[Jurassic (S)Park!] History, technique and listening of the legendary Bose 901 speakers

Jurassic (S)Park! Bose Direct/Reflecting® 901 (1 series) - Part I

Birth of a controversial legend

Product: Bose 901 loudspeakers
Manufacturer: Bose - USA
Approx. price: from €500 to €1500 depending on status/conditions
Reviewer: Lucio Cadeddu - TNT-Audio Italy
Published: June, 2024

Our Jurassic (S)Park! section aims to rediscover old HiFi components, both immortal classics and components that would have deserved a different success and reception. It's the turn of a component that somehow belongs to both of these categories, the Bose Direct/Reflecting® 901 speakers.

Since it is a “Gold Classic”, venerated or despised in a ferocious way, it is necessary to approach it with a spirit free from prejudice or fanaticism, as we always do, and dedicate a larger than usual space to its analysis. This treatise will be therefore divided into two parts: this first section, dedicated to the history of the project, its characteristics, hints on availability and maintenance, and the second part entirely devoted to sound performance and some advice for obtaining the best out of these unusual speakers.

To do this - after a long and exhausting search - I had to get my hands on a first series Bose 901, in almost “like new” conditions, which hasn't been an easy task, given that it is a speaker from around 55 years ago. It was my intention to deal only with the first 901 series, the one conceived, imagined and designed by Amar Bose himself, the one that started a series of modern incarnations, more or less successful than the first one. Only in a first series (and, to some extent, in the second) one can understand what the scope of Bose's revolutionary project was, how different it was from all that that had been done up to then, to understand why it was loved and hated so much. And, above all, why it has been mostly misunderstood.

Having found an almost perfect pair in a small vintage HiFi shop in Frankfurt, I checked that it was 100% original. Products like these undergo modifications or repairs that make their historical value close to zero. The model I purchased had been professionally restored to its original specifications, without modifications. The only concession to the aesthetic restoration, irrelevant for sound purposes and removable, is the presence of small upper and lower frames around the cabinet (see pics), in walnut of the very same colour.

Amar Bose and the birth of the 901 project

Before dealing with this speaker, I need to spend a few words on the engineer and professor Amar Bose. Son of a Bengali exile and an American, he was born in Philadelphia on November 2, 1929. Amar Bose began very early to dedicate himself to electronics and audio in particular. Already at the age of 13 he used to repair radios and electric model trains in the basement of his parents' house during the Second World War. After graduating from MIT, he obtained at the same prestigious university a master's degree and a doctorate in electrical engineering. To celebrate the title, he purchased his first HiFi system, which he found rather disappointing, a fact that pushed him to direct part of his research towards acoustics. Due to his abilities, he was called to work at MIT in 1956 where he taught until 2001. During his activity of research at MIT he developed several technologies that were later patented. Strengthened by these patents, he founded in 1964 - at the age of 35 years and with the financial help of some of his colleagues - the Bose Corporation. He dragged into his adventure one of his best students, Sherwin Greenblatt, with whom he actually shares the credit and paternity of the 901 project.

He was named Inventor of the Year in 1987 by the Intellectual Property Owners Association for his acoustic waveguide speaker system and has held numerous patents in the fields of acoustics, electronics, non-linear systems and communication theory.

Among other things, in 1986 he invented noise-cancelling headphones, which were used to protect the hearing of pilots Dick Rutan and Jeana Yeager during the record non-stop round-the-world flight aboard the Voyager.
In 2004, after 25 years of research, he designed a revolutionary electromagnetic automotive suspension system that combined comfort and control. Here you can found a rather amazing comparison video with a traditional suspension. In 2007 Amar Bose was included in the list of the richest men in the world, drawn up by Forbes magazine, and in 2011 he gave to his "Alma Mater", i.e. MIT, a substantial part of his share package, thanks to which the institution acquired, despite remaining without voting rights, the majority of the share capital. He died at the age of 83 on July 12, 2013 in Weyland.

