What does a Hexfred do?

Amplitude of the spikes produced by rectifier diodes is not as dangerous as the rate of change (dv/dt).  Very narrow spikes will couple to any LC circuit causing ringing, even if inductance and capacitance are extremely small.  Pretty much everything has inductance and capacitance including components like resistors or capacitors, including power supply caps.

rodman99999,   Fig. 1 is incorrect.  For that to work properly it needs grounded center tap of transformer.  

There are Schottkys operating at higher voltages.  60V would not be enough for most audio application since each diode in bridge rectifier is subjected to double reverse voltage making it 30V max.  In addition we need to count for some overvoltage making it probably useful to about 20V. 

I believe Fig 1 was simplified, and only represents how a transformer’s secondary connects to a diode bridge, as stated. "There are Schottkys operating at higher voltages." I have no idea, how old "QuikNote 108" might be. Actually, my only interest, was the paragraph immediately under Fig 1(which had no expiration date). That Senior Applications Engineer did mention, "With regards to Schottky rectifiers concerning LOW VOLTAGE applications....... almost all solid state preamplifiers, CD players, digital processors, tuners, high end surround sound, low voltage class “A” power amps and tube filament supplies." I have no idea what specific components the author had in mind and- I suppose, "low voltage" could be considered, a relative/subjective term. Contact General Semiconductor, for further information.