Bose 901 equalizer built from scratch

leesonic

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OK, so I didn't build it completely from scratch, but I was lucky enough to find an unpopulated circuit board. I managed to get the resistors in yesterday while my daughter was taking a nap. I used 1/2w ones as per the Bose service manual, but I used 1% metal film from Mouser. The smaller resistors are actually 0.6w ones, still 1% metal film, there were some values I couldn't find in the 1/2w series I was using.

More pics as the build progresses...

Bose901S1EQ-01.jpg
Bose901S1EQ-02.jpg
Bose901S1EQ-03.jpg
 
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Looks great so far. Would be interesting to compare your "optimized" one with a good, working "original" with oem components still within spec.

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Looks great so far. Would be interesting to compare your "optimized" one with a good, working "original" with oem components still within spec.

Good luck with that. The guy I got the circuit board off specializes in restoring these EQs. On his web page, he showed some examples of components that test WAY out of spec, and not just capacitors either. The sad thing is, there are probably hundreds if not thousands of Bose 901 speakers not giving their best because the equalizer is still running on tired old components. And it's a shame, because I like the 901s, there are a lot of Bose haters out there, but it's a pretty remarkable design.

Lee.
 
I suppose the EQ is used to beef up the low and high end from those small speakers and attenuate some of the mid-range for a more even response.
For those that have a set of 901's without the BOSE equalizer, couldn't a 10 to 31 band stereo EQ be used to achieve similar results?
BTW, considering how long some of these have been around, your equalizer built with modern components should sound a lot better:music:
 
...........I like the 901s, there are a lot of Bose haters out there, but it's a pretty remarkable design.....Lee.


You are so right about your statement here, Lee, But I'm definitely not one of them. Have three pairs of Bose 901's Series V and two pairs of Bose 802 Series II.


P.S. as for the 'Active Equalizer(s)' well I sent all four back to the manufacture in South Carolina for a total refurb.
 
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For those that have a set of 901's without the BOSE equalizer, couldn't a 10 to 31 band stereo EQ be used to achieve similar results?

From what I understand, it's possible with a multi band eq, but the Bose eqs are specifically tailored to the response of the drivers. There were recommended settings somewhere around, but I can't think it to be better than the oem so far as representing the sound as intended by the designers.
 
I used the EBay guy that is up in Mass somewhere that specializes in redoing and upgrading the 901 eq. Same guy?
Anyways....a big difference was made.
 
Options to Bose EQ?

Did a quick search on the web and yes, using a 10 band, it has been done successfully. The extreme low and high end need a boost of close to 12 db so an EQ with even a 6db boost/cut would work fine, all one would need to do is drop the mids and simply follow the recommended curve. Maybe not better than using the BOSE EQ, but certainly better than without one at all, so at the least it seems like a good option for those that can't get their hands on the real deal.
 
I would look for a good, pro parametric eq, ideally with two or three stackable bands...if such a beast exists, that is. :scratch2:
 
Did a quick search on the web and yes, using a 10 band, it has been done successfully. The extreme low and high end need a boost of close to 12 db so an EQ with even a 6db boost/cut would work fine, all one would need to do is drop the mids and simply follow the recommended curve. Maybe not better than using the BOSE EQ, but certainly better than without one at all, so at the least it seems like a good option for those that can't get their hands on the real deal.

Ehhh... Kinda, but not really/exactly.
The Bose Active EQ also provides some not insignificant preamp gain - more than you could expect from a graphic EQ.
And either the EQ is right, or it isn't. These units are different - and should be thought of differently - than outboard equalizers. They are really an intrinsic part of the speakers, no different - in a way - than the crossovers in conventional speakers.

Having worked at the Bose factory, 901's are pretty much the only Bose product I have any respect for. Those & them big honkin' 1800 power amps (which is also what they used in the test booth on all the speaker lines).
 
I managed to get the resistors in yesterday while my daughter was taking a nap. I used 1/2w ones as per the Bose service manual, but I used 1% metal film from Mouser. The smaller resistors are actually 0.6w ones, still 1% metal film, there were some values I couldn't find in the 1/2w series I was using.

Bose901S1EQ-03.jpg
Any chance you'd post a parts list? WHAT "VERSION" EQUALIZER IS YOURS? Far as I know, Series I and II share an equalizer, Series III and IV share an equalizer, and V and VI share an equalizer.

I put fresh electrolytic caps in my 901V equalizer, but I didn't do anything with the other caps, the silicon, or the resistors.

Bose_901V_Equalizer_04.JPG


Bose_901V_Equalizer_05.JPG
 
Do you have the schematic for the unit that you're building?

Yes, I do, do you need one? It's a series 2 board by the way.

Some of the values were very specific, like 39.2k for example. Bose specifies close tolerance resistors in key areas of the circuit, I went with 1% across the board. I also am replacing all of the signal path electrolytics with film caps.

Lee.
 
Any chance you'd post a parts list? WHAT "VERSION" EQUALIZER IS YOURS? Far as I know, Series I and II share an equalizer, Series III and IV share an equalizer, and V and VI share an equalizer.

I put fresh electrolytic caps in my 901V equalizer, but I didn't do anything with the other caps, the silicon, or the resistors.

Not wanting to criticize, but it looks like you have used caps more usually used for crossovers, I've used those Erse Audio ones for my own crossover work. I think I would have gone with Nichicon KWs for the power supply caps, and low leakage KLs everywhere else there were caps in the signal path. Maybe even using film caps if physically possible?

I can post a parts list, but it would be different to your EQ.

Lee.
 
Here are the caps and transistors installed. Where Bose used a ceramic cap, I used a ceramic cap. Electrolytics were Nichicon VX series (their only axial lead cap I think), except for the four that are in the signal path, I managed to squeeze in Cornell Dubilier films there, and I also used these elsewhere.

Bose901S1EQ-04.jpg

Bose901S1EQ-05.jpg

Adding switches and input/output jacks. The rotary switch was an Alps piece that Mouser had, the two push button switches were some pulled from an old receiver.

Bose901S1EQ-06.jpg
 
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Not wanting to criticize, but it looks like you have used caps more usually used for crossovers, I've used those Erse Audio ones for my own crossover work. I think I would have gone with Nichicon KWs for the power supply caps, and low leakage KLs everywhere else there were caps in the signal path. Maybe even using film caps if physically possible?
You're correct. I could have chosen better--and yet I'm still satisfied with the outcome.

I can post a parts list, but it would be different to your EQ.
Your parts list won't help me much, but it'll sure help others with the Series II (and maybe Series I) equalizers.
 
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