SOUND FOUNTAIN
  

hobbyists's views for hobbyists
How to Use an Electronic Crossover Network
Adding a Subwoofer - Building a 3 Way Active Loudspeaker



INGREDIENTS

Building an active loudspeaker system involves various ingredients and a number of choices:

1. The availability and choice of a suitable electronic crossover unit or module.
2. The determination of how to divide the frequency band (the number of ways per channel).
3. The choice of the appropriate loudspeaker units in relation to efficiency, frequency response (bandwidth) and linearity.
4. The choice and availability of power amplifiers

Units, cabinets, electronic crossover and amplifiers, all these are interrelated. So we have to deal with many variables which make the road to harmony, excellent transient response and efficiency a difficult one.

ADVANTAGE

The main advantage of an active system is that the loudspeaker units are directly connected to the amplifiers without the use of filter sections with capacitors, coils and resistors. Especially the omittance of coils used in passive filters (which are placed in series with the voice coil of the loudspeaker unit), make the units do their job in an optimum manner. Coils in series with the voice coil generally deteriorate the behavior of the unit. That is why designers often use coils with a ferrite core or a specific transformer.
The proper loading and filtering is translated in a cleaner, clearer and better controlled sound. Only of course if the volume for each unit (acoustic loading) is well chosen. This volume does not need to be the volume calculated by the program for filter and crossover.
Another important advantage is that one can choose units which are generally not used in commercial systems: 38 cm bass speakers, horn loaded bass and/or midrange speakers, high efficiency ring radiator tweeters, ribbon tweeters, etc.
Also speaker units can be selected which are built with specific materials for cones and magnets. Paper for instance has gone out of use. Paper has the advantage that it does not store energy as some modern materials do. Paper gives more detail as it does not smooth out (equalize) the curve.
Units with heavy magnets are expensive. Units with alnico magnets are expensive. So these are hardly used in commercial systems. When constructing or assembling an active loudspeaker system, such units can of course be incorporated. And if they are too expensive, high quality second hand units may be acquired.

DISADVANTAGE

A disadvantage of building an active system is that units have to be chosen which do have rather linear response characteristics because the aim is to avoid passive components which are generally used to linearize the frequency responses and the impedance curve.
Remember: most electronic crossovers have slopes of 6, 12, 18 and only in some cases 24 dB per octave. That's it. No 9 dB or 16 dB. The correction of the impedance of the units in an active system is of course possible by inserting a well calculated network in parallel (a capacitor, a resistance and sometimes a coil). The effect of such a correction is that the high end of the curve is less free sounding. That is why it is better to choose a unit which has an impedance curve which is rather straight far into the high end of the curve. If you want to know what I mean just check the impedance curve of a few units and compare the impedance curves of for example Visaton WSP21S and Visaton W200S.
Whether it is advisable to apply these corrections depends on the configuration and also can be a matter of personal judgment.

BEST SYSTEM: 2 WAY

The best loudspeaker system is a 2-way system whether it is a passive or an active system. In a 2-way system a wide frequency band is reproduced by a low-mid unit, starting at the fundamental frequency up about 3000 or 4000 Hz. (or sometimes even higher).
The advantage is that a harmonious buildup of the sounds of instruments is not disturbed by filter section early in the audio band, nor is it disturbed by phase shifts caused by the mechanical and electrical differences in woofers and midrange units. The ear is most sensitive to irregularities in the low and mid frequency band up to at least 2000 Hz.
That is why the best system is a 2-way system. That is also why in olden days the best 3-way systems left lows and mid frequencies in tact and filtered at around 600 (and sometimes at 900, 1500 or even at 2000 Hz.) and the crossover frequency of the tweeter was at 3000 Hz. or 5000 Hz. In certain cases even 7000 Hz. was chosen if a the crossover point between woofer and mid band unit was high. Then specific tweeters could be used like the Decca Super Tweeter and Fostex Ring Radiators. Such crossover frequencies guarantee a harmonious build up in the low and lower mid sections and keep the midband open and whole.

