Our realtor friend knows my interest in audio, so she brought me a whole Bose 901 Series III speaker set that was abandoned in a property. The 901 speakers were re-foamed, however a couple needed redoing due to voice coil rubs. The set is from 1978, so the equalizer is by now in need of refreshing. I record here the process for the future 901 owner who may find this useful:
Replace all electrolytic capacitors:
There are 10 capacitors to replace, all 35V rated (or use higher):
2 - 470 uF One was upgraded to 3300 uF, other remains at 470 uF, make sure bigger cap clears cover, mine did not!
4 - 10 uF These were replaced with Nichicon FW
4 - 1 uF - these were replaced with film caps, it improved sound a bit
Replace 14 small signal transistors:
12 - BC239, replaced with BC547, available at Mouser.com: NPN
512-BC547CTFR
BC547CTFR
2 - Motorola 2N3906, replace with complimentary pair to BC547, the PNP BC557 - Note pin orientation different than Motorola, turn 180 degrees. These are PNP
512-BC557BTF
BC557BTF
Replace 2 rectifier diodes:
1N4002
After replacements, I noticed residual power noise, that is why I upgraded the 470 uF cap to 3300 uF, and installed a 2-3 watt 0.47 ohm resistor between the rectifiers and the first cap, had to cut the trace. See picture.
Replace 8 RCA phono connectors:
These were upgraded to gold as the aluminum connectors were oxidized and sometimes failing to make a solid connection. Available from Mouser:
568-NYS367-0
RCA PANEL JACK GOLD
1568-NYS367-2
RCA PANEL JACK GOLD
Sound is now crisp and clean with no lose connections. I hope this will help the next Bose 901 Series III owner.
Replace all electrolytic capacitors:
There are 10 capacitors to replace, all 35V rated (or use higher):
2 - 470 uF One was upgraded to 3300 uF, other remains at 470 uF, make sure bigger cap clears cover, mine did not!
4 - 10 uF These were replaced with Nichicon FW
4 - 1 uF - these were replaced with film caps, it improved sound a bit
Replace 14 small signal transistors:
12 - BC239, replaced with BC547, available at Mouser.com: NPN
512-BC547CTFR
BC547CTFR
2 - Motorola 2N3906, replace with complimentary pair to BC547, the PNP BC557 - Note pin orientation different than Motorola, turn 180 degrees. These are PNP
512-BC557BTF
BC557BTF
Replace 2 rectifier diodes:
1N4002
After replacements, I noticed residual power noise, that is why I upgraded the 470 uF cap to 3300 uF, and installed a 2-3 watt 0.47 ohm resistor between the rectifiers and the first cap, had to cut the trace. See picture.
Replace 8 RCA phono connectors:
These were upgraded to gold as the aluminum connectors were oxidized and sometimes failing to make a solid connection. Available from Mouser:
568-NYS367-0
RCA PANEL JACK GOLD
1568-NYS367-2
RCA PANEL JACK GOLD
Sound is now crisp and clean with no lose connections. I hope this will help the next Bose 901 Series III owner.
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Electrolytics and "mechanicals" such as connectors , maybe even switches and pots are worth replacing, age and corrosion are enemies.
Transistors? ... not that much.
In any case, congratulations on reviving an old warrior.
If you found it, please post schematic for reference.
Transistors? ... not that much.
In any case, congratulations on reviving an old warrior.
If you found it, please post schematic for reference.
Ditto. I fully respect the art and pleasure of restoring an old piece of gear.
That said, you could get better sound with a MiniDSP 2x4HD, plus it has digital inputs, and it would only take about 15 minutes to get the EQ within the ballpark.
I wrote a sidebar about the Bose 901 EQ (the engineering of the 901 EQ system relative to the low port tuning Fb of the small drivers is really quite ingenious) in this article in AudioXpress: "The DSP Assisted Reflex" https://audioxpress.com/files/attachment/2721 page 43 of the magazine / PDF p. 7 - the 901 is Exhibit A of this technique and Bose executed it quite well in the analog domain back in the day.
That said, you could get better sound with a MiniDSP 2x4HD, plus it has digital inputs, and it would only take about 15 minutes to get the EQ within the ballpark.
I wrote a sidebar about the Bose 901 EQ (the engineering of the 901 EQ system relative to the low port tuning Fb of the small drivers is really quite ingenious) in this article in AudioXpress: "The DSP Assisted Reflex" https://audioxpress.com/files/attachment/2721 page 43 of the magazine / PDF p. 7 - the 901 is Exhibit A of this technique and Bose executed it quite well in the analog domain back in the day.
I saw that DSP device, would probably not be as noisy as the original Bose equalizers. Thanks for the replies, all great information, especially for folks that get a 901 set without the equalizer. I don't have the schematic, searched all over and could not find one.
