scope probing; approaches for balanced vs unbalanced signals

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buildafriend

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This thread is kind of newby, please excuse my audio measurment newbness.

In what ways would you approach probing around in a balanced circuit that would differ from probing around in an ubalanced circuit? What kinds of things would you expect to see besides a bit more noise? I think things might be going in and out of phase. Must you use a balanced test signal to observe what is happening inside of a balanced circuit?

Thanks in advance and regardless


 
buildafriend said:
In what ways would you approach probing around in a balanced circuit that would differ from probing around in an ubalanced circuit?
If it's a true balanced non-floating circuit, where both voltages are identical except there of opposing polarity, you may test them unbalanced separately. If it's a floating signal (meaning that what ever you do to one of the legs reflects on the other) you must probe balanced.
What kinds of things would you expect to see besides a bit more noise?
Not necessarily more noise. But you have to make sure the scope is perfectly balanced; if it is not (which is easy if you don't set the vernier in calibrated position, the CMRR would not be good and it may show. Or may not...
I think things might be going in and out of phase. Must you used a balanced test test signal to observe what is happening inside of a balanced circuit?
Not necessarily. If the unit's input stage has good CMRR, it shouldn't make a difference.
Just two examples: take a BA6A; you can feed it balanced or unbalanced, it won't change a thing, because the input transformer splits the signal in two exact halves.
Take a Cohen or SSL9k mic pre; the signal on the front transistors will be similar to the input signal. If and only  if the input signal is balanced, the signals on transistors will be balanced too.
If the input signal is unbalanced, the signal on the transistors and on the opamps after them will be  poorly balanced. That's because the full mechanism of CM cancellation happens after them.

Now tell us what you have in mind. What piece of gear do you want to investigate?
 
thank you very much abbey road. I have learned some of these concepts in school but taking it to the workbench is a different experience than reading a text book. you are very helpful.

I have been probing through an API 312 clone.




 
buildafriend said:
thank you very much abbey road. I have learned some of these concepts in school but taking it to the workbench is a different experience than reading a text book. you are very helpful.

I have been probing through an API 312 clone.
API312's signal pat is unbalanced throughout. Only the input and output are balanced.
 
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