2 WAY

So your active system could well be a 2-way system with one or two 7 or 8 inch woofer/midrange unit(s) plus a tweeter. The determination of the crossover frequency depends on the capabilities of the chosen tweeter (linearity and power handling) as well as the technical specifications of the low-mid unit and especially the linearity of the frequency curve. In this way you will avoid all the pitfalls which present themselves when constructing a three-way system with three amplifiers and three different units for low, mid and high frequencies.
The choice of units for an active 2-way system can be inspired by existing DIY sets which do not use complicated filters but serial filters or simple sections with 12 and/or 6 dB slopes. If an existing DIY design from a shop is chosen as a basis of your active system, see to it that the volume for the woofer/low-mid range unit is recalculated as you leave out the passive filter section with the coil in series. The volume should be less.
Many high fidelity enthusiasts stick to the classic configuration of two satellites and one (or if the budget and the room allows) two sub woofers. Especially analog fanatics go by this configuration to which extra speakers can be added and used when viewing movies.
In all cases the main speakers can consist of various transducers in appropriate cabinets.

SIMPLE

If you want to go active by just adding a subwoofer to a pair of satellite speakers, you can start by using existing 2-way systems and combine these with just one subwoofer, home built or manufactured commercially. This is the most common configuration. There are many options.
Not every sub woofer sounds the same. The best options are the closed box systems, the front loaded (band pass) systems and the Janis subwoofer principle (which is actually also a band pass system). The advantage of the band pass system is that it sounds like a closed box systems with a continuously increasing excursion going down from 100 to 20 Hz. for example. These systems do need woofers with strong magnets and these are of course expensive. That is why so many designers use units in bass reflex systems which are cheaper. But bass reflex (ported) systems have an irregular excursion curve with a dip in the 60 to 80 Hz. region or there about. There the excursion is minimal.
Back loaded horns do not go deep and if you want to reproduce low frequencies through a horn, the mouth of the horn should be extremely large. Two of these can cover the entire wall of your listening room.
Also transmission line systems have a peculiar behavior. In the region of 60 to 80 Hz. there is a slight phase shift in most systems. And again the cone excursion is restricted in that area. There the sound is slender if compared to a big unit in a closed box and in many transmission lines the lowest frequencies are emphasized and are slow.
There are also designs for open baffles. But the side panels have to be very large (as in the case of the horns). And there are various dipole designs.
Each and every box, baffle, column or screen has to reproduce sounds which in real life are produced by large instruments (organ, grand piano, big drum, an large orchestra), or by instruments with a entirely different shape (violin, cello, flute, etc.), it certainly is no mean task to get a loudspeaker system"right".

TO BOOST OR NOT TO BOOST THAT IS THE QUESTION

Many hobbyists and audiophiles use 24 dB Linkwitz filters to separate the woofer completely from the transducers for the mid and high frequencies. It is up to you if you want to opt for such steep filter slopes. It may be that when gaining more experience while assembling an active system you will find out that less steep slopes may result in better phase and a more harmonic and lively sound.
Also you are free to correct the frequency curve of the unit in the subwoofer enclosure, and to boost the lowest frequencies as some designers do propagate. This is however a method which I personally do not like at all. Better is to give the unit its appropriate volume and curve, as the parameters enable you to calculate.
Also Motional Feedback circuits do hamper transient response. Feedback correction tops of the pulses of the woofer. And as the lows are the basis on which the harmonics are build, the feedback correction can be heard in the rest of the frequency band, in the total sound picture.

