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Old 1st February 2013, 05:59 PM   #1
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Default LT with gain-dependent boost limiter - a new idea?

I've been SPICE modeling what (to me) is a new version of the well-known "Linkwitz Transform" biquadratic filter that includes a boost limiter (not a volume limiter). My circuit uses a different type of biquadratic filter circuit than what SL uses. It still has a single op amp to create the filter itself, but since the response of the filter can not provide boost (only cut) it is followed by a gain stage.

It turns out that the shape, specifically the low frequency boost, can be modulated. A detection circuit looks at the final output signal and when it exceeds some threshold the low frequency boost of the biquad filter is reduced (at low frequencies) while the high frequency gain remains unchanged. This permits the power of the amplifier and the driver excursion capability to be used more efficiently. At low input signal level the lowest frequencies are boosted as much as possible, up to the full power input level for the amp, and as the input signal level is raised the LF boost is tapered off. This is done without introducing extreme distortion at the expense of reducing/compressing the output at the lowest frequencies on peaks.

About the only similar thing I have seen out there is a circuit (US Patent 4481662, download at PAT2PDF - Free PDF copies of patents: Download and print!) by Bag End for their ELF subwoofer line that used a double integrator type of boost circuit, with a limiter. Supposedly the ELF systems were pretty well received. My approach is related in theory, but is implemented differently.

As part of the modeling that I have done, I've looked at the distortion under dynamic signal conditions and it looks to be no more than -40dB, sometimes below -60dB dominated by 2nd order and falling off quickly. The driver will probably exhibit much more distortion.

Anyway, is this really something new? I have some PCBs already made that I can build up and test if there is enough interest. I would also be willing to post some modeling results.

-Charlie
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Old 1st February 2013, 06:13 PM   #2
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Watching with interest - if you get it working nicely, I think you might be able to sell a few boards.
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Old 1st February 2013, 07:51 PM   #3
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The attachment shows an example of the boost reduction that the circuit can achieve with increasing input level, so as to keep the max output level from exceeding a threshold. The "flattest" curve corresponds to the highest input level, while the curve with the highest low frequency gain corresponds to the lowest input level.

It's not perfect in that it also slightly alters the filter shape, but it's only by about 2dB or less at the upper knee in the response, and is IMO low enough to not worry about.

-Charlie
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File Type: jpg boost_limiting.jpg (64.0 KB, 98 views)
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Old 2nd February 2013, 12:22 AM   #4
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Attached is an example of the distortion spectrum under very strong boost limiting conditions for a 20Hz tone burst. Distortion components are -50 or better (fundamental is slightly off scale and is about +2dB). Note, this is SPICE modeling using TINA, not a "real world" circuit.

Edit: I should add that when I obtain the same spectrum on the stimulus (input signal) I get similar distortion numbers, so it looks like the circuit does not add much distortion.
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File Type: jpg spectrum_under_strong_limiting.jpg (65.1 KB, 92 views)

Last edited by CharlieLaub; 2nd February 2013 at 12:31 AM.
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Old 2nd February 2013, 04:29 AM   #5
jamikl is offline jamikl  Australia
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If I am reading this correctly it sounds like you have something that I was looking for many months ago. If I remember correctly Don Hills was looking at the idea. I wanted something that would boos the low end of two Beta 15As in an open baffle at lower volume levels but would reduce that boost as the volume was turned up and therefore hopefully preventing the the Eminences exceeding xmax too much as the volume was increased. Will this circuit you have developed do that? jamikl
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Old 2nd February 2013, 06:20 AM   #6
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This is great, I'll follow this thread with big interest!
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Old 2nd February 2013, 05:44 PM   #7
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Quote:
Originally Posted by jamikl View Post
If I am reading this correctly it sounds like you have something that I was looking for many months ago. If I remember correctly Don Hills was looking at the idea. I wanted something that would boos the low end of two Beta 15As in an open baffle at lower volume levels but would reduce that boost as the volume was turned up and therefore hopefully preventing the the Eminences exceeding xmax too much as the volume was increased. Will this circuit you have developed do that? jamikl
Yes, that's it exactly. I will try to post additional plots that make this more clear.

-Charlie
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Old 2nd February 2013, 06:29 PM   #8
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Quote:
Originally Posted by CharlieLaub View Post
I've been SPICE modeling what (to me) is a new version of the well-known "Linkwitz Transform" biquadratic filter that includes a boost limiter (not a volume limiter)
About the only similar thing I have seen out there is a circuit (US Patent 4481662, download at PAT2PDF - Free PDF copies of patents: Download and print!) by Bag End for their ELF subwoofer line that used a double integrator type of boost circuit, with a limiter. Supposedly the ELF systems were pretty well received. My approach is related in theory, but is implemented differently.

Anyway, is this really something new?
Charlie,

Sounds like what can be done with the dynamic EQ in the Behringer Ultradrive Pro DCX2496.
"The DYNAMIC EQ influences a specific frequency range of the signal, depending on the volume level. It can either boost or cut this frequency range, depending on the gain setting made by the user. If the gain control is set to lower the respective frequency range in level, and if a preset THRESHOLD is exceeded, the EQ modifies the sound by reducing the gain of the frequency range. The amount of gain reduction applied is defined by the RATIO value. When the signal drops below threshold again, the frequency range is ôsmoothed outö again, i.e. the DYNAMIC EQ stops processing the signal."

Unlike the Bag End device which is fixed for a single application, the DCX2496 can be adjusted for any desired boost/cut frequencies and thresholds.

Art
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Old 2nd February 2013, 07:05 PM   #9
epa is offline epa  Netherlands
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also available in sigma studio for powersoft dsp board(dynamic bass).

sort of loudness for the lowes
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Old 2nd February 2013, 07:17 PM   #10
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Very nice idea and is probably of great interest to the OB crowd. I know I sometimes worry about my woofers bottoming out when wanting to really crank out the tunes.

Additionally, this circuit would help with the "best response curve for hearing" that I found a few years ago. Click the image to open in full size.
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