help my converter is oszillating

I build a output stage for CD, its exactly like the outputstage from the passlabs DAC manual with the following differences. Its unbalanced, the attentuator is at the output and i use sepparate +/- 24 volt regulators for both channels. The circuit makes music without distortion, but has one big problem, it makes a 300 Hz tone on both channels, same frequency and same level. It makes no difference if the Pcm 63 DA is connected or if i put the level or the bias up or down, or change any resistor values, the tone is allways aproximatly 10 DB below the music.
Does someone have any ideas what causes the noise?
I have found the failure, it was the 7824 Voltage regulator, which oszillated, i put a 10 ohm resistor between the regulator and the 22oo uF capacitor and the oszillation stopped. But i still dont understand why it was oszillating, the 300 Hz was even in the first capacitor after the bridge, so the 7824 oszillated in both directions, input and output.