Stereo to Mono switch with phase inverter Circuit.

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Stereo to Mono switch with phase inverter Circuit.

Unread postby Dane » Wed Aug 10, 2016 4:22 am

Hi Everyone,

I am building a switch box to set my audio input levels from stereo to mono.
I know that for the best result a true mono mix of the audio should be made, but then still one of the stereo recording inputs should be phase inverted, something this switch also should do.

Can I get some feedback/check on this circuit?

Please note that I am only familliar with electronics to a very small degree, and only understand some basic principles.
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stereo-mono-circuit.png
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Re: Stereo to Mono switch with phase inverter Circuit.

Unread postby Soulbear » Thu Aug 11, 2016 5:04 am

Hi Dane,
Looking over your Circuit, if I'm reading it correctly, it looks like in Switch Position 1 the Stereo Signal passes through unaltered. In Switch Position 2, the Signal is Summed to Mono in a similar way to the Circuits within this PDF, but with the addition, that Left Output is in Anti-phase or Opposite Polarity to the Right Output. Just out of curiosity, might I ask what is the reasoning for this Polarity change?
Regards :wink: :P :D Soulbear
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Re: Stereo to Mono switch with phase inverter Circuit.

Unread postby Stevie342000 » Thu Aug 11, 2016 6:31 am

Soulbear wrote:Hi Dane,
Looking over your Circuit, if I'm reading it correctly, it looks like in Switch Position 1 the Stereo Signal passes through unaltered. In Switch Position 2, the Signal is Summed to Mono in a similar way to the Circuits within this PDF, but with the addition, that Left Output is in Anti-phase or Opposite Polarity to the Right Output. Just out of curiosity, might I ask what is the reasoning for this Polarity change?
Regards :wink: :P :D Soulbear
Stereo to Mono Summing.pdf


There may be a further issue that needs research not sure it applies to all signals or just to tape but if you are summing tracks to mono you end up with a 2-3dB lift at around 700Hz and this may be where the phase error comes in and thus why the anti-phase is useful. This may have only applied to 3-track tape as stereo was intended to be L-C-R. It was usual to record left & right as normal from microphones and a single microphone was used for the centre track. Thus you got a 3 channel stereo mix, mono mix and a stereo mix. It was usual to blend a bit of the centre track in to the final mix to fill out the whole in the middle. Take a look at Mercury Living Presence recordings, RCA Living Stereo, Decca UK etc, mostly all 3 track recordings from 1954 onwards, they knew stereo was coming so the major labels had a backlog of stereo tapes in the archive ready for when stereo entered the mass market.
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