Peerless/SEAS monitor with XLS-12 subwoofer

                    

 

Introduction

This system can be considered an upgrade of the system I built for my brother. The system is bi-amped, a hybrid active/passive system. Between pre- and power amplifier a box with crossover electronics provides four monitor channels and two subwoofer channels. With the active crossover in place, the monitor woofers and the subwoofers will reach their maximum excursion limit at a total sound pressure of 105 dBSPL (because of room lift down to 20 Hz in most living rooms).

The two additional monitor channels can be used to drive surround speakers for movies and music DVDs. I do not use a center channel, I cannot find a good location for an identical monitor in the center position on my TV set. I do not feel I'm missing something since imaging is very good. I realise this set-up falls short when watching movies with more than two people. I also do not use a separate subwoofer channel, the two main channels provide plenty of bass and when I select 'center no' on my surround processor the bass of this channel is also directed to the main channels. Recently I discovered that when I select 'subwoofer no' that the bass is even getting too much especially with DTS (I therefore select 'subwoofer yes' without connecting the subwoofer channel to a power amplifier and subwoofer). So it's not 5.1 surround, but 4.0 surround effectively. Still I'm very satisfied. For me, the system should be excellent for stereo listening in the first place and very good for DTS and Dolby Digital. The power amplifier used by me is the Rotel RB-976MkII, which provides 6 channels of 75 watts into 4 ohms (and 60 watts x 6 into 8 ohms). This is barely enough to reach 105 dBSPL with this loudspeaker system, but it is good enough for me (105 dBSPL is too loud anyway regarding the relationship with my neighbours). The newest Rotel equivalent would be the RMB-1066. A nice alternative would be to use solid state amplifiers for the subwoofers and tube amplifiers for the monitors. At the moment I drive my front left and right monitors with the AMP1-B power amplifier, based on the class T 'digital' power amplifier TA2022. I consider it to be a major upgrade with respect to the Rotel RB-976MkII.

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Monitor drivers

The monitor drivers are the Peerless 850488 midwoofer (a.k.a. HDS 134) and the SEAS Millennium tweeter. The Peerless 850488 resembles very much the famous VIFA P13WH-00-08 in terms of speed and smooth frequency response, but it has lower distortion and higher excursion capability. This is important for high volumes, the 850488 has to produce frequencies down to 100 Hz with lots of dynamics. The SEAS Millennium is one of the best tweeters available. Distortion is very very low and its sound character has something that I can only describe as "right", very natural.

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Monitor enclosure

For the enclosure I took another approach when compared to the enclosure of my brothers system. It should be not too heavy, but still very well damped. Also, I wanted to use an internal transmission line which has inherent damping of standing waves. This allows for a not too tight stuffing with long hair wool to prevent a 'stuffed' sound. The enclosure is made of 12 layers of 18 mm MDF and a 13th layer which serves as the back panel. The top, bottom and sides are finished with 4 mm MDF. The front is finished with 6 mm MDF. Here you find the word document containing the dimensions of all layers. Building the enclosure means that you have tot do a lot of sawing, cutting and fixing with polyurethane construction kit. Each layer is hold together with only 2 screws during 4 hours to let the construction kit dry, generating an MDF/polyurethane/MDF constrained damping layer. For each new layer, the 2 screws are removed first and used to fix the next layer. The resulting enclosure is very stiff and well damped. The back panel is lined with rubber foil and fixed with 12 screws to the enclosure, to be able to do repairs and tweaks to the crossover.

Number of layers fixed, view from behind

Number of layers fixed, front

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Monitor damping

The end of the transmission line (just behind the tweeter, see the word document for the parts in dark grey on page 13) is stuffed with 6 gr of open cell foam (two small pieces of Pritex), the rest of the enclosure is stuffed with 46 gr of long hair wool. The wool filling is tight at the upper part starting at the open cell foam and more and more loose across the back of the enclosure down to the woofer.

Monitor finished

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Monitor crossover

As you can see, the electrical slopes are second order. The acoustical slopes, i.e. the combined response of the electrical crossover and driver responses, are third order in the crossover region. The crossover point is at 2000 Hz.

