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Old 13th October 2003, 09:58 AM   #1
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Question Diode Question

Hi all high hand,

I have some question in Diode.

Someone tell me that speed of diode (used as rectifiers) will affect the amp performance.

High speed (or even ultra high speed) diode will help to improve the high tone.

And one type of diode call "Fast soft-recovery" type is even better.

Q1. Would any high hand can explain more detail on the speed of diode?

Q2. Also how fast is considered as high speed/ultra high speed?

Q3. What is the safety factor on voltage? (e.g. My circuit design is 280V, what is the diode voltage requirements? 600V safe enough or not?)

Thank you for your help.

Regards,
Tuna Fish
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Old 13th October 2003, 10:07 AM   #2
dhaen is offline dhaen  Europe
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Hi Tunafish,

The diode issue is about reverse-recovery noise. This is generated when the diode turns off. The noise can get into audio signal path. It is normally measureable with a scope.
I don't think there is much point in going for just "hi speed" diodes now. There are cheap excellent soft-recovery diodes for high voltages, and Schottky's for heater supplies.
The diode's rating should be at least 1x the AC peak + the DC voltage. So in your case, 600v is just enough. 800V would be better.
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Old 13th October 2003, 10:52 AM   #3
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John,

Thank you for your help.

Would you explain further on :

Q1. How the high speed soft recorvery diode can help on pre/power amp performance?

Q2. How fast is fast?

Q3. What is "Soft recovery"?

I find some "Fast soft-recovery controlled avalanche rectifiers" the reverse recover time is 30ns (up to 600V) and 75ns (up to 1000V).

Q4. Is it fast enough?

Thank you for your help in advance.
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Old 13th October 2003, 11:39 AM   #4
dhaen is offline dhaen  Europe
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Tunafish,

1. You had better ask others what they think diode noise sounds like. I can measure it, but am unsure whether I can hear it.

I am not an expert on diodes, but this is what I think:
2. I don't think that speed helps much for audio. The faster the diodes switch, the narrower the noise energy puse, but the harder it is to stop the noise from leaking out from the rectfier circuit.
Fast diodes are produced mainly as efficient rectifiers in high frequency circuits such as Switch Mode Power Supplies.

3. I don't know the definition of "soft recovery" but do know that they produce much less noise.

4. IMO It doesn't matter. (Perhaps someone will point out a reason why it might )

Incidentally, back in the late 1960's I used to use a bank of standard diodes (1N914) as a varactor to multiply 432MHz *3 to 1296 MHz. I could get about 100mW. This shows how powerful the effect is, and how even a slow diode an produce large reverse energy.

Edit:
Although I'm not sure that I can hear the difference, I do use "quiet" diodes. It's just good engineering practice to minimise noise sources.
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Old 13th October 2003, 12:26 PM   #5
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Interestingly, diode noise does not give the subjective impression of extra noise, so, yes in this respect John is certainly right - no noise to hear It changes the ease of reproduction and particularly the high frequencies - previously squashed, compressed, muddled sounds now become much clearer. I am not so sure about any positive subjective effects in the bass though. Interesting again is that the subjective sound of vaccum rectifiers may be approximated but not reached even by the best Schottkys.
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Old 13th October 2003, 12:43 PM   #6
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Quote

Someone tell me that speed of diode (used as rectifiers) will affect the amp performance.


Well, if you don't mind my .02 worth here goes... I purchased some empty tube bases and installed a diode in each and replaced my 5AR4's just for the hell of it to see what difference it made on the bass response.

With the 5AR4's the bass was nice, what you would expect from a tube amp kinda slow and tubby. When I installed the diodes in place of the rectifier tubes the bass became quicker but slightly less. I switched back to the tubes after this because I have become used to the sound of the tube rectifiers.

This was performed on some modified Quicksilver 8417's but I expect the same results on other amps. I would assume that tube like performace could be derived from diodes depending on the type and speed. Making a power supply is certainly cheaper using diodes over the more expensive tube.
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Old 13th October 2003, 12:46 PM   #7
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Hi,

Diodes have been discussed indepth here:

High Speed Diodes

Cheers,
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Old 13th October 2003, 12:53 PM   #8
SY is offline SY  United States
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burnedfingers: You've actually changed a couple of variables. You've changed the B+ and the power supply impedance. Your sonic observations are exactly consistent with that- I doubt that "diode noise" had much to do with it.

FWIW, I can measure diode switching noise in my amp using a scope, but for the life of me, I can't seem to find it on the rails or in the signal path. The cap filters do a pretty good job of shunting it to ground.
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Old 13th October 2003, 12:53 PM   #9
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And here a good post yesterday at the AA by Gary Pimm. Don't know if that is against the rules or spirit of discussion boards..

"Reverse recovery is a term that describes the SS diodes turn off characteristics.
Basically, a SS diode uses minority cariers (holes) to cary the current across half of the PN junction. When the diode is reverse biased at the turn off point of the waveform the diode will conduct current backwards untill all the minority cariers (holes) are swept out of the PN junction. This reverse current is what generates the reverse recovery noise. Different diode constructions have different characteristics.

Another thing to note is that during diode turn on there is also a voltage spike where the forward voltage of the diode is higher than the nominal ~.7 volts. This lasts for the duration that it takes to inject the minority cariers (holes) into the PN junction. This mostly applies to high frequency switching supplies but may be of interest to folks trying understand all about SS diodes.

Here are some links showing reverse recovery info.

Good scope shots of reverse recovery
http://www.avtechpulse.com/appnote/techbrief9/

Lab to measure reverse recovery and good description
http://howard.engr.siu.edu/staff1/ha...LABS/Exp1.html

Good information from IXYS on FRED's. Click on "Characteristics and Applications of Fast Recovery Epitaxial Diodes"
http://www.ixys.com/tecnot01.html

A .pdf file from General Semi on diode applications in high end audio. I can't find this on the General Semi page now so the link points to the .pdf on my web page.
http://www.pacifier.com/~gpimm/general_semi_quik108.pdf

Hope this helps,

Gary "

The last link might answer your question on how fast fast is.
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