Tech­ni­cal Talk The Bose 901 Speaker Sys­tem

Ju­lian Hirsch’s re­view of the Bose 901 in 1968 helped set off one of the great­est and long­est-last­ing au­dio­phile de­bates.

Sound & Vision - - CONTENS -

There may be no sin­gu­lar prod­uct in

mod­ern au­dio his­tory that has gen­er­ated more ac­co­lades, de­ri­sion, or pure con­tro­versy than the Bose 901 loud­speaker. In­tro­duced in 1968 by a then four-year-old con­cern named af­ter its MITe­d­u­cated founder, the 901 nei­ther looked, nor sounded, like any speaker that had come be­fore it. With its pen­tag­o­nal cabi­net that faced eight of its nine iden­ti­cal 4-inch, full-range driv­ers at the re­flect­ing wall be­hind the speaker, its de­signer Amar Bose sought to have it mimic the way we hear in con­cert halls and im­bue its sound with a giant sound­stage and spa­tial re­al­ism that was un­sur­passed.

Be­yond any suc­cess of its spa­tial trick­ery, the 901 had its is­sues— the com­bi­na­tion of its small cabi­net and un­usual dis­per­sion pat­tern re­quired equal­iza­tion at both ends of the fre­quency spec­trum, and it was (not sur­pris­ingly) room and place­ment sen­si­tive. Some so­phis­ti­cated audiophiles be­moaned a per­ceived lack of de­tail and veiled qual­ity to its sound. J. Gor­don Holt, found­ing edi­tor of our high-end sis­ter pub­li­ca­tion Stereophile, noted in a 1971 com­men­tary (avail­able at stereophile.com) that the 901 “pro­duces a more re­al­is­tic sem­blance of nat­u­ral am­bi­ence than any other speaker sys­tem, but we would char­ac­ter­ize it as un­ex­cep­tional in all other re­spects.” My own men­tor, Harry Pear­son, Jr., told me in the early 1980s that he bought a pair of first-gen­er­a­tion 901s af­ter read­ing the pos­i­tive re­views in the main­stream au­dio press and was so dis­ap­pointed that it prompted him to found The Ab­so­lute Sound as an al­ter­na­tive voice.

In the leg­end and mythol­ogy of the Bose 901, the re­view we’ve reprinted here, writ­ten by Ju­lian Hirsch for Hifi Stereo Re­view’s Septem­ber 1968 is­sue, looms large. It has been sug­gested by some ob­servers that few fac­tors be­yond Bose’s own ad­ver­tis­ing con­trib­uted more to the speaker’s huge com­mer­cial suc­cess. While the re­view re­tained Hirsch’s usual dis­pas­sion­ate and pro­fes­so­rial voice, it was cer­tainly as close to a rave as he ever got. In 1998, when SR cel­e­brated its 40th an­niver­sary and Hirsch was asked to re­flect on the most note­wor­thy prod­ucts he’d en­coun­tered, he cited the 901 right along­side such clas­sics as the orig­i­nal Shure V15 car­tridge, the Marantz 10B tuner, and the Dy­naco A-25 book­shelf speaker. Back in ’68, the 901 re­view ap­peared with­out fan­fare and was mixed among the sev­eral fea­tured each is­sue in Hirsch’s “Tech­ni­cal Talk” depart­ment, which al­ways be­gan with a brief es­say (not re­pro­duced here), fol­lowed up by a hand­ful of prod­uct tests.

The 901, with pe­ri­odic im­prove­ments, re­mained con­tin­u­ously avail­able from Bose right up un­til 2017, when the com­pany fi­nally sus­pended pro­duc­tion.— Rob Sabin

DE­PEND­ING on one’s view­point, the Bose 901 speaker sys­tem might be con­sid­ered a revo­lu­tion­ary approach to sound re­pro­duc­tion, or sim­ply a work­able com­bi­na­tion of well-es­tab­lished (and some­times dep­re­cated) tech­niques. The Bose 901 en­clo­sures house nine small, spe­cially de­signed driv­ers that have 4-inch cones and pow­er­ful mag­netic struc­tures. Eight of the driv­ers are an­gled to the rear, while the ninth is mounted on the front of the en­clo­sure fac­ing the listening area. This ar­range­ment is in­tended to achieve ap­prox­i­mately the same ra­tio of direct to re­flected sound that ex­ists in the con­cert hall.

The 901’s cab­i­nets are quite com­pact, mea­sur­ing 12-3/4 inches high by 20-9/16 inches wide when viewed from the front. Seen from the top, the rear of the en­clo­sure forms a “V” of about 120 de­grees. Ba­sic to its op­er­a­tion is the re­quire­ment that it be mounted with

the “V” fac­ing the wall, the apex be­ing about 12 inches from the wall. When a pair of 901s are so in­stalled, the sound ap­pears to be uni­formly dis­trib­uted across the wall be­tween the speak­ers com­pletely free of any “hole-in-the-mid­dle” ef­fect. Since only 11 per­cent of the sound is ra­di­ated di­rectly for­ward, it is al­most im­pos­si­ble to lo­cal­ize the source.

