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Old 22nd April 2010, 06:30 PM   #1
tomchr is offline tomchr  United States
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Default Why FRED?

Folks,

Why are FRED diodes used in the Tubelab circuits rather than the "normal" 1N400x series for example?

I take FRED is short for Fast Recovery Epitaxial Diode, correct? If this is the case, I'm guessing the FREDs are used because of the fast recovery. But what is the advantage of the faster recovery and how does it impact the measured performance of the amp (as well as perceived differences in the sound quality)?

~Tom
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Old 22nd April 2010, 07:50 PM   #2
tomchr is offline tomchr  United States
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I should clarify... Why use the FREDs rather than "normal recovery" diodes on the BIAS supply? I'm referring specifically to their use in the TubelabSE.

~Tom
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Old 22nd April 2010, 08:14 PM   #3
DougL is offline DougL  United States
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Just for background:

High Speed Diodes
Fast Rectifiers for Power Supplies
Appropiate cabling quality & Regulators
FREDs on PSU
Tube vs. Semiconducter rectification
FREDs? Replace 1N4003 rectifier diodes?
Cree diodes
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Old 22nd April 2010, 08:54 PM   #4
tomchr is offline tomchr  United States
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Doug! That's good stuff.

I'm somewhat familiar with reverse recovery from designing switchmode supplies some 12~13 years ago in college. I suppose I could dig out the old books and start reading up on it again, but, frankly, I'm too lazy... As far as I recall, the deal is that it takes a while for the carriers in the diode to recombine after the diode is reverse biased. This will cause a reverse current to run, and sometimes the magnitude of this current is an issue. When the diode finally recovers, some shut off abruptly, others softly. If the dI/dt (change in current vs time) is high, there's a good chance it'll excite the parasitics of the rectifier, transformer, PCB, etc. and give a high-frequency 'buzz'. This 'buzz' may dissipate power in places you don't want it to and wreak havoc. Selecting a diode with desirable reverse recovery characteristics and using snubbers will limit the havoc as a diode with desirable reverse recovery will create less 'buzz' and the snubber provides a safe place for the power of the 'buzz' to be dissipated. Of course, the 'buzz' power may also be radiated as an EM wave that can cause interference elsewhere.

I'm still curious as to why FREDs were used in the Tubelab SE. Maybe it's more a question for George directly - but I figured there'd be some general interest in the answer.

~Tom
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