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SD mixpre impedance vs. active blanced output into a 744T


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i'm hoping someone will help me out to understand the difference (if there is a difference) when using an older SD mixpre which has outputs that are impedance balanced, vs. an newer mixpre that has active balanced outputs when using it as a front end to feed signal into ch 3/4 of a 744T.

i've got a few other questions:

are there differences in quality, level, rf/interference rejection or other?

is active-balanced and balanced the same thing?

when going through an xlr how does impedance balanced avoid interference by having only one voltage line when compared to balanced that has 2 lines?

i've read the paper on the differences (http://www.sounddevices.com/tech/balancing.htm). so for impedance balanced outputs, if "This topology has equal impedances from pin 2 to ground and from pin 3 to ground; however, only pin 2 is driven with signal voltage", can someone explain how driving only pin 2 is different from an unbalanced line where pin 2 is also driven with voltage.

they say "It provides common-mode rejection of unwanted noise into a balanced output just like an "active" balanced or a transformer balanced output stage." i guess maybe i don't understand what "common-mode rejection" means.

sorry if this seems rudimentary to some, i just would like to understand it better.

cheers,

P.

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" is active-balanced and balanced the same thing? "

sort of...

they are both balanced so yes, but there is also transformer (passive) balanced, which is also balanced, but different.

in balanced audio wiring, the audio signal is carried on the two (balanced) conductors, with a separate ground/shield that is not part of the audio path. (compared to unbalanced, where the shield/ground is a part of the audio path, completing the audio circuit carried on the single audio conductor.

for common mode rejection, google is your friend, as is wikipedia.

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for common mode rejection, google is your friend, as is wikipedia.

The most interesting (and applicable) part of the Wikipedia page for balanced line:

Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by differential signaling, transformer coupling or by merely balancing the impedance in each conductor.

The sort of balancing that SD is talking about is the last type. The more "traditional" is the first type.

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And because I'm feeling geeky, here's a little article (linked from the Wikipedia page above) that goes into more detail on the science behind why it's the balanced impedance that gets rid of the unwanted RF or other EMI, and not the differential nature of hot/cold signaling.

http://www.midasprosound.com/cms/cmsfiles/pdf/__www.soundcraft.com_extra_product_pages_audio_balancing_i.pdf

This looks to be lifted from Soundcraft, the other white papers are here for more propellerhead fun.

http://www.soundcraft.com/support/white_papers.aspx

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" a MIx-Pre D makes a first class front end for inputs 3 and 4 on a 744T "

as does an original MixPre, or even the Shure version...

Why stop? Please keep listing every mixer that would make a similar first class front end for inputs 3 and 4 on a 744T.

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Why stop? Please keep listing every mixer that would make a similar first class front end for inputs 3 and 4 on a 744T.

I think the Senator's point was that the MixPre has been available in several different forms over the years, all very similar, each offering almost identical performance but slightly different features: MixPre / MixPre-D / Shure FP24. I was told by a Sound Devices booth rep that the preamp sections in all their analog mixers were essentially the same; the 788's are different in terms of the digital control and signal flow.

It'll sound fine as a mike preamp for a 744 -- I bet the room noise on an actual real-world location will be more audible than any preamp noise.

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