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). In the very first issue of High Fidelity Magazine, published in the Summer of 1951 (Vol.I, No. 1), is a description of 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 (1951)

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.

JOHN MAROVSKIS

Already in 1970 John Marovskis from the Bronx (New York) designed his extraordinary Janis W-1 subwoofer. The story goes that he made some 50 cabinets before he arrived at the excellent sonic performance of his system which has 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. He called this elelctronic crossover unit with amplifier, Janis Interphase with a fixed crossover frequency of 100 Hz. Both low pass and high pass sections filtered with 18 dB slopes.
Later he designed a smaller Janis W-2 to add to his catalog, and also another version of the Interphase. The crossover frequency was also fixed at 100 Hz. This Janis Interphase, type 3A, filtered the low pass section (Janis W1 or smaller W2) and the high pass section (satellites) also with slopes of 18 dB per octave.
Another, most important feature of the Interphase modules 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.
Dutch importer Jan Endenburg told in an interview in 1986 that he had read about the Janis W-1 system in 1970 in an article written by Bert Whyte (published in AUDIO) and when he was in New York he called John Marovskis asking where he could hear the Janis subwoofer. Marovskis asked Jan Endenburg where he was calling from. "I am in New York", Endenburg said. John Marovskis invited him to his home and demonstrated the systems set up with Quad ESL loudspeakers. That is how Endenburg's firm Engasound became the importer for the Netherlands already in the early 1970s. It was Jan Endenburg who explained to me the importance of phase and he pointed out to me that optimum phase coherence could also be obtained by using a 12dB low pass for the Janis W1 and a 6dB high pass filter for the satellite speaker as was achieved by the than cherished and famous Symmetry Electronic Crossover. But I found that the 18 dB slopes were much cleaner and also the Symmetry did not have this variable phase.

JANUS INTERPHASE + SUBWOOFER

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 the Janis sub woofers. They make the mistake of using high current amplifiers (NAD, Rotel, Bryston, etc.) that get their power out of the large toroidal transformer instead of large capacitors. Toroidal transformers cannot deliver enough current and are soon exhausted because they need to deliver current directly out of the AC mains. In the past I have tried a Tandberg TPA 3003 power amplifier for the satellites. They did not match at all. The sound of the Janis Woofers became realistically impressive only when using amplifiers that really go with the nature of the Janis Interphase. For instance when an Amcron power amplifier was used to furnish the power to the satellites. The Janis worked also perfectly well with mono blocks from French manufacturer Elipson which were actually designed by Setton. Just to mention some historical units. If you combine the Janis Interphase with the right amplification for the satellites, the music will get its impact and slam. Not only when playing Tchaikovsky's 1812, but any record from your collection.
In recording studios in Hilversum (The Netherlands) Janis Subs were used in the nineteen seventies en eighties. Below the frequency response curve of the Janis W-1 (from the literature provided by John Marovskis).

Another favorite bass system from the past 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.)

OTHER SUBWOOFERS

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). The firm Thommessen, also from Germany, used to make the Subway Active Crossover. But they have stopped the manufacture.

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 LuxKit 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 (LuxKit 2003) 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: Initially the Marchand electronic crossovers were designed with 24 dB or even steeper slopes and they use 12AX7 tubes. The steep slopes are not to everybody's taste. However, now the website says that 6, 12, and 18 dB slopes can also be used by inserting appropriate circuit boards.

The Lux Kit also works with circuit boards, but the advantage of the Lux Kit is that switching from one crossover frequency to another, and from 6 dB to 12 dB, is possible without changing a circuit board. This means that the effect of a different crossover frequency can be heard immediately and helps to determine the best crossover points for the loudspeaker units and the final values of capacitors on the circuit boards.

CONFIGURATIONS

Various configurations are possible. As said the simplest is in fact a 2-way active system consisting of a subwoofer and two satellite speakers. Satellites can be full range speakers or 2-way, 3-way and even more complicated designs.

More experience and a good ear is necessary to establish an active 3-way system where all three transducers have to be adjusted: frequency, power level, and phase.