QUALITY

A higher level of quality can be achieved when using commercially manufactured 3-way systems as satellites, instead of two-way systems. The better quality is specifically attained with satellite three-way systems which have rather large 12" or two 10" woofers in parallel. I have discovered that large woofers for the lower mid section do follow up the frequency band of the subwoofer in a much better way than small sized units do. Large woofers provide a far more realistic sound.
The choice of crossover frequency will be around 100 Hz. Such a system can later be updated by adding a second subwoofer.
Although it is said that frequencies under 150 Hz. are non-directional, it does matter whether one or two subwoofers are being used.
Another option is to go active with the small satellite 2-way systems and omit the specific subwoofer, but combine 2-way satellites with large woofers as in a full blooded 3 way system. These woofers have to have their appropriate (well calculated) cabinets, one for the left and one for the right channel. The satellite 2-way systems can be home built and optimized in relation to the woofer. The appropriate turnover-frequency should be determined in practice but could be situated in the region from 350 to 600 Hz.. Such a system has the advantage that it leaves at least three octaves of the fundamentals intact.

CLASSIC 3-WAY

The most original and most difficult option for assembling/constructing an active loudspeaker system is not to use existing 2- or 3-way satellites with subwoofers, but to calculate and build the cabinets for the low, the mid and the high frequency bands oneself. Whether the choice is for incorporating a subwoofer or not, the midrange units must have the ability to perform well up to round and about 3000 or 4000 Hz. A very musical system can consist of a 12" woofer, one 5" or 7" midrange unit and a quality unit for the high frequencies. Crossover frequencies could be 600 and 3000 Hz. in case of a dome-tweeter or horn loaded ribbon tweeter. In the event a ring radiator like one manufactured by Fostex is chosen, the crossover frequency for the treble should be 7000 Hz. With such a system a very harmonious and lifelike reproduction can be achieved especially if you choose high efficiency units. That is the classic way.
There are many options and many possibilities and also many pitfalls. But you can begin by adding a subwoofer and crossover to an existing system. And that can give you already increased listening pleasure, a lot of fun and it is a learning school for gaining knowledge. After some time you can continue from there and expand your active system. Or not. It is up to you.

COMPLICATED

It takes a lot of calculation and experimentation to find the best units for a three way system with crossover frequencies of 100 Hz. and, say, 4000 Hz. as the mid band speaker should well be able to reproduce the most important section of 100 to 200 Hz. as well as the higher frequencies.

You see: the possibilities are many. The question remains: What is a good option?
Whatever configuration is opted for: the quality of the final result depends on your own experience and expertise which you will gain in practice. Do not think that you will be ready in a couple of months. An active system is not easily built. To give more insight in the matter the following topics will be considered:

1. Subwoofer and the Electronic Crossover Module
2. Amplifiers, Loudspeaker Cable and Interconnects
3. Woofer, Low-Mid Loudspeaker Unit and High Frequency Unit.

Subwoofer and Electronic X-Over


LABYRINTH, AIR COUPLER, KELTON, HITACHI, JANIS

Subwoofers have existed since the early days of high fidelity. Even before World War II (according to Jean Hiraga in his excellent book 'Les hautparleurs' - Editions Fréquences. Paris, 1980) a man named Olney proposed a loudspeaker specifically designed for the reproduction of the low frequencies. Mr. Olney used a labyrinth (which was later tapered by Dr. Baily and became known as the 'transmission line' loudspeaker). The first issue of High Fidelity Magazine in 1950 describes another low frequency system: the 'air coupler', which is a long slim cabinet with a loudspeaker mounted on the back panel, radiating into it and just above the speaker unit is a wide port.

Click on the drawing to view a larger image.

The cabinet is 6 feet tall (long). The Air-coupler is meant to reproduce the frequencies from 20 Hz. up to around 350 Hz. The cabinet was complemented by a two way speaker or a dual concentric or a loudspeaker unit like a Goodmans Twinax or some similar construction. Both air-coupler and full range cabinet had their low pass, respectively high pass filters. I have scanned the drawing from my (rare) copy of the first issue of High Fidelity magazine (summer 1951) and have put the drawing on another page. Just click on the drawing above. Maybe it could inspire you to build an air coupler and find the best crossover frequency.

