>\n

The tweeter section consists of the ScanSpeak D3004\/664000 \u201cBeryllium\u201d 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.<\/p>\n

The \u201cBeryllium\u201d tweeter has a neutral effortless sound characteristic and with great high frequency resolution, but without ever sounding exaggerated or harsh.<\/p>\n

For further details see:
\nScanSpeak 15W\/4531G00<\/a>
\nScanSpeak D3004\/664000<\/a><\/p>\n

Cabinet:<\/span><\/h3>\n

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The Revelation Two \u2013 Monitor MkII<\/span><\/strong> uses an 11 liter \u201cDayton Audio\u201d enclosure. This is a very nicely built curved cabinet with a 25mm (1\u2033) MDF front-baffle. The enclosure walls are made of 18mm MDF as well as a 15mm brace from the bottom to the top.<\/p>\n

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Cabinet drawing:Dayton 11 liters Curved Cabinet #302-711<\/a><\/p>\n

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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 \u201cbreathe\u201d properly, don\u2019t forget to chamfer the baffle.<\/p>\n

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Box simulation:<\/span><\/h3>\n

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There are three different box tuning options in order to satisfy personal bass preferences and room setups.<\/p>\n

Option 1 (reference):<\/strong>
\n11 liter bass-reflex box with Fb = 44Hz (anechoic response)
\n-6db = 39Hz
\n-12db = 31Hz<\/p>\n

Option 2:<\/strong>
\n14 liter bass-reflex box with Fb = 39Hz (anechoic response)
\n-6db = 35Hz
\n-12db = 28Hz<\/p>\n

Option 3:<\/strong>
\n14 liter closed box with Qt = 0,7 (anechoic response)
\n-6db = 51Hz
\n-12db = 34Hz<\/p>\n

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.<\/p>\n

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.<\/p>\n

Cross-over design:<\/span><\/h3>\n

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The Revelation Two \u2013 Monitor MkII<\/strong> BE<\/strong><\/span> uses a true symmetrical second-order Linkwitz-Riley (LR2) filter topology.<\/p>\n

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 \u201cLadder Delay Network\u201d filter circuit, also often called an \u201cAll-pass Network\u201d, \u201cLattice Network\u201d or \u201cPhase Control Circuit\u201d.<\/p>\n

The mid-woofer cross-over filter section is very simple and consists of a large coil (L1<\/strong>) and two response shaping filter circuit (C1+L2+R1<\/strong>) and (C2+L3+R2<\/strong>) which shapes the cross-over slopes to a LR2 roll-off with a targeted 3kHz cross-over point. The large inductor (L1)<\/strong> tunes the design with a full \u201cBaffle Step Compensation\u201d (BSC).<\/p>\n

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

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The tweeter cross-over filter section consists of a single tweeter padding resistor (R3<\/strong>) and a second-order electrical filter (C3+L4<\/strong>) 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<\/strong>) can be changed to adjust the tweeters top frequency to personal preferences.<\/p>\n

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C8<\/strong> = 1.5uF -> +2.5dB @ 20kHz
\nC8<\/strong> = 2.2uF -> +4dB @ 20kHz<\/p>\n

(C6+L7+R4<\/strong>) flattens out the tweeters impedance peak at the resonance frequency and it\u2019s necessary to shape the filter to a LR2 filter slope, as well as to avoid \u201cringing\u201d at the resonance frequency that could cause audible distortion. It\u2019s ok to use a 68uF value for (C6<\/strong>) 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.<\/p>\n

(C4+L5<\/strong> and C5+L6<\/strong>) contains the \u201cLadder Delay Network\u201d 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.<\/p>\n

The tweeter is connected with reverse polarity and as can been seen in the schematics, the tweeter isn\u2019t connected directly to ground. It\u2019s of the outmost importance that the tweeter is connected exactly as shown, otherwise the delay network will fail.<\/p>\n

Cross-over simulation:<\/span><\/h3>\n

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Top:<\/strong> Tweeter-axis, 15deg off-axis response.<\/p>\n

Left:<\/strong> Tweeter-axis, 15deg off-axis response, reverse polarity.<\/p>\n

Right:<\/strong> Individual driver unit phase tracking.<\/p>\n

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

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Impedance measurements:<\/span><\/h3>\n

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Blue<\/strong>=C8 = 2.2uF, Green<\/strong>=Without C8<\/p>\n

4 Ohm impedance minimum @ 200Hz.<\/p>\n

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Frequency measurements:<\/span><\/h3>\n

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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.<\/p>\n

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

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Frequency response 15deg off-axis:<\/strong><\/p>\n

Blue:<\/strong> Without C8
\nRed:<\/strong> C8=2.2uF<\/p>\n

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(click on picture to zoom)<\/p>\n

Upper Left:<\/strong> Tweeter-axis @ 2m, 0, 15 and 22.5deg off-axis.
\nUpper Right:<\/strong> Tweeter-axis @ 2m, 0, 15 and 22.5deg off-axis C8=2.2uF<\/strong>.
\nLower Left:<\/strong> Tweeter-axis @ 2m, 30, 45 and 60deg off-axis.
\nLower Right:<\/strong> Tweeter-axis @ 2m, 30, 45 and 60deg off-axis C8=2.2uF<\/strong>.<\/p>\n

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Distortion measurement:<\/strong><\/span><\/h3>\n

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Measurement setup:<\/strong><\/p>\n