Bass Alignment filter for B&W Matrix 805

I've been trying to find a Bass Alignment filter for my B&W Matrix 805s, but have been unsuccessful. While searching I came across this thread That got me thinking maybe I could build one. I'm not too savy in the engineer department (though I would love to learn), but I can read a schematic and operate a soldering iron, multimeter, and scope. I wanted to see if someone here could help me out on the design of a BAF for my speakers. From what I understand, the BAF is a high pass filter with a bump at the speakers port frequency. I'm thinking a bump around 40 Hz, with a rolloff of the low side, would do the trick Anybody want to help me out with this?
What you want is 6dB boost around 50Hz or so, like this (ignore the opamp type) -


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It´s a Sallen and Key high pass filter with a Q of about 1.94.
In combination with the 4th order alignment of the speaker you
get an approximate B6 alignment. (lower bass & subsonic protection).
Use of an (unity gain stable) opamp is ok, but a good discrete buffer
would be possible also.
The 50 Hz mentioned are way to high, it´s more around 35Hz, have a look at
these measurements (Fig.2):
B&W Matrix 805 loudspeaker Measurements |
For some reason the notification email that you'd replied didn't arrive - hence tardy reply.

What type of Op amp should I use, and what +/- voltages do I need to supply to the op amp?

The opamp chosen depends on what quality level your system is. A decent mid-range choice would be NE5534 (or NE5532 for stereo). If you choose 5534 don't forget the 22pF comp capacitor. Power supplies depend on what its feeding - into a poweramp then +/- 5V should be sufficient as poweramps have nominal 1VRMS sensitivity normally.

As regards the cut-off frequency, I was going from memory of the original alignment, not measurements. If you look at the original Matrix 805 owner's manual you'll see it states 'Sixth order Butterworth alignment 42Hz cut off'. Since the speaker alone has 4th order alignment I take it the 42Hz figure is with the BAF in-circuit.
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Although anything Ken Rockwell writes should be taken with a grain of salt to say the
least, he has the B&W Bass Alignment Filter user manual online:
B&W Bass Alignment Filter

The manual has some errors, but I think they are obvious.

For the Matrix 805 the filter frequency appears to be 36Hz.
A resulting f3 of 42Hz for the whole system as indicated by abraxalito sounds reasonable.
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To me this is confusing but perhaps I'm missing a vital piece. To get a new f3 of 42Hz the peak boost would have to be above this frequency no?

I suggest suck it and see - to extend the original 50Hz boost freq down to ~35Hz, put a 220nF in parallel with the existing 470nF in both positions.
In the case of an exact B6 (Butterworth) Alignment there would be f3=fb=ff, without the filter the response would be down to -9dB at fb. If I remember correctly this would
require a driver Qts of 0.39. Other Alignments (QB6 "quasi" Butterworth) are possible
and were described by Thiele & Small. Those with the filter frequency above f3 do not
provide a boost, they actually lower the reponse of a high Q speaker to a flat B6 like
response. These type of alignment is not commonly used. Any alignment for a speaker with
a Qts lower than 0.39 would have a ff below f3.

A more practical approach is described in a paper by D.B. Keele (attached).
In his case ff is always fb (which is good because at fb almost all output comes from
the port, so there is no added speaker excursion at the boost frequency) and the applied
boost is always limited to 6 dB.


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Emulating using software EQ

Hi, I've got hold of a pair of 805 Matrix speakers and I'm really pleased with them. They could do with a bass boost and I've been looking at Windows software alternatives of doing this. The is a program called Equalizer APO which enables you to create what seem to me to be really complex filter arrangements. I had a basic idea about gain and frequency before but Q and different filter types were a mystery until a couple of hours ago. I've been playing around with editing an existing bass boost function in the program and it's made a difference already. Currently I've got a 'PK' filter giving a 6dB boost at 50Hz with a Q of 0.5. I need to read up on low pass filters but I imagine using one of them would improve things?

If you understand this all better than I do it would be interesting to see the results others get!
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All very interesting. I haven't done Butterworth Filters in years. :)

So what B&W did with their add-on box of tricks is to add a second order electrical filter to a fourth order bass reflex to make a 6th order bass filter. :cool:

Bit of a job to deconstruct this:

An externally hosted image should be here but it was not working when we last tested it.

Vishay Precision Group - Foil Resistors - Case Study: Upgrading High-Pass Alignment Filter for Audio Systems

Then you can find B&W's resistor settings here for the Matrix series:
B&W Group North America Service & Support - B&W Archive Manuals & CAD Drawings

Presumably you could fiddle with the box alignment for different 4th order results too. As noted, you could build your own 2nd order electrical filter based on Don Keele's work too.

I'm not sure it's really worth the effort. If you think that closed box is second order, reflex generally 4th order at whatever steepness you fancy, then even steeper 6th order is pretty small beer in the overall scheme of things. :D