Stereo Width Experiments

Submitted by Link555 on Tue, 05/04/2021 - 07:10

I am getting interested in building a hardware stereo image widener circuit.  I have seen a few ideas floating around based on the Middle(M) +Side(S) technique.  The basic premise can be summarized into the following equations:

Encoding M and S

Middle  = L+R

Side  = L-R

Middle Circuit

Translating that into basic op-amp circuits, the Middle signal could be created with a summing amplifier.

Non Inverting Summing Amplifier









I am not sure if a non inverting summing amplifier will be as stable as inverting amplifier, but its a simple to test to find out.  If Rf is a short and Ri is open then the Middle = Left + Right.   

Side Circuit

For the side signal a difference amplifier can be used.

Difference Amp

If all the Resistors are the same value then Side output = Left-Right


Decoding M and S

The basic equations are as follows:

L = M+S

R = M-S


Decoding can be done using  the same summing and difference circuits, just with different inputs.

Decode Left

Decode Right

Blog comments

You need a bit more circuitry if you want to handle balanced inputs and outputs. You also have to cater for the overall 6dB gain that you get if you go LR -> MS -> LR. You can do this by giving each route a gain of -3dB, or just arrange the first unit to have -6dB and the second 0dB. This avoids possible internal overload.

I've built lots of different types of M-S en/decoders, but the one that proved most robust in practice was a passive box using 1:1+1 transformers. It was quite critical on the type of construction that the transformer used, as the splitting/recombination tended to break down at low frequencies.

You might also look at using THAT1240 and 1246 devices, since they have internal resistors of the necessary precision.

M-S decoder/encoder