The inductors are standard air core. All capacitors are standard Intertechnik Audyn Cap MKP QS 400 V. I have checked and tried very high quality and expensive capacitors, but was not able to hear a "better" or more natural sound. If you insist you could use e.g. Auricap or Intertechnik Audyn Cap PLUS. It is up to you whether you consider it worthwhile to spend extra money on this component. Given the price of the truly excellent tweeter and keeping things in perspective, it should be no problem. The 0.82 mH inductor can be fixed to the flat bottom of the enclosure at the location closest to the back panel, the 0.39 mH inductor can be fixed in the upper corner region of the back panel. Look at the word document page 13 to see what I mean. The location of the capacitors is not critical. The 13.3 µF capacitor is obtained by parallel mounting 10 µF and 3.3 µF. The 2 µF capacitor is made from two 1 µF capacitors.

Below the anticipated response at the listening position. Also the total monitor impedance is shown.

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Subwoofer

The subwoofer is just a bigger version of the subwoofer that I built for my brother. The driver is a real long stroke subwoofer, the Peerless XLS-12 which is well known for its low distortion. The internal volume of the subwoofer enclosure is 50 liters which gives the 12" woofer enough room to breath. If you make the box not too tall, not taller than 24", internal standing waves will occur above 250 Hz and no internal damping with glas fibre or open cell foam will be needed. Maximum bass dynamics are obtained this way.

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Active crossover (including subwoofer equalisation and correction for room modes)

The schematic shows an RF noise suppression filter at the inputs, an input buffer with active bafflestep compensation, the active crossover between monitors and subwoofers, equalisation of the subwoofers for almost flat response down to 20 Hz, three notch circuits (for correction of room modes) and additional noise suppression for a symmetric DC power supply. Don't be frightened by the complexity, if you leave out the notch circuits and stick to stereo listening, only six dual OpAmps (OPA2134NE) are needed for building this circuit for both the left and right channel. The anticipated total system response at the listening position is shown below. Between 30 Hz and 20.000 Hz the response is +/- 2.5 dB, mainly due to diffraction.

The notch circuits are room specific and should only be implemented if needed and after a high resolution measurement of the response below 200 Hz at the listening position. Feel free to contact me for advise on component values if you want to implement notch circuits for your own situation.

The low frequency equalisation is a slightly modified version of the THOR design to obtain even flatter bass response. It also includes gain control (10 k variable resistor) for matching the monitor output level depending on the gain of the power amplifiers, driver sensitivity and room lift. Since I use identical power amplifiers, I have to correct only for driver sensitivity and room lift.

When trying to find a good place for the subwoofers, start at the opposite room corners of the same wall. The crossover between monitor and subwoofers is acoustic third order, therefore it is rather arbitrarily whether you connect the subwoofers in or out of phase. Just try and listen to get the tightest bass. The third order acoustic crossover gives flat power response and seemless integration between subwoofers and monitor. If the subwoofer level is set at the correct volume, you won't hear them until low frequency content is being sent to them.

Below you see the ugly electronics. The orange components are 56 nF styroflex capacitors. The input buffer opamps and output highpass filter opamps for the monitors are placed close to the connectors. The input noise filter is hard-wired directly at the input connectors. This way of circuit layout, combined with the OPA2134 opamps yields a no-compromise buffer/filter for the monitors. At the other end of the case you see the outputs for the subwoofers and the power supply connector. The rather big yellow components are the 10 µF capacitors of the DC supply filter.

     

     

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The sound

The sound is clear, transparent, crisp, detailed, airy, breathy, with depth, spacious, delicate, smooth, sweet, punchy, tight and weighty. Just kidding with this glossary of audiophile terms, but I am very satisfied with the result. Designing, building and optimizing the system was lots and lots of fun. Building two extra monitors will give you a Home Theatre system if you can miss the center channel. I hope that you will have the same amount of pleasure if you take the leap and decide to build this system!

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