The 'air coupler' (in two versions with a length of 6 feet and 8 feet respectively) as pictured in High Fidelity Magazine Vol. 1 No. 1 (1950)

In the nineteen fifties Kelton designed a system which was further developed by Hitachi. This idea became the basis for the bandpass subwoofer of today.
In the mid nineteen seventies John Marovskis from the Bronx (New York) designed his extraordinary Janis W-1 and W-2 subwoofers. The story goes that he made some 50 cabinets before he arrived at the excellent sonic performance of his systems which have a flat frequency response from 30 to 100 Hz. (-3dB). He designed an electronic crossover module with a 60 Watt amplifier to go with the handsomely finished cabinet. This unit, the Janis Interphase, filtered the low pass section (Janis W1 or smaller W2) and the high pass section (satellites) at 100 Hz. both with slopes of 18 dB per octave.
Another, most important feature was the continuous variable phase adjustment of the subwoofer (0-180º) in order to achieve a seamless and harmonious integration of subwoofer and satellites, independent of the subwoofer's position in the room.
In the nineteen seventies the Dutch importer of the Janis subwoofers in the Netherlands (Jan Endenburg of EngaSound) pointed out to me that optimum phase coherence could be obtained by a 12dB low pass for the Janis W1 and a 6dB high pass filter for the satellites as was achieved by the than cherished and famous Symmetry Electronic Crossover. But I found that the 18 dB slopes were much cleaner. Once in a while you read a rather negative review about the Janis subs. But these are always written by people who do not know how to use them. They make the mistake of using high current amplifiers (NAD, Rotel, Bryston, etc.) that get their power out of the large toroidal transformer and when that one is exhausted, directly out of the AC mains. In the past I have tried a Tandberg power amplifier for the satellites. The sound became realistically impressive when for instance an Amcron power amplifier was used, or mono blocks from French manufacturer Elipson. Only then the music got its impact and slam. Not only when playing Tchaikovsky's 1812, but any record from the collection.
Another favorite bass system was the Hartley sub woofer which was combined with de Decca Ribbon high frequency unit and the Quad electrostatic speakers (ESL). Impressive as well. (See: The Decca London Ribbon Horn Tweeter.)

In the last 15 years many well known manufacturers of expensive subwoofers have forgotten about the importance of the continuous variable phase feature or just do not realize its importance and only adorn their modules with a phase-switch with the two positions of 0º and 180º.
A few years ago I had a severe discussion with the manager of a well known English manufacturer of big subwoofers. Even their most expensive subwoofers can only be integrated with great difficulty into a given high fidelity stereo-set of speakers. At times it is even impossible to find a seamless transition of these subwoofer with the satellites. Most people think that a boom is what it is all about. This British manufacturer certainly have not done their homework. I have witnessed the horrible sound produced by expensive Avalon loudspeakers (and other high-end loudspeaker systems) in various living rooms because the electronics of the expensive subwoofer(s) did not have the continuous variable phase control. Disconnecting the subwoofer brought back the harmonious sound and the slam of the Avalons. Even today, many years after my conversation, these subwoofers still do not have the variable phase control but just a phase switch.

Yet there are electronic crossover modules on the market for filtering the subwoofer and correcting the signal via the continuous variable phase control from 0º to 180º. Best known are those of Detonation (search with Google) and Thommessen, both from Germany.

THE CROSSOVER MODULE

If you decide not to add a subwoofer to satellites but you want to assemble a full blooded 3-way system you will have to make your choice with care. There are several x-over units on the market. There are units made especially for studios and for PA purposes.
Try to find a high quality module for real high fidelity application. Three-way modules do not have the adjustable phase control for the subwoofer. In that case you have to find/calculate the best position of the units in relation to each other as you would do when designing a 3-way passive system with stepped/recessed baffles. You may even look for a vintage unit on eBay or Audiogon.
I myself use a vintage crossover unit: the Lux Kit 2003 from Luxman which was a present from one of my mentors, Mr. W. de Haan, well-known icon of the hifi-history of the Netherlands, not only because he imported for a long time Tandberg and later the Mission and Cyrus products, but also because he is above all a first rate technician.

