**I've looked at the VAB84 again and realized that it is a completely different animal than the LF blending of earlier Neumann "elliptical equalizers."**
Phase and polarity differences (Left relative to Right) are what make stereo.

Pan something hard Left and the Side information will be the opposite polarity in Right relative to Mid to produce full cancellation.

By definition the "Side" component contains out-of-phase material.

Another curious property of the MS Matrix is that when the polarity of Side is reversed, Left and Right swap.

When simple LF blending occurs in an "elliptical equalizer," Side information that gets blended will partially (or fully) self-cancel.

If a LF element is Side-heavy (wide) and gets progressively blended to mono it will lower relative to the overall mix.

It's not fully mono compatible.

I don't know what the filter order is in the VAB84 and my analysis is early and may be fluid.

It looks like the VAB84 converts Side information to Mid below a certain frequency and then adds it to Left and Right.

**The VAB84 low pass filters out everything in the side channel above "x" Hz.**

It then recombines the low frequency Side information with reversed polarity into Left and Right so that it reinforces Mono, not cancels it.

That's how it converts Side to Mid.

An MS Matrix *subtracts* Side to increase width or separation.

The VAB84 does the opposite: It *sums* Side into Left and Right to reduce separation.
*Neumann VAB84 Matrix. Vertical is Difference or Side.*
In a conventional MS Matrix Mid would be extending into the summation stages, not Left and Right, and the polarity of Side feeding Left and Right would be reversed from the above figure.

In the VAB84 Left and Right are fed into the summation stages and the polarites of Side feeding Left and Right, which has been filtered, is the opposite of what it is in an MS Matrix.

The VAB84's filter is dynamic. Assume for the sake of discussion that the filter is static.

The broadband Left and Right inputs pass through to their respective outputs. (Ignoring the "RIAA" weighting filters.)

The filtered side information in the VAB84 then is summed into Left and Right but in an opposite polarity of what it would be in an MS matrix.

Rather than subtract Side from Mid to reconstruct Left and Right Side is added to Left and Right at low frequencies to reinforce mono, and decrease LF separation.

It appears that LF elements hard-panned no longer drop out of the mix due to phase cancellation but fold down to mono.

The VAB84 is "mono compatible."

**I haven't fully proven this but what I did do is a quick setup with the MS board similar to the topology of the VAB84.**

I used a second order filter (50 Hz -3 dB) and was able to obtain a 12 dB/octave crosstalk curve.
Look at the second-order 12 dB/octave crosstalk/blending curves:

*VAB84 Emulation Crosstalk/Blending Curve Using the Precision MS Matrix*
*VAB84 Emulation Frequency Response Curve Using the Precision MS Matrix*
Update:

**I've found that I can also do third-order low frequency cross-talk curves.**

This is with a 50 Hz corner frequency.
*VAB84 Emulation Crosstalk/Blending Curve Using the Precision MS Matrix With An 18 dB Per Octave Slope*
The cutoff frequency was set at 50 Hz.

The slope is steep enough that it can be set much higher without affecting mid-band separation.

So the question becomes "how do I make the MS board do this?"

It did after all generate those curves.

Turns out it may be pretty easy but I think there may be more than one way to do it.

Update 2:

This is the crux of the Neumann EE77's LF blend.

It's 6 dB/octave.

150 and 300 Hz appear to be the 3 dB points.

*Neumann E77 Low Frequency Blend*