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Old 12th January 2020
  #10
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Brian M. Boykin's Avatar
 

Quote:
Originally Posted by Cabirio View Post
Note that the value of the cap is meaningless without knowing the value of the feedback resistor. As a rule of thumb I use 22p for 10k and scale it accordingly, so 100p for 2k2, 4p7 for 47k, etc. This sets a -3dB point around 700kHz and a drop at 20kHz of around -0.003dB, hardly noticeable. Designs where this cap is so large that it limits the bandwidth significantly (i.e. audibly) are rare, and if you come across one, they've probably done it for a reason (see John's edit above), e.g. oscillation due to layout that couldn't be cured another way, so reducing it may get you in trouble.

The one change that may be beneficial is to replace class 2 ceramic caps (most old disc caps with no black colour band on top are) with NP0 / C0G of the same value. The former suffer from aging (they lose capacitance over time) and have a large voltage coefficient (dependance of capacitance on voltage) which may introduce distortion, while NP0 / C0G don't have these problems and modern ones are as small as old class 2 ones.
Great info. Up until now I’ve road the coattails of the original designer using values they used for the feedback resistor in parts of the design they added them. It’s served me well. They picked those values for a reason so I tend to stick with them. I’ve never changed an existing feedback cap, only added them. I never use ceramics, only NP0/C0G, however, I’ve never changed ceramic disks. I’ll start doing that. There so cheap.