Homemade summing box?

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robertm2000
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Homemade summing box?

Post by robertm2000 » Sat Sep 29, 2012 8:15 am

Has anyone made a homemade summing box? I seem to remember that if you

-take plain resistors,
-put them in series with the hot wire of audio outputs, and
-tie the leads of the resistors together on the side of the resistors opposite the input - the wire from the audio output,

you can combine several outputs without having interaction between them. In other words, mix them.

Has anybody tried this?
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Nate Dort
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Re: Homemade summing box?

Post by Nate Dort » Sat Sep 29, 2012 9:15 am

robertm2000 wrote:
Has anybody tried this?
Tons of gear "sums" signals together like this internally. It's called passive summing. It's a perfectly fine way to do it, assuming there's some sort of makeup gain available afterwards if you need it.

An even better way to do it is to add an active summing stage so you've got a low-impedance output to drive the next piece of gear. An inverting summing amplifier would be the best choice as the Thevenin equivalent resistance of one channel doesn't effect the level of the other channels, unlike a non-inverting summing amplifier. You can then put another inverting amplifier after it to bring it back to the correct polarity and make it variable to adjust the makeup gain. Pretty simple with one dual opamp.

rockstudio
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Post by rockstudio » Sat Sep 29, 2012 9:26 am

I built a passive summing box that had output transformers. For some reason the device had a buzz, so I put it away and haven't fixed it yet.

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Post by qbert1 » Sat Sep 29, 2012 10:14 am

I used this web thread to build mine:

http://www.groupdiy.com/index.php?topic=16731.0

It works as advertised on complex mixes (I used Millennia Media pres to bring it back up to level). Small mixes, say a jazz combo, didn't seem to benefit. Also, in one environment I've used it, it developed a ground hum. Not sure why, there may have been something in the electricity or air causing that - I just mention it so you're informed.

Honestly, for what I do, I'm not sure I'd replace it if it was lost.

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Post by Nate Dort » Sat Sep 29, 2012 10:14 am

rockstudio wrote:I built a passive summing box that had output transformers. For some reason the device had a buzz, so I put it away and haven't fixed it yet.
Probably not enough drive current to push the transformer.

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Post by dfuruta » Sat Sep 29, 2012 10:24 am

You probably want to sum the "cold" input (xlr pin 3) also, to stay balanced.

But if the goal is really to just mix the channels without interaction between them, why bother doing it analog? Staying in the box will be way more accurate than a passive device like this...

robertm2000
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Post by robertm2000 » Sun Sep 30, 2012 4:39 am

I'm going to give it a try and build a passive combining network.. Resistors at Radio Shack are cheap! Thanks to all for the info.
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Post by ashcat_lt » Mon Oct 01, 2012 10:45 am

Here's mine:
Image
8)

10 pairs of unbalanced ins. It's got a passive output which gets the signals before it hits the onboard makeup gain stage, then a variable gain stage, then a set of four independent outputs each with its own volume control. It takes 12VAC wallwart power and internally converts to +/-12VDC bipolar supply for itself, with two "aux power" outputs. It's pretty specifically built for my situation, but works pretty well.

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digitaldrummer
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Post by digitaldrummer » Mon Oct 01, 2012 2:14 pm

I built two of them based on this:

http://www.forsselltech.com/media/attac ... hsum_2.pdf

the pictures in the group DIY link look nice and clean! much better than my first attempt that's for sure.

and since I had room in the box, I also added a monitor switcher (for active or self-powered monitors). In one of the units I used a toggle switch (so you can choose between two pairs of monitors), and on the other I used a 3-way rotary switch so I can select from 3 different pair. That is the unit I use in my studio today.

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Last edited by digitaldrummer on Mon Oct 01, 2012 2:28 pm, edited 1 time in total.
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Post by digitaldrummer » Mon Oct 01, 2012 2:23 pm

don't forget about this one too:

http://www.diyrecordingequipment.com/st ... mming-kit/

simple and cheap.

Mike
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