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Product: Bose 901 loudspeakers (1st series)
Manufacturer: Bose - USA
Approx. price: from €500 to €1500 depending on status/conditions
Reviewer: Lucio Cadeddu - TNT-Audio Italy
Published: June, 2024

In Part I of this series of articles devoted to the legendary Bose 901 loudspeaker you will find the history of the project and its designer, the cornerstones of the Direct/Reflecting® idea, a summary of how to recognize the different 901 series over the years and a list of clichés, often incorrect, that crowd the web and audio fans' - often fallacious - memories.

In this second part I will focus exclusively on advice related to room placement and active equalizer settings, leaving the listening notes to a later Part III. I would like to point out once again that the object of this series of articles is the first series of 901, the original project by Amar Bose and Sherwin Greenblatt, his partner at Bose Corp. and his former student at MIT.

Installation and positioning in the listening room

As already written in Part I, the 901s are by no means traditional speakers. Their correct operation is based on two unavoidable principles: the eight drivers located in the “V” of the cabinet must be facing the rear wall and the 901s must play coupled with their electronic equalizer only. Any deviation from these two principles will destroy the sound of these speakers completely, without exception.

I start with some notes on listening room placement, publishing a screenshot of the original manual. In summary: the 901 can be positioned on the long side of the room or on the short side (I recommend using the freest and largest wall you have), the important thing is that the wall behind the speakers is mainly reflective and free from furnishings. This is because the reflected sound produced by the sixteen rear drivers must reach the listener by reflection from the surrounding walls. The distance from the walls is quite flexible, with the following constraints: the “V” of the back of the speaker must be at a distance from the rear wall between 30 cm and 45 cm. In any case it must never be less than 15 cm or more than 60 cm (refer to the pic below for measures in inches). There are no particular restrictions on the size of the listening room, the important thing is that you must follow these instructions carefully. Of course, a large room will allow the 901 to express themselves at their best. I have also tested them in rooms under 20m² without any problems.

The height distance from the floor must be between 90 cm and 120 cm, because this is the average height of instruments in a concert hall. In any case it cannot be less than 30 cm. Let's say that a speaker stand, even if not the original Bose “tulip” one, of reasonable height, will be sufficient. As a guideline, I would use supports that allow the (seated) listener's ear to be aligned with the center of the front full-range driver, to optimize the perception of the high range. Some of the extension is lost by listening off axis. Do not place the speakers on the floor and, if possible, avoid furniture shelves! The 901s produce a very heavy bass range, hence the whole furniture structure would resonate. Don't forget that the 9 full-range speakers move, at a given membrane excursion, the same air as a single 32 cm woofer (!!!), without the typical diaphragm break-up problems of such large woofers. If desired, the 901 can be hung from the ceiling with chains, so that the height is still that foreseen by design. The connection cables must have a section of at least 18AWG (1 mm²). Any good electrical cable will do but, clearly, today you can use high quality cables, which were not available at the time.

Connecting and adjusting the electronic equalizer

The connections of the active equalizer are intuitive: it must be inserted between preamp and power amp or between the pre out and main in connectors of an integrated amplifier. Refer to the original manual for any other situations or needs that may arise.

These notes refer to the first series, later active equalizers are substantially different and the instructions in the original manuals, still available on the Bose website, must be followed accurately. When the controls positions are centered on the black dot, i.e. the “reference” position, the system response is that envisaged by the design. However, personal tastes or particular environments may require different adjustments. The Below 40Hz switch, if positioned on DECREASE, attenuates the low range below 40 Hz. At the time it may have been necessary to limit the background noise (rumble) generated by certain turntables. Being an electronic filter, it acts precisely and without phase rotations in the range above 40Hz.

As for adjusting the high range, you can opt for the DECREASE or NORMAL position (always the one with the dot). There are 5 different positions, marked with progressive numbers from 1 to 5. The flat response is obtained with the switch in NORMAL and the selector in position 4. The other settings generate an increase/decrease in the mid-high range starting from 450Hz in the DECREASE position and from 1200Hz in the NORMAL position. The graphs below explain more than many words.

As far as we can tell from all of the above, therefore, the unfortunate phrase No lows, no highs, it must be a Bose which has often accompanied the comments on the sound of the 901 (or Bose speakers in general) demonstrates how often these speakers have been listened to without their electronic equalizer and/or positioned incorrectly (often with the V facing the listening point or placed on the floor). The high frequencies can be increased as desired with the controls, and as for the bass, I can already anticipate that they are powerful and deep! Therefore, claiming that the 901 has neither bass nor highs is a bold and extremely inaccurate claim.

I recommend to avoid decreasing the bass range below 40 Hz, given that today the rumble is a non-existent problem with digital sources and greatly reduced with analogue ones, and to start listening with the high range controls in flat, increasing or decreasing the level if the listening room requires some fine tuning in the high range. Bear in mind that these are not real tone controls, but electronic adjustments that move the intensity of the mid-high range up and down, as a whole, from a certain frequency onwards. The tone controls on amplifiers, however, act like a bell graph starting from a very precise central frequency, typically 10 kHz on the high range and 100Hz on the low range.

The first graph below describes both the effect of the Below 40Hz switch and the boost/attenuation in the medium-high range with the switch in the NORMAL position. The second one, instead, describes the decrease in the mid-high range with the switch in the DECREASE position (except for the 5 position, which generates a boost above 5 kHz).

Driveability

The first series of 901s do not have a very high sensitivity (just over 80dB), but the load is easy on the amp, thanks to the fact that the passive equalizer is absent. At the time amplifiers were less performing than modern ones, therefore I would say that a good modern amplifier, of 40-50 watts per channel, is more than enough to provide full sound for even a fairly large room. Contrary to many speakers of the same period, the 901 can handle enormous amounts of watts without evident compression or distortion (and, believe me, I tried to stress them quite heavily), therefore very, very high sound pressures can be generated, provided the amp is of adequate power. Many speakers from the same period, with acoustic suspension like these 1st series of 901s, go into trouble with modern musical programs which are rich in low frequencies. It is easy to reach the physical limits of a normal woofer of that age. Not so the 901, hence you can have a lot of fun, with the most popular genres. Even if they were designed to mimic the sound of a concert hall (hence for classical music) they also get along very well with rock, pop or electronic music from the modern era, as we will see in the third episode dedicated to the listening test.

In conclusion, to be able to claim to have heard what a first series 901 can (and cannot!) do, it's necessary to follow all the above recommendations. Better, much better to refrain from making hasty judgments if these guidelines are not respected. It is a revolutionary, courageous and “wild” design, it deserves all our respect.

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Copyright © 2024 Lucio Cadeddu - editor@tnt-audio.com - www.tnt-audio.com

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