[Gvt1911]

Looking to replace stock diodes in preamp in HV power supply. Reading good and bad about Schottky and HexFreds. What do you guys use or like the best? Don't want any switching noise. Recommendations please !
Thanks !!!

[Niteshade]

Standard silicone are my favorite. The type doesn't matter as long as the filtering section is up to snuff. Are you having problems?

[BobM]

Cree Schotky's

[JoshK]

Cree Schotky's

Yeah, that is what I'd opt for too.  I'd make sure the heater diodes are schottkys too.  If not the switching noise can couple to your B+ through capacitive coupling. 

[avahifi]

Just make sure you replace diodes with the proper type, voltage rating, and current rating.

For example, for diodes used in basic AC line rectification, make sure they have at least three times the normal AC input from the power transformer.  Diodes can take an over-current for a bit of time, but are VERY sensitive to an over-voltage.

For example, if your power transformer is putting out 300V AC into the diode bridge or thereabouts, you better have 1000V rated diodes to assure reliable operation.  Why?  To protect the diodes from that occasional high voltage spike on your AC power lines.  In general most power supply diode failures occur in late spring, when thunderstorms and lightning abounds.

Of course the diodes better be rated for more than continuous current requirements of the circuit too or you will have found another way to cook your amplifier. 

I am not all certain that FRED or Schottky diodes are a cost effective improvement in raw power supply applications.  The signal in is at 60 Hz, pretty slow.  It is pretty easy to throw a scope across the output of the rectifier and see if any high frequency switching noise exists.  Normally an appropriate high voltage disc capacitor across the bridge will take care of this very well.

I have yet to hear of any true double blind tests of identical circuits, one with normal silicon diodes and the other changed only with so called premium high speed diodes, being conducted.  I have been at this for 50 years.  Sugar pills abound in high end audio and cost you way more money that you would believe.

Best regards,

Frank Van Astine

[Gvt1911]

Thanks for the input on diodes. I am not having any trouble at all with the standard silicon diodes that are in there now, just trying to get the most out of this preamp. Had many suggestions from other parties that just did not make any sense. I think Frank maybe correct and I should leave things be.   I would hate to go thru all the trouble and have little if any improvement. Unit has a single 100 mfd cap as a filter for the HV supply and the sound is decent for what it is. Any other suggestions would be appreciated. 

[JoshK]

Well I've seen scope shots from various diodes in some circuits.  Not saying you can't snub it out, you can, but you don't have to with schottkies.  CREE makes 1200V schottkys.

On costs, as a DIYer, I don't see the big stink over spending $50 on parts that you know will make it easier to design around.  For a manufacturer, I get why you'd rather design smart than throw money at it.  HEXFREDs aren't very expensive last time I checked.  The most expensive CREEs at the premium parts shops are like $12/ea, so $50 for the bridge.  You can get them cheaper elsewhere if you shop around. 

Tube rectifiers, like GZ34 command more than that these days and that doesn't include the heater windings, sockets, etc.

With all the money thrown into this hobby, usually well north of 10 big ones, I don't see being cheap about $50 worth of parts.  But that is me.

I am not saying you will necessarily hear a difference.  I don't claim that.  But the hope is that an almalgamation of small improvements gets you somewhat closer to a higher performance level.

I still believe that a good circuit trumps good parts.

[BobM]

True Josh - a self-modder may not cringe at a $50 parts swap, but a manufacturer must translate that $50 parts cost into about a $300 increase in the retail price. So there's a definite balance of cost vs benefit for someone whose products usually sell in the $1k-2k range.

[bluemike]

For solid state rectification Fairchild Stealths are my favourite fwiw

[JoshK]

For solid state rectification Fairchild Stealths are my favourite fwiw

I think those are HEXFREDs iirc.  Very bespoke.  They can handle mass current which I don't think many schottkys can.  I am not up on all that is available.  But, I'd still advise to use a snubber with HEXFREDs.  If I am not mistaken they just push the ringing up higher in freq where it is easier to filter, but it can still couple via capacitive coupling to other things in the audio chain, especially with toroid transformers.

[Gvt1911]

Thanks for the input on the diodes. I have a very simple power supply that consists of the transformer going into the 4 diodes thru a resistor thru a 100mfd 450 vdc electrolytic cap going to the preamp board. High voltage is 375 VDC. On the filament side ; out of transformer into 4 diodes into a 4700mfd 25 vdc cap going to board. Very simple. I was looking to upgrade any parts that would be benificial to this supply. Any advise ?
Thanks again.

[JoshK]

Thanks for the input on the diodes. I have a very simple power supply that consists of the transformer going into the 4 diodes thru a resistor thru a 100mfd 450 vdc electrolytic cap going to the preamp board. High voltage is 375 VDC. On the filament side ; out of transformer into 4 diodes into a 4700mfd 25 vdc cap going to board. Very simple. I was looking to upgrade any parts that would be benificial to this supply. Any advise ?
Thanks again.

My opinion, take it fwiw.  I'd do the filament supply with schottky's first.  They aren't expensive.  See if you hear any difference, if not, stop there.  You likely cleaned up your B+ a little by virtue of crud not coupling to your B+.   

Secondly, I'd replace teh resistor after the B+ bridge with a choke.  That matters way more than diode types.  However, you'll need to do a little PSU modeling to make sure it is an all-else-is-equal.   I can help you with that if you can measure things in your current PSU.