Frequency dependent limiter to avoid one excursion above x max - diyAudio
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Old 11th April 2016, 02:23 PM   #1
hahfran is offline hahfran  Germany
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Default Frequency dependent limiter to avoid one excursion above x max

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 ?

Last edited by jazbo8; 12th April 2016 at 05:36 PM. Reason: typo
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Old 11th April 2016, 03:01 PM   #2
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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
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Old 11th April 2016, 05:43 PM   #3
Ron E is offline Ron E  United States
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accelerometer feedback and pid control?
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Old 12th April 2016, 08:35 AM   #4
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by phase_accurate View Post
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.
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Old 12th April 2016, 09:19 AM   #5
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Sounds like you are after a modified active loudness control?
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Old 12th April 2016, 11:31 AM   #6
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Quote:
That had indeed improved the bass quality of a 2 way active bookshelf speaker but commercial 2 way speakers do much better.
Which one are you talking of ?

I don't see any particularily special components used in the KLH circuit BTW.

Regards

Charles
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Old 12th April 2016, 04:37 PM   #7
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by billshurv View Post
Sounds like you are after a modified active loudness control?
no it just limiting of cone excursion below xmax
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Old 12th April 2016, 05:03 PM   #8
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A limiter cannot improve bass quality unless the speakers are undersized and forever been driven to nasty distortion? You want the protection circuit to stay out the way unless you are 'banging head' surely?
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Old 12th April 2016, 05:30 PM   #9
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Quote:
Originally Posted by hahfran View Post
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:
IDEA: Linkwitz-Transform LADSPA plugin with lookahead boost control

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.
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Old 12th April 2016, 06:31 PM   #10
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I'd do a 1 pole band split at maybe 100HZ, and use a real-time compression below 100HZ and a slow attack compression above 100HZ, and ideally make it so neither circuit does anything until you're within 6-9dB of clipping. Trying to simplify the function further may be a waste of time.
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