Dear friends:
I am not a very active member of this forum but it is one of my favourite sources for information. Now I have a doubt and although I believe I'm acting in reverse, I will explain this later, I feel like I want to open a thread and get your opinions and ideas.
When I say in reverse I mean I should first experiment and then ask.
Going to the subject. I have read and studied a lot about balancing and unbalancing audio lines. I am not going to explain what is all about, most of you must be fully documented. But I have found something it is giving me some headaches and before I go any further I need your ratification or disagreement.
Briefly speaking, to me, an electronically balanced line (transformerless) is a line in which hot and cold wires transport the same signal with a 180º phase difference, or said in a different manner, same amplitude but different polarity.
A few weeks ago I got on my workshop a mixer (DENON) and the owner was refering some kind of distorsion on the master output. I discarded the human factor as I know him very well and he is of the very few who knows what he has in his hands and how to use it. I put a 1 Khz signal on one channel and measured the output. "Gazimba" (as Dr. Sheldon Cooper would have said) The level at the pin 2 (hot) of the XLR plug was a 30% over the pin 3 (cold). Due to accumulation of work, I had to leave this on the shelf to attend other urgent issues.
On the following days, some other products came, and something I am not often checking, and is the symmetry of the balanced signal on the output, was the main objective in most mixers. To my surprise I have found in two DJM 600 mixers from PIONEER the same behaviour. A difference on the amplitude of a 30 % from negative(cold) to positive(hot) wires in both of them.
At this point I have the suspect of this, rather being a problem, to be a technique which I can't understand. I have been reviewing some service manuals of different brands but unfortunately the only one that shows level and signals does not do on the Master section.
So I am now waiting for your ideas, thoughts and masterliness.
Juan
I am not a very active member of this forum but it is one of my favourite sources for information. Now I have a doubt and although I believe I'm acting in reverse, I will explain this later, I feel like I want to open a thread and get your opinions and ideas.
When I say in reverse I mean I should first experiment and then ask.
Going to the subject. I have read and studied a lot about balancing and unbalancing audio lines. I am not going to explain what is all about, most of you must be fully documented. But I have found something it is giving me some headaches and before I go any further I need your ratification or disagreement.
Briefly speaking, to me, an electronically balanced line (transformerless) is a line in which hot and cold wires transport the same signal with a 180º phase difference, or said in a different manner, same amplitude but different polarity.
A few weeks ago I got on my workshop a mixer (DENON) and the owner was refering some kind of distorsion on the master output. I discarded the human factor as I know him very well and he is of the very few who knows what he has in his hands and how to use it. I put a 1 Khz signal on one channel and measured the output. "Gazimba" (as Dr. Sheldon Cooper would have said) The level at the pin 2 (hot) of the XLR plug was a 30% over the pin 3 (cold). Due to accumulation of work, I had to leave this on the shelf to attend other urgent issues.
On the following days, some other products came, and something I am not often checking, and is the symmetry of the balanced signal on the output, was the main objective in most mixers. To my surprise I have found in two DJM 600 mixers from PIONEER the same behaviour. A difference on the amplitude of a 30 % from negative(cold) to positive(hot) wires in both of them.
At this point I have the suspect of this, rather being a problem, to be a technique which I can't understand. I have been reviewing some service manuals of different brands but unfortunately the only one that shows level and signals does not do on the Master section.
So I am now waiting for your ideas, thoughts and masterliness.
Juan
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The most important feature of a balanced output is impedance symmetry.
Output voltages that are symmetric WRT ground are desirable but by no means essential, and some devices also balance an unbalanced output by just putting a resistor between ground and the cold line (which is useful because you can connect an unbalanced input without risk of damaging the output).
As for the 30% difference, things like that do occasionally happen right from the factory - even engineers don't always keep the difference between inverting and noninverting amplifier gain in mind (the +1 for noninverting). If you're just going for the same resistor values, this easily happens. If in doubt, check the schematic (or part markings) and do the math. Actually it's not even that dumb an idea from an impedance balance POV, as both sides would have near-identical output impedance up to very high frequencies at the same noise gain (while identical signal gain on inverting and noninverting output would mean inequal noise gain and as such high-frequency rise in impedance occurring earlier on the inverting side).
Such amplitude imbalance in itself does not cause distortion (otherwise you would not be able to connect an unbalanced output to a balanced input), though balanced amplitudes would allow exploiting distortion cancellating in the input stage. A look at nonlinearity will be needed to determine the cause of the problem.
Output voltages that are symmetric WRT ground are desirable but by no means essential, and some devices also balance an unbalanced output by just putting a resistor between ground and the cold line (which is useful because you can connect an unbalanced input without risk of damaging the output).
As for the 30% difference, things like that do occasionally happen right from the factory - even engineers don't always keep the difference between inverting and noninverting amplifier gain in mind (the +1 for noninverting). If you're just going for the same resistor values, this easily happens. If in doubt, check the schematic (or part markings) and do the math. Actually it's not even that dumb an idea from an impedance balance POV, as both sides would have near-identical output impedance up to very high frequencies at the same noise gain (while identical signal gain on inverting and noninverting output would mean inequal noise gain and as such high-frequency rise in impedance occurring earlier on the inverting side).
Such amplitude imbalance in itself does not cause distortion (otherwise you would not be able to connect an unbalanced output to a balanced input), though balanced amplitudes would allow exploiting distortion cancellating in the input stage. A look at nonlinearity will be needed to determine the cause of the problem.
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