A "flat" response outside is likely to be very bass boosted in the room. That is a bandpass filter.Īs far as measurement, there is a vast difference between in room measurement and outdoor measurement. The equivalent would be a highpass (aka "subsonic filter") and a LP ("bass boost"). 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 manual did indeed have curves, so you could match them. Today I'd just take *any* inexpensive EQ, Behringer is a best bet, and dial in the EQ settings. the highs had two settings, iirc, more and even more. The Bose 901 requires about 12dB of bass boost at about 35Hz, iirc. 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. 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. And that involves engineering from scratch, or mathematically analyzing the schematic diagrams. WIthout the bose equalizer unit, you are back to using a bunch of drivers to derive a flat frequency curve by electronics. You could build an anechoic chamber to do this, but a quiet spot in the open air is a lot cheaper in my location. I would suggest buying quality microphone and doing frequency sweeps in the open air to determine what frequency compensation is required. 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. I had a friend that bought a pair of 901's in 1969 and we listened to them extensively. Trouble is, I don't know enough about electronics to break down the circuit and figure out exactly what its doing to the waveform. I am pretty sure I know enough about electronics to do that. 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. That way I could power it off of a 12V wallwart instead of building a dedicated PS. 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).Īlso, I would like to use op-amps to simplify things if possible. 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. 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. My goal is to make an EQ box for less than $100, as that is about the going rate on the 'bay. I am thinking an EQ box, even a homemade one, will make them easier to sell. I am curious how they were meant to sound and, if nothing else, they are an interesting piece of audio history.ģ) I know that the EQ curve can be approximated by a regular EQ, but I don't have one of those. I made the purchase with the intent of restoring and selling them.Ģ) On the other hand, these are supposed to be one of the best speakers that Bose ever made. I am thinking about trying to build the equalizer for them myself.īefore I start, let me get a few things out of the way.ġ) I am aware that no other company faces the amount of derision from audiophiles and audio-snobs on the net. I picked up some Bose 901 original's yesterday, but they came without the equalizer that is so important for making them sound right.
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