Bose 901 equalizer built from scratch

[Pete B]

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.

Yes, I'd like to have a copy, is it scanned or online?
I have a very old homemade pair that need an EQ.

Nice work!

 

[robgmn]

I would look for a good, pro parametric eq, ideally with two or three stackable bands...if such a beast exists, that is. :scratch2:

I was thinking the same thing, but along the lines of a digital one.
JRiver has one built in and you can dial it in to the "n"th degree...

 

[GD70]

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.

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.

That looks great! Very neat and tidy!
Glenn

 

[Pete B]

Transformer looks like a perfect fit, did you find the exact one and have the part number?

 

[leesonic]

Transformer looks like a perfect fit, did you find the exact one and have the part number?

The transformer was the only thing left on the board when I got it.

Lee.

 

[leesonic]

I figured I would add to this thread, rather than start a whole new one. I just restored a Series 2 equalizer for a customer. All new 1% metal film resistors, film caps in place of electrolytics in the signal path, new caps everywhere else. Those resistors especially are a b*tch to get out, it seems like Bose used some kind of cutter that crimped off the end of the leads. You have to physically pull each component through the board, even after all the solder is gone.

Before.



After.





Lee.

 

[Copa1934]

Damn nice! But how did it sound afterwards? How close to finishing you DIY EQ?

 

[leesonic]

Damn nice! But how did it sound afterwards? How close to finishing you DIY EQ?

Sadly, I didn't get much listening time with either. I saw a pair of Dynaco A50s on Craigslist that the owner would consider swapping for some Bose 901s. I had a pair of Series 1 901s, and this bare EQ board (the DIY EQ), and asked if he would consider a straight trade, which he did. I listened to the 901s and my DIY EQ for about four hours while I was tidying up downstairs, and we did the swap later that day. After he been using it for a little while, he asked me if I would restore the Series 2 EQ he had, which I did, and you can see pictured here.

I figured it's easier to find a pair of Bose 901s than Dynaco A50s. Despite all the haters, I actually like the 901s, the earlier models especially.

Lee.

 

[Copa1934]

Sadly, I didn't get much listening time with either. I saw a pair of Dynaco A50s on Craigslist that the owner would consider swapping for some Bose 901s. I had a pair of Series 1 901s, and this bare EQ board (the DIY EQ), and asked if he would consider a straight trade, which he did. I listened to the 901s and my DIY EQ for about four hours while I was tidying up downstairs, and we did the swap later that day. After he been using it for a little while, he asked me if I would restore the Series 2 EQ he had, which I did, and you can see pictured here.

I figured it's easier to find a pair of Bose 901s than Dynaco A50s. Despite all the haters, I actually like the 901s, the earlier models especially.

Lee.

I can't say I've seen, let alone heard, any Dynaco speaker. I think that would confirm, at least in my area, the lack of availability of these speakers. Even 901's pop up from time to time locally.

 

[DavidR623]

My series 6 EQ had surgery and the power supply caps were replaced with Panasonic FM Gold and Elna Simlic II in the signal path. They made a (better) difference; especially in the upper treble and definition.

Superb job on that S2EQ board leesonic.

 

[DavidR623]

.................................................................................................................................. Despite all the haters, I actually like the 901s, the earlier models especially.

Lee.

The acoustic suspension were better. Don't worry about the Bose haters.
It only matters what it sounds like to YOU.

Bose made 2 very good speakers. The 901 and the 601 (especially Series 2 for both models). If the 901 was total junk do you think it would be THE most successful speaker ever produced. Most speakers ever made and for the longest period..........and they made money doing it.

Rebuild those old EQ's for the Series I and II and they just get better. I just picked up a pair of series 2 901s. They've become quite popular and demand good money if in good condition. I bought a pair back in college and they were late production Series II and Bose began using the new drivers destined for the Series 3. The foam rotted about 15 years later. Bose replaced them with Series 6 for $350 around 1990. The ones I just picked up have the CT drivers with cloth surrounds.

 

[DavidR623]

Does that guy in mass sell any. How do I contact them

Looks like leesonic does first class work. I've used the guy in MA and he does good work BUT he does not want to bother with the 1% film resistor upgrade. I wish I had seen this thread 3 weeks ago. I would have asked leesonic for a quote.

