Audiolics Anonymous Chapter 43
Marchand Electronics XM-44 Active Crossover
Article By Bill Gaw
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  First, for most of you this column will be purely a learning experience because the equipment I am discussing today, specifically, an active loudspeaker crossover that cannot be used by the vast majority of you who have one of the 98% of the loudspeakers available that have built in passive crossovers. For those hobbyist who "roll their own" that want to reach the ultimate in loudspeaker design, read on...

Crossovers are electronic circuits that divide the full frequency audio signal into two or more frequency bands of information and transmit them to the different drivers of your loudspeakers. Thus, in a two-way crossover only low pass frequencies go to the woofer, which will then not distort the highs, and high pass frequencies to the tweeter so the lows will not blow out the driver. Sometimes one or several bandpass crossovers allows the middle signals to go to midrange drivers (three-, four-way, etc.). Not only do they divide the signal, but depending on how many components are used, they can determine how rapidly the signal is attenuated (the slope) and where in the frequency range this occurs (crossover point). It can also affect the relative volume of each side of the crossover that is allowed to go to the loudspeaker (attenuation or loudness) and how the two drivers interact at the crossover point (damping and polarity). 

First order crossovers use one stage giving a gentle slope that allows perfect time alignment and correct absolute phase between the drivers, but make the drivers carry signals that only decrease by 6dB per octave. Thus a tweeter is made to carry some information from the lower octaves and the woofer from the upper octaves, which may cause distortion. Passively this is done using an inductor in series or capacitor in parallel for the low frequencies and the opposite for the highs. Second order crossovers decrease the signal by 12dB per octave by adding a cap in parallel for the low frequencies and an inductor in parallel for the highs, which decreases how much out of band information the driver has to process, but unhappily run the drivers out of phase at the crossover point. This can muddle the sound for those who can hear phase information.

Third order using three stages gives 18dB slopes but really gives out of phase information. Fourth order bring both drivers into proper time alignment and rapidly drop off out of band information, but require four stages that add to the complexity and signal loss... and cost. Finally there are even higher crossover points, such as the eighth order 48dB that really give rapid drop-off slopes and proper time alignment but add even more complexity and are very difficult to design both for the passive and active types, and usually cost a fortune. Thus, there is only one loudspeaker manufacturer I know that uses 48dB passive crossovers in their loudspeakers to great advantage. 

Passive crossovers that are used in the vast majority of loudspeakers today go between the amplifier and loudspeaker drivers and use inductors in series and capacitors in parallel to direct the low frequency information, and caps in series and inductors in parallel to direct the high frequency information plus resistors (or potentiometers or T-pots) in series or parallel to vary the amount of power to each driver to balance their outputs.

Advantages of the passive type are that:

1. The loudspeaker manufacturer can build and tune the circuitry to the drivers he is using.

2. 2. Can adjust the crossover to reproduce the sound he feels his loudspeaker should have.

3. Only one amplifier and loudspeaker wire and interconnect is needed for each loudspeaker.

Disadvantages are that:

1. Large inductors and capacitors which can carry the current from the amplifier need to be used and very good ones are much more difficult and expensive to build than small signal ones.

2. A large amount of the current produced by the amplifier is used up as heat in the crossover circuit. Thus…

3. A larger amplifier is needed to run the loudspeaker.

4. The signal can only be decreased in volume, and damping is impossible.

5. Time alignment becomes a problem as the signal has to transverse sometimes very large inductors, which slows the signal to the woofer compared to the tweeter.

Active crossovers are built into or go between the source component and the amplifier, and use op amps or tube circuits with resistors and capacitors to perform the same function. The advantages of the active type are:

1. Small signal resistors and capacitors can be used which are much easier and less expensive to build to high specs than inductors or high voltage caps.

2. No power is lost in the circuitry, and they can also be designed to actually boost the signal strength.

3. Time alignment is not a problem.

4. If you are building a preamp for your particular loudspeaker with sufficient circuit stages, you can adjust the circuitry to do the crossing over.

5. The ability to adjust the crossover to obtain the sound that you want from the loudspeaker in your room.

6. One can use lower power amplifiers as no current is lost, and each amplifier can be tailored to its frequency range.

Disadvantages are:

1. If you are not contemplating number 4 above, you will need another piece of electronics containing one to several stages of op amps or tube circuits which may add distortions to the signal.

2. A separate amplifier for each driver

3. Extra interconnects with their RCA's

4. The ability to really screw up the sound and go crazy trying to get the best from the loudspeaker.

The vast majority of loudspeaker manufacturers prefer passive crossovers, usually second or third order except for very high end ones such as the first order VMPS or the now defunct Dunlavy, and the fourth order B&W 800's, or the Joseph Audio eighth order. Only a few, especially pro audio types use their own active crossovers with built-in amplifiers, thus imposing complete control over the sound of their loudspeakers. Finally, Meridian actually uses built-in digital active crossovers with power amplifiers. In all, the manufacturer keeps complete control over tuning the loudspeaker, except for possibly volume control of the various drivers. 

