The most advanced magnet wire that never was is Rectangular Anodized Aluminum Clad Copper (ACC). The primary reason is the lack of demand from the loudspeaker industry. The Aluminum Oxide Insulation layer will not melt below 2000 degrees C and Aluminum has a melting point of 658 degrees C. Anodized Aluminum Coils can survive temperatures of up to 500 degrees C for extended duration. In contrast conventionally insulated Aluminum or Copper Wires have a max operating temperature limitation of 180 degrees C – 240 degrees C for polyimide enamel. Copper Clad Aluminum (CCA) magnet wire is commonly used to realize audio transducer voice coils. The most desirable characteristic is that it's a low mass magnet wire that can be soldered to braided lead dress wires typically called "tinsel."
The Type I (Chromic) anodizing process forms a thin layer of Alumina (ceramic), roughly 2.5 µm (0.0025 mm) thick. In contrast enameled magnet wire insulation is 0.06 to 0.11 mm thick. Additionally, the anodized insulation has high thermal conductivity, 30 to 35 W/mK. In contrast enameled insulation has a thermal conductivity of about 1 W/mK. In effect the anodized insulation is inert. However most importantly is the differences in "Packing Factors." Rectangular wire with anodized insulation is the SOA for voice coil products. BOSE Corp. was the first to use Rectangular Anodized Aluminum magnet wire for voice coils. One problem with Anodized Aluminum magnet wire is that it cannot be effectively soldered. BOSE addressed this problem by using mechanical wire crimps to terminate the Anodized Aluminum magnet wire to the tinsels. Anodized Aluminum Clad Copper would offer a solution. The ACC wire would need to have the aluminum cladding mechanically stripped but then soldering would be conveniently facilitated. Winding rectangular wire works best in a single layer. Voice coil winding companies have developed the ability to wind a single layer of wire on both the inside and outside of the voice coil former or bobbin and this avoids any wire crossover that would require a grove the the motor assembly's gap plate. This was how BOSE addressed the crossover issue. The inside | outside winding inherently removes the need for for a wire crossover. Another issue with a single layer voice coil is low resistance, Re. The inside | outside winding would effectively double the wind length and thus double Re. The BOSE 901 transducers were about 0.7 ohms DC, where 9 x 0.7 = 6.3 ohms (series connection). BOSE disclosed their helical voice coil details in their 4.5 inch transducer patent filed in 1976 (49 years ago), in US4061890. Frankly speaking that voice coil was superior to anything being used today. The Anodized Aluminum wire was manufacturer and supplied by H.P, Reed Co. I believe that they went out of business and were subsequently sold and they no longer offer Anodized Aluminum magnet wire.
The technology to clad copper with a thin layer of aluminum has existed for more than 60 years. US2947069, filed in 1956, reveals the secret. I can remember reading about the ACC voice coil development back in the day. However, I can not identify sellers of Rectangular Anodized Aluminum Clad Copper (ACC) wire today, never mind voice coils using it. The crimping process on Anodized Aluminum wire and foils has been accepted by magnet coil manufactures and some use welding to terminate their coils. The added crimp's dead mass is not an issue for them. Crimping audio transducer voice coils adds mass that serves no purpose other than to connect and can be related to a failure mode(s).
The last BOSE transducer to use the single layer Anodized Aluminum helical voice coil was Manta. A reverse motor super slim woofer that was developed just before I went to work there. BOSE's cost to manufacture the Manta voice coil in 1998 was US$10, which was considered very high back then. At that time the NdFeB magnet was being imported from Germany's Magnequench. Cost concerns killed the highest performing voice coil in the industry.
Note the small Neo motor in the front in the picture below and the dust cap on the center of the spider in the picture above. Manta was invented by James Scanlan from MIT, the Transducer Engineer that I replaced in 1995. The patent related to the diaphragm attachment to the frame, US5625701. Jamie Scanlan was a student of Dr. Bose at MIT. When he interviewed me in 1995, he told me that I would last about 3 years. I lasted just about 3.5 years before moving to San Diego to join Thilo and his mother at TC Sounds. Jamie visited me in Thailand a few times after I left P.Audio in 2002. After his last visit, I moved from Bangkok to Phuket and I have not seen or heard from him since. He now lives in San Francisco and does software. Frankly, he was a super smart guy and his eyes seemed to sparkle through his glasses back then. That was the guy I was expected to replace. Five years before that, I was driving trucks for UPS. When I got to BOSE in 1995, I was like a hungry animal in a technical sense and there was plenty to eat!
Manta came in 8, 10, and 12 inch versions. Nominal impedance was 2 ohms and all versions used the same spider, dust cap, voice coil, terminals, and Neo magnet assembly. Only the frames and cone/surround assemblies were different. There were two frames sandwiched. One to fix and support the motor and the other to fix and support the soft-parts. The design was well received throughout the company.
So then the most advanced voice coil possible for audio transducers would be an update to Manta's but wound inside | outside with Rectangular Anodized Aluminum Clad Copper (ACC) magnet wire. Then why is no one doing this?
