Bose 901 series I active equalizer DIY - diyAudio
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Old 15th October 2013, 03:51 AM   #1
amerloc is offline amerloc  United States
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Default Bose 901 series I active equalizer DIY

I picked up some Bose 901 original's yesterday, but they came without the equalizer that is so important for making them sound right.

I am thinking about trying to build the equalizer for them myself.

Before I start, let me get a few things out of the way.
1) I am aware that no other company faces the amount of derision from audiophiles and audio-snobs on the net. I, too, am one of those snobs. I made the purchase with the intent of restoring and selling them.
2) On the other hand, these are supposed to be one of the best speakers that Bose ever made. I am curious how they were meant to sound and, if nothing else, they are an interesting piece of audio history.
3) I know that the EQ curve can be approximated by a regular EQ, but I don't have one of those. I am thinking an EQ box, even a homemade one, will make them easier to sell.

My goal is to make an EQ box for less than $100, as that is about the going rate on the 'bay. I have a little experience with a soldering iron, but the only circuit I've ever built from scratch is a c-moy headphone amp.

The original EQ had a couple of switches: a below 40Hz bass-reduction switch (read bass-boost off), a treble reduction switch (ditto), and a couple of treble "contour" settings on a rotary switch. The only feature I am interested in keeping is the below 40Hz switch since this allows the speakers to be used with lower powered amplifiers (<200W).

Also, I would like to use op-amps to simplify things if possible. That way I could power it off of a 12V wallwart instead of building a dedicated PS.

My first step will be to try to simplify the circuit as much as possible--focus on one channel and remove the parts of the circuit that will be switched off. I am pretty sure I know enough about electronics to do that.

Trouble is, I don't know enough about electronics to break down the circuit and figure out exactly what its doing to the waveform. Any suggestions on how best to proceed?

The SM is available at hifiengine.com. Here is the schematic: http://i1051.photobucket.com/albums/...c.png~original
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Old 15th October 2013, 06:42 AM   #2
Mooly is offline Mooly  United Kingdom
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Old 16th October 2013, 03:08 PM   #3
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Well, sorry you invested in these. I had a friend that bought a pair of 901's in 1969 and we listened to them extensively. I was totally unimpressed. The problem could have been the five sided bedroom his father had built for him, but I much preferred my pair of LWE III's in my square bedroom. The LWE III's were cheaper, too.
I would suggest buying quality microphone and doing frequency sweeps in the open air to determine what frequency compensation is required. You could build an anechoic chamber to do this, but a quiet spot in the open air is a lot cheaper in my location.
WIthout the bose equalizer unit, you are back to using a bunch of drivers to derive a flat frequency curve by electronics. And that involves engineering from scratch, or mathematically analyzing the schematic diagrams. As you have surmised, op amp equalization would be a lot cheaper than buying a lot of non-polar electrolytic caps and large current inductors.
Picking the design might be easier if you had a 16 band or 20 band equalizer to set up a curve with knobs, before commiting to the soldering iron.
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Last edited by indianajo; 16th October 2013 at 03:11 PM.
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Old 16th October 2013, 03:22 PM   #4
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I had the "good fortune" to install a pair of these in live performance venue. As I recall, the service manual had a picture of the frequency response and the schematic of the eq in it. See if you can not find the manual. E
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Old 16th October 2013, 06:04 PM   #5
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Oh gee.

This is pretty simple.

The Bose 901 requires about 12dB of bass boost at about 35Hz, iirc... the highs had two settings, iirc, more and even more.

Today I'd just take *any* inexpensive EQ, Behringer is a best bet, and dial in the EQ settings. The manual did indeed have curves, so you could match them.

You DO NOT want a "shelving" EQ, which would be a straight LP (low pass), since that will put in way too much subsonic gain, and cause the drivers to exceed their excursion capabilities. The equivalent would be a highpass (aka "subsonic filter") and a LP ("bass boost"). That is a bandpass filter.

As far as measurement, there is a vast difference between in room measurement and outdoor measurement. A "flat" response outside is likely to be very bass boosted in the room. Also the distance between the 901 and the wall has a definite effect on the bass.

So, there are a variety of free software that will do acoustical measurements, and you can use almost any microphone you like, assuming it is reasonably decent. The Panasonic WM-61a is like a few $$ and is very flat. Much info on using it, especially on Sigfried Linkwitz' site. A straw, battery, and a connector and you have a measurement system.

Since you don't care much about absolute and true readings, all you need to do is to A) get the bass and highs to be approximately "in line" with the midrange level and B) have it sound ok in your room, to your ears.

I would suggest measuring these drivers at a very close distance to the drivers, and as far as possible from any surfaces. Even within 6" is fine... keep in mind that for highs, small movement of the mic position changes what you see. Ur trying to get things "in the ballpark".

So, you would dial in ur boost on your adjustable equalizer and watch the effect on the curve. Changing the slope matters, as does the center frequency, and "Q" or width of the EQ.