His fame among audio enthusiasts - and not only - is inextricably linked to the Direct/Reflecting 901® speaker system. Dissatisfied with the performance of the speakers on the market at the time, he began to think of a completely different approach that would revolutionize the idea of frontal sound emission and perception of distortion.

Regarding the latter, during a lecture at MIT, Amar Bose gave the following example: “You can put engineers in a room and give them the task of lowering distortion, and they will do this merrily for years. But there's a basic question: does this mean anything with respect to perception? The distortion - though you can read it on meters - maybe isn't audible. Well, if it isn't audible, what are you doing that's worthwhile here except achieving some particular engineering goal?”

These words, perhaps, should be carefully read by the new champions (should I say defenders?) of technical measures at all costs, for whom 10 lives would not be enough to reach Amar Bose's scientific and technical goals, in terms of publications and patents.

[Bose 2201]

Comparing the sound he heard in concert halls with that reproduced at home, he formulated the initial idea behind the Direct/Reflecting® project: the sound in a concert hall reaches the listener mostly reflected from the walls (89%) and not directly from the instruments (only 11%). The first prototype was therefore a sort of pulsating sphere (a quarter of a hemisphere, actually, see photo) which had drivers radiating in every direction. The 2201 speaker (year 1966) is therefore the first model that embodied the idea of Direct/Reflecting. It consisted of 22 five-inch drivers (hence the model name/number) and was designed to be placed in the corner of a room, using reflections from the walls and floor to spread the sound. The system included tone controls and a switch to adjust frequencies below 50 Hz. Plus, it was active, thanks to a built-in 50-watt transistor amplifier. It could be used in mono or in pairs for stereophonic sound which, according to Bose documentation, eliminated the problem of having to sit exactly in the center to enjoy correct sound. You can still find the original manual here, something worth reading carefully.

[Bose 2201 drivers]

Despite the technical efforts and the originality of the design, due to its unusual shape and high cost, the 2201 was a commercial disaster: Bose stopped production after three or four years. In a 2004 interview he said: “I would have been fired hundreds of times at a company led by professional managers. But I didn't go into business to make money. I went into business to do things no one had done before.” [from: Clynes, Tom (July 15, 2013). "The Curious Genius Of Amar Bose". Popular Science.]

Hence, having abandoned the idea of the pulsating sphere, in 1968 Amar Bose designed and produced the first series of 901 Direct/Reflecting® loudspeakers. The number 9, as in the case of the 2201, indicated the number of drivers used. Having abandoned the quarter hemisphere to its fate, the 901 used one more conventional pentagonal shape, with passive operation (no internal amp, that is). An external active equalizer, to be placed between pre and power amp or between the pre out and main in sockets of an integrated amplifier, completed the project. Without this equalizer the 901 speaker works, but does not follow in any way the design specifications. The equalizer serves precisely to rebalance or, better yet, compensate, the response of the full-range speakers and create a specific frequency response, albeit deliberately not flat. Thanks to the equalizer you can also adapt the response curve to a specific environment.

The cornerstones of the 901 project, revolutionary for the time and some still absolutely valid today, are the following:

Passive speaker with 9 small full-range drivers, of 4 inches in the first two series and subsequently 4 and a half inches (11.5 cm); no passive crossover, in order to eliminate phase errors, crossover interference, sensitivity drop and amp driving issues.
A single front driver and 8 rear-firing drivers, facing the rear wall. In this way, according to Bose, the correct percentage of reflected sound and direct sound is achieved, as in concert halls.
9 small and light full-range speakers can easily reproduce high frequencies and, at the same time, have a radiating surface equal - indeed slightly greater - than that of a single 32 cm woofer.
The active equalizer compensates for the frequency response of a small full range driver, both in terms of low and high frequencies. However, the frequency response was not made linear - even though it could be done - because the aim was to imitate as much as possible the acoustics of a concert hall. This compensation occurs upstream of the amplifier. The latter therefore drives a practically resistive impedance, given that there is no passive crossover. The full-range drivers are connected in series/parallel to obtain a very easy, practically constant 8Ω impedance (drivers were wired in series from the third generation onwards)
First scientific approach to the problem of the acoustic interaction between room and speakers. Positioning must be subject to some constraints, without which the magic of Direct/Reflecting® will not work. This was essentially big news for the time. Besides the Klipschorn, and later Allison designs, no loudspeaker designer was too concerned about where to place the speakers in the listening room. The 901s had specially designed supports, as well, but they could also be suspended from the ceiling. More on this in Part II of this treatise.

