I’m working on a Bose 901 Series 1 equalizer for a friend. The 1970s-era film capacitors have failed (like I’ve seen before) and the equalizer has begun to introduce distortion instead of remove it like it’s intended. As expensive, top of the line audiophile speakers they definitely shouldn’t sound like that, but fortunately it’s a pretty easy fix.
The Bose 901 Series 1 equalizer was offered in two production runs, which can be interchanged and either will work fine with a pair of Series 1 speakers, but they have slightly different components. I spent some time with the circuit schematics and identified how to tell the two apart – it’s extremely simple!
Along the back wall you can see the row of silver capacitors used in the power supply filtering. These are all 100uF 25V electrolytic capacitors, which help ensure the preamp transistors have clean power. On the Second Production of the Series 1 equalizer, there are 7 of these 100uF capacitors. On the First Production, there are only 3 of these capacitors. See schematic snips, and click through for full sized images:
Both models of equalizer also use different signal capacitors. The First Production run uses a total of 18: (.1 x 2, .047 x 2, .015 x 6, .01 x 4, .001 x 2) while the Second Production run uses only 12: (.1 x 4, .047 x 2, .015 x 6).
The equalizer is a necessary component of the Bose 901 speaker system, Bose’s highest-end hi-fi equipment. The speakers are specially designed in a way that requires the signal to be equalized and pre-amplified, and if you don’t use the Active Equalizer they’ll really sound pretty bad. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, but the lack of using an equalizer might be where derogatory slogan “Bose: No Highs, No Lows” came from.
With these very obvious circuit differences, it should be easy to tell which model you’re servicing and which parts to replace. Keep in mind, though, this article only applies to the Series I Active Equalizer. The Series II equalizer is compatible with the Series I speakers, but has a different circuit schematic.