Simple Sub-Woofer Driver

Graham Slee wrote: OK, but 24dB/octave will sound really weird phase-wise if the sub has a shallower slope. Maybe a choice between 12dB and 24dB? Frequency sweep 50Hz to 120Hz? I'll see how easy that is. Best to make it a post volume stereo pre-out with a back panel mono-sum switch. The relative level would have to be adjusted on the active sub (or the additional amp driving a passive sub).

A very well thought out "lowest common denominator" !
Graham's intuitive design technique is inspiring.

My question is: how much phase shift is there with a 12dB or 24dB preamplifier crossover? and what effect will it have on the sound above 2X the crossover frequency?
(Xover @ 100 Hz, 2X would be @ 200 Hz, just below voice frequencies that start at ~ 300 Hz)

The 0.707 Q says it's Sallen-Key which has zero ripple, and checking the Linkwitz site, it indeed is, and two are placed in series to get the 24dB. This type of filter is often used in high quality audio, and the 12dB (2nd order - unity gain) was used in the MA230 high-pass filters.

Just using stereo speakers I found that any high-pass filter didn't fit my tastes at all. However, that was early 90s before bass subs became popular, and I'd never used one.

To sweep between frequencies (in hardware) would need an eight-gang potentiometer, so it would need to be a number of switched fixed frequencies, maybe at 1/3rd octave spacing.

I think Tony may be right...it can be quite complex to integrate subs and a 'simple sub woofer driver' may create as many issues as it solves. There are a lot variables which can have a bearing on this:

- Sub brand and configuration, compatibility with the 'sub driver' chosen by Graham
- Variable phase adjustment, not just a 180 degree switch
- Crossover frequency
- Crossover slope
- Time alignment
- Sub location relative to listener & main speakers, floor, wall and ceiling construction.

I am afraid that without addressing time alignment in particular, no matter how integrated the other factors are, adding a sub cannot be 'seamless'. The converse is true though - once frequency/crossover/phase/alignment are all correct, the sub literally 'disappears'

A subwoofer driver could potentially be quite complex and TG reminded us that the thread title includes the word 'simple'

As far as I can see, some multi channel AV amps include high-pass filters but I'm not aware of this for 2 channel where a basic (full frequency?) mono or stereo RCA pair 'sub-out' is more common so maybe take the same route?