Home loudspeaker builders have the option of using either passive or active units to control their creations, and with the new software programs, and with the genius of most tinkerers, can certainly obtain the best sound available at a significantly reduced cost. Witness my finding of a superb home loudspeaker in the EDGE room at the CES, (see show reports by clicking here), which they used to demonstrate their multi mega-bucks pre-amplifiers and amplifiers to great acclaim. 

For horn enthusiasts, active crossovers are the ideal way to go as they allow us to use flea powered 2A3 or 300B amplifiers for their superb mid and high ranges and push-pull tubes or solid-state for the bass. One gets the best of speed, transparency, control and drive by not putting passive components between the amplifiers and drivers. At least these were my findings when I decided to build my own horn systems using Bruce Edgar's designs. While he prefers first or second order passive crossovers between his horns and active between the horns and subwoofers I found that going the active route using fourth order actives of proper design, allowed me to use SET tube amps for the upper end and solid-state "Class A" low powered amplifiers for the woofers, thus maximizing the best attributes of horns and amplifiers while giving them the soundstaging of the best single driver designs. Don't believe me? Come over for a listen sometime.

Anyway, while auditioning many actives there was only one company, Marchand Electronics, that had crossovers which both sounded great and easily allowed me to vary the frequency, slope, damping and volume. I first purchased their XM-9 24dB solid-state three way about six years ago for use on my main loudspeakers, from which I learned the perfect settings for them. Then, I was turned on to the Marchand XM-26 24dB tube crossover that had that certain rightness of tubes with horns. When Allen Wright built my RTP-5 pre-amplifiers with phono, I had him design the correct parameters into the pre-amplifiers circuitry and have been extremely happy, up till now, with the synergy. The XM-9 was shifted to control my side horns and one channel of the XM-26 was used for the center horns, which are a match for the main horns except for their 80Hz rather than 50Hz extension. The second channel of the XM-26 was then used to separate out subwoofer information for the center channel. Imagine my joy when Steven R Rochlin, our Fearless Leader (editor's note: and Head Toilet Bowl Scrubber) asked me if I'd like to review Mr. Marchand's newest incarnation, the XM-44 Active Crossover.

This unit is the Swiss Army Knife of crossovers. The price is very reasonable as he has designed the unit to use one chassis, power supply, and circuit boards to do 2-, 3-, and 4-way crossovers, both single ended and balanced, and able to be configured for anything from 6dB to 48dB slopes. Thus you can have it designed in any configuration you wish, and can reconfigure it at any time if your loudspeakers change. On the front there is an on-off switch, stepped attenuators of 12dB or 24.5dB or 1 dB steps for each output, and a summing switch for those with single subwoofers. 

He has gone back to his solid-state circuitry, using high quality op-amps (OPA- 2134 PA), resistors and caps throughout the active circuitry and power supplies. There is his standard PS-10 power supply, which while small compared to many high-end units, is definitely adequate for the job. For those who prefer batteries, he also has a supply that will run off of a 12-volt gel cell or car battery. The circuit boards are designed such that any of the above mentioned variables could be accommodated. Maximum gain of +5dB is available from each circuit and minimum can be as low as you request. While damping is fixed with this unit, slopes and crossover frequencies can be very quickly changed by the use of very inexpensive ($7 kit and $10 assembled) plug in frequency modules, and one can even have different slopes and crossover points for the high and low pass. One can even build in notch filters for any room or loudspeaker peaks if one desires.

Best of all, you can either have Mr. Marchand build it for you or you can do it yourself. Prices are as follows for the single ended unit using 12 position attenuators:

  Type                         Kit          Fully Built
2 channel 2-way          $399           $799 

2 channel 3-way          $499           $999
3 channel 2-way

2 channel 4-way           $599          $1,199
4 channel 2-way


Obviously, balanced circuits and 24dB or 48dB attenuators add to the cost.

I had Mr. Marchand build mine as I wanted to test a factory built unit figuring I could use the other empty board area to experiment later with kit building. To match my loudspeakers, I had the low pass crossover point at 600Hz and the high pass at 350Hz as I had found in the past that this matched perfectly the natural rolloffs of my horn drivers, which would allow me to test the unit against his XM-9 and 26dB units and the crossovers in my pre-amplifiers and had him make up 24dB and 48dB slope frequency cards. Also, I had him make up 24dB at 80Hz card to try the unit out with my subwoofers.