The Luxman Crossover was available in the nineteen seventies. It has 8 tubes of the type 12AU7 (ECC82). This unit has slopes of 6 and 12 dB. The turnover (crossover) frequencies can be adjusted easily by inserting small circuit boards with high quality capacitors. Even then the turnover frequency can be adjusted in 5 steps of 1/8th of an octave of that frequency. This means that each step is 1/8 of twice the chosen basic frequency or 1/8 of half that frequency. In other words: 5 steps of 1/8th of an octave which makes the Luxman 2003 crossover unit extremely versatile and lets you immediately know what happens if you change the crossover frequency and the level of one or more sections. No other crossover has these flexible adjustments for fine tuning in order to produce beautiful harmonics. See for schematics and details at the end of this page.

Note: the Marchand electronic crossovers are designed with 24 dB or even steeper slopes and they use 12AX7 tubes. The steep slopes are not to everybody's taste.

TUBES, HARMONICS, CHASSIS, RESONANCES

As with all cabinets and chassis, also the Luxman shows some microphony when tapping on the housing.
The level depends on the way the chassis is built (weight of the material, strength and sturdiness of the
construction), on how tight the tubes fit in the sockets, and were the crossover is located.
I have heard tube amplifiers constructed by hobbyists and manufacturers alike which were too heavy and placed the mechanical resonance (resonant frequency of the unit) too low and made the sound muddy to such an extent that they began to sound like solid state amplifiers.
The schematics of the Luxman do not prescribe the way the cabinet should be built. So it is up to the hobbyist to decide what housing he will build and how he will go about it.
I myself am against too heavy housings for tube amplifiers. I myself am against the silicone tube rings or whatever rings are often used to suppress resonances. They not only dampen the resonance, but do make the sound un-tube-like and less harmonious. But I must emphasize that these opinions/findings are strictly my personal preferences.
Many tube amplifiers of today sound neat but are too heavy in the lower region and specifically in the bottom.
One of the features of good constructions is the airiness of the sound, specifically tube amplifiers and valve crossovers It is wise to seek the best construction and weight of the housing of any amplifier and especially the Luxman filter.
The total weight of the original Luxman is about 6 kg. which is some 13 pounds. Weighing the parts (transformer, circuit boards, components, capacitors, potentiometers, etc.) and subtracting this from the 6 kg. can give some indication about the weight to be chosen for the cabinet of the X-over.

THE DESIGN

So, you should decide on what configuration you are going to make: just add a subwoofer to existing satellites or design a real active loudspeaker system.

Let us start with the subwoofer. There are many subwoofers on the market. When surfing on the world wide web you will encounter many different propositions and plans to build a subwoofer yourself. Read these plans carefully and find out about efficiency, power handling, frequency band, linearity and size of the cabinet, and then decide what way to go. If you are into classical music and into jazz and the refined reproduction of analog material, I advise not to use any electronic bass boost. You just should strive to select a bass unit and construct the appropriate cabinet so that low frequencies will be reproduced without any artificial compensation. That is only if you have rather sensitive ears. If so, you may have heard that boosting and correcting the low frequencies (as in motional feedback systems for example) yields to a completely different (unnatural) low frequency reproduction.

When planning to build a subwoofer you should look for 12" or even 15" speaker units that have a low resonance frequency and the appropriate Qts for functioning in a closed compartment. Try to find units that do not have a foam suspension which will deteriorate in time. The unit should have a rubber roll surround which can stand the test of time. Yes, they are expensive. I was lucky to buy a pair of second hand 1976 Goodmans 3-way systems which had 12" woofers in a volume of 53 liters. The suspension of the woofers is rather stiff, but as I could hear before I bought them, the resonance frequency seemed relatively low. The units weigh some 10 lb. each. Since the basket is of steel most of the weight goes into the magnet.