Bose 901 1st series - rear view (N.B. small walnut slats are an aesthetic add-on. No influence on sound and 100% removable)

Taking into account Amar Bose's philosophy, referred to all of the above, no technical specifications were declared, except for dimensions, weight and nominal impedance (8Ω). No reference to sensitivity, frequency response or power handling. As for amplification, the only mention concerns the fact that the 901s work well even with low power amplifiers, despite being able to handle very high watt output. In the 2nd series, a few years later, the technical data are enriched by power handling in continuous regime (sinusoidal, 50 watts RMS) and peak (400 watts, for less than 5 seconds). The amplifier must have a power between 25 and 270 watts on 8Ω and a damping factor of at least 40. The drivers have an excursion of ±6.26mm, and a 270 gram magnet. In the 3rd series the specifications are further enriched and changed: an amplifier power between 10 and 250 watts (peak); recommended power for high dynamic listening: 70 watts continuous. The manuals for all 901 series can be found on the official Bose website (search for "901 manuals").

The Bose 901 were produced continuously from 1968 to 2017. This is the chronology of the different series, together with the changes made:

First series: 1968-1972. Full-range drivers made by the American company CTS, selected and coupled by Bose, with fabric suspension. Acoustic suspension cabinet, fibreglass lining, series/parallel connection of the drivers for a total impedance of 8Ω.
Second series: 1972-1976. Full-range drivers made by the American company Rola, blue membrane. The rest of the features are substantially identical to the first series. In the last units of the second series 901 the drivers had foam suspension. The cabinet finish has been improved, but essentially it remains identical. As in the first series, the position of the front driver is not central, but offset. In fact, the speakers are identified with One and Two instead of right and left. Later, series marked "Continental" were been created, with a curved front, also available in white lacquer. The equalizer was modified, as well.
Third series: 1976-1978. Drivers made directly by Bose with foam suspension, blue membranes and helical voice coil, resulting in a higher sensitivity. Driver impedance is 0.9Ω each, thus the drivers are connected in series. There is a third connector which was supposed to connect to the "spatial receiver". Matrix internal structure with bass reflex. The front drivers are centred and no longer asymmetrically placed.
Fourth series: 1978-1983. Same drivers, with slight changes. Some changes also to the equalizer, to fix what was defined as a slight accentuation of the midrange. In fact, the third series was short-lived, only two years.
Fifth series: 1983-1987. Changes to the foam suspension of the drivers, which had caused problems; an antibacterial treatment to prolong its life has been applied. A sort of damping treatment is applied to the membrane, to improve the mid frequencies. Temporary addition of bronze-coloured aluminum side panels, subsequently removed. The third connector for the spatial receiver has been eliminated.
Sixth series: 1987-2010. Substantially identical drivers, small aesthetic changes.
Sixth Gen II series: 2010-2017. New drivers, with fabric suspension, like in the first series. New equalizer.

In general, it is not a good idea to use equalizers from different series, because the correction curves are specific to each model. The biggest changes occur with odd number series, therefore: the first and second series have few differences, third and fourth few differences and so on. The third series is certainly the one that differs the most from the original idea: bass reflex instead of acoustic suspension, drivers with foam suspension connected in series, matrix structure of the interior of the cabinet.

Drivers with foam suspensions, of course, deteriorated easily over time. Re-foaming is expensive, given that it has to be applied to 18 speakers! Consequently, the series with drivers equipped with fabric suspensions (the first two or the last one) would be preferable at least for this simple reason. To easily recognize the various series and their differences, the reference site is Bose 901 Series - Generational History edited by Daev Roehr. The info therein have been integrated and enriched with those from other enthusiasts' pages and the original owner's manuals. There is also an international Facebook group devoted to the 901.

