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#1000675 - 09/21/10 07:07 PM Anyone made a passive summing device?
Randyman... Offline
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Registered: 03/30/02
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I'm interested in analog summing - and knowing me - I'd prefer to DIY something simple than spend thousands on a brand name box with bells and whistles I don't need and can't afford. I realize an active gain stage will be needed for make-up gain - and I have plenty of nice Preamps that will easily fit the bill.

I'm looking to make a simple DB25-to-L/R XLR passive (resistive) summing cable that will take the 8 Outputs from a DA converter (RME ADI-8 or SSL AlphaLink) and sum them to the stereo pair of XLR's - the XLR's will then feed a high-quality Mic Preamp for make-up gain and return to the AD Converter to be re-recorded as the Main Mix. I won't need any Mono/Stereo switches or expensive trim-pot controls - just a fixed level summing network for 1+3+5+7 to Left XLR, and 2+4+6+8 to Right XLR.

I'd also like to see how much more difficult it would be to expand to 16 channels (2x DB25's). I'm assuming the resistor values would change, but that's probably all that would be different aside from more soldering.

So - what is a good starting point (and ballpark resistor values for a professional Line Level DAC)? Is it as simple as Line Level Inputs > Resistors in series > tie resistor outputs together and done? Or would this involve something a bit more involved than that (like multiple H-Pads summed together)?

I'm betting this can be crammed into an adapter cable opposed to building something in a project box / rack chassis to save space. I don't see why this would sound any different from any of the $,$$$ summing boxes that have nice gain-stages built in?

Any bright ideas on where to start? Thanks!

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#1000818 - 09/22/10 02:09 PM Re: Anyone made a passive summing device? [Re: Randyman...]
Randyman... Offline
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Registered: 03/30/02
Posts: 8670
Loc: Houston, TX
Anyone?



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#1000831 - 09/22/10 03:19 PM Re: Anyone made a passive summing device? [Re: Randyman...]
LakeStone Karl Offline
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Registered: 06/26/02
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I'd start with a fistfull of 2K 1% resistors and see what happens.
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#1000840 - 09/22/10 03:39 PM Re: Anyone made a passive summing device? [Re: LakeStone Karl]
Randyman... Offline
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Registered: 03/30/02
Posts: 8670
Loc: Houston, TX
Fistful - Nice!

Is the general concept to simply wire the resistors "in series" with each input (one for each leg of the balanced signal) and then tie (parallel) the outputs of the resistors together (keeping the 2 legs seperate, of course)? Or is an "H-Pad" type network needed where you'd add a shunting resistor across the series resistors? What kind of dB loss are we talking per input?

I obviously need to do some research - but I'd love to get pointed in the right direction - and you seem like the one to help \:p

I plan on using descent resistors (1% 1/4 or 1/2 watt - I'll try to find "Low Noise" resistors suitable for high-end gear)

Thanks for the reply, LS Karl!

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#1000846 - 09/22/10 04:11 PM Re: Anyone made a passive summing device? [Re: Randyman...]
bear Offline
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Registered: 10/25/99
Posts: 5319
Loc: abq,nm,usa
http://www.twin-x.com/groupdiy/albums/userpics/balancedmixnetwork.pdf

http://www.rhythminmind.net/presetblog/2009/11/diy-12-to-2-passive-analog-summing-panel/

http://www.gearslutz.com/board/geekslutz-forum/440806-diy-12-2-passive-analog-summing-panel.html


Edited by bear (09/22/10 04:17 PM)

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#1000847 - 09/22/10 04:25 PM Re: Anyone made a passive summing device? [Re: bear]
Randyman... Offline
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Registered: 03/30/02
Posts: 8670
Loc: Houston, TX
Peeking now - Thanks bear! I've got 40 channels of DA outputs and I'd like to use more of them (Only using 20 as of now) \:\)
_________________________
Audio + PC is the place to be
Randy V.
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#1000852 - 09/22/10 04:42 PM Re: Anyone made a passive summing device? [Re: Randyman...]
Randyman... Offline
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Registered: 03/30/02
Posts: 8670
Loc: Houston, TX
Looks like my loose assumptions were correct! It appears that simply using ~10K resistors (still researching the exact values I'll use) in series with each input's leg and then using a single shunt resistor on the output is the ticket (basically an H-Pad with summing between the leg resistors and the shunt resistor). The Shunt might not even be required since I'm feeding from discrete DAC outputs and not from a Mixer's "busses" that can feedback through each other per se.

I think I'll be able to wire the resistors into large-bodied DB25 shells (two DB25's for 16 channels) and then marry the 2 connectors sums together and shunt at the XLR connector - so this should be able to take on the "Cable" form factor I was hoping for (I'd prefer not to use a project box and XLR inputs as all of my converters have DB25's and I'm short on rack space).

Thanks for the excellent links - bear. Much appreciated

I'll wrap up my resistor value research and order the parts ASAP. Who needs a $4,000 Tube Tech summing box when you have a "Fistfull" of nice resistors and a good Mic Pre (I'll likely use an ADL600 or my SCA Neve clones for gain) \:p

Any suggestions on the largest bodied DB-25 Solder type connectors that are available (so I have ample room to cram everythnig in there)? I'll only need a single mic cable exiting the connector - but will need enough room for 16 resistors and some short wiring inside the shell. I've got XLR's coming out my ears, but no DB25's on hand...

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Audio + PC is the place to be
Randy V.
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