 

[leesonic]

Fixed broken picture links.

 

[Bobbyd38]

You're correct. I could have chosen better--and yet I'm still satisfied with the outcome.


Your parts list won't help me much, but it'll sure help others with the Series II (and maybe Series I) equalizers.

Lee, I have to rebuild my series II 901 EQ, can I get a copy of your parts list? Thanks, Bob

 

[Jdaubenspeck]

Hi Lee, I have a Series II EQ that I need to rebuild and was interested in getting a copy of your parts list. Also would be interested in knowing where you sourced the parts. I have gone to Mouser, but was wondering if you would recommend something different. Thanks, Jamie

 

[Audiokingman]

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.

Anyone know what value(s) are the 4 yellow caps on the left?

 

[Schurkey]

From my notes, many years ago:

C3 and C4 470uF 50V (Blue) P13134-ND 0.87 ea.` $1.74



C5 and C6 001uF 50V (Yellow) P4675-ND 0.68 ea. $2.72
C15 and C16
That is, 1uF, not 0.001


C101 and C301 010uf 35V (Blue) P14629-ND 3.84 ea. $23.04
C141 and C341
C147 and C347
That is, 10uF, not 0.010uF


All original capacitors rated for 85 C.

12 pieces $27.50



From Polk Forum

http://www.polkaudio.com/forums/showthread.php?66220-Reminising-Bose-901-s/page2

For a premium upgrade, the two 470uf get 35volt 1000uf Panasonic FM Series Caps, the four 1uf get the same value 50 volt Panasonic FC Series, and the remaining 10uf caps get 25 volt Elna Silmic IIs. The two quad op/amps rearly fail, howerver when they do, they are replaced in pairs with identical models. For the dual op/amps, if they fail, I prefer TL072CPs instead of the 4559s. The TL072s were designed as audio amplifiers, while the 4558 and 4559 (interchangable) are general purpose high frequency amps.



Integrated Circuits

LS404CB (2) 14-Pin filter/equalizer

RC4559N (2) 08-Pin high-performance unity-gain op-amp

 

[Audiokingman]

From my notes, many years ago:

C3 and C4 470uF 50V (Blue) P13134-ND 0.87 ea.` $1.74



C5 and C6 001uF 50V (Yellow) P4675-ND 0.68 ea. $2.72
C15 and C16
That is, 1uF, not 0.001


C101 and C301 010uf 35V (Blue) P14629-ND 3.84 ea. $23.04
C141 and C341
C147 and C347
That is, 10uF, not 0.010uF


All original capacitors rated for 85 C.

12 pieces $27.50



From Polk Forum

http://www.polkaudio.com/forums/showthread.php?66220-Reminising-Bose-901-s/page2

For a premium upgrade, the two 470uf get 35volt 1000uf Panasonic FM Series Caps, the four 1uf get the same value 50 volt Panasonic FC Series, and the remaining 10uf caps get 25 volt Elna Silmic IIs. The two quad op/amps rearly fail, howerver when they do, they are replaced in pairs with identical models. For the dual op/amps, if they fail, I prefer TL072CPs instead of the 4559s. The TL072s were designed as audio amplifiers, while the 4558 and 4559 (interchangable) are general purpose high frequency amps.



Integrated Circuits

LS404CB (2) 14-Pin filter/equalizer

RC4559N (2) 08-Pin high-performance unity-gain op-amp

Thank you

 

[Coytee]

Likely a dumb question.... (because I know that PART of the answer is you might enjoy the process of doing this!)

Could you/someone instead, simply insert an active crossover into the food chain and now, you can DSP your processed curve to your liking?

(any reason that couldn't work?)

Just asking to be curious, I too, sometimes enjoy a project when a different path might also work.

 

[Ibemiked]

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

I have the 901 II’s and when I first got them the EQ wasn’t working properly. I tried running them with a Fisher EQ-Z1 10 band & Luxman C-1010 preamp and they sounded “ok” at best. Once i got the EQ repaired it was night and day! It also helped considering I was still using the same EQ & preamp + the Bose EQ. Just as important as the EQ for the 901’s is placement! If you don’t have them properly placed they will not sound correct. They need ample space on both sides a good 12” from the wall and should be either 36” from the ground or ceiling. When properly placed & powered (Atleast 100W) they sound great.