Some urban myths

The Bose 901s have a nearly infinite power handling, so much that you can plug them directly to the 230V (110V) electrical outlet and they won't break apart. False.
They must be installed with the 8 drivers facing the listening point, i.e. the vertex of the pentagon must face the listener. False. It's exactly the opposite!
They can be used without the Bose equalizer, just put any other equalizer and adjust the sound to your liking. False. That equalizer is an integral part of the 901 system and cannot be eliminated, nor replaced.
Any Bose equalizer of any series can be be used interchangeably. Partially false. It is good practice to use the equalizer intended for that specific series of speakers.
The 901s require very powerful amplifiers. False. Especially the bass reflex series have excellent sensitivity (89-90 dB/w/m) and they sound loud even with few watts of output from the amp. Even the 1st series are not hard to drive, due to the absence of crossover. The sensitivity is much lower, so a good 50 watts per channel may be needed if listening at high volume is your habit.
The 901s are the best speakers in the world, because they have been the best selling ones for decades. False. The statement makes no sense for any type of speaker, by the way.

Availability

A first or second series of Bose 901, 100% original and full working, is not easy to find, not even in the USA. The price depends a lot on the condition of the drivers and of the cabinet. Let's not forget that we are talking about products from around 50/55 years ago! Taking a look at the online ads, the price range is a very wide, between 500 and 1500 Euros. Consider this estimate is referred to year 2024. The equalizer costs around €300. The original Tulip stands, though not strictly necessary, can be found around 150-250 Euros. There are several issues you might need to face: the drivers can belong to different series and/or might have been re-foamed in a wrong way; the equalizer might be missing or belong to previous or subsequent series. An accurate check is essential.

Reasons for commercial success and controversies

The success of the 901 was rapid and certainly due also to a famous review that appeared in September 1968 in the magazine HiFi/Stereo Review (now Sound & Vision, which republished it 50 years later, in 2018) written by Julian Hirsch. For him, who was not prone to hyperbole, the 901 was among the best speakers ever, regardless of price. A short time later, in 1970, the 901 was one of the most popular speakers sold on the market, even if one of the most expensive.

Despite the market success, other magazines reviewed the 901 quite negatively, in particular several criticisms came from Stereophile and The Absolute Sound. This triggered the war between admirers and detractors of the 901. Bose's aggressive marketing then did the rest.

The bottom line is that many enthusiasts have their idea about the sound of the 901 simply based on "hearsay". Very few audiophiles, nowadays, can say they have actually listened to the original 901 project, whose sound has changed over the years, among other things. Not only that, but at the time they were made, the end of the '60s, both the sources and the amplifiers available were unable to make this design perform at its best.

For all these reasons it becomes extremely interesting to listen to the 1st series of 901 today, with modern ancillaries, so to better understand their strengths and weaknesses, free from technical conditioning and prejudices. And this is the purpose of these two episodes I have devoted to the 901. I will investigate the sound, the pros and cons and the tricks to make perform them at their best in the second part. Stay tuned!

Did you enjoy this article? You can support our free, advertising-free magazine with a small Paypal donation using the button below. Thank you!

DISCLAIMER. TNT-Audio is neither a shop, nor a HiFi company or a repair laboratory for HiFi components. We don't sell anything. It is a 100% independent magazine that neither accepts advertising from companies nor requires readers to register or pay for subscriptions. If you wish, you can support our independent reviews via a PayPal donation. After publication of reviews, the authors do not retain samples other than on long-term loan for further evaluation or comparison with later-received gear. Hence, all contents are written free of any “editorial” or “advertising” influence, and all reviews in this publication, positive or negative, reflect the independent opinions of their respective authors. TNT-Audio will publish all manufacturer responses, subject to the reviewer's right to reply in turn.