Does it matter what type of capacitor is used in the feedback path of an op amp to limit audio bandwidth?

I have always used cheap Radio Shack ceramic caps reasoning that I shouldn't care about distortion at frequencies above the audible range as long as it serves it's purpose to stabilize the circuit. But it so happens that I have some fancy 47p film caps (polystyrene I think) and I'm wondering if they would actually be better somehow. I must ask because I don't have the equipment necessary to actually measure a difference.

  • \$\begingroup\$ Don't use ceramic caps in audio paths. Sources: Douglas Self, John Linsley Hood, Morgan Jones, all passim. \$\endgroup\$ – user207421 May 26 '13 at 3:08
  • \$\begingroup\$ 95% of all consumer and pro-audio out there uses just ceramic (NPO & COG type) and polarized aluminum electrolytic caps in the audio path and it sounds fine. Film caps are measurably better, but not always pragmatically better. \$\endgroup\$ – user3624 May 26 '13 at 3:43
  • \$\begingroup\$ @DavidKessner So take it up with Self, JLH, Jones. The OP is looking for 'better somehow'. These distinguished authors think so. I didn't actually mention electrolytics. \$\endgroup\$ – user207421 May 26 '13 at 10:48
  • \$\begingroup\$ @EJP Don't take it personally. I know you didn't mean Al. Elec., but I included them because using polarized caps in a non-polarized way is also considered bad. I should mention that I have been designing pro-audio and audiophile sound equipment for the past 16+ years, and I have measured the audio performance of various caps. But all you have to do is open up lots of consumer and pro gear to see that what I am saying is true. \$\endgroup\$ – user3624 May 26 '13 at 15:33

Assumption: By ceramic capacitors, you mean the low cost, no-name generic Y5V ones such as are cluttering up my parts boxes - not NP0/C0G capacitors with tight specifications

Generic ceramic capacitors (not NP0 / C0G) may exhibit some fairly strong non-linearity, with temperature, frequency and voltage. That last, the voltage related non-linearity, is easily handled by using ceramic capacitors rated for an order of magnitude or more greater than the voltage they will be used for, i.e. a 50 or 100 volt cercap for a 5 Volt circuit (Thanks to David Kessner for pointing this out).

Also, generic types of ceramic and mica capacitors act as tiny microphones, changing their capacitance and inducing voltage variations corresponding to incident sound.

This is a function of the piezoelectric effect of some dielectric materials: The accumulation of electric charge across such materials due to mechanical stress, in this case ambient sound. In effect, the capacitor behaves as a piezoelectric sensor, which is not its desired function on a feedback loop.

Both these factors, non-linearity and microphonic behavior, contribute to generic ceramics being not recommended for audio feedback paths, especially where aural quality is important. You should be fine with an NP0 / C0G ceramic capacitor with tighter tolerances. If you don't know what ceramic capacitor you have, it is safe to assume they aren't ideal.

While mathematically, a fractional percentage of distortion appears insignificant, the human ear can often distinguish very minuscule distortion in sound, especially on stringed instrument pure, extended notes.

Besides the scientific concerns above, there is also an entire school of thought among DIY audiophiles, who would swear by various types of capacitor, provenance of the power supply, the color of the PCB, or even the cloth weft and weave of a cable cover, as factors contributing to a non-quantifiable "feel" of a sound system.

  • \$\begingroup\$ Why would some ceramic caps have piezoelectric microphonics, and others not? \$\endgroup\$ – Scott Seidman May 26 '13 at 11:12
  • \$\begingroup\$ Nearly all types of capacitors other than liquid electrolyte ones have some level of microphonics - not solely piezoelectric in nature. Some ceramic dielectrics show a greater piezoelectric effect than others, to answer your question. Then again, varying layer orientation in multilayer chip capacitors can counter this effect mechanically to an extent, so the same dielectric could conceivably be used to make capacitors with different microphonic behaviors. \$\endgroup\$ – Anindo Ghosh May 26 '13 at 12:15
  • \$\begingroup\$ Piezo effect brings up interesting questions. In addition to deformation inducing a charge, a charge induces a deformation. I suppose, oriented in certain ways, a deformation will change capacitance, causing a nonlinearity with voltage. \$\endgroup\$ – Scott Seidman May 26 '13 at 13:29
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    \$\begingroup\$ @ScottSeidman Ouch... A multiple feedback filter in a single capacitor! That made my head spin - I was actually using FilterPro to put together a quick MFB low-pass filter when I read this. \$\endgroup\$ – Anindo Ghosh May 26 '13 at 14:00

Polystyrene capacitors work better even than C0G/NP0 ceramics on that position. You can test this out yourself. You already have polystyrene, now you just need to find a 47pF C0G ceramic capacitor and switch between the two. Have in mind that the same polystyrene type capacitors can be outperformed by polypropylene when you start working with bigger values (nano farads).

If you want a cheatsheet of some sort you can check out a book on this subject by Cyril Bateman. He measured and published THD of all capacitor types, and in practice, you do want to avoid high THD parts. Also, have in mind that even if electronic components don't have really high THD by nature, it can get high when you stack up a lot of them in a circuit.


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