1. No patent - The coil concept is public domain and unfortunately the loudspeaker industry is patent driven. Similar coils are available; this would be a wire update.
2. Cost
3. Specialized winding processes that include the need for an induction heater(s) to activate the polyimide adhesive coated former during winding
4. Industry attitude
What is the most important component of a moving coil audio transducer? Many transducer experts would reply, the voice coil.
The Type I (Chromic) anodizing process forms a thin layer of Alumina (ceramic), roughly 2.5 µm (0.0025 mm) thick. In contrast enameled magnet wire insulation is 0.06 to 0.11 mm thick. Additionally, the anodized insulation has high thermal conductivity, 30 to 35 W/mK. In contrast enameled insulation has a thermal conductivity of about 1 W/mK. In effect the anodized insulation is inert. However most importantly is the differences in "Packing Factors." Rectangular wire with anodized insulation is the SOA for voice coil products. BOSE Corp. was the first to use Rectangular Anodized Aluminum magnet wire for voice coils. One problem with Anodized Aluminum magnet wire is that it cannot be effectively soldered. BOSE addressed this problem by using mechanical wire crimps to terminate the Anodized Aluminum magnet wire to the tinsels. Anodized Aluminum Clad Copper would offer a solution. The ACC wire would need to have the aluminum cladding mechanically stripped but then soldering would be conveniently facilitated. Winding rectangular wire works best in a single layer. Voice coil winding companies have developed the ability to wind a single layer of wire on both the inside and outside of the voice coil former or bobbin and this avoids any wire crossover that would require a grove the the motor assembly's gap plate. This was how BOSE addressed the crossover issue. The inside | outside winding inherently removes the need for for a wire crossover. Another issue with a single layer voice coil is low resistance, Re. The inside | outside winding would effectively double the wind length and thus double Re. The BOSE 901 transducers were about 0.7 ohms DC, where 9 x 0.7 = 6.3 ohms (series connection). BOSE disclosed their helical voice coil details in their 4.5 inch transducer patent filed in 1976 (49 years ago), in US4061890. Frankly speaking that voice coil was superior to anything being used today. The Anodized Aluminum wire was manufacturer and supplied by H.P, Reed Co. I believe that they went out of business and were subsequently sold and they no longer offer Anodized Aluminum magnet wire.
The technology to clad copper with a thin layer of aluminum has existed for more than 60 years. US2947069, filed in 1956, reveals the secret. I can remember reading about the ACC voice coil development back in the day. However, I can not identify sellers of Rectangular Anodized Aluminum Clad Copper (ACC) wire today, never mind voice coils using it. The crimping process on Anodized Aluminum wire and foils has been accepted by magnet coil manufactures and some use welding to terminate their coils. The added crimp's dead mass is not an issue for them. Crimping audio transducer voice coils adds mass that serves no purpose other than to connect and can be related to a failure mode(s).
The last BOSE transducer to use the single layer Anodized Aluminum helical voice coil was Manta. A reverse motor super slim woofer that was developed just before I went to work there. BOSE's cost to manufacture the Manta voice coil in 1998 was US$10, which was considered very high back then. At that time the NdFeB magnet was being imported from Germany's Magnequench. Cost concerns killed the highest performing voice coil in the industry.
Note the small Neo motor in the front in the picture below and the dust cap on the center of the spider in the picture above. Manta was invented by James Scanlan from MIT, the Transducer Engineer that I replaced in 1995. The patent related to the diaphragm attachment to the frame, US5625701. Jamie Scanlan was a student of Dr. Bose at MIT. When he interviewed me in 1995, he told me that I would last about 3 years. I lasted just about 3.5 years before moving to San Diego to join Thilo and his mother at TC Sounds. Jamie visited me in Thailand a few times after I left P.Audio in 2002. After his last visit, I moved from Bangkok to Phuket and I have not seen or heard from him since. He now lives in San Francisco and does software. Frankly, he was a super smart guy and his eyes seemed to sparkle through his glasses back then. That was the guy I was expected to replace. Five years before that, I was driving trucks for UPS. When I got to BOSE in 1995, I was like a hungry animal in a technical sense and there was plenty to eat!
Manta came in 8, 10, and 12 inch versions. Nominal impedance was 2 ohms and all versions used the same spider, dust cap, voice coil, terminals, and Neo magnet assembly. Only the frames and cone/surround assemblies were different. There were two frames sandwiched. One to fix and support the motor and the other to fix and support the soft-parts. The design was well received throughout the company.
So then the most advanced voice coil possible for audio transducers would be an update to Manta's but wound inside | outside with Rectangular Anodized Aluminum Clad Copper (ACC) magnet wire. Then why is no one doing this?
1. No patent - The coil concept is public domain and unfortunately the loudspeaker industry is patent driven. Similar coils are available; this would be a wire update.
2. Cost
3. Specialized winding processes that include the need for an induction heater(s) to activate the polyimide adhesive coated former during winding
4. Industry attitude
What is the most important component of a moving coil audio transducer? Many transducer experts would reply, the voice coil.
Last edited:
