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Old 6th August 2014, 04:09 AM   #1
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Default Standard Silicon Diode Rectifier's VS Ultra Fast

Okay I finally happened to try this swap out in a guitar amp/Plexi style circuit.....dumped the old standard silicon 1N4007s.... the
Ultra fast UF4007s were/are around same price so figured I give them a try...Just wow....I thought it was all hype/myth you wouldn't even notice the difference....

To me there is a Huge difference....first thing right off the bat was the response, its more touch sensitive/exact. I'm really impressed actually, just improved on a fuller/dynamic sound.

Last edited by AudioFreak88; 8th August 2014 at 01:31 AM.
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Old 6th August 2014, 07:03 AM   #2
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Yes, UF4007 emits much less noise at its switch-OFF.
A snubber circuit can remove much of the remaining noise (caused by leakage inductance of the transfomer): connect 47 ohm 1/2W resistor and 22nF 1500V (yes, really 1.5kV rated) across the trafo secondary.

Sometimes, the difference between rectifiers is accentuated by the electrolytic power supply capacitors. They age quickly in a hot amp, so if they are more than 8-10 years old, replace them with Nichicon or Panasonic parts, and you'll certainly notice the difference.
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Old 7th August 2014, 05:29 PM   #3
sjs is offline sjs  United Kingdom
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Rod, and how would an SiC such as the Cree C4D02120A stack up?
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Old 7th August 2014, 06:06 PM   #4
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Being majority carrier only devices, Schottky diodes are "noiseless". Snubbing is not needed.

IMO, it makes great sense to use high PIV Schottky diodes in new designs and builds. There may be problems in retrofitting the TO220 cases into existing equipment. UFnnnn parts are drop in replacements for their 1Nnnnn counterparts.
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Old 7th August 2014, 06:28 PM   #5
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It's true that Schottky diodes are majority carrier devices, but they have a large non-linear (vs. voltage) capacitance, which is an excellent generator of noise/garbage/harmonics on its own. Some snubbing will still be needed.
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Old 7th August 2014, 06:56 PM   #6
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Yes, snubbing is still recommended, even though the conducted and radiated emissions from SiC rectifiers is lower overall, because of the lack of reverse recovery current pulses.

The major culprit is the leakage inductance of the transformer, which stores energy, and releases it at the moment the rectifier current passes through zero. The high capacitance of the diode hurts, too, as Wrenchone says. The snubber catches the pulse and turns it into heat in the resistor.

Don't hold back from trying the Cree SiC rectifiers, though. Keep their wiring (trafo-rectifier-cap) very short, use a snubber, and a modest-sized C1, to get best results.
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Old 7th August 2014, 09:24 PM   #7
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Diodes optimized for active PFC applications are a good bet, as they are designed for low reverse recovery charge under conditions of hard switching. Standard ultrafast rectifiers will not necessarily the best , but they are much better than normal recovery diodes.
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Old 8th August 2014, 01:30 AM   #8
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Everything just feels far more snappy/quicker/precise response, Really liking that....upper frequencies are more interesting too, more smoother/transients etc....and just overall sustain/definition/Dynamic's has improved greatly...I can't get over how big of a change it really is...I figured there wasn't going to be much of a difference...true that high end "hash" has been reduced greatly....

Last edited by AudioFreak88; 8th August 2014 at 01:32 AM.
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Old 8th August 2014, 08:20 AM   #9
sjs is offline sjs  United Kingdom
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If the transformer leakage inductance releases energy when the rectifier current passes through zero, would you expect any difference between a full wave rectifier with centre tap or a bridge rectifier without?

I have always tended to go with full wave as I dont like the idea of the bridge switching noise into the ground buss, but maybe I am miss thinking this :-)

I tend to use choke input for ht, but even here I have seen benefits from using snubbers, even with valve rectifiers.
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Old 8th August 2014, 09:05 AM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by AudioFreak88 View Post
Everything just feels far more snappy/quicker/precise response,
Just because they're called 'fast rectifiers' doesn't mean they make the amp go any faster. They just produce less noise. Your amp has become less noisy. Nothing else has changed, other than your perception.
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