The unit was first used using the full range output of my pre-amplifiers, comparing the sound to built in active crossovers of the pre-amplifiers. For the life of me, using a pair of Allen Wright's silver foil interconnects, I had a difficult time distinguishing between the two. Except for its crossover functioning, it was a close to a straight wire without gain as I could tell. Only the slightest decrease in soundstage transparency was evident. The unit was also dead quiet. I could hear no increase in background noise with my ear against the 110dB efficient mid horn. So far so good.

The greatest revelation came when I put in the 48dB slope cards. For years I had been hearing a slight hardness in the sound at high volume that I attributed to my room. With the 48dB slope, this significantly decreased. Most likely the hardness was coming from the mid driver, which is supposed to be cut off at 600Hz to 800Hz, but which I had to drive down to 350Hz to cross over properly to my woofer horns that have a natural rolloff at that point. Obviously the mid driver, a TAD 4001, was probably distorting some signal below 350Hz. While not an improvement caused by the Marchand unit, it did let me correct the problem. I guess now I'll have to redo the slopes built into my preamp, or maybe bypass the preamps crossovers and use a Marchand unit. Surprisingly, using the 48dB instead of the 24dB card did not add any discernible decrease in transparency.

Next I used the 44dB in place of both my XM-9 and 26dB units on my center and rear loudspeakers. As each is a match for my main loudspeakers except for their 80Hz low rolloff, I can use them as well as the main loudspeakers for sound comparison. While the 44dB did not have quite the tube magic that the 26dB does, I actually preferred its more open and clean sound. My XM-9 may be getting old, or Mr. Marchand may have changed the circuitry, but the 44dB sounded more open and clean. Thus, in my system the 44dB was a better match than either the XM-26 tube or XM-9 SS units. The XM-26 may be a better match for those systems using primarily solid state, as the tubes may add that certain something to the sound that tubes do so well.

Finally, I placed the unit as my active crossover for my center channel, using the 48dB at 350Hz/600Hz side for the mid-woofer crossover and the 80Hz for the subwoofer. And there it will stay. Using frequency sweeps with my IMP MLSSA unit, I got an extremely close match in sound between my main loudspeakers using my preamps' built in crossovers, and the center loudspeaker with the Marchand. With six channel SACD and DVD-A's, the front of the room produced the most open and balanced sound stage I have yet obtained with my setup. Where before the soundstage was somewhat constricted in the center, and male voices sounded somewhat chesty with the XM-26, with the XM-44dB the center extended as far back as the corners of the soundstage coming as close to a two channel soundstage as I have heard. Also, with proper adjustment of the volume of the center compared to the main loudspeakers, using the Smart Devices Circle Surround unit on those two channel recordings where the violins and cellos seem to come from a clump near the loudspeakers, now they extend across the front almost to the center of the stage. 

Disadvantages: While his other units came with excellent instruction booklets describing the circuits and how to build the kit, the booklet with the Xm-44 was lacking in detail. I understand that it was a quickie preliminary one and will be improved in the near future. Also, I would have like to have a wider range of volume control of the individual channels and a master volume control for the input that would have allowed me to drop a pre-amplifier from my system. Finally, I would have loved to have a damping control at the crossover point, as on the XM-9.

In summary, if you are a hobbyist who wants the best from your home built loudspeakers, the Marchand XM-44 solid-state active crossover is the best that I have had in my system. If you get the kit, which is not that difficult to build from the looks of it, you will have several hours of fun building it, will have a superb crossover with which you can spend minutes or hours tuning your loudspeakers to their maximal potential, and at a price which can't be beat. You'll probably spend more for interconnects attached to the unit. 

Also do yourself a favor and use the 48dB slopes and buy several frequency cards around what you think the optimal crossover point is to dial in your loudspeaker. The money spent here will be piddling compared to most other high end equipment, and will make your loudspeakers, the weakest link in any audio chain, play to their maximum potential. Mr. Marchand won't be getting this unit back and may be in for another order for my main loudspeakers.

Specifications

Frequency response: DC to 100kHz (+/- 1dB)

Crossover Frequency: 20Hz to 20kHz

Insertion Gain: +6dB with level controls at maximum
                      0dB (unit gain) with level controls at center

Filter Slope: 24dB/octave standard, up to 48dB/octave

Harmonic Distortion at 1 KHz: Less than 0.001%

Signal-To-Noise Ratio: Better than 110dB

Input impedance: 30K

Output load capability: 1 KOhm minimum

Output Impedance: 50 Ohms

Maximum Input Voltage: 5 V RMS

Power supply requirement: 117VAV/230VAC 1A fuse

Dimensions: 17 x 3.5 x 11 (WxHxD in inches)

Shipping weight: approx. 15 pounds (7 Kg)

Company Information

Marchand Electronics Inc. 
P.O. Box 473 
Webster, NY 14580

Voice: (585) 872-0980 
Fax: (585) 872-1960 
E-mail: info@marchandelec.com
Website: www.marchandelec.com