Any hints on limiting or compressing sub bass?

[abbey road d enfer]

]It's trivial for closed box.

In my 12may post on http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=735, U3b does this.  It is a LP filter which matches the Q & frequency of the sealed box speaker.  The relationship is exact for a closed box.

Agreed. I have to admit I have just had a quick glance at it; I'll take some time to read the whole thread.
I was under the impression that you used a 2nd-order LPF attempting to model the slope of the first part of a BR excursion curve (between DC and Fb). That's what a number of designers of high-power systems try to do.

 

[ricardo]

I use an excursion-driven variable HPF; when "limiting" occurs only the lowest spectrum is turned down, the rest is still there to fool perception.

This is indeed one correct way to do what's required. 

Abbey, can you post (even a rough) circuit of how you implemented your sliding HPF?

It was trying to do this nicely in Jurassic times that I stumbled upon what is probably the most important part of my Powered Integrated Super Sub tech.

I was under the impression that you used a 2nd-order LPF attempting to model the slope of the first part of a BR excursion curve (between DC and Fb). That's what a number of designers of high-power systems try to do.

A Bass Reflex is much harder to match and I've spend many happy (!?!) hrs trying to do this with both passive & active circuits.  But exact matching is rarely necessary.

 

[abbey road d enfer]

I use an excursion-driven variable HPF; when "limiting" occurs only the lowest spectrum is turned down, the rest is still there to fool perception.

This is indeed one correct way to do what's required. 

Abbey, can you post (even a rough) circuit of how you implemented your sliding HPF?

See attached.

It was trying to do this nicely in Jurassic times that I stumbled upon what is probably the most important part of my Powered Integrated Super Sub tech.

Please let us know more about it...

I was under the impression that you used a 2nd-order LPF attempting to model the slope of the first part of a BR excursion curve (between DC and Fb). That's what a number of designers of high-power systems try to do.

A Bass Reflex is much harder to match and I've spend many happy (!?!) hrs trying to do this with both passive & active circuits.  But exact matching is rarely necessary.

Exactly. I have used successfully the principle here for BR and 6th-order boxes. There is no point following the exact curve because when the excursion is minimal, the impedance is also minimal and thermal protection will take precedence. Another reason is that the combination of mfg tolerances and temperature-related variations conjure to make vain any attempt to increase operational accuracy.

 

[FETlife]

Thanks all, very interesting, much appreciated.

Thanks all, much appreciated.

Haven't got anywhere with the FETs but think I've found a compromise with the THAT circuit. As I understand it, C2 sets the attack and C4 sets the release, can I then simply multiple their value with R10 to find my time constants? LINK: http://www.thatcorp.com/datashts/dn129.pdf

Sort of but not quite that simple.

C4  with R10 is the slow time constant and C4 with R8 is the fast time constant,

The extra op amp sets a  threshold by the DC current flowing done R9 that the op amp opposes through D4. When the difference between the "slow gain control" current and the DC threshold current from R9 is overwhelmed by the signal level current from R8, diode D4 becomes unclamped and the output of UC2 starts ramping up at a rate limited by C2. So while C2 slew limits the side chain the attack time is still set by C4.

JR

I see, thanks, what's the advantage of doing this? Easy threshold adjustment?

I wonder if tacking a C to ground (with R in parallel) after the op amp would give any useful control of release time.

 

[JohnRoberts]

Thanks all, very interesting, much appreciated.

Thanks all, much appreciated.

Haven't got anywhere with the FETs but think I've found a compromise with the THAT circuit. As I understand it, C2 sets the attack and C4 sets the release, can I then simply multiple their value with R10 to find my time constants? LINK: http://www.thatcorp.com/datashts/dn129.pdf

Sort of but not quite that simple.

C4  with R10 is the slow time constant and C4 with R8 is the fast time constant,

The extra op amp sets a  threshold by the DC current flowing done R9 that the op amp opposes through D4. When the difference between the "slow gain control" current and the DC threshold current from R9 is overwhelmed by the signal level current from R8, diode D4 becomes unclamped and the output of UC2 starts ramping up at a rate limited by C2. So while C2 slew limits the side chain the attack time is still set by C4.

JR

I see, thanks, what's the advantage of doing this? Easy threshold adjustment?

and independent attack/release. The standard RMS detector circuit has single cap attack/release.

I wonder if tacking a C to ground (with R in parallel) after the op amp would give any useful control of release time.

More likely you would create an oscillator. There is a negative feedback path back to the - input via C4 and R10 so a cap to ground after the op amp would introduce phase shift that could impact stability.

JR

 

[FETlife]

Was thinking more put the cap at the +in pin of U2D, the buffer. Is that what you meant?

 

[JohnRoberts]

Was thinking more put the cap at the +in pin of U2D, the buffer. Is that what you meant?

YES... the path back to the - input of U2C passes directly through the + input of U2D...  Put a cap to ground there and see what happens. 

JR