The distortion circuit of the DST-5A has soft-clipping characteristics of the transfer function, but the clipping can be made harder and lower in level thanks to the two HARD CLIP buttons.
If only one HARD CLIP button is pressed in then the clipping is asymmetrical and there's more even harmonic distortion. The top or the bottom half of the waveform can be hard-clipped individually as indicated on the front panel.
With both HARD CLIP buttons in - the clipping is symmetrical, harder and has more harmonics.
The TONE knob works in conjunction with "Pre" and "Post" enable switches.
There are two EQ circuits operated by one knob - pre-distortion and post-distortion. The EQs have individual enable switches, but their operation is complementary: for example in the clock-wise position of the TONE knob the pre-distortion EQ boosts high frequencies and the post-distortion EQ cuts high frequencies by the same amount, so the frequency response is not changed. It was called pre- and de-emphasis in the old days. The effect on the sound below the threshold of clipping is none, but it changes dramatically the tone of the clipping. At the counter clock-wise position of the TONE knob the "pre" EQ boosts lows and the "post" EQ cuts lows.
Any single EQ stage (pre or post) can be enabled .
The MIX knob starts at the DRY setting and goes through the WET position to the Super WET mode, where the input tone is subtracted from the distorted signal, leaving only the distortion components.
The DRIVE knob controls the amount of distortion by varying the level going to the clipping stage, but it has also a built-in volume compensation at the output of the stage with the LEVEL knob acting as a level trim.