Effects of el. capacitor load on distortion
In my posts #1 and #14 the 10uF/100V and 4.7uF/6.3V capacitors were loaded with input impedance of the E1DA Cosmos ADC, which is approx. 830 ohm at the input divider setting used. This input impedance results in some LF roll-off, which makes -0.4dB for the 10uF cap and -1.3dB for the 4.7uF cap, both values taken from real life measurement. The measured plots are shown below, in a comparison with the loopback response.
This roll-off would not have much effect on audibility, but has an effect on capacitor distortion by increasing ac voltage across the capacitor.
To verify if there is any capacitor distortion if the capacitors are loaded with high impedance, thus no roll-off in audio band is added, my Audio Buffer was inserted into the measuring loop, behind the capacitor, ensuring that load impedance is now 100 kohm. My Audio Buffer is described here:
Measurements into 100 kohm load
The same set of measurements as in post #14 was performed, using the Audio Buffer in the loop, as a capacitor load. Same capacitors were used and measurements with them compared to the loop with Audio Buffer (without capacitors).
We can say that:
- overall distortion has increased a bit due to the Audio Buffer inserted into the loop
- all 3 measurements overlap and we can see that the capacitors now do not add any distortion
- low frequency distortion of electrolytic capacitors has disappeared by using high impedance load (100k input of the Audio Buffer) and the LF distortion is now lower than the distortion with capacitors loaded directly by E1DA input
- there is still some slight rise in distortion in all 3 plots below 70Hz, most probably due to input electrolytic capacitors in E1DA, that are loaded with quite low impedance of the input opamp of the E1DA ADC. This might be a space for further improvement of E1DA
- my measurements also demonstrate why some of us (me,
@restorer-john ) complain on too low input impedance of SOTA ADC and preamps
Edit (Jan 13, 2023): as a comparison, I am adding distortion plot of 100nF X7R capacitor loaded with 100 kohm, measured under same conditions as the graph above