Simon Lewis Posted March 24, 2017 Share Posted March 24, 2017 GL series auxes have (from memory) impedance balanced outputs. Link to comment Share on other sites More sharing options...
kgallen Posted March 24, 2017 Share Posted March 24, 2017 impedance balanced outputs. Which I believe are the words used to describe an output stage that is not truly differential (which would be an opamp with differential outputs). I've seen this implemented in Soundcraft desks as the cold being a 75 ohm resistor to ground and the hot being the op-amp output through a 75 ohm resistor, thus giving a 150 ohm "impedance balanced" output. I'm not suggesting there's anything wrong with an output stage built like this, just that it ends up being ground-referenced as flagged by timsabre. Link to comment Share on other sites More sharing options...
paulears Posted March 25, 2017 Share Posted March 25, 2017 My thinking is the there isn't an earth reference at the headphone end, is there? The amp is on their belt or somewhere similar - and if the unit had to sit on a metal surface that caused grief, you could always wrap it in gaffer? For what it's worth, my Soundcraft analogue mixer is happy with pins 2 or 3 being linked to 1 on input and outputs, and the iPad app I wired yesterday linking 1 and 3 on my S16s works fine - not tried doing it to the outputs, but will try soon. Link to comment Share on other sites More sharing options...
Simon Lewis Posted March 25, 2017 Share Posted March 25, 2017 The is nothing wrong with an impedance balanced output. Although there are some caveats, it fulfils the requirement for balanced line inasmuch each each signal line has the same impedance to ground and therefore induced noise signal should be picked equally. Of course, the physical arrangement of the two lines (twisted pair, starquad arrangements) will further help ensure noise signals are picked up equally and in the same polarity on each line. Only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal. Therefore, it can make no difference whether the desired signal exists entirely on one line, as a greater voltage on one line than the other, or as equal voltage on both of them (IEC 60268-3:2001). Although connecting tip and ring of the aux send to the headphone amp input will circumvent the desk to amp earth connection, I would suggest it will not offer the best chance of noise reduction as the headphone amp input will not be differential and therefore able to reject common mode noise. Therefore, a suitable transformer at the far end would be good, or if using a DI box backwards, the slight possibility of transformer saturation and level matching issues (line input into the "mic level output") a pair of passive back to back DIs could be used, if these were cheaper to source or were already available.A single 1:1 transformer would otherwise be good. Link to comment Share on other sites More sharing options...
timsabre Posted March 25, 2017 Share Posted March 25, 2017 err... confusing brain now but I think taking pin 3 and 2 of the signal with a floating pin 1 would cancel out common mode noise, wouldn't it? Not sure you'd call it differential but it would have that effect. Link to comment Share on other sites More sharing options...
Simon Lewis Posted March 25, 2017 Share Posted March 25, 2017 No - as far as I understand it, the input does need to be differential. There may in some instances be a benefit in "telescoping" the shield, but in this specific situation, a transformer at the far end is going to be the best solution.However, it would help to know if noise was actually a problem with these headphone amps. Wiring signal to both tip and ring of the amp input would give them sound in both ears. If the system was quiet enough, then the added cost of transformers wouldn't be needed? Link to comment Share on other sites More sharing options...
sunray Posted April 7, 2017 Share Posted April 7, 2017 http://www.maplin.co...-isolator-vw43w Two isolating transformers in a can rewire it for one winding having XLR on 2 & 3 NO ground, output as unbalanced with a phono to jack adaptor. Otherwise open the can and use one transformer. Big Clive dot com (of this parish) did a youtube vid of opening one of these. added; Have used loads of those but not from that source - too dear! Used to get them from CPC till they stopped them.The video is right on the buttonHave found these to be quite useable:http://cpc.farnell.c...m-to/dp/AV25544 stereohttp://cpc.farnell.c...m-to/dp/AV25543 monobut have not tried to open one yet. The unbalanced (3.5mm) wire is not particularly strong for when it comes to changing the plugs. Link to comment Share on other sites More sharing options...
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