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I'm designing an audio mixer and took good care to create a relatively low THD signal path. At the final output stage I'm measuring a a THD of 0.06%, (10 Vpp, sine @ 1 kHz.)

This is measured directly at the op-amp output. After that I added a resistor to protect the output, a 360 Ω 0805 SMD resistor.

When I measure after the resistor I found that the THD jumped up to 0.7%.

First I thought it was because of a bad resistor, so I replaced it with a high-quality Susumu thin film (RG2012P-361-B-T5). This did not fix the issue, unfortunately.

Lowering the resistance does make it better, I'm at 0.15% with a 51 Ω resistor, but I would really like to add some output resistance.

The output after the resistor is floating when nothing is connected. There is a nominal 10 kΩ - 100 kΩ input impedance in the next device.

I'm really scratching my head as to what could cause this.

Updated schematic and measurement

edit:

Turned out to be a small non-linearity by a component in the signal path I initially overlooked.. Moving the RHS of R2 to the other side of R34 fixed the issue (ie zero output impedance config), as proposed by danmcb.

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  • \$\begingroup\$ Exactly how are you measuring it? \$\endgroup\$ Commented Oct 5, 2022 at 6:54
  • \$\begingroup\$ Look into the input impedance of whatever the measuring device is. Sounds like it is a non-linear impedance. If necessary, use a low distortion amp as a buffer ahead of it. \$\endgroup\$
    – user16324
    Commented Oct 5, 2022 at 7:03
  • \$\begingroup\$ I've added my measuring set up to the question. Thanks for the suggestion will try with a buffer \$\endgroup\$
    – Matt
    Commented Oct 5, 2022 at 7:18
  • \$\begingroup\$ Is the distortion analyzer grounded in the same way in both cases? Does the box labeled "device under test" include capacitance between the opamp's negative input and ground that could make it unstable? \$\endgroup\$
    – bobflux
    Commented Oct 5, 2022 at 7:47
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    \$\begingroup\$ Thanks, will try it! Yes I've read Self's book, really amazing book \$\endgroup\$
    – Matt
    Commented Oct 5, 2022 at 8:04

2 Answers 2

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Well, without anything connected, the voltage would be identical. What is different is your measuring device and/or "floating". One guess would be that your THD is due to overshoot of the opamp, and your measuring device adds enough additional capacitance to squash it when connected directly.

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  • \$\begingroup\$ Ok good suggestion, I'll try adding additional capacitance to see if that does anything. I'd think 100p is already quite a lot, but there's a VCA stage before it that requires quite a bit of capacitance to function correctly. \$\endgroup\$
    – Matt
    Commented Oct 5, 2022 at 7:21
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If your scope is grounding the right hand side of R34 then you have applied a 360 ohm load to U1B and at 5 V peak that will be demanding 5 / 360 = 14 mA from the op-amp. That might explain it.

Instead connect a 10k resistor from the right hand side of R34 to ground and measure the voltage across it. That will resemble a real load more accurately.

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  • \$\begingroup\$ If the input probe for my analyzer has 1MΩ impedance wouldn't that prevent loading? Or am I misunderstanding how that works? \$\endgroup\$
    – Matt
    Commented Oct 5, 2022 at 7:55
  • \$\begingroup\$ It wasn't clear from your diagram what the split in the probe wiring is. If it's not probe and earth then forget my suggestion. I was just concerned that you had grounded the right-hand side of R34 through the scope while the probe was connected to the left. \$\endgroup\$
    – Transistor
    Commented Oct 5, 2022 at 10:23

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