Planar magnetic circuit composition

What is the typical conductor composition and in what thickness, applied to the surfaces of isoplanar drivers? How much gauge is required for adequate power handling?
Could silver with rhodium plate be applied with good weight proportion too?

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What do you have in mind?

I have in mind just the ideal of a circuit of silver from end to end. So not compromising isoplanar drivers’ surface conductor topology in terms of it not being silver. And if it IS silver… better have it rhodium plated while at it. To have stable smooth surface topology. Or maybe gold-plated for more mellow effect? How the silver is served is a matter of taste.
There is the objection that it needs to be served in specified varieties to satisfy it not degrading.

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Have you visited here?

I think it would add too much weight.

Well it might seem trivial to peck at a magnetic circuit’s composition so trivially, but maybe not as Borresen &co have made it clear to use whole lots of cast silver in their (dynamic) motors to ever lose some more inductance.
The inductance problem is already mostly solved for the isoplanar driver by design. Maybe leave it as it is or maybe an exotic solution involving particle accelerator sputtering would befit planars as well.

Could you briefly describe the material composition and topology of the planar drivers used in the Aspens?

We use aluminum traces (17-20 micron) bonded to a 12 micron Teonex (PEN) polymer planar magnetic drivers.

Though silver has the highest conductivity and things like gold, copper and silver have higher conductivity than aluminum, their density (specific gravity) is much higher than aluminum. Aluminum has a specific gravity of 2.7 where copper, silver and gold are 8.96, 10.49 and 19.32. Aluminum has by far the best combination of conductivity/density and so the resulting mass of the “coil” is the lowest.

The motor force generated by a planar magnetic motor is rather weak. Since SPL is proportional to air volume acceleration, and moving parts Acceleration is Force divided by Mass, the mass has to be low for planars tp generate sufficient acceleration for good SPL/sensitivity. This makes the other conductor materials unsuitable for most applications within planar magnetic drivers.

We bond a layer of aluminum foil to a polymer backing and then the foil is caustically etched away to form the diaphragm “circuit” in a similar manner to how a flex circuit is made. However, flex circuits for other electronic use are made with copper traces and so it’s a little hard to find planar diaphragm vendors (because the chemicals and process used for etching is a bit different). For this reason, historically a few planars have been made with copper traces. However, it results in much lower sensitivity.

There is essentially no inductance because our planars only use about 4 turns of wire in their flat “coil”. that is sandwiched with a clamshell of magnets giving a symmetrical “push/pull” structure.

There is no need for sputtering or vapor depositing of the conductor as it is far too expensive and the adhesive layer works quite well. Slightly lower mass could be achieved with some special (and very expensive) equipment that can directly cast the polymer onto the aluminum without the need for adhesive. I know of a fellow in Russia (using polyamide and aluminum) doing this and I believe Audeze was doing this on some of their planar magnetic headphone transducers but it is relatively rare.


Speaking specifically about Magnepan, I wonder what the “specific gravity” was of their older models that used actual copper wire glued to the mylar?

There are still people out there who prefer the sound of the older copper wire models, and it must have been light enough since that design is what put Magnepan on the map to begin with.

I haven’t done any R&D work with large planar magnetic “woofer” panels because they have significant issues with size/polar response and dynamic range. There may be some instances where the increased mass of copper might be useful in getting a lower resonance or some other factor but, generally, aluminum is better for planars by a wide margin.

I do think that open baffle dipole bass can be done better with dynamic “cone” woofers. I’ve been thinking about doing some high Q woofers for a personal/R&D open baffle project, as open baffle bass can be quite good in sound quality because of the reduced room interaction (though it has other significant compromises)

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Hello Chris!

I couldn’t agree more, on both accounts.

I’ve had or heard older Maggies , and my current 1.7i’s have much better transient attack, are capable of better detail retrieval, and seem as though they can play louder more effortlessly, and cleanly.

And yes, I 110% agree on dipole bass with cone woofers. I have built many OB systems over the years, one pair being H-frame OB subs with two 15’s per side, flat down to 20 Hz, loads of impact and slam with minimal EQ (they measured 24"W x 24"D x 40"H), using drivers with a Qts of 0.97 and an Xmax of 6mm. They didn’t have much excursion but they were so efficient at 96dB per driver, they didn’t need it, not to mention the size of the baffles.

I ran that design by Siegfried Linkwitz and Nelson Pass (this was back around 1999), and they both said pretty much the same thing… “If you can handle having two refrigerators sitting in your listening room, they’ll perform without additional EQ or monopole subs”, and they were right!

I ran that setup with a pair of Magnepan MGLR-1’s. The OB subs never used more than about 15 watts at full tilt.

Anyway… I hope you post something about your personal/R&D OB project.

I too want to build some OB subs again as well. You just can’t beat OB bass done right.

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Thanks for sharing that.

The OB system I was thinking of wouldn’t be flat to 20 Hz (maybe around 25 Hz in-room) because I want something very manageable in size, along the lines of the ~18" wide baffle speakers (of spatial, emerald physics and others). However, my woofer is a lot different because the most important efficiency is down at resonance and below the dipole baffle frequency (where the system needs an effective 6 dB per octave boost), so I am doing kind of larger/modern take on the woofers that David Graebener and Jim Croft developed for the Carver “Amazing Loudspeaker”. They have an extremely light moving system but motor force is lower than on a high sensitivity driver (because mot BL causes more electrical damping and reduces bass output at resonance). The Q of these woofers is around 2.0 but that gives better extension. Excursion is around 16 mm on-way and this only takes about 100 watts to get to that in an open baffle (with a pair of these 15’s). A dipole system like this needs 4X the volume displacement than a sealed woofer for a given SPL and so I’m designing for the lowest mass with good excursion. We’ll see how it goes!


You’re welcome, sir!

I’m really looking forward to your project, that is if you post about. I sure hope you do.

It’s been a good number of years since I’ve had true OB bass in my system. The last time I heard excellent OB bass was at the first Florida Audio Expo I went to back in 2020. MC Audiotech was there with their Forty-10 loudspeakers that utilize two 18" pro woofers in a ripole setup. Man, that system really opened my eyes up again to quality OB bass. And that’s what I want again in my system. No matter what traditional subwoofers I’ve owned, built, or heard at any price point, never compared to my huge, ugly OB subs or those in the Forty-10’s.

Also, not that it matters, but a correction on the drivers I used back in the day… Pyle Pro PPA15’s. They had a Qts of 0.67, an Fs of 27.6 Hz, and were rated at 90dB.

And I was just thinking, I still have those drivers, I still have my Emotiva amp, and I still have my dbx DriveRack Venu 360. Hmm…

You are there really enjoying a great treat only obtainable with the unique benefits of dipolarity.
Why even do monopolar dispersion? It can be a waste.