I take questions from readers often, but they don’t always make it to a Mailbag segment. I’ve got this one a few times lately, though, so I figured I’d answer this one publicly.

Jim from Ohio writes,

I bought a set of Bose 901 Series II speakers on base back in the ’70s, and have had them in storage for the past 20 years. Somewhere along the way the equalizer got lost, but I have an old Kenwood equalizer. Can I use that instead? Is the Bose equalizer really that important?

Thanks for writing, Jim! Can you use that Kenwood graphic equalizer in place of the bose? Short answer: It’s not recommended.

Bose used a really interesting design concept for their 901 speakers. Each contains a set of nine, 4 1/2″ full range drivers wired in series-parallel. There’s no crossovers or other passive components inside – just a bunch of individual speakers wired together to give the right impedance. About 11% of the sound is radiated from the single front-mounted speaker, and the remaining 89% of the sound is radiated from the back. Designed to be placed in corners of rooms, the Direct/Reflecting design produces a ridiculously wide, lifelike sound field which packs more of a punch than you might think from speakers that size.

When you think about the speaker design, the equalizer makes a lot of sense. These are full-range speakers, but are only loaded with 4 1/2″ high-excursion drivers whereas a normal speaker might have an 8″, 10″ or 12″ (or even multiple!) subwoofers, a midrange, and a tweeter. The 4 1/2″ drivers are very midrange sized; to achieve the highs and lows, you need to apply some serious curve shaping to the incoming signal to make up for the physical limitations of those drivers.

Enter the Active Equalizer. Without it, you’re left with the sardonic description, “Bose: No Highs, No Lows”, and for good reason. The Active Equalizer applies a pretty serious amount of emphasis to the low and high ends – as much as +18 dB to the low end. Obviously, this requires a pretty powerful amplifier to drive that power – +18 dB corresponds to a 63x increase in required power at that frequency. But that’s beside the point.

You could look up the service manual (or some of the curves I’ve published previously) for a Bose 901 Series I/II equalizer, but you’d need to shift the curves somewhat to make it work. I’ve never seen an equalizer which offered more than +12 dB of gain – which represents only a 15.8x increase in level at that frequency. If you use an off-the-rack equalizer, you’re just not going to get the boost you need for it to sound right if you leave it zero-centered, and if you offset the curve, you’re going to lose about 8-10 dB of gain in the midrange to accommodate the full travel. Which means, you’ll need to turn the volume up that much more and risk running into distortion.

The Bose 901 Series I and Series II equalizers are interchangeable as they both produce the same frequency response curves, but if you really can’t come up with one, it is possible to use two equalizers daisy-chained together. You’d have a range of +/- 24 dB in each band which would cover the full range of the 901 Active Equalizer. On the extremes, you’d have both equalizers gained up; in the middle, you’d only have the first gained up slightly and the second left flat. (Decibels add arithmetically: +12 dB on one equalizer and +6 dB on the other gives a total of +18 dB.)

So, in conclusion, while you could kludge it together, I wouldn’t recommend using an equalizer other than the Bose unit with the 901 speaker system. You can pick up an authentic model on eBay for $100-200, depending on condition, or you could always pick up a parts unit and have it restored by Rain City Audio. It might seem like a little bit of money, but for speakers like this, it’s worth it to get it right rather than trying to rig something up that’s just not going to be as good as they deserve.

See also:

Everything that goes into an overhaul for a Bose® Active Equalizer

Which revision of the Series I active equalizer do I have?