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Line Magnetic Audio LM 755i Field Coil Speaker Review

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Line Magnetic Audio LM 755i field coil speaker

Back on January 15th and 16th Matt Rotunda of Pitch Perfect Audio hosted the first ever Greenhaus. It was Pitch Perfect Audio’s homage to the Monkeyhaus events that John DeVore of DeVore Fidelity held in New York. You may have read some of Art Dudley’s stories about these events. Matt wanted to do something as a special treat for Jonathan Halpern of Tone Imports and John DeVore who were visiting Pitch Perfect after leaving CES 2012.

It was at this event on Monday night that I first saw and heard the LM755i speakers in their beautifully crafted wooden cabinet of Line Magnetic Audio’s own making. It’s a single driver system using LMA’s replica of the famous Western Electric 755A drivers down to their hammertone paint finish. The original WE 755A was an 8″ alnico magnet speaker designed shortly after World War II. Line Magnetic Audio reproduces this speaker and builds on its virtues by using a field coil topology instead of a fixed magnet. Their field coil drivers have a power supply that uses 300Bs to power the magnets. The result is the LMA 755i which is 92dB efficient at 8 ohms. The 755A drivers are placed in a cabinet built by LMA; the cabinets are just plain beautiful in a very retro way. I have a beautiful, floor standing, 1937 Philco floor standing radio in my home. This radio cost over $300 in 1937, and is drop dead gorgeous. While the LMA cabinets are not quite as ornate at the Philco, they look a lot like it.

Line Magnetic Audio is a highly respected Chinese audio company. It’s not your typical “China Come Lately HiFi Company.” They have a well known history for repairing and restoring classic Western Electric gear. The fact that the owner of LMA is an ardent collector of historical audio components is part of the reason these replicas are so faithful to the original; the attention to detail is stunning. They produce amazing reproductions of Western Electric’s electronics, horns and speakers; some of the latter have been adapted like these to use field coils. There’s not a lot about this company in English on the web, especially info on design and specs on these speakers. The only thing I could find on frequency response was for the driver itself, which is 70Hz~13KHz. I promise you in their side vented cabinets, they go much lower than 70Hz. My guess with my ears would be somewhere in the mid 40Hz region.

To spend a lot of time talking about frequency response, soundstaging, transparency, or almost any of the words we audiophiles and especially reviewers like so much is to miss the point of these speakers. I would go so far as to say these aren’t really audiophile speakers, what they are all about is beauty and music.

Why Field Coils?

Almost all modern speakers are permanent magnet speakers. That is, they have a fixed magnetic field created by a magnet. Early on, they were typically Alnico but most often now they are ceramic. With a fixed magnet speaker the voice coil is suspended in a gap between the poles of the magnet. The voltage that is applied to the voice coil causes it to move in and out.

A field coil speaker uses two coils; the voice coil and a field coil. Instead of a magnet, DC is applied to the field coil creating a magnetic field. This takes the place of the permanent magnet in creating the fixed magnetic field. Field coil-based drivers done well, which is a no easy job, are claimed to drastically reduce distortion levels, and are able to control the driver much more accurately. Drivers in speakers vibrate up to thousands of times per second. Some claim that permanent magnets actually lose strength slightly with each vibration, causing a loss of low-level information and a blurring of the signal. Field coil drivers, with their own power supplies should not lose strength and so have much less distortion than their permanent magnet counterparts.

Well, I’m in over my head with all this technical talk, but one thing I know is that they are significantly more expensive to build than permanent magnet speakers, just like Alnico magnets have gotten too expensive for most speakers. The other thing I can say is that at least a couple of the best speakers I have ever heard use field coils. That is not to say only field coils are among the best; I love Lowthers and Quad 57s as well. No, I’m not saying that field coils are the only way to go, but I am saying that cost and difficulty in manufacturing are the reasons most of us had never heard of a field coil speaker until lately. Permanent magnets are always cheaper than a well-built field coil driver and do not need a power supply.

The Power Supply

As you have probably gathered by now, like electrostatic speakers, field coil speakers have to be plugged-in to work. As I have already mentioned, this is done by using some kind of power supply that is plugged into the wall and into the speaker. In the case of the LM 755i, Line Magnetic Audio has chosen to build a power supply named the LM Audio PR-3 that uses 300Bs to power the magnets. I don’t know why, but they do sound better than the less expensive transistor power supply you can order instead. You can’t step up from the transistor unit to the tube power supply though, because the drivers have to be built for whichever power supply you choose.

