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In Reply to: Rectifiers---fast/slow---SS/tube??? posted by Hans on April 12, 2000 at 00:22:50:
okey, I went to International Rectifier, www.irf.com , and looked at some data for HEXFREDs.The clue is not that they are very fast, but that they: (from their own description) 'In addition to ultra fast recovery time, the HEXFRED features extremely low values of peak recovery current and does not exhibit any tendency to 'snap-off' during the 'tb' portion of recovery.'
This gives less noise, (RFI and EMI), than regular SS diodes.
This is excactly what tube diodes are good for, the soft on/off characteristics, and explanes their superior 'sound'.
Everytime I see the use of HEXFREDs it is always the 'ultra fast' feature which is mentioned, and that's what puzzled me, but it's the 'ultra soft' feature which should be mentioned.
So now I see how HEXFREDs, just like tubes, are better rectifiers.
Yas, it is the lack of "switching nasties" (back current before switching) that is good with hexfred (and diode och full wave valves, not mercury though). That is why caps across the diodes helps. Who needs to be fast at 50-60p/s?
One difference btw valves and sand is that valves have quite a voltage drop when the current rises (drop as a close to linear function of durrent). So valves does introduce a sound, but as valves are mainly for valve amps, and valve amps usualy runs class a coz valve users care about sound, and as class a amps uses more or less a constant current (with active load / regulation / linear PP it is a constant current) it doesn't really matters. But for an AB amp it is a completly different matter... [PSU is also a bit easier to filter with sand compared to valve rectification, as valve rect. don't like large caps but rather chokes (but they also have a appealing sound).]
/Pär
agree. But: valves aren't more difficult to filter, rather they are more expencive to filter as well as SS, due to the need for chokes...
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