Context: I am trying to design a solid-state microphone preamp using dedicated audio op amps (ex: API 2520).
- flat ≈60-66dB of gain from 20Hz to 20Khz
- Limit changes in phase shift as much as possible as the frequency varies (ideally within +/- 5 degrees).
Setup: I am currently experimenting with classic non-inverting closed loop op amp designs: Output feeds back to the negative input of the op amp using a voltage divider to adjust gain (with a capacitor in parallel of the first resistor to reduce parasitic oscillations).
See https://www.electronics-tutorials.ws/opamp/opamp_3.html for diagram with notations.
Ex: values for the gain-setting voltage divider: Rf: 10KΩ, R2: 10Ω or Rf: 100KΩ, R2: 100Ω.
I have a good oscilloscope which can produce frequency / gain-phase response diagrams by scanning a frequency range and measuring the gain and phase shift at a set number of points in the frequency range (ex: 100 points).
Problem: There is little phase shift (+/5 degrees) at "low" gains (<40-50dB) - this is good. However, when the gain is set to a "high" value (ex: 60dB or more), the deviation to linear phase increases to unacceptable values between 10kHz and 20kHz (ex: 20+ degrees @ 20kHz).
The issue increases as the gain increases (ex: 66dB) and is also present when the capacitor in parallel with the first resistor (Rf) of the feedback voltage divider is removed entirely.
This bad as it will introduce distortion for signals where high frequencies are important (ex: cymbals, pianos, woods, etc...)
I am operating under the assumption that the op amps are good since the API 2520 is a professional audio classic and I was able to reproduce the issue with different op amps (2 original API 2520, and 4 "good copies").
- Am I missing something in my design or this part of typical op amp limitations / imperfections? (Apparently John hardy gets to +/-2 degrees of phase deviation by using servos: http://www.johnhardyco.com/M-1details.html, I don't mean to do as good as a real professional and more elaborate product but I would like to explore simple ways to mitigate this phenomenon).
- If this is classic problem? If yes, what are the typical approaches to address this issue?
Edit: based on popular demand, I am adding 6 pictures of the scope at Gain=0, 40 and 60dB for frequency ranges 20Hz-20MHz and 20Hz-20KHz.