An in­trin­sic part of the Bose 901 sys­tem is an ac­tive (ten-tran­sis­tor) equal­izer that han­dles both chan­nels; it com­pen­sates for the high­fre­quency losses in­her­ent in the re­flect­ing process and also flat­tens out the bass re­sponse. (The un­com­pen­sated bass re­sponse is down be­cause of the nat­u­ral bass roll-off re­sult­ing from the very small vol­ume of the en­clo­sure.) Housed in a wal­nut cabi­net 2-13/16 inches high by 9-1/4 inches wide and 6-3/4 inches deep, this self-pow­ered equal­izer unit is con­nected ei­ther be­tween the pream­pli­fier and power am­pli­fier or in the tape-mon­i­tor­ing sig­nal path of the am­pli­fier or re­ceiver. In the lat­ter case, the am­pli­fier’s tape-mon­i­tor switch is left set to TAPE. So that the tape-mon­i­tor func­tion would not be lost, Bose has built it into the equal­izer. A tape recorder can be con­nected to the equal­izer and the usual mon­i­tor­ing switch­ing per­formed through it.

There are five con­trols on the equal­izer, four rocker switches and one five-po­si­tion ro­tary con­trol. One rocker serves as an on-off switch, an­other as the tape-mon­i­tor switch, and the third as a low-cut fil­ter that pri­mar­ily af­fects fre­quen­cies be­low 40 Hz. This is in­tended to re­duce rum­ble or acous­tic feed­back. The fourth rocker switch in­ter­acts with a ro­tary five-po­si­tion tre­ble-contour con­trol. When the rocker switch is set for NOR­MAL, the ro­tary switch pro­vides a boost po­si­tion, a flat po­si­tion, and three po­si­tions of de­creas­ing high-fre­quency re­sponse from the speak­ers. When the rocker switch is set for TRE­BLE DE­CREASE, it in­tro­duces a de­pres­sion in the re­sponse be­tween 2,000 and 6,000 Hz. The five switched con­tours then not only af­fect the very-high-fre­quency speaker per­for­mance, but also the fre­quen­cies be­tween 500 and 2,000 Hz that are not af­fected with the rocker switch in its NOR­MAL po­si­tion. In all, ten dif­fer­ent high-fre­quency/mid-range re­sponse con­tours are avail­able.

For those who have well-trained hear­ing and mu­si­cal judg­ment— plus the urge to tin­ker—it is pos­si­ble to cor­rect for poor record­ings to a re­mark­able de­gree with the equal­izer con­trols. Most peo­ple will prob­a­bly pre­fer to leave them in their NOR­MAL settings.

The ac­tive equal­izer in­tro­duces no per­cep­ti­ble dis­tor­tion. We mea­sured its dis­tor­tion at less than 0.13 per­cent for any out­put un­der 3 volts, which is greater than would be re­quired with any am­pli­fier we know of. The out­put sig­nal is of ap­prox­i­mately the same level as the in­put sig­nal.

In the Au­gust, 1968 Tech­ni­cal Talk col­umn, I com­mented on the dif­fi­culty of de­scrib­ing speaker per­for­mance in purely ob­jec­tive terms. The Bose 901 is a per­fect illustration of this prob­lem. Af­ter a cou­ple of months of liv­ing with a Bose 901 sys­tem, I am con­vinced that it ranks with a hand­ful of the finest home speaker sys­tems of all time. Be­cause of its un­con­ven­tional mode of op­er­a­tion, I rather doubted that any fre­quency-re­sponse measurements I could make would ac­count for the re­mark­able re­al­ism of its sound. Dif­fi­cult as it is to mea­sure the out­put of a sin­gle direct ra­di­a­tor in a nor­mal liv­ing room, it is well-nigh im­pos­si­ble to mea­sure an al­most per­fectly dis­persed sound pat­tern such as that of the 901 with­out strong in­flu­ence from the ef­fects of room acous­tics. Nev­er­the­less, a mea­sure­ment was at­tempted.

We placed the speaker in the rec­om­mended po­si­tion rel­a­tive to the wall. We did not have the equal­izer in the sig­nal path for our fre­quency-re­sponse and tone-burst measurements, but mea­sured the equal­izer

re­sponse sep­a­rately and added it to the speaker re­sponse to ob­tain the final curve. Ten mi­cro­phone po­si­tions were used, and their read­ings aver­aged. Har­monic dis­tor­tion was mea­sured at a 1-watt drive level with the equal­izer in­stalled.