Qts = 0.30
Vas = 199 l
Fs = 31 Hz.
Re = 7.19
Le = 3.93
Qms = 1.83
Qes = 0.36
Spl = 90 dB
Bl = 13
Diameter = 26.4 cm
Sd = 0.550 cm2
Pe = 75 Watt
Z = 8 Ohm

There are speaker units which are especially suitable to be used in band pass enclosures I wanted to see if these Goodmans woofers could do the job as well. A friend measured both units and came up with interesting parameters and to my surprise the resonance frequency was 30 Hz. and the QTS had a nice value of 0.30. I would have welcomed a slightly higher QTS and a lower Fs though. In order to calculate a bandpass system I used Juha Hartikainen's valuable WinISD beta-program which you can download from the web. It is a rather simple but effective program which also can calculate closed boxes and vented enclosures.

Hartikainen's program makes it possible to determine the desired frequency response by altering and adjusting the volumes of the front and rear chambers just by moving the mouse over the drawing. Since a computer program is infatigable one can go on calculating and by trial and error come up with many possible solutions. The aim is threefold.

First it is important to change volumes in order to come up with a low -3dB frequency which is situated around 30 Hz. or possibly lower.
Second: it is important to find an upper -3dB frequency situated around 100 Hz. to go with it.
Third: in all cases the curve should be as linear as possible and at the same time the impulse should the best you can get. If the configuration you calculate shows a bad impulse and a frequency characteristic with the shape of a saddle, than calculate again and again.

Most subwoofer designers choose a frequency curve which is linear within 3 dB. This is all right if you have a large listening room. In small rooms the long wavelengths of low frequencies cannot establish themselves. So what is the use of reproducing 20 or 30 Hz. when you have to sit in the kitchen or in the hall in order to hear them properly? There is a second problem. When adding a subwoofer to a satellite system there is a possible side effect and that is that the sound in the vicinity of the crossover frequency gets a slender character, especially in cases a satellite with a relatively small bass-midrange unit is used. To avoid this one has to doctor around and find the best integration without changing the strength around the crossover frequency. In Hartikainen's program you can alter the system's resonance frequency and come up with a different port length and characteristic. This all helps to find out what the best rear and front volumes are. The final adjustment of the port length should be done when setting up the system in practice. I calculated a volume of 29 liter for the front and 58 liters for the rear volume. The volumes were actually 30 and 60 liter because the bracing and the woofer's baffle take up some of the volume,

I decided to position the units on oblique baffles. This would help minimizing standing waves. It would also position the coils somewhat farther towards the front of the box for better alignment with the other units. The Goodmans woofers only have four holes for fixing. Four clamps were added for rigid fixing of the units. The box is well braced using metal strips glued and screwed on wooden sticks to minimize panel resonances. The unit is braced as well by means of a beam supporting the magnet. The front of the box is detachable so the unit can be screwed on the baffle and damping material can be adjusted. Each subwoofer has 2 vents which can be adjusted as well in order to achieve a seamless integration.

 

 I was lucky to buy a pair of second hand 1976 Goodmans 3-way systems which had 12" woofers in a volume of 53 liters. The suspension of the woofers is rather stiff but the resonance frequency is relatively low from what I could hear. The unit weighs some 10 lb. Since the basket is of steel most of the weight is of the magnet.
A friend measured the parameters and to my surprise the resonance frequency was 30 Hz. The speakers could be used in bandpass subwoofers. For calculation I used Juha Hartikainen's valuable WinISD beta-program which you can download.
Hartikainen's program makes it possible to determine the desired frequency response by altering and adjusting the volumes of the front and rear chambers. I choose a volume of 29 liter for the front and 58 liters for the rear volume.

The volumes were actually 30 and 60 liter but because bracings and the woofer's baffle, the volumes were calculated slightly less: 29 and 58 liter respectively.
I decided to position the units on oblique baffles. This would help in minimizing standing waves. It would also position the coils farther towards the front of the box for better alignment with the other units. The drawing shows the small difference according to the inclination.
The units had only four holes for fixing. Four clamps were added for rigid fixing of the unit.
The box is well braced using metal strips glued on wooden sticks to minimize panel resonances. The unit is braced as well.

The front of the box is detachable so the unit can be screwed on the baffle and damping material can be adjusted.

