Frequency dependent limiter to avoid one excursion above x max

hahfran

Member
2007-12-19 8:53 pm
Frequency dependend limiter to avoid cone excursion above x max is a circuit to which many active bookshelf speakers owe excellent bass quality.
However whatever...it always comes as multiband limiter. Cone excursion rises
with falling radiation resistance thus near and below resonant frequency small
voltages already achieve x max while say 200 Hz above the resonant frequency voltages equivalent to some hundred watts achieve x max.
Therefore a multiband limiter is required.
But even reducing to 3 frequency bands the limiter is still a bunch of hardware.
Has anyone seen a simple but effective solution ?
 
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Look at US patent US4327250 A.
This is a circuit that was used by KLH for active bass equalisation. The pole frequency and Q of a 2nd order highpass, that is placed before the power amp driving the speker, is controlled according to a signal that is derived from the output of the power amp. This signal is first voltage divided and then lowpassed in order to mimic the excursion function of the woofer. The output of the filter is then sent into a level detector that controls the excursion-reducing highpass filter.
KLH used a quite simple lowpass that didn't mimic the notch in the excursion function of a reflex box accurately. Circuits used by PA companies do however mimic the excursion function a little closer in order to not give away too much headroom. This could be achieved with a 2nd order lowpass and a notch filter.
Alternatives to the JFET for the frequency control would be OTAs (most of them are obsolete) or LDR/LED combinations (soon obsolete as well).

Other possibilities would be the use of a subtractive crossover (B&O once used that IIRC)combined with a VCA or soft-clipper for the low-channel.

Regards

Charles
 

hahfran

Member
2007-12-19 8:53 pm
Look at US patent US4327250 A.

This could be achieved with a 2nd order lowpass and a notch filter.
Alternatives to the JFET for the frequency control would be OTAs (most of them are obsolete) or LDR/LED combinations (soon obsolete as well).

Other possibilities would be the use of a subtractive crossover (B&O once used that IIRC)combined with a VCA or soft-clipper for the low-channel.

Regards

Charles

hi thanks. The US patent is nice but requires special parts. My first attempt was a subtractive crossover and a soft clipper with CMOS analog switches.
That had indeed improved the bass quality of a 2 way active bookshelf speaker but commercial 2 way speakers do much better.
 
Frequency dependend limiter to avoid cone excursion above x max is a circuit to which many active bookshelf speakers owe excellent bass quality.
However whatever...it always comes as multiband limiter. Cone excursion rises
with falling radiation resistance thus near and below resonant frequency small
voltages already achieve x max while say 200 Hz above the resonant frequency voltages equivalent to some hundred watts achieve x max.
Therefore a multiband limiter is required.
But even reducing to 3 frequency bands the limiter is still a bunch of hardware.
Has anyone seen a simple but effective solution ?

I am working on something (using DSP) that is similar to, or at least related to, what you are interested in. See this thread:
http://www.diyaudio.com/forums/pc-b...rm-ladspa-plugin-lookahead-boost-control.html

The idea is that if you can reduce the LT boost, you will reduce power (and therefore excursion) and this mainly will happen at the lowest frequencies first. When more boost reduction is needed, the edge of the band in which the boost reduction is happening increases in frequency. This is done by dynamically changing the output between various levels of LT bass extension as a function of the signal level.

This also lets the user use LT correction that is somewhat excessive, meaning that so much LT boost is used that very little input signal is needed to achieve Xmax at the lowest frequencies (when there is no correction applied). As the drive level increases, the relative amount of power at low frequency is reduced but power to the higher frequencies is allowed to increase. With a closed box driver, this should result in protection against excessive cone excursion if the system is set up properly.