Balancing Unbalanced Audio Inputs

Question

I'm wanting to nicely balance some unbalanced inputs to a power amp for good CMRR, and because I'm feeling too lazy to start trying to closely match pairs of resistors, I'm looking at using a hi-fi balanced line driver IC like this.

I know this can do the job for me, but is it appropriate or overkill? It's designed to drive balanced outputs of a device, including long cable runs, and this is obviously not needed because I'm using it to buffer / balance the input of a device.

Is there a more appropriate monolithic IC for me? (again, ignoring the obvious op amp solution that requires matched resistors)

Answer 1

The simplest way to balance an unbalanced signal is with an isolation transformer. Unfortunately, a transformer that works well across the entire audio frequency range is going to be expensive ($>100 from Jensen).

Another way to do it is to use a chip similar to the one you found from THAT Corp. Is it overkill, probably. But it'll be cheaper and perform better than a transformer (assuming that you don't really need the isolation that a transformer will give you).

The "typical" way would be to use two op-amps. One is set as a mostly-unity gain non-inverting buffer and drives the + input of the amp. The other opamp is set as a unity-gain inverting buffer and takes it's input from the other op-amp and drives the - input on the amp. Vary the gain on the first op-amp if you need to.

Of course, the real question for this is: do you need to use a balanced signal or can you get away with going unbalanced straight into the amp? Of course, a balanced input will give you better audio quality but the op-amps or THAT chip will degrade quality. If you don't do a good job, the net effect could be a decrease in quality. And then you have to wonder if the change in quality (for good or bad) is even noticeable to your ears. It's very likely that simply going unbalanced into the amp is going to be the better choice.

Answer 2

Oh, ok. The TDA8920 has a differential input. This is good for two things:

1. Cancelling out interference in the cable
2. Cancelling out common-mode ground loop voltages

For #1, you'd need a balanced line feeding it. This means the source impedance for the positive and negative signal pins needs to be equal, so that any interference creates identical voltages on each, which can then be cancelled out by the diff amp. (The signals do not need to be equal and opposite, despite popular opinion.) But since your signal is already unbalanced, you won't get this benefit.

The #2 case only matters if the source is grounded through the third prong on the power cord. If the amp and source are grounded through different outlets, there can be significant differences in their reference voltages. Then you can get 60 Hz power line hum in your amp, because ground loop currents flowing through the cable's shield will cause the amp to measure relative to a fluctuating ground. If so, you want to keep the power amp's ground disconnected from the source's ground, except the unavoidable connection through the wall. The ground from your source should connect to the - input of the amp, and nothing else.

On the other hand, if the source is floating, this will cause problems because the common mode voltage will be too great to cancel. To handle both cases (floating source and grounded source), you can use a ground lift switch on the input, or compromise with a smallish resistor (100 Ω) between the amp's ground and the source ground:

And follow the NXP schematic and PCB layout as closely as possible, including all the filter components shown above, or you'll get distortion, noise, EMI, etc.