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Regaussing Woofers

Alnico Magnets and Degaussing (Losing their magnetic field)

Alnico magnets, by their nature, are easy to degauss (demagnetize) with drive. They will not change with time and their dependence on temperature is really small – maybe 1% at 100 degrees Fahrenheit. Alnico stability and resistance to back EMF is really good. This is why they make very good sounding magnetic structures. Unfortunately, given a big enough pulse of magnetic energy, they will degauss by up to 3 dB. The sensitivity to degaussing is dependent on the specifics of the magnetic circuit and the length of the coil providing the field. Underhung woofers, midranges, tweeters, and compression drivers do not have sufficient back EMF fields to push the operating point of the structure below the knee. They are essentially stable regardless of input signal. The short-gap, long-coil speakers are the ones that have a problem. A woofer can take a hit of up to 3 dB, if a big enough hit of current takes place. 1.5 dB to 2 dB is more common. The effect does not get better or worse with time, it solely depends on how much current is driven through the coil. The more current, the more field. Once the field is bigger than a certain number, some amount of degaussing occurs. It is permanent (until externally recharged) and will only increase if a larger sustained current hit occurs.

Therefore, if you have a qualifying alnico woofer and you have played it loudly, you have some degaussing. You can have the unit recharged, and it will be fine until you play it again. Exceed the critical level, and it will start degaussing again. If you never do, it won’t ever degauss. Most of these designs trace back to the 50’s and 60’s where 15 – 30 watt tube amps were the rule. They didn’t have the current capability to hurt anything. With the advent of big solid state amps, the current levels went up and the problem started to surface.

Most of the qualifying motors will lose 1 – 1.5 dB, unless they are pummeled. Some of the older 3″ motors with really short magnets will typically be around -3 dB down. They go really easily.

Great Plains Audio can regauss your alnico magnets, if they need it. Regaussing is included free when we replace diaphragms and cones.

Altec Lansing Valencia

Altec Lansing Valencia: Improving a Classic

The Altec-Lansing Valencia 846B is an American audio classic that inspired the development of the legendary Model 19. It shares many similarities with the newer and larger Model 19. There were two versions of the Valencia: the 846A and 846B. For our testing purposes we chose the 846B due to its impedance, and that it was a tuned port enclosure. We had two goals with our Valencias: 1. Make them perform like a Model 19 and 2. Make them outperform a Model 19. Many Altec Model 19 fans will immediately point-out the differences between these two loudspeaker system, and claim......

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Altec Lansing Model 19 Cabinet Plans

Follow the links below to download and print sections of the Vintage Altec Lansing Model 19 Cabinet Plans for use in your wood shop. Refer to the Model 19 Cabinet Plans page for NOTES that accompany the plans. Altec Lansing Model 19 Front View Altec Lansing Model 19 Side View Altec Lansing Model 19 Top Down View Altec Lansing Model 19 Bottom Up View Model 19 Material & Finish Data The Model 19 was available in three materials &  finishes as indicated below:   Materials and finishes were applied per the chart below:   Thanks to Art J. for his Photo-Shop efforts in producing these jpg images....

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Altec Lansing Compression Driver Rear Loading Caps

In the spring of 2008 I was loaned a pair of Altec Lansing Super Duplex 604E for testing and research about possible crossover improvements. Unfortunately, one of the drivers had sustained damage to a compression driver binding post during shipment, and as I removed its rear cap for an inspection of the damage I discovered that the 604E had been equipped by Altec with compression driver rear loading caps. I remember the first time I removed the rear cover of an Altec Lansing small format compression driver (1” dia. throat) and saw a black plastic factory loading cap mounted over......

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Altec-Lansing 604E Super Duplex

I received an unexpected opportunity to perform electro-acoustic measurements on a pair of 604E Super Duplex speakers in early 2008 thanks to a long-time owner seeking improvement in their sound. He had been using the original Altec Lansing N-1500A series type crossovers, and wondered if an improved crossover implementation might make them sound better. I had wondered that too. The drivers had original factory cones and voice coils, as well as identical production date codes of 391611, signifying the 11th week of 1966. My initial measurements suggested weakened magnets, so the drivers were sent to Great Plains Audio in Oklahoma......

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Altec 604-8K

Altec Lansing 604-8K Duplex Studio Monitor

In early 2008 I had the opportunity to evaluate a loaned Altec Lansing 604-8K utilizing the CLIO system. The driver was an early unit (serial # 3403, date stamped DEC 1982) and was equipped with the original factory low frequency cone and high frequency diaphragm. This model was manufactured by Altec from the early 1980s thru the company’s demise in the late 1990s and was marketed for use as a studio monitor, musician’s stage monitor and a premium ceiling speaker. The 8K had 8 ohm impedance voice coils and was supplied with a dual EQ passive crossover that had first been introduced for use......

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Phase Correction for Altec Ferrite Duplexes

Ferrite Magnet 604s As I mentioned on the Home Page, horizontal separation between voice coil gaps for ferrite magnet Altec Lansing 604 & 904 duplexes is 2.8125”. This distance produces 112 degrees of phase shift between the driver’s acoustic centers at 1500Hz. The Altec supplied passive crossovers for most of these units were essentially the same as for the ALNICO 604-8H, namely third order high pass and second order low pass sections at 1500Hz with MF and HF EQ circuitry. Might there be a better circuit to phase correct ferrite motor Altec Duplexes than the 8H filter? A good question –......

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Altec-Lansing 604-8H Crossover Revelations

Enter the H Model... In early 2004 I acquired a pair of 604-8Hs, the last ALNICO magnet duplex model Altec produced before they switched to ferrite magnets. It was also the first Altec duplex model that combined the Tangerine phase plug with a constant directivity high frequency horn and a crossover that utilized dual equalization of the compression driver output. I swapped out my 604Bs for these Hs using the same cabinets, and was immediately impressed  by the differences in output - well controlled upper mids, significantly more high frequency extension, improved imaging detail and more apparent bass & mid bass. What was in......

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Altec-Lansing 604B

Phase Shift Between Drivers

I acquired a pair of 604Bs in the early 1990s, and had them refurbished with new 604E low frequency (LF) cones and new 16 ohm high frequency (HF) diaphragms at the Altec factory in Oklahoma City, Oklahoma. I subsequently used them with Altec N1500A crossovers for years in a secondary audio-visual system. (Follow the Vintage Altec Lansing N1500A schematic link for a look at that vintage series type crossover) My main system had horn loaded Altec 515-8Gs and 909-8As with custom phase corrected passive crossovers, and the 604Bs seemed to lag behind them in a number of sonic areas. In......

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About Us

Great Plains Audio was born out of the Altec Lansing corporation in 1998. It carries forward the loudspeaker design and manufacturing tradition that began with Western Electric in the early 20th century. Great Plains Audio is the only place where the legendary Western Electric/Altec Lansing sound reproduction technologies live on.

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7127 Northwest Third St. Oklahoma City,
Oklahoma, USA, 73127