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Question regarding balanced / unbalanced audio circuits

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Hello Eevbloggers.

I'm currently working on developing a very specialised piece of professional audio equipment.
While thinking about the circuits some questions did arise that I have not been able to answer for myself yet.

Basically the device will have several balanced inputs and several balanced outputs. Internally all connections will be
unbalanced - because the device will also have to interface to unbal. equipment - , so I need a BALUN / UNBAL circuit.
For the balanced inputs I chose TI's INA217, because it is easy to use and the datasheet promises excellent properties.

Here's the basic circuit I came up with, basically just what TI say in the datasheet. Pin 1 & 8 are unconnected to give
a gain of 1.

When I apply 1,55V (+6dBu) at the bal. input I suppose I'll also get 1,55V at the unbal. output.
But what would happen if I were to connect the - bal. and the GND line at the input to connect an unbalanced device to it, as is common practice? Would that mean I'd loose 6dB?

I haven't tested any of this yet, it's early days and this just popped into my head.

Also, a very basic question: How do I measure across a balanced line using a mV meter that only has a negative and a positive terminal, e.g. your run-off-the-mill multimeter? I suppose across the hot/cold leg, right?
Same problem, just the other way round: How do I get a signal from my signal generator in? Will I have to make up a circuit to balance the sig. gen's output, or is there an easier way?
Maybe I'm just thinking to complex, that is often my problem  ;).

Thanks in advance

Forget about the GND terminal completely for your input signal. Think of it like a transformer, where the signal is applied across two terminals and there is a high impedance between each terminal and 'ground' and a low impedance between the two terminals.
The INA217 is an instrumentation amplifier, replacing the SSM2017 device. The GND connection in your circuit is probably meant for the 'shield' connector, as it looks like you plan to use phantom power over the balanced pair, otherwise it's not really necessary since all of the signal is carried in the balanced pair. You also take your measurements directly across the pair, since the circuit GND has nothing to do with the balanced input signal.
Your signal generator probably has a balanced output (ie not referenced to any chassis eartch etc), so it can be connected across the input. If this is not the case you can use an audio transformer to isolate the signal and make it fully balanced for testing.
If you did connect the cold input to the circuit GND point, then you would reduce the output by 6dB.

You might find this app note useful.

@Dr. Geoff: Thanks. That explains it perfectly.

@Alex: Interesting read, I wish I could splash out on some Jensen transformers but alas, will have to live with the INA217.


--- Quote from: david77 on September 19, 2011, 12:07:35 pm ---splash out on some Jensen transformers

--- End quote ---

I believe that is the purpose of that app note, but you can filter out the contribution of the marketing department. Some topologies are interesting.


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