The subwoofer has 2 vents and is mounted in an iron frame together with the units for mid and high.
Qts = 0.30
Vas = 199 l
Fs = 31 Hz.
Re = 7.19
Le = 3.93
Qms = 1.83
Qes = 0.36
Spl = 90 dB
Bl = 13
Diameter = 26.4 cm
Sd = 0.550 cm2
Pe = 75 Watt
Z = 8 Ohm

Amplifiers, Units and Power

PHASE AND FREQUENCY RESPONSE

The choice of amplifiers depends on several criteria.
1. Efficiency of the loudspeaker units.
2. Size of the room.
3. Crossover filter.
4. Available budget.

LOUDSPEAKER UNITS

If you use loudspeaker units with an efficiency of 85 to 88 dB and they can handle enough power, it is advised to use power amplifiers with a high power capability. That means not just Watts, but how these are generated and eventually stored is important. Only amplifiers with large capacitors for storage of lots of energy will do the job.
Amplifiers with large toroidal transformers and just two capacitors with low values do not pump enough power in 85 to 88 dB units. Yes, they generate power. But the power is mainly drawn from the big transformer, i.c. from the net. The effect of these amplifiers (most of which are in the non-clipping category) can not only be measured but can be heard as well. In heavy and loud, complex passages in classical symphonic music the attack of drums, brass and cymbals should be instantaneous and direct. In other words: if the conductor's baton says "now!", it means "now!" for all musicians involved. When using low efficiency speakers with amplifiers which draw their energy from the transformer, such complex passages do not sound instantaneously but there is a small time delay which in commercial loudspeakers get larger and is really noticeable when complex passive, energy-absorbing filter sections are being used. Many times the sound of the kettle drum which needs more energy comes at a different time than the sound of a cymbal. So it is advised to use these amplifiers with big toroidal transformers in conjunction with relatively high efficiency speakers of 90 to 93 dB.

If you use units with an efficiency of 95 dB and higher, you also should choose your amplifiers with care. Especially high efficiency units make it possible to use small power transistor amplifiers or valve (tube) amplifiers which are fast. Power amplifiers with many hundreds of Watts and large capacitors for energy storage, generally mean overkill for these units.

The most important criterion for choosing amplifiers for an active loudspeaker system is that they should be of the same electronic design. A manufacturer like Cabasse for instance, who have a long history of designing active loudspeaker systems, may choose power amplifiers with different output power for the low-, mid- and high frequencies. But what all these amplifiers have in common is the same design philosophy. This results in phase coherence of the amplifier throughout the entire system.

So the best option is to choose the same type of amplifier for all loudspeaker units. Most of the time you will choose amplifiers from the same manufacturer and even with the same reference number.
When choosing amplifiers in relation to the efficiency of the units you can find the optimum balance of speed and power capability.

THE ROOM

It goes without saying that small listening rooms ask for small speakers with an efficiency around 88 dB and relatively small amplifiers. The sound generated by high efficiency units and big amplifiers does not fit in a small room. In very large rooms big units and amplifiers with a lot of energy are a necessity. In average rooms units with an efficiency of 90 to 93 dB will work wonders. These numbers are just an indication. Experience will tell you what the best system is for a given listening room.

