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I have designed a 2nd-order active Sallen-Key high-pass filter for use in an audio circuit. An LTspice schematic is shown below, where VDD is 5 V.

enter image description here

I have also breadboarded the circuit using low quality components (X5R caps and 1/4 W, 5% resistors). Using an Audio Precision audio analyzer, I drove the breadboarded circuit with a 1 Vrms differential signal swept from 20 Hz to 20 kHz and measured the following response:

enter image description here

The sweep looks good in my opinion and the -6 dB cutoff point is at 3.16 kHz (very close to expected 3.18 kHz). I measured the THD+N via the Audio Precision using a 1 Vrms signal and a 200 kΩ measurement load. Below about 12 kHz, the THD+N is pretty bad (0.5% - 0.7%). Above 12 kHz, the THD+N is very close to the datasheet of the TL084 opamp being used (THD+N ~= 0.0006%).

I suspect that the higher value tolerance of the resistors and low-quality dielectric material of the capacitors may be to blame here, but does anything else in the circuit diagram / design seem like a likely cause for undesirably high THD+N?

Is it even necessary for me to worry about the THD+N of this circuit? I've only ever measured THD+N on full-bandwidth amplifiers before, not filters.

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    \$\begingroup\$ Try larger C5.. \$\endgroup\$
    – Mattman944
    Commented Sep 2, 2022 at 23:17
  • \$\begingroup\$ 12 nV / rt(Hz) @ 1kHz what do you see? \$\endgroup\$ Commented Sep 3, 2022 at 0:56
  • \$\begingroup\$ Your highpass filters pass high frequency distortion and noise but reduce the fundamental frequency. Your 5V supply voltage is much less than the 10V recommended on the datasheet. \$\endgroup\$
    – Audioguru
    Commented Sep 3, 2022 at 0:57
  • \$\begingroup\$ Measure THD vs Vout @ 10kHz \$\endgroup\$ Commented Sep 3, 2022 at 0:59
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    \$\begingroup\$ Look at the residual waveform (output from the AP). If it looks like harmonics, suspect the ceramics per Russell's answer. If it looks like noise bursts, suspect instability instead. \$\endgroup\$
    – user16324
    Commented Sep 3, 2022 at 10:04

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The number one suspects are the ceramic capacitors. Here is a SE link that shows the severe voltage dependence that ceramics have. The X5Rs are some of the worst, especially in the small SMD sizes.

For any analog signals audio or otherwise, use NPO (Thanks Spehro Pefhany) ceramic or film capacitors instead. Ceramics are fine for bypass and decoupling. Just be sure to increase the value to compensate for voltage dependence. Manufactures don't all specify so you may have to measure voltage dependence.

The common VCC/2 bias will cross-couple at low frequencies. May not cause THD but interference may increase. Better to buffer the voltage divider (VD) with a unity gain amp or have separate VD for each stage.

@Audioguru makes a good point. You need to keep the signal from VCC and GND for good THD, even with rail-to-rail amps. Where you work with single supply, there are lots of modern choices better than the TLO8x.

Whether breadboarding or production: Always put a decoupling capacitor from VDD to GND as close to the chip as possible. A quad amp needs a bigger cap than duals or singles. Always tie off unused amplifiers to prevent coupling through the supply rails.

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    \$\begingroup\$ +1 NP0 50V 1nF ceramic caps are easily available in sizes as small as 0603, and they have almost no voltage coefficient. The bypass caps should not matter for voltage coefficient, but I also don't like the coupling. \$\endgroup\$ Commented Sep 3, 2022 at 3:31
  • \$\begingroup\$ Thanks @SpehroPefhany. Forgot the NPO. And yes the bypass caps are fine. I'll fix \$\endgroup\$
    – RussellH
    Commented Sep 3, 2022 at 3:53
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    \$\begingroup\$ Did you try 10V for the supply instead of the 5V that barely works with an old TL084 quad opamp? Are the other three opamps in the package properly deactivated? \$\endgroup\$
    – Audioguru
    Commented Sep 3, 2022 at 16:43
  • \$\begingroup\$ Thanks @Audioguru: Very important comment \$\endgroup\$
    – RussellH
    Commented Sep 3, 2022 at 17:45
  • \$\begingroup\$ "The X5Rs are some of the worst" The higher-K stuff is even worse, like Z5V. Those are atrociously bad if they see almost any non-zero voltage across them. If you can keep the capacitor voltage down to a few mV, then they are not super-bad, not great either. They make decent varicaps, and are available in a huge range of values. \$\endgroup\$ Commented Sep 6, 2022 at 20:25

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