Each speaker’s power supply has a small knob on the front left corner and a meter in the center of the top plate; this enables you to vary the voltage. The sound get more relaxed as you lower the voltage, and the highs and lows will be more extended and detailed as you raise the voltage. These speakers are so relaxed though that I can’t imagine why you would ever want to listen at anything but full voltage.

Review system and setup

It just happened that the LM 755i were here at a time when I had a lot of equipment to play around with. I listened to them in my reference system: Shindo 301 Vinyl Playback System, Shindo Giscours preamp with an Auditorium 23 Homage T1 SUT, Wavac EC300B amp, and all Shindo cabling. I also listened to them with the AMG Viella “V12” turntable with cartridges from Benz, London Decca, and Allnic. I used the Allnic and standard Auditorium 23 SUT for some of the cartridges. I also had on hand phonostages from Allnic, Fosgate, and Pass Labs. The LM 755i easily let me hear the differences between these components.

Of course set up is a little different from that with most speakers because you have to find a place to plug the power supplies in and a place to sit them. Other than that the setup is no different from other speakers. One of word of advice: Let the power supplies and field coils be on for at least 48 hours before moving them around too much. They do open up and get more vivid during this time and continue to improve for a couple of more days. My pair was already broken-in so I can’t say how long it takes to break in a pair from scratch. (Matt Rotunda from Pitch Perfect Audio recommends against leaving the power supplies on all the time. -Pub.)

Placement is the very key as it is with most speakers. If you put them too close to the corners they are overly warm and a little thick sounding. In my room, I ended up with them about seven feet apart, with the back of the speaker about three feet from the rear wall, about nearly two feet from the side walls, and with toe-in that was aimed at my shoulders. This setup gave me the best combination of bass and openness. They are also very susceptible to toe-in and distance from the listener; if not properly set up, you will not get a focused center. Still, I did not find them difficult to set up; by the third day I had finalized their placement and did not have to think about it again. I should mention that I did find them to sound better set on a hard surface compared to carpet. I used some 3/4 inch red oak as stands.

4 Responses to Line Magnetic Audio LM 755i Field Coil Speaker Review


  1. Jackson says:

    Thanks for this review. It’s too bad you didn’t have some LM amps, like the AM2 (300B SET) to mate with these. I find it frustrating that speaker reviews don’t often enough lead discussions (or recommendations) into acceptable matching components. A sensitivity rating of 92 leads one to think that it may be comfortably powered by low-powered amps, but at what limit? That’s borderline efficiency, not like your Teresonics, and in a rating area where not all speakers are created alike. One 92db speaker may open up with a 1-2 watt amp, while another may require much, much more. Is the AM2 at only 6 watts enough? Inquisitive readers want to know more.
    You mention testing with a GM70 amp and a 300B amp. Which tubes did you prefer? Did you try any other amps, and if so what amp seems to be the best match?
    These are questions I like answered when I read a speaker review.
    Thanks again,
    J.B.

  2. Jack Roberts says:

    J.B. Thanks for reading the review. I did have a LM amp on hand, but not one more powerful. I also had Carver’s 20 watt tube amp. The Wavac EC 300B was the best by far and had plenty of power. I could drive them louder than I could listen with no clipping that I could hear. My Wavac EC300B has NOS Western Electrics for power and drive tubes. The GM 70 amp was a Shindo at Matts place and I’m not sure which tubes they use. I did not use the GM 70 for the review.

  3. Roger Hebert says:

    Hi Jack,

    Thanks for the informative review. I have a technical question about why the speaker is recessed into the cabinet, other than the obvious reason to insert the grill.

    It seems to me that many speaker companies would consider this a bad idea, since they proclaim to flush mount and round or cut the corners of the cabinet to stop diffraction.

    Do you have any thoughts on this in regard to this speaker ?

    Regards,
    Roger

  4. Sameer Kumar says:

    Hi Jack,

    A great review and it helped me a lot to get clarity on my future speaker choice. I have recently auditioned LM 755 at Bangalore, India and instantly ordered a pair. I concur with each and every view point expressed by you. I have auditioned LM 755 using LM 210 IA 300B and other LM amps. I am presuming that you have tested your LM 218 IA 845 22w amp on these LM 755s by now. I wish to know your opinion on this combination. I have the option to pick their higher version of 845 i.e LM 219 IA.

    Thanks DAGOGO…..Thank you, Jack.

    Regards,
    Sameer

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