What does a Hexfred do?

Amps using Silicon diodes can sound harsh and
brittle because the diodes are slower in response than a tube rectifier(they're so nice). By replacing the diodes with FREDs or Schottkys your amp
will have a smoother/sweeter top end, more detailed tone definition and be a touch more responsive. FRED’s are
much faster than the cheaper diodes that most amp manufactures use. Although they seem kind of simple, rectifiers are not well understood from an RF-emission standpoint. If you think
of them as really crude Class B (not Class AB!) high-power output stages, the mental picture will get clearer.
The rectifiers commutate the AC waveform just like a fast mechanical switch, and like a mechanical switch, the
zero-crossing region is not dealt with cleanly. Conventional silicon diodes actually knock a hole out of the
waveform thanks to charge-storage effects. Fast/soft recovery diodes(or zero recovery SIC Schottkys) do a much smoother/quieter job of it. It would be nice if the power supply filter caps in your amp could rid the DC output of all of the garbage created, but they don't.

BTW: There are manufacturers that appreciate the diffs that better rectifiers provide. ie: Note the upgrades that are available from Rogue(http://www.rogueaudio.com/The_Magnum_Series.htm)

and Cary Audio(http://www.caryaudio.com/upgrades-and-modifications/)

Hexfred? Poor Fred, he didn't understand why the bad witch cast that awful spell on him.

Is this why my ARC CL-60 always had a nasty "buzzsaw" ghost riding the waveform of vocals? It has tube output stage but solid-state rectifier.

It was either this, a Classe DAC (solid-state) or digital jitter (maybe all three?)
that just drove me up the wall.

I hear it from time-to-time with my current system but to a much lesser degree.

Can't the demons just leave me alone???

Rectifier conducts until amplitude of AC voltage is lower
than capacitor voltage (just after peak of the sinewave). At
this point diode is reverse polarized and diode current drops
to zero. Unfortunately it will conduct for a moment in
opposite direction and then it will recover back to zero.
Ideal diode should be very fast to minimize current spike in
opposite direction and very slow to recover to zero. Ratio
of time (on negative side) after the peak to time before the
peak is called "Softness Factor" (RRSF - Reverse
Recovery Softness Factor). You want fast diode to minimize
negative current spike, but not the one that snaps back fast
(you don't want narrow spikes).

I used six each of the following, in my Cary SLM-100s. High
voltage SiC Schottkys weren't available at the time.
(http://www.digikey.com/product-detail/en/DSEP12-12A/DSEP12-12A-ND/560364)

HEXFRED rectifiers are about the fastest rectifiers and the softest recovery- just about the closest you can get to a tube rectifier, without the voltage drop of a tube rectifier.

We've been using them for about 20 years.

Sonically they sound smoother. This is because they make less commutation (also known as 'switching') noise for the reasons mentioned above.

Saw this on YouTube a few days ago. Talks about Schottky diodes:

https://www.youtube.com/watch?v=bXEyCf1P0UU&t=2s

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.