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John Moore

Passive Summing - stereo to mono?

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I'm thinking of building a stereo to mono summing box so that I can send a mono signal and was wondering if anyone has done this... any suggestions/refinements? I'm probably going to make it along the lines of this one from the Rane Note "Why Not Wye":


I'd be using it to sum the 2-channel output of my mixer, since I want to keep both channels separate to the recorder for ISO's. It doesn't necessarily have to be a balanced signal since it's just the scratch track. What do you think?


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The one you showed might not sufficiently isolate the two main channels.  The Sound Devices one would work for you but, be aware, your mono output is at mic level.  If that's okay, try that one.  Also, be aware, that both diagrams are for connecting to unbalanced outputs.

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Hi All

I am new to the forum but have been following this with interest since January 2K11. I had the problem of summing stereo outputs from a sound module but also had an isolation issue as well. My take on the matter of stereo-mono patches are as follows:

1. Stereo-mono is really a problem of mixing two channels of audio

2. Y adapters are not at all an option (as per the Rane recommendation) - we don't bridge amp outputs using Y connectors,  do we, as this can cause disastrous loading effects.

3. Passive Mixing is one up on the cost chain. However, passive mixing can reduce signal/noise ratio. There are two main types:

3a.  Resistor mixing is a good low cost solution for most applications. However, the con is that there will always be some attenuation and the "mixer" performance (frequency response and attenuation) will depend a whole lot on the output and input impedances of the equipment being interfaced.  As John Blankenship pointed out there can be isolation issues and even the Rane note talks about channel crosstalk (poor isolation between stereo channels).

3b.  Transformer mixing: if you could get your hand on a quality pair of audio transformers you can do a good passive mix without compromising signal level and signal/noise ratio too much; this will also give very high isolation between channels and between the equipment being interfaced.  Its as easy as connecting each channel to a transformer primary and taking the mono out from the series connection of the secondaries.  Alternatively a single dual winding audio transformer should work. I see that some products such as the Whirlwind PodDI seem to provide solutions along this line. I said seem because they may actually be Y-connecting L and R or using a resistive mixer (as does the Radial Engineering ProAV1) before inputting to the transformer for isolation. From the little description given,  RAPCO Horizon appears to have done it right by first isolating the stereo signals in their TDI-100.  You can also Y  the outputs of two passive DI boxes, one connected to each L/R Channel. There are low cost dual DI boxes around.

4. Active Mixing: For applications where we want the mono into a mixing console (with no mono switch), the cheapest way to do this is to  use two free line level channels on the console. If space is an issue here are a lot of analog audio summers around. Most are expensive  but I can't see why any of the many mini mixers around (Behringer, ART, Rolls etc) cannot do the job, particularly for general purpose applications.

Any  of the above would work once one is aware of all the  pros and cons and is comfortable with the resulting performance.  My pick (haven't tried it yet) is to use the Active Mixing approach (coming to think of it I have a free small 4-channel mixer lying around in the church I support) or the transformer approach. 


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