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My summing circuit using Edcor mixing transformers Audio Interfaces
Old 23rd August 2007
My summing circuit using Edcor mixing transformers

First-time poster here. I was browsing the Edcor website one day and noticed that they sold mixing transformers for cheap (approx. $10 per transformer). After searching the web and not finding anyone else who's tried them, I built a simple 8 (4L, 4R) into 2 summing circuit using the Edcor WSM-6400 and WSM-6410 mixing transformers (see attached image). If any of you hit the JLM Audio forum I've posted this over there as well with some more ramblings.

I haven't boxed it yet (I'm going to use this as the summing buss for a mixer/summing box hybrid), but it works fine. I used 4 WSM-6410s (two unbalanced inputs, one unbalanced output) into 2 WSM-6400s (two unbalanced inputs, one balanced output), wired directly to each other, with some input and output jacks. There are no other components in the circuit. The jacks shown in the photo aren't permanent, I used some Neutrix patch bay cards since they were available and relatively easy to work with and attached them with quick-disconnects. Anyways, I think it sounds excellent (certainly better than summing through the Behringer, which you can see in the picture). Also, it doesn't really need makeup gain -- my unscientific tests (i.e., looking at the meters on the behringer) show very little signal loss (Edcor quotes .25 db per transformer), which is cool because a.) I don't need a makeup gain amp and therefore b.) there isn't an amplifier to clip.

I don't know how this would perform under other circumstances -- I've read elsewhere that mixing transformers have their own set of design issues. Perhaps I've been lucky in not encountering anything noticeably wrong about the sound; I'm using a MOTU 896HD for D/A conversion and it hasn't complained so far about driving the circuit. The input impedance is unknown to me; I'll have to check to see if my meter reads AC resistance. I suspect it's farily low, though, so that's something to keep in mind.

For those considering simple 8 channel summing circuits, I can say that this is a possible option and may work well for you. It's more expensive then a few resistors and some connectors, but it doesn't need makeup gain so that offsets the cost somewhat. One issue to consider, though, is that if you need more than 8 channels you'll have three transformers in the signal path for each channel instead of two. I'm tempted, when I get some more money, to try that and see what happens. Perhaps a hybrid circuit would work better in that case -- sum each 8 channel circuit at the final output using resistors. Or maybe it would be best to stick with resistors since at that point the cost of the transformers (approx. $120) may outweigh their benefits. Another option would be to tie two resistors to each input leg on the WSM-6410s, in which case you could expand this 8 channel circuit to 16 with fewer components at the cost of greater signal loss.
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My summing circuit using Edcor mixing transformers-mixer_full.jpg  
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