Shelving EQ

[JohnRoberts]

However, the shape of the curves is different because one is due to a pole and a zero separated in frequency and the other is due to a coincident pair. I do not like shelving curves because they continue to boost outside the audio band which can introduce unnecessary noise necessitating the use of additional HPF or LPF. Instead I now use low Q (sorry) bell curves which have the advantage of attenuating out of band signals.

Cheers

Ian

I have mentioned this before. Back in the day I added an extra HPF rolloff to the basic Baxandall shelving EQ. A characteristic of the Baxandall topology is the input impedance becomes lower resistance as more boost is commanded (the wiper of the boost/cut pot is held at a virtual earth by negative feedback). Judicious sizing of a capacitor added in series with input of the bass leg can work serendipitously with the varying input resistance to create a sliding HPF. It is therefore possible to deliver full bass response when set flat or attenuating bass, with additional bass rolloff as more bass boost is commanded (exactly when you do not want the excessive infrasonic bass boost ).  IMO this is superior to adding an additional fixed HPF that is always in the path, even when not boosting the bass.

JR 

 

[abbey road d enfer]

I have mentioned this before. Back in the day I added an extra HPF rolloff to the basic Baxandall shelving EQ. A characteristic of the Baxandall topology is the input impedance becomes lower resistance as more boost is commanded (the wiper of the boost/cut pot is held at a virtual earth by negative feedback). Judicious sizing of a capacitor added in series with input of the bass leg can work serendipitously with the varying input resistance to create a sliding HPF. It is therefore possible to deliver full bass response when set flat or attenuating bass, with additional bass rolloff as more bass boost is commanded (exactly when you do not want the excessive infrasonic bass boost ).  IMO this is superior to adding an additional fixed HPF that is always in the path, even when not boosting the bass.

Then technically it's not a shelving EQ anymore. It is a bell EQ where the bandpass filter is not the usual biquad. The notion of center frequency is definitely valid, although variable. Specifying the characteristics becomes an exercise in subjectivity. 

 

[JohnRoberts]

Then technically it's not a shelving EQ anymore. It is a bell EQ where the bandpass filter is not the usual biquad. The notion of center frequency is definitely valid, although variable. Specifying the characteristics becomes an exercise in subjectivity. 

Semantic distinction... Behaves very much like a true shelf for cut, and scraping off some extra low bass when commanding large amounts of bass boost is a good compromise IMO.  I would not want a  Shelf EQ to deliver full boost at DC, and I never designed  a fully DC coupled audio path (audio is AC). 

Of course opinions vary...

JR

 

[abbey road d enfer]

Semantic distinction... Behaves very much like a true shelf for cut, and scraping off some extra low bass when commanding large amounts of bass boost is a good compromise IMO.  I would not want a  Shelf EQ to deliver full boost at DC, and I never designed  a fully DC coupled audio path (audio is AC). 

Of course opinions vary...

JR

Exactly. It shows that things cannot be taken for granted. Behind a simple input stage is hidden a band-pass filter...  8)