With a modern EQ system in theory you can do a better job than Bose originally did. Especially since you can dial it in precisely for your room and your ears.

Keep in mind that you NEED POWER to run 901s properly.
I suggest 200watts RMS minimum, IF you like to run at SPLs that anyone would consider not background levels.

As you increase the boost, the requirement for amp power goes up considerably.

Keep in mind that for each 3dB of boost (EQ) you need to DOUBLE your amplifier power. Doing the math you can see that *if* you had to run at say 4 watts average to make 90dbSPL, 9dB of boost requires 32 watts. That doesn't sound too bad. But that's at 90dB. If you want peaks at 100dB you have to go up 9dB from there. So 9dB up from 32 watts is 64,128, 256watts!

Hope this helps...

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Old 16th October 2013, 06:17 PM   #6
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I thought you need 10X the power to add 10 dB, and that hasn't yet factored in the power compression loss.

Most people, when using a simple EQ method, seem to set it 'smiley face'
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Old 16th October 2013, 06:38 PM   #7
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Also, while I am still thinking about this...

...there are multiple threads on the 901s here, check them.

...one idea is to eliminate the need for HF EQ by installing an array of tweeters. One would make a box that sits on top of the 901s and has some combination of reflecting and direct tweeter... one would probably put a pad on the front firing tweeter in the case where one used say 2 rear facing and one front facing. But as the number of rear facing goes up the need for padding the front one decreases - but since tweeters are typically more sensitive (higher output for given input) one might need to pad the entire tweeter array or use a separate amp for the tweeters (not a bad idea). Depending on what one measured, and where the xover point is it may be sufficient to use a small inductor of proper value to roll off the 901.

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Old 16th October 2013, 11:23 PM   #8
amerloc is offline amerloc  United States
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Thanks for all the input! These are some interesting ideas. I found the service manual and it has a chart with some response levels at various test frequencies. I'm not 100% how to interpret this, but I used these numbers to approximate a curve on a software equalizer. They sounded okay, but none too impressive. Of course I had to tone down the bass since I only have about 80W to play with right now. I need to put my 200 watter under the knife before I can use it again.

@indianajo, I'm not too invested in these so far. Picked them up for $20 so don't feel too bad about my investment. I could easily sell the stands alone for twice that.

@bear, I'm not sure exactly what you mean by a "shelving" EQ. I thought that basically all EQ's (graphical EQ's, not tone control boards) were a series of band-pass filters. Are you saying that in some EQ's the lowest freq tone control is just a simple low-pass filter? If this is the case, maybe one solution would be to run it through a preamp that has a rumble filter.

Also, I think the curves in the manual are only meant to illustrate how the various "contours" on the EQ affect the sound. As I understand it, these "contours" are simply additional equalization on top of the standard equalization that give the speakers a somewhat flat response.

I thought it might be interesting to try to build the dedicated hardware EQ since the circuit itself didn't seem too complicated. Especially with most of the switches removed. I did make a simplified circuit diagram: http://i1051.photobucket.com/albums/...b.png~original I was hoping to be able to reverse engineer it and use some op-amps, but I suppose it wouldn't be too hard to replicate as-is. On the other hand, maybe it just wouldn't be worth the effort.
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Old 17th October 2013, 02:41 AM   #9
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Quote:
Originally Posted by Cal Weldon View Post
I thought you need 10X the power to add 10 dB, and that hasn't yet factored in the power compression loss.

Most people, when using a simple EQ method, seem to set it 'smiley face'
Sure but you still double the power to go up 3dB.
So it works out that 100x the power you go up 20dB.

Here's a chart:

Watt to dBm conversion table
Power (mW) Power (dBm)

0.00001 W -20.0000 dBm
0.0001 W -10.0000 dBm
0.001 W 0.0000 dBm
0.01 W 10.0000 dBm
0.1 W 20.0000 dBm
1 W 30.0000 dBm
10 W 40.0000 dBm
100 W 50.0000 dBm
1000 W 60.0000 dBm
10000 W 70.0000 dBm
100000 W 80.0000 dBm
1000000 W 90.0000 dBm
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Old 17th October 2013, 01:32 PM   #10
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The contours show, irrc, the different settings of the switches.
That means EQ is either "normal" or "more".

The curves are the actual EQ applied, not a depiction of how it sounds.

Assuming you followed the "normal" curve (lowest boost) you ought to get reasonably good sound, assuming you have positioned the speakers properly in the room - that means about 10-20" from a flat relatively hard wall, not much between the speakers.

You can duplicate the original box, but that seems foolhardy today since you can do a better job with a DSP based EQ (like a Behringer) than the original.
Even a good parametric equalizer would likely be better than the original.

You could get improved distortion figures using modern opamps in the original EQ circuit.

The 80watts ought to be sufficient as long as you don't try to crank up the volume. They will sound a bit thin unless you have sufficient bass boost.
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