Help determine output Impedance (before adding Impedance balanced resistors)

It's been a while since I've been around & could use some help determining the output Impedance before adding an Impedance balanced output (resistors) to this circuit. Also, what to do with the 10k to GND... should I remove it?

I've been told 33.3r, 50r.... & I want to say 100r because the resistors are separated by the IC. ???

I'm all ears & thanks.

100r/3 = 33r. The opamp outputs are at virtual earth, so from the other end we're looking at three paralleled and grounded resistors.

R17 should be left where it is, it makes sure C6 is at 0 VDC even with no load connected.

Samuel

Samuel Groner said:

100r/3 = 33r. The opamp outputs are at virtual earth, so from the other end we're looking at three paralleled and grounded resistors.

R17 should be left where it is, it makes sure C6 is at 0 VDC even with no load connected.

Samuel

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Thanks for your help & teaching me something.

*How (if at all) does the 10k to GND effect an "Impedance balanced output"?
Can I just add a 33.3r from pin 3 (output jack TRS/XLR) to GND, leaving the 10k?

Last question,
is 33.3r too low for an "Impedance balanced output"? I think the lowest I've see is about 47r but 68r seems to be more common.

How (if at all) does the 10k to GND effect an "Impedance balanced output"?

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The 10k resistor appears in parallel with the 33r33…, but as 10k is much, much larger than 33 Ohm it hardly has any effect.

Can I just add a 33.3r from pin 3 (output jack TRS/XLR) to GND, leaving the 10k?

Click to expand...

Sure.

Is 33.3r too low for an "Impedance balanced output"? I think the lowest I've see is about 47r but 68r seems to be more common.

Click to expand...

There is no fundamental lower limit; the resistor just needs to be large enough to sufficiently isolate capacitive loads. This limit decreases when paralleling opamps.

Samuel

What Sam said...

If you want to get tweaky, back in the day I was always exploring things past just making them functional, a practical target for build out resistance that I often used was 51 ohms. This is near the characteristic impedance of some audio cables. I am not going to claim transmission line effects are significant at audio frequencies, but if we must pick a value out of the blue, why not use one that has a theoretical basis, however thin?

Another consideration to make your impedance balanced output symmetrical, you arguably need a capacitor in series with the grounded leg, to deliver the same impedance all the way down to DC...  I am not sure that even I would add the cap there since it is arguably adding some potential in-band funniness, to perfect out of band balance... So take this as informational, not specific advice.

Just a little TMI...

JR