AnalogSteph
Major Contributor
50 Hz indicates either ground loop action or capacitive coupling, both of which are topological (potentially including the measurement setup) and as such should be unaffected by filter capacitance.
That said, since the AX-396 shares a PCB with the -496 which comes with 10000µ/63V caps, it would be quite easy to upgrade the stock 6800µ/50V parts. As a rule of thumb, you need 8200µ for power bandwidth to extend down to 20 Hz with 4 ohm loads.
BTW, as a general tip, I would advise using the CD Direct function not so much for a regular CD player but rather something of somewhat lower and variable output. The CD input amp has quite a bit of gain (3.55x), not the most headroom (~2.2 dB tops) and would be breaking a bit of a sweat even at 2 Vrms due to being fairly heavily loaded by the feedback network (conditions are similar to the NwAvGuy O2 gain stage, including the NJM2068 opamp). Not to mention that the opamp's nonlinear Class B output stage currents are being dumped into signal ground as there is no dedicated power ground connection to go along with the +/-15 V supply... that said, we're close to the back panel so currents would be travelling via the chassis instead, so probably not a super big deal.
That said, since the AX-396 shares a PCB with the -496 which comes with 10000µ/63V caps, it would be quite easy to upgrade the stock 6800µ/50V parts. As a rule of thumb, you need 8200µ for power bandwidth to extend down to 20 Hz with 4 ohm loads.
BTW, as a general tip, I would advise using the CD Direct function not so much for a regular CD player but rather something of somewhat lower and variable output. The CD input amp has quite a bit of gain (3.55x), not the most headroom (~2.2 dB tops) and would be breaking a bit of a sweat even at 2 Vrms due to being fairly heavily loaded by the feedback network (conditions are similar to the NwAvGuy O2 gain stage, including the NJM2068 opamp). Not to mention that the opamp's nonlinear Class B output stage currents are being dumped into signal ground as there is no dedicated power ground connection to go along with the +/-15 V supply... that said, we're close to the back panel so currents would be travelling via the chassis instead, so probably not a super big deal.