High Speed Diodes
Since there has been a lot of discussion about the merit of high speed diodes, I thought that the subject deseved it's own thread. The following application notes gives an excellent overview of the design parameters for high speed soft recovery diodes and how they can introduce RF noise into an audio circuit. Don't get freaked out by the semiconductor physics but pay attention to the definition of terms such as reverse recovery time and recovery softness. Appendix 1 is very useful for understanding how to compare parameters of diodes from different manufactures.
The following are also very useful:
Grey would have a fatal stroke over me posting references this technical, but he does not seem to be around lately. You don't need to understand all of the information in articles to get some very useful imformation out of them. Required reading for power supply perfectionist in my opinion.
* fast recovery epitaxial diode
Also -- another link
Linear Technology discusses EMI/RFI from fast acting diodes in Application Note 70 -- note at www.linear.com you can search the apnotes numerically by going into "Publications"
To the extent I can follow these paoers, I keep hearing a voice in my head ask: "But doesn't an old fashoined bridge rectifier with RC snubbers do about as well?"
EMI/RFI from fast acting diodes
Thanks jackinnj. I figured the other kids would come out and play
on this thread and I don't think read this one.
"doesn't an old fashoined bridge rectifier with RC snubbers do about as well?"
I don't think so. There is no reason you can't use snubbers with the FREDs also which is kind of implied in the references. Another good one that discusses snubbers is:
Also an-15 from:
The oldfashioned Way
I replaced all 1N4002 powersupplydiodes in my Philips CD-650 by 11DQ10 Schottky's and I removed all snubber caps across the diodes. This includes the diodes for the +32V for the display.
It made a dramatic difference to the sound. Voices became much less screamy. All music became more "musical".:idea:
alfsch did some tests here and found the BA159 to perform surprisingly good.
They are easy to get here and are cheap.
Schottky diodes are very fast and can create lots of RFI as well.
Snubbers without resistors tend to move the ringing to another frequency and can create an even more troublesome undamped ringing. This was not meant as a snub to your post Elso, but as a snubber post to you.*
For some infor on snubber design:
I wonder if anybody is noticing a trend here?
* I believe that might qualify as a pun but I guess I will be forgiven....... eventually.
When building an amp, I look at what's on the DC rails with a scope and a coupling cap while signals (music or test, depending) are running through it. I can't ever recall seeing artifacts attributable to the use of slow diodes (like 4007) actually appearing on the PS rails. For 50/60 Hz power supplies, I wonder if it's worthwhile to go high-speed. Has anyone got DC rail waveforms or spectra which show some kind of significant difference? If there really is something there, I'd certainly upgrade the 4007oids I usually use.
If snubbed, What values of R's and C's do you suggest?
The capacitors across the diodes...
For the record...my experience is that the capacitor across the rectifier diodes allways degrad perfomance...sorry!!
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