Balanced output w/ MC33079

[MisterCMRR]

I'd highly recommend the THAT1646 for the balanced output (as abbey road said, they overcome problems with DRV134, SSM2142, and others of that ilk) and use one of the THAT1200 (depending on how much gain you want) for the balanced input. The 1200 series will give very high CMRR whether the source is perfectly impedance balanced, "sort of" balanced, or completely unbalanced (no other circuit topology does that). Full disclosure: I invented and patented the circuit for the 1200 series.

The "impedance balanced" output is a good second choice output stage because it can drive an unbalanced input with no issues whatsoever. Note that the grounded leg of this circuit must duplicate all components between op-amp output and signal output connector pin to preserve impedance balance across frequency. This includes the coupling capacitor, which was omitted in circuit posted by abbey road earlier.

The IEC standard test for CMRR of a balanced input is shown in the attachment. I suggested this test in 1999 and it was adopted in 2000. This test clearly reveals the weakness of the popular single op-amp, four-resistor balanced input stage!

 

[JohnRoberts]

I like using differential amps, but not for raw exposed inputs.

Modern IC solutions for balanced inputs/outputs are quite good.

JR

 

[abbey road d enfer]

Note that the grounded leg of this circuit must duplicate all components between op-amp output and signal output connector pin to preserve impedance balance across frequency. This includes the coupling capacitor, which was omitted in circuit posted by abbey road earlier.

That's correct. I goofed when I doctored the schemo.
One should always turn the brains on before typing/drafting.

 

[Newmarket]

The "impedance balanced" output is a good second choice output stage because it can drive an unbalanced input with no issues whatsoever. Note that the grounded leg of this circuit must duplicate all components between op-amp output and signal output connector pin to preserve impedance balance across frequency. This includes the coupling capacitor, which was omitted in circuit posted by abbey road earlier.

Yes. But capacitor often omitted in commercial designs. Bear in mind capacitor tolerances esp for electrolytics and use same batch and/or test for best results.

 

[Bo Deadly]

Yes. But capacitor often omitted in commercial designs. Bear in mind capacitor tolerances esp for electrolytics and use same batch and/or test for best results.

Impedance balanced is ideal for simple inexpensive outputs in which case I'm not super convinced the extra cap is critical. At 60Hz the impedance is still less than the resistors so there will be decent rejection more or less depending on the other end. If I want immaculate CMRR, I'll use the 1646. Although I just noticed the price has come up a little. I seem to recall they were in the $4 range in quantity at one point. Right now I'm seeing $5.50 @10 on Mouser.

 

[abbey road d enfer]

Impedance balanced is ideal for simple inexpensive outputs in which case I'm not super convinced the extra cap is critical. At 60Hz the impedance is still less than the resistors so there will be decent rejection more or less depending on the other end.

Degradation of CMRR is related to the absolute difference between legs, so whatever the nominal impedance, for a given difference, the degradation is the same.
In other words whether the resistors are 22 ohms or 220 ohms, the effect of having one 100uF cap will be the same.
Of course, the ratio of nominal input Z vs. nominal output Z will directly impact CMRR. So regarding the entire connection, 22r is better than 220.

 

[Newmarket]

Impedance balanced is ideal for simple inexpensive outputs in which case I'm not super convinced the extra cap is critical. At 60Hz the impedance is still less than the resistors so there will be decent rejection more or less depending on the other end. If I want immaculate CMRR, I'll use the 1646. Although I just noticed the price has come up a little. I seem to recall they were in the $4 range in quantity at one point. Right now I'm seeing $5.50 @10 on Mouser.

Agree it's effective without the capacitor. And I've done that in some of my own DIY interfaces etc. But the extra cost is minimal for DIY so I'd tend to do that now.