The PP500 phono preamplifier, built around the JRC4580 op-amp, originally uses 3.3 kΩ resistors and 4.7 µF capacitors in the feedback network. While this design is stable, it can result in a lower output level when used with medium-output MM cartridges such as the Audio-Technica AT3600L.
To increase the gain and improve the low-frequency response, a parallel RC modification was applied. In this case, a 1.5 kΩ resistor in parallel with the original 3.3 kΩ resistor, combined with a 10 µF capacitor in series, was added. This effectively adjusts the feedback loop, increasing the overall gain while maintaining the low-frequency cutoff around 10 Hz, ensuring