soft clipping from current limited power supply

[thoglette]

NAD amps in the early 80's had a soft clip circuit. the soft clipping was done on the input, and kept the amplifier out of saturation, as well as producing a more tolerable sound than hard clipping.

They also used undersized transformers (e.g. 2030 IIRC) resulting in sag resulting in less heat in the output transistors => lower costs for the same peak power.

If you look at the "creative clipping circuits" thread hereabouts you'll also see the limiter circuit that the Vox SS amps use to use to avoid nastiness.

Important bit: The problem with trying to use the PS to "soft clip" is if that's all you do the standard feedback architecture of SS amps will bring all the ugliness back

Read your Hamm and Crowhurst. JAES 1973 21(4) and 1957 5(4) respectively.

 

[wg_ski]

Tube circuits tend to have “soft clipping” because of gain compression near the top end of their transfer curves. They start to limit (distort) long before the output comes to a hard stop limited by either voltage or current. Triodes more so than pentodes, as you can get incrementally more output if you can drive the bejeezus out of it (positive grid current). NFB reduces the gain compression region, coming to that hard stop much quicker. Tube amps with NFB don’t have much - 6 to 12 dB is common. Solid state amps usually have 20dB or more, resulting in almost no gain compression before it hits the hard stop.

To implement soft clipping in a solid stare amp it needs to be done ahead of the NFB-controlled power stages. The limiting thresholds can be set to start before the amplifier clips, coming to the final limit value approximately where the amp actually does clip. DSP is your friend here.