One visitor of my pages wondered if a combination of a 3-way active system could work together with a separate 2-way active crossover and subwoofer. That is of course possible. But the success depends on the quality of the electronic crossover units, and again the knowledge and perception of the hobbyist. The more adjustments will have to be made, the more difficult it is to optimize such a system. No active system will be assembled overnight, not even the simple 2-way configuration, let alone such a complicated system using two active crossover units.

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 LuxKit 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 Electronic Crossover filter (Lux Kit).
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.

CHECKING TUBES

Since the Luxkit has 8 tubes, and brands and quality of tubes are not the same, it is important to know what tubes to get and if the quality is right. Testing tubes is necessary to find out if your Chinese and Russian tubes deliver what they promise and if old Philips, RCA, Telefunken, etc. specimen are still working well. At right the EMC 213 tube tester which I use. It is a simple to handle, very practical and effective unit to measure the quality of tubes. And you will be surprised how often newly manufactured tubes perform badly. Throw the bad tubes away so you will not be accidently use a less performing one. But the quality measurement does not say all. You will have to experimently find the best sound by rearranging the tubes in the crossover. Always of course after listening tests and evaluating each section of the audio band. Before you rearrange make sure the tube crossover is switched off and cooled down during several minutes.

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.

WIN ISD BETA

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 31 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. Also by changing the frequency of the system. 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 be 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.

FREQUENCY CHARACTERISTIC

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.

 

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

This is the edited graph of the calculation done by Win ISD Beta for the Goodmans woofer. For the volume of the closed chamber I choose 59 liter. The corresponding volume for the front chamber is 29 liter. This is the graph for the system resonance of 65 Hz.

The program makes it possible to change the volumes and view the corresponding graphs and thus determine a preferred system. The actual response at -3 dB in this case is 35 to 125 Hz. The equalization lowers the efficiency (sound pressure) of the chassis which was 90 dB originally and now is 86 dB.

Naturally different volumes can be calculated with different system frequencies and the corresponding graphs make the changes visible.

Should these graphs (response curves) always show a straight line? That is for you to determine. And always in relation to the mid range and high frequency sections (the satellites). At left the curve I choose several years ago. The curve is far from a straight line. In most listening rooms the lower frequencies are being amplified. The level depends on size, volume, measurements, and shape of the room.

The above graph shows the response of the same sub woofer (Goodmans in cabinet) but with a system resonance of 76 Hz. and vents of 5.5 cm in length and of 60 mm in diameter. Now 100 Hz. is just under 90 dB, the original efficiency level of the Goodmans loudspeaker unit. And in this case 50 Hz. and 125 Hz. are at -3 dB, whereas the sound pressure at 30 Hz. is -11 dB.

This curve is of the sub woofer after filtering with the Luxman Electronic Valve Crossover or any other crossover unit, x-over frequency 100 Hz. The curve has changed. The result is that 40 Hz. is -3 dB. and 30 Hz. is - 8 dB. Increasing the length of the port(s) will straighten the curve from 30 to 50 Hz. and dynamics in that region will be improved.

Of course it is up to you to define the best system frequency in relation to the rest of the active loudspeaker system. These are just examples to show you what you can do by altering the frequency of the system in relation to the filter.

 

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.

CHOICE OF MODULE

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 but an electronic filter with only 24 dB slopes is not versatile at all and soon takes away the hobbyist's enthousiasm. Slopes of 6 dB/oct. mean that the phase is correct (coherent). It is still is necessary to find the correct vertical and horizontal distances between the cabinets and the best acoustic load for the mid range units. 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 settings of 6 and 12 dB (like the Lux kit) 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

Owner's and Service Manual of Luxman 2003 Electronic Valve Crossover in English on 24 pages, schematic diagram, edited scans of the circuit boards for the power supply, the low, mid and high sections to be used for etching the boards, plus suggestions and considerations. The price: $25 / Euro 20. Plus $14 for shipment outside the European Community and $10 for EU countries.

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, maybe I can be of help. The cost of such a transformer -wound to specification- would be approximately Euro 55 (shipment not included).

Click to order the Manual and Info.

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