It was no sur­prise to find that the final re­sponse curve was not as flat as some we have mea­sured. There ap­peared to be a broad rise of about 5 or 6 db in the 130- to 250-Hz re­gion, al­though we could not de­tect its pres­ence by ear. The out­put fell smoothly above 1,000 Hz to –7 db at 6,000 Hz, then rose to the 1,000-Hz ref­er­ence level be­tween 10,000 and 15,000 Hz.

The low-fre­quency har­monic-dis­tor­tion measurements were af­fected by the speaker and mi­cro­phone place­ment. The dis­tor­tion was 7 per­cent at 20 Hz, and reached max­i­mums of 12 per­cent at 30 Hz and 10 per­cent at 50 Hz. It was con­sid­er­ably lower at other fre­quen­cies in the bass range. (As a point of ref­er­ence, the bet­ter acous­tic-sus­pen­sion speak­ers have about half as much mea­sured dis­tor­tion at sim­i­lar drive lev­els.)

We lis­tened to the Bose 901 in sev­eral listening rooms, which ranged acous­ti­cally from ex­tremely hard and bright to quite dull. It was com­pared in A-B tests with sev­eral of the bet­ter speaker sys­tems at our dis­posal. The Bose 901 had an ut­terly clean, trans­par­ent, and ef­fort­less sound. Its clar­ity and def­i­ni­tion when re­pro­duc­ing com­plex or­ches­tral pas­sages were, in the writer’s opin­ion, un­sur­passed by any other speaker he has heard. This im­pres­sion was con­firmed by its tone-burst re­sponse, which was uni­formly ex­cel­lent across the fre­quency spec­trum. Its low-bass re­sponse was dif­fi­cult to credit to such a com­pact sys­tem. It had all the room-fill­ing po­tency of the best acous­tic-sus­pen­sion sys­tems, com­bined with the taut­ness and clar­ity of a full-range elec­tro­static speaker. The spa­tial dis­tri­bu­tion, which brings an en­tire wall alive with sound, con­trib­utes greatly to the sense of re­al­ism.

There is, un­for­tu­nately, a se­ri­ous ob­sta­cle to the universal ac­cep- tance of a speaker such as the Bose 901. The 12-inch gap nec­es­sary be­tween the apex of the speaker and the wall places the front of the speaker about 30 inches from the wall. Book­shelf mount­ing is gen­er­ally im­prac­ti­cal, and it may be dif­fi­cult to in­stall the 901 in the cor­rect lo­ca­tion with­out dis­turb­ing room decor. Many po­ten­tial users will be forced to de­cide be­tween style and sound.

Elec­tri­cally, the Bose 901 is rather in­ef­fi­cient, and the 18 db of bass boost sup­plied by the equal­izer re­quires huge re­serves of am­pli­fier power if loud low-fre­quency pas­sages are to be played. To a lesser de­gree, the same prob­lem ex­ists at the very high fre­quen­cies. Bose rec­om­mends am­pli­fier power rat­ings from 20 to 200 watts per chan­nel, into 8 ohms. We have used it suc­cess­fully with am­pli­fiers at both ends of this range. Un­like most speak­ers, the 901 sounds as good at a whis­per as it does at a roar, but if you are ever tempted to turn up the vol­ume a bit, an am­pli­fier with a con­tin­u­ous power rat­ing of at least 60 watts per chan­nel is strongly rec­om­mended. A pos­si­ble com­pro­mise is to use the “be­low 40 Hz” roll-off in the equal­izer, which re­duces low-fre­quency peak-power re­quire­ments by 8 db and has lit­tle au­di­ble ef­fect. In­ci­den­tally, don’t worry about over­load­ing the 901. The in­di­vid­ual driv­ers can each handle 30 watts with­out dif­fi­culty, and few of us are likely to be able to ap­ply more than 270 watts to each chan­nel.

In the final analysis, the judg­ment of a speaker must be sub­jec­tive and per­sonal in na­ture. I have, on oc­ca­sion, warmly praised speak­ers that I con­sid­ered to be out­stand­ing per­form­ers. Ev­ery­thing I have said in the past is still valid. Nev­er­the­less, at this mo­ment, I must say that I have never heard a speaker sys­tem in my own home which could sur­pass, or even equal, the Bose 901 for over­all “re­al­ism” of sound. My part­ner, Glad­den Houck, con­curs to the ex­tent that he con­sid­ers it a very fine sys­tem, cer­tainly the equal of any­thing at or near its price. The Bose 901 sys­tem, con­sist­ing of two speaker units and the equal­izer, is priced at $476.

Our orig­i­nal re­view of the Bose 901 in Septem­ber 1968 was one of sev­eral that month in Ju­lian Hirsch’s Tech­ni­cal Talk depart­ment.

The uni­formly ex­cel­lent tone-burst re­sponse of the Bose 901 is il­lus­trated by the os­cil­lo­scope pho­tos of tone-bursts at (left to right) 130, 1,000, and 9,500 Hz.

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