I have a digital recoder similar to this.
It works fine on - batteries. Then I build my own -3V DC power unit for it. It still works fine with that but there is hum *if I hold the recorder while recordin*. If I don't actually touch the recorder the hum is not there. I guess it's mains hum as there is no hum when run on batteries..
The psu I built is spot-on the correct voltage and easily provides the current demands of the recoder (500mA at most). The transformer I used is screened and the screen to tied to ground. One side (+) of the DC output is also grounded. It uses a LM337 regulator chip an has the usual configuration. There is a 1u tant capacitor across the output (as per design notes for this chip).
Any ideas how to cure(!) this hum problem appreciated.