An introduction to Bose 901 speakers: A classic design, updated several times since its introduction in May of 1968, and still in production 45 years later.
There are detail differences among the various versions, chiefly the cabinet and grille styling and the internal structure of the cabinet. Most have a wood or wood-looking cabinet; a few were built having a glossy "piano black" finish. There was a (now discontinued) version of the Series VI that came with its own amplifiers--and a $3500 list price.
Series I----1968 to 1973, acoustic suspension (sealed cabinet), drivers use 4 fasteners each to hold them in place
Series II---1973 to 1976, acoustic suspension, improved high frequency response. Drivers use 4 fasteners.
Series III--1976 to 1978, Acoustic Matrix (ported cabinet--takes less amplifier power to drive to the same volume level) Drivers use 3 fasteners.
Series IV--1978 to 1983, Acoustic Matrix, improved high frequency response. Drivers use 3 fasteners.
Series V---1983 to 1987, Acoustic Matrix, drivers use 3 fasteners.
Series VI--1988 to present, Acoustic Matrix, drivers use 3 fasteners. This is current production, as of April 2013 its $1,398.95 list price includes two speakers and an active equalizer, with shipping and sales tax additional, of course. Bose has periodically put the speaker system on sale for about a hundred bucks off, and some eBay vendors are not charging tax and shipping, which helps a lot.
Dr. Amar Bose practically built his company on the reputation of these speakers. Although he now seems to concentrate on selling tiny speakers, headphones, and expensive table radios which DON'T sound all that good, the 901 series is a different animal in that sound quality was a real, driving force in the design and development of the speaker system. Since the current production is the Series VI; and it has been since '88 (TWENTY FIVE YEARS!), I suspect that there is presently z-e-r-o engineering effort expended on the 901 series speakers, Bose finding much greater profit in the cheap and tiny stuff.
There are several things that make the 901 series speakers so special. The unique Direct/Reflecting design of the speaker causes most (89%) of the sound produced to be directed to the rear of the cabinet, away from the seating position. This sound bounces off the back and side walls before reaching you. Depending on your preferences, this is either "Stereo Everywhere" if you like the effect, or "Stereo Nowhere" if you don't care for it. The use of nine identical 4 1/2 inch drivers, rather than the more traditional "big" woofer, "little" tweeter, and "just right" midrange cones is another design choice. Nine small drivers, acting in unison, have about the same air-moving capacity as a single large woofer and mid-sized midrange cone. On the other hand, it's tough to get a 4 1/2 inch driver to put out high frequencies. And so here is a trade-off. Using multiple, identical drivers allows Bose to totally eliminate a passive crossover network, but instead, a special, purpose-built Active Equalizer is needed to extend the frequency response of the speaker unit.
You will want a Bose 901 active equalizer to go with your 901 speakers. It is contained in a small box that is powered by normal household current (120 volts AC, at least in the US and Canada) and connects to your receiver or preamp by plugging into the tape loop or external processor jacks using two pair of interconnect cables. (It's simple and easy) The equalizers are somewhat specific to the speaker series model--a Series IV speaker should use a Series IV equalizer. The 901 Series VI owner's manual says the Series VI equalizer is also compatible with the Series V, but NOT COMPATIBLE with Series I, II, III, or IV. A reading of the Series IV owner's guide seems to suggest that the Series IV equalizer is compatible with Series III speakers. A Series II equalizer seems to be compatible with Series I speakers. I cannot recommend mis-matching the speaker series and the equalizer series, so if you choose to do so, (i.e., Series II speakers with a Series IV equalizer) it would be wise to contact Bose first. Be sure to check the auction to see if the correct active equalizer is included. If not, equalizers are a common item on eBay.
Another option would be to use an antique Bose receiver, which will have an equalizer for an older model 901 (probably series III or IV, requiring Series III or IV speakers to be used) built-in. I've found references to two different receiver models: the Spatial Control Receiver (100 watts/channel) and the Model 550 (40 or 50 watts depending on the source of the info--and info is kind of hard to come by for these receivers!) Those receivers weren't too common when new, and that was a long time ago--around '79--'83 give or take a couple of years. I'd be concerned that the electronics are getting old enough that they may not be reliable anymore. The 901 Series III and 901 Series IV have three speaker cable terminals. There is the usual + and -, but in addition they have one labeled *. This third terminal is unused (and un-needed) unless you have one of the special antique Bose receivers. This third terminal is used as part of an "effects" device. You could vary the apparent width of the soundstage with an adjustment on the receiver. This third terminal disappeared on the Series V speakers, by that time Bose had quit producing the special receivers.
Bose model 108787 "Spacial Control Receiver" photos courtesy of online_enterprises; used with permission.
Aside from a Bose 901 Active Equalizer, either a stand-alone unit or integrated into a Bose receiver, you must be concerned about older Bose 901 speakers having problems with deteriorated (rotted) foam on the actual speaker drivers. I think this is a problem mostly with the Series III and Series IV speakers. My Series V speakers have more of a rubber than a foam surround, but perhaps the surrounds were replaced by a previous owner. Repair kits are readily available on eBay to repair speakers suffering from foam rot. The price of the kits isn't bad, but it would be a labor-intensive job to re-foam the 18 drivers in a pair of speakers. It is possible to buy aftermarket speaker drivers that are intended to replace the Bose drivers. Since they're sold as a replacement for all of the 901 speakers from Series III to Series VI, they are clearly NOT exact-replacements.
The Bose 901s were legendary for their ability to handle large amounts of amplifier power. For a long time, Bose claimed that the 901 could take "unlimited" amplifier power, providing that a fuse of a specified value (a "3-amp, quick acting" fuse such as a Buss AGC-3 or Littelfuse 312-003) was inserted into each of the + speaker cables. This was back in the days when a "120 watt" receiver was considered hugely powerful. Today, watts are cheap and readily available, and Bose is now claiming that the latest 901 (Series VI) will handle up to 450 RMS watts per channel, or 250 watts as tested by the IEC method . First Guess: That's about what a Series IV or V will take, too.
The 901 design--particularly the ones new enough to use the molded plastic Acoustic Matrix internal structure (Series III and newer) produce big sound even with low-powered receivers or amps. They are very versatile with regard to the amount of watts you have available to feed them!
The Bose 901 is small and (relatively) light for a quality speaker, easily portable, handles massive power reliably, and doesn't sound bad at all. I would say that it has sound quality equivalent to, or better than 90% of the speakers sold in big-box retailers like Best Buy or Circuit City, and FAR better than ANYTHING sold by the likes of Sears, Wal-Mart, Target, and Sam's Club. Which means, a properly functioning pair of 901s with the active equalizer is better than what is in 95% of the homes in America right now.
The 901 is probably at its best when used as a very high-quality public address speaker system, or when used to provide sound reinforcement for such music-intensive public venues as skating rinks and teen discos. My 901 Series Vs are powered by an $11 bought-from-eBay antique JVC 35 watt receiver , and hang from the ceiling of my garage where I listen to them as I work on my car. I'm nowhere near running that 35 watt receiver even moderately hard, (The volume control is set at the 3 or 4 mark out of 10) yet the speakers fill 1000 square feet of garage + the open-air driveway with good sounding music loud enough to make the neighbors complain (or dance, depending on what CD is in the player.)
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