Series I schem. in post #12 here: https://www.diyaudio.com/community/threads/bose-901-series-i-active-equalizer-diy.244191/
jeff
I like more the restoration of the old EQ. You have the freedom to adjust the sound using the knobs.
I am curious if a low drop regulator would further help to get the noise down and could be integrated?
I am curious if a low drop regulator would further help to get the noise down and could be integrated?
How often would you need to adjust the sound? Granted, the miniDSP’s Device Console is more work and gear to use, but would offer far more granular control of not just the 901s, but up to 2 independently EQd subs, should in room measurements suggest their effectiveness.
edit: were Amar around, I’d not be surprised if he would jump all over the idea.
Thoughts?
edit: were Amar around, I’d not be surprised if he would jump all over the idea.
Thoughts?
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I used MiniDsp and it had good sound quality.
But liked most Behringer DEQ2496 for auto EQ and adjustability without PC.
Today smartfone EQ is easiest available technology.
But liked most Behringer DEQ2496 for auto EQ and adjustability without PC.
Today smartfone EQ is easiest available technology.
Easiest available perhaps, but I’d like to see one as flexible as the combination of REW and miniDSP. Yup, it’s a few hundred dollars all in, but who here hasn’t spent more than that on “audiophile grade” snake oil cabling and other assorted ephemera?
Apologies for digging up an oldish thread. I recently got hold of an early set of 901s, but no equaliser.
In the UK at least MiniDSP seems to be either unavailable or prohibitively expensive.
I’ve been wondering about one of these (ADAU1701 based) or another similar low cost DSP processor. Any thoughts?
https://www.ebay.co.uk/itm/256270357227
In the UK at least MiniDSP seems to be either unavailable or prohibitively expensive.
I’ve been wondering about one of these (ADAU1701 based) or another similar low cost DSP processor. Any thoughts?
https://www.ebay.co.uk/itm/256270357227
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A WiiM streamer $90 in the USA could be pressed into service to do the job. It has 10-Band and parametric digital EQ, and analog inputs and outputs. Not ideal but do-able and affordable.
If your main source is streaming through Airplay, then so much the better.
If your main source is streaming through Airplay, then so much the better.
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You can order directly from MiniDSP in HK, that is what I have done. You will have to pay import duty/VAT on the purchase, but the MiniDSP is a great option. The Behringer DEQ probably is too, and probably more readily available new/used in the UK and can be modded for improved performance.
@chrisb To your question seems highly likely, I knew doctor Bose professionally.
Kevin, setting aside my admitted fraught feelings over the 901 design in general, I’d imagine that Dr Bose would have embraced any technology that could reliably and affordably achieve his desired goal. I wouldn’t be surprised if he’d applaud the granular level to which modern acoustic measurement and signal processing gear and software can mitigate many of the issues with which varying room environments impart on any type of speaker, and certainly designs that rely on the room for their performance.
I just had a thought of the adventure to be had implementing the full suite if Dirac with 901s, 🙂
I just had a thought of the adventure to be had implementing the full suite if Dirac with 901s, 🙂
Although I am a big fan of EQs being it the sophisticated ones named or today also possible choice the ones on smartfones (would be good enough for a 901 Bose) I would always go for the original Bose EQ and do the modernization (recapping and the like) because simply its original and nice to look to.
I'd prefer to use an original Bose EQ, but they are not easy to find in the UK, and expensive to import. The same goes for MiniDSP or the Dayton equivalent. I suspect it's partly a currency issue.
There are ways around it in the purely digital domain (i.e. using a computer or a smartphone) but I'd like a solution with analogue i/o so that the speakers can be partnered with a conventional amp and analogue sources (as originally intended) - in other words, something that effectively works the same as the original Bose box.
Looking around online it seems that the ADAU 1701 based Wondom DSP and Sigma Studio ought to do what I need it to do, it's also a very low cost option - the version with the case is neat but there are cheaper board-only versions too:
https://store.sure-electronics.com/product/AA-AP23122
I plan to experiment with the approach, but it's likely to sit on the back burner for a while.
There are ways around it in the purely digital domain (i.e. using a computer or a smartphone) but I'd like a solution with analogue i/o so that the speakers can be partnered with a conventional amp and analogue sources (as originally intended) - in other words, something that effectively works the same as the original Bose box.
Looking around online it seems that the ADAU 1701 based Wondom DSP and Sigma Studio ought to do what I need it to do, it's also a very low cost option - the version with the case is neat but there are cheaper board-only versions too:
https://store.sure-electronics.com/product/AA-AP23122
I plan to experiment with the approach, but it's likely to sit on the back burner for a while.