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Mackie 1604VLZ-Pro - are balanced outputs really balanced?

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plz:
Has anyone actually checked this? According to the manual (pg10) & every review I've seen, all outputs are said to be balanced... but I was checking my new studio wiring with a scope and found nothing was coming out of any of the TRS cold (ring) connections except for the Main outputs, i.e. the direct outs, aux sends, subs and the control room outputs all appeared to be unbalanced.
I checked the circuit diagram in the user manual and sure enough, all outputs apart from the main outs only feed the hot output; the cold appears to be connected to earth via a resistor.
Am I missing something or is there some deception going on?  ???

RoadRanger:
No deception - just confusion. Those outputs are "impedance balanced" - the theory is that the resistor matches the impedance of the cold output pin to the impedance of the hot output pin, hence a balanced input will work almost as well with it as a true balanced output :).

plz:
Many thanks RoadRanger - it's all becoming clear now. Have heard the term before but never understood how it worked. So the fact that the cold wire can now 'float' means interference noise on the cable will still cancel out at the balanced input end... but you sacrifice the signal-to-noise benefits of the larger voltage swing from two out of phase signals. It's a nice simple trick with one resistor - seems the Allen & Heath mixer does the same thing but the manual uses the terms 'differentially' and 'impedance balanced' so is a lot clearer (although not as humorous as the Mackie :) ).

This has scuppered my idea of verifying the new wiring by checking the hot & cold levels at the patch panel end - if there's no signal on the cold when it's behaving normally, how can you check it's wired ok? The non-pedants might say who cares; you wouldn't hear any difference and it's only the noise immunity which would suffer, but that's just me :facepalm:

Greg C.:

--- Quote from: plz on August 19, 2013, 07:09:26 AM --- So the fact that the cold wire can now 'float' means interference noise on the cable will still cancel out at the balanced input end... but you sacrifice the signal-to-noise benefits of the larger voltage swing from two out of phase signals. It's a nice simple trick with one resistor - seems the Allen & Heath mixer does the same thing but the manual uses the terms 'differentially' and 'impedance balanced' so is a lot clearer (although not as humorous as the Mackie :) ).

--- End quote ---

Impedance balanced outputs have been used for decades now to save money and provide comparable CMRR of active balanced outputs. balanced lines are not about having both hot and cold be active, they're about having matched impedance relative to ground. Most people don't need the extra 6dB of gain that active balanced outputs provide. As for degraded S/N, that may not be true. In theory you would lower the noise floor by 6dB with double the voltage swing. However, the active output circuit on the cold line adds some noise, so it's probably a wash.


--- Quote from: plz on August 19, 2013, 07:09:26 AM ---This has scuppered my idea of verifying the new wiring by checking the hot & cold levels at the patch panel end - if there's no signal on the cold when it's behaving normally, how can you check it's wired ok?

--- End quote ---

You can measure the resistance from the hot and cold lines relative to ground if you want to make sure your line is balanced. If you want to make sure your point to point polarity and lines are correct, get something like a Rat Sniffer/Sender pack with the appropriate adapters in and out of your patch bays to make sure your wiring is good.

plz:

--- Quote from: Greg C. on August 19, 2013, 10:19:34 PM ---Impedance balanced outputs have been used for decades now... ...balanced lines are not about having both hot and cold be active, they're about having matched impedance relative to ground.

--- End quote ---
Thanks for the clarification Greg. I've used them for years but never fully appreciated them - it was only this recent re-wiring exercise which showed up my lack of knowledge. When the previous layout was wired, a cold connection was missing on an active balanced output - the signal was still there but the level had dropped and I spent ages tracking it down; it was then that I incorrectly presumed all balanced outputs were differential. This time round I tried to be clever by checking all hot & colds first - that'll teach me :-[


--- Quote from: Greg C. on August 19, 2013, 10:19:34 PM ---You can measure the resistance from the hot and cold lines relative to ground... ...get something like a Rat Sniffer/Sender pack to make sure your wiring is good.

--- End quote ---
These sound good ideas so I'll get on with the testing.
Thanks to all for your quick help on this - the Mackie is exonerated.
Phil

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