Elliptic Equalizers Using MS Matrix and LRS Width Controller

Construction information for DIY projects, including the MS Mid Side Matrix, Elliptic Equalizer, Mastering Console, Phono Transfer System, Insert Switcher and the Dual Class-A Amplifier. You can post your baby pictures here.
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mediatechnology
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Elliptic Equalizers Using MS Matrix and LRS Width Controller

Post by mediatechnology » Wed May 13, 2015 12:39 pm

This thread shows how to use the Precision MS Matrix and Stereo Width Controllers as simple Elliptic Equalizers to provide Vertical crossover for vinyl mastering and "bass-to-mono" subwoofer steering.

Background Information

There is a long and winding thread that contains a lot of information about using the MS board to provide mono bass: viewtopic.php?f=6&t=622
Inn the thread linked to above the LR +/-S Stereo Width controller was born based on the Neumann VAB-84's Left, Right, Side approach.
The Width Controller discussion thread spun off here: viewtopic.php?f=6&t=681

There are also threads about building the MS Matrix and Stereo Width Controller.
The MS Matrix has a build thread located here: viewtopic.php?f=7&t=262
The Stereo Width Controller's build thread is at this location: viewtopic.php?f=7&t=731

This thread is about using the MS Matrix or Stereo Width controller with simple passive networks in the inserts to provide Elliptic EQ.


To make the MS or Width Controller into an Elliptic Equalizer all one has to do is filter Side.

In the MS domain, the Side filter requires a high pass response.
In the LR +/-S world, where Side is either added or subtracted from Left and Right, (not Mid) a low pass response is required.

The filters for a first-order 6 dB per octave Elliptic EQ response can be simple passive networks:

The THAT1646 Side Output on either board that drives these network have high current outputs with a 50 Ohm source impedance.
The THA1246 Side Insert Return that receives the filter outputs has a 24K Ohm typical differential load impedance.

This is a high pass EE filter for the Precision MS Matrix Side Insert:

Image
MS Matrix Side Insert Filter for Elliptic Equalization

The crosstalk response showing "bass-to-mono:"

Image
MS Matrix Elliptic Equalization Response With a 75 or 150 Hz HP Filter in the Side Insert

The Stereo Width Controller can also be made into an Elliptic EQ by placing a low pass filter into the Side Insert:

Image
A Simple Passive Low Pass Filter in the Side Insert Turns the Stereo Width Controller into an Elliptic Equalizer

The filter shown above is either 150 or 300 Hz.
A 75/150 Hz filter can be made by making the caps 1 uF.
Other values of capacitance can be bridged across tip and ring to form a low pass filter in the side insert.
When computing resistor values for custom filters include the internal 25 Ohms per leg (50 Ohm total) output resistance of the THAT1646.

If the Width control is set to mono, the LF separation will converge to mono.
Intermediate settings of the Width control allow permit some LF separation within the pass-band of the filter.

The result is a low frequency crossover to mono (to reduce Elliptic vertical modulation) at low frequencies similar to the Neumann EE-70 or EE-77.

Image
Crosstalk curves with the Stereo Width Controller configured as a 150 Hz or 300 Hz Elliptic Equalizer.

This plot is a 300 Hz EE curve with the Width Control varied:

Image
Crosstalk curves with the Stereo Width Controller configured as a 300 Hz Elliptic Equalizer showing Variable Bass Width.

The MS Matrix or the Stereo Width controller with passive filters in the Side Insert provide a simple way to emulate the Neumann EE-70/EE-77 elliptic EQ.

This thread shows how to use a HP filter in the Stereo Width Controller's side insert to expand width in the midrange and above: viewtopic.php?f=7&t=788&p=9284#p9284
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