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robertm2000
audio school
Joined: 21 Oct 2011
Posts: 9
Location: Longview WA
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 Posted: Sat Sep 29, 2012 11:15 am Post subject: Homemade summing box? |
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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
tinnitus
Joined: 08 Mar 2004
Posts: 1030
Location: Denver
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| Posted: Sat Sep 29, 2012 12:15 pm Post subject: Re: Homemade summing box? |
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| 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.
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rockstudio
gettin' sounds
Joined: 15 Dec 2003
Posts: 120
Location: Brunswick, GA
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| Posted: Sat Sep 29, 2012 12:26 pm Post subject: Re: Homemade summing box? |
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| 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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qbert1
gettin' sounds
Joined: 06 Dec 2005
Posts: 109
Location: Woodstock, IL
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| Posted: Sat Sep 29, 2012 1:14 pm Post subject: Re: Homemade summing box? |
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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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Nate Dort
tinnitus
Joined: 08 Mar 2004
Posts: 1030
Location: Denver
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| Posted: Sat Sep 29, 2012 1:14 pm Post subject: Re: Homemade summing box? |
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| 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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dfuruta
buyin' gear
Joined: 08 May 2009
Posts: 503
Location: mpls
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| Posted: Sat Sep 29, 2012 1:24 pm Post subject: Re: Homemade summing box? |
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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... |
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robertm2000
audio school
Joined: 21 Oct 2011
Posts: 9
Location: Longview WA
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| Posted: Sun Sep 30, 2012 7:39 am Post subject: Re: Homemade summing box? |
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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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ashcat_lt
buyin' a studio
Joined: 04 Jan 2007
Posts: 929
Location: Duluth, MN
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| Posted: Mon Oct 01, 2012 1:45 pm Post subject: Re: Homemade summing box? |
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Here's mine:
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
carpal tunnel
Joined: 24 Nov 2005
Posts: 1537
Location: Austin, Texas
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| Posted: Mon Oct 01, 2012 5:14 pm Post subject: Re: Homemade summing box? |
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I built two of them based on this:
http://www.forsselltech.com/media/attachments/8chsum_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.
Mike
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Last edited by digitaldrummer on Mon Oct 01, 2012 5:28 pm; edited 1 time in total |
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digitaldrummer
carpal tunnel
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Location: Austin, Texas
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