CROSSOVER UNIT

Even if you have selected loudspeaker units with linear frequency curves, it is of the utmost importance what X-Over unit you chose. If you just add subwoofers to existing loudspeaker systems, than the earlier mentioned Thommessen subwoofer crossover or the Detonation will do the job. But if you go for a 3-way system, than it is important to chose a quality unit which enables you to vary the crossover frequencies and the slopes. There are units with slopes of 6 and 12 dB like the Luxman 2003 tube cross-over. There are others which also sport 18 dB and sometimes even 24 dB (Linkwitz) filtering. The advantage of steep 24 dB slopes is that the unit just gets that part of the audio band which it is meant for and causes the least distortion. The sound becomes clean. But in my view too clean. I myself have always found that 24 dB is too steep a slope and makes it more difficult to build a harmonious sound pattern without leaving details out. Slopes of 18 and 12 dB, or 12 and 6 dB are better. Using 6 dB and/or 12 dB slopes as these are provided by the Luxman make attaining phase coherence much easier. Using just 24 dB slopes means that you have too choose the units with as much care. The slope of 6 dB means that the phase is correct (coherent), but it still is necessary to find the correct vertical and horizontal distances between the cabinets. The satellite you choose should be versatile, yes, but foremost the X-over should be well designed and if possible have discrete circuitry or just have not too many operational amplifiers (opamps) and integrated circuits (ICs).
If you have a X-over with with settings of 6 and 12 dB you can make a perfect setup with very good phase, filtering the subwoofer (or woofer) with 12 dB, the midband with 6 dB high pass and low pass slopes, and the tweeter or midhigh/high section again with 12 dB.

BUDGET

An important ingredient is the available budget. If the budget is low, the best thing is to start with relatively cheap power amplifiers with the same reference number of a given brand. Maybe you will find them in the second hand marketplace. If after some time when you become more experienced and more knowledgeable and your active system reaches a high level of quality, than more expensive power amplifiers may be chosen. Remember that cheap amplifiers will hardly be able to drive a subwoofer in the lowest register due to little capacitance in the power supply. Also I do not advise the use of high current amplifiers (large toroidal transformer and just a two low value capacitors). When high dynamic levels are asked for the transients are smeared out and in fact can not be called transients!

CABLES

Important is also the choice of cables: interconnects and loudspeaker cables. How many times did you read in threads or mailing lists that someone combined cables of different make and bi-wired a loudspeaker system? And how many times that person described how the sound benefited from this? If different cables and/or cables from different brands can be combined and give a correct sound, it is a chance of one in a hundred. In an active loudspeaker system the same type of loudspeaker cable has to be used for all sections. There may be some difference in internal wiring, connecting the binding posts with the unit inside the cabinets, but even then one has to choose wires which are from the same manufacturer. Using Cardas wire - as a suggestion - of different diameter for internal wiring of low, mid and high frequency unit, will lay the basis for smooth transition from one section to another.

The various amplifiers should also be connected with the same type of interconnect. Optimum slopes and crossover frequencies can only can be chosen if there is no influence brought about by different cables.

© R.A.Bruil, October 10 2002


How to order the Owner's and Service Manual.

Owner's and Service Manual of Luxman 2003 Electronic Valve Crossover in English on 24 pages: $25 / Euro 20. (Prices can vary slightly as the conversion rate of Euro and USD can change.)

The Luxman 2003 Electronic Valve Crossover with 8 x 12AU7 (ECC82) is a fine unit and can certainly be built by the amateur with knowledge of electronics and experience in building valve components. The Luxman 2003 is a 3-way crossover with many possibilities. The low pass and high pass crossover frequencies are variable and can be adjusted in relation to the qualities and behavior of each driver. This means that sections can be adjusted in such a way that they overlap each other or that a gap between two sections can be made in case of an irregular frequency curve of one or more units, combinations of 12 and 6 dB slopes can be made with minimal phase shift.

This Owner's and Service Manual contains all schematic diagrams and values of resisters and capacitors.
It includes images of the 4 circuit boards (3 for the individual channels and 1 for the power supply).

As an extra I have edited the images so they can be used for etching the circuit boards in case you would like to overhaul your unit or build a unit from scratch. I have also added extra information about the unit and how to make the prints for the selection of the crossover frequencies. But you also can etch prints yourself or have them etched.
The manual also explains the functioning of the x-over:
* selecting frequencies,
* polarity of amplifiers,
* polarity of speaker units in relation to the chosen slopes of 6 and 12 dB
* relation to phase.

If you want to build a unit from scratch and need the appropriate power transformer as indicated in the schematics, I can be of help. The cost of such a transformer -wound to specification- is approximately $50 (shipment not included).

Click to order the Manual and Info.

 


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