MS Matrix inside Audio Development mixer

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timtom

Member
Joined
Apr 27, 2016
Messages
20
Location
Berlin, Germany
Hey everybody,

I want to add a MS matrix inside my AD mixer to use only 2 channels with my stereo M/S mic instead of 3 channels, for recording straight to a Nagra tapemachine. I was planning the following:
- Inserting the Matrix after the EQ section.
- make it switchable.

I used this schematic and designed this board.

M-S_Simple.jpgsimpleMS_pcb.jpg
The mixer is giving +/-12V.

How should the ground of the power supply and the audio be connected ? (I connected them all together)

I powered up a prototype board and I get very loud noise at 20/30hz.
 

timtom

Member
Joined
Apr 27, 2016
Messages
20
Location
Berlin, Germany
The +/-12V should be clean enough as they are coming directly from the mixer.

The 20-30hz are also present without the nagra tape machine.

I'll add the 100nf caps and check again.
 

abbey road d enfer

Well-known member
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Jan 22, 2008
Messages
12,622
Location
Marcelland
Hey everybody,

I want to add a MS matrix inside my AD mixer to use only 2 channels with my stereo M/S mic instead of 3 channels,
You have a streo mic that uses 3 channels?
How should the ground of the power supply and the audio be connected ? (I connected them all together)
The rule is "Ground follows signal", so you should connect it at the insert send/return ground.
I powered up a prototype board and I get very loud noise at 20/30hz.
How have you evaluated the frequency? have you measured it with an oscilloscope?
If it's really that low, it's a sign of "motorboating", a problem that's usually due to inadequate rail decoupling.
 

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