[Jens]

There was some talk about this quite some time ago, but I don't seem to be able to find the thread.

What's the difference between these two types of diodes, and does anyone have experiences with variations on this theme in the AKSA amps and preamps?

[NealH]

A fast recovery diode stops conducting quickly when a reverse charge in imposed across it's junction.  The quicker the better, as this limits the voltage transient and associated ringing resulting from a change in current across the internal inductance of the diode and it's leads (di/dt).  In practice, even with a super fast recovery silicone diode, such as that exhibited by the popular 1N914, a voltage transient will still be created since the recovery event is fast (di/dt again).  Our semiconductor industry addresses this by designing a "fast-soft recovery" characteristic into some of their diodes and these have generally garnered wide acceptance.  Snubber networks further mitigate the negative effects of  di/dt.  

A schottky diode incorporates a different junction topology which produces a  lower voltage drop associated with it - thus lower power loss and better efficiency.  The recovery is fairly fast, and happens to exhibit an indigenous  fast-soft recovery characteristic making it near ideal for many switching applications.  But, in general it is the low forward drop that generally marks this device for a specific circuit application.   Schottky diodes have a couple of undesirable attributes.  They exhibit a  higher leakage current than their silicon junction brethren, as well as limited blocking voltge due to their junction structure.  These two characteristics often negate it from being used in many applications.

[ginger]

The latest technology is Silicon Carbide Schottky Diodes. These use majority carriers (technobabble speak for electrons rather than some other technobabble concept like holes or electron/hole pairs). To cut to the chase, these do not exhibit any reverse recovery at all. They should therefore be better than Ultrafast or standard Schottky or in fact anything else currently available. I believe Hugh has ordered samples (from CREE) for evaluation. They aren't cheap.

So Hugh - anything to report yet?

Cheers,
Ginger

[AKSA]

Great answers to Jens question, guys, many thanks.

Ginger, I've ordered a few SiC diodes from Cree;  nothing has arrived yet.

They are incredibly expensive, around $US15 EACH!!  They would have to be damn good for my interest to be maintained.........

Cheers,

Hugh

[Jens]

Thanks, all - very good technical explanations.

However, what does this technospeak mean in terms of sound - specifically, what are the advantages/disadvanges of using either of the buggers in the AKSA gear - sonicwise?

The new Schottkys sound very interesting, although the price is very heavy. Still - if they turn out to be very good, it might be something worth considering.

[JoshK]

This is a great read!  This is something I know zip about.  What are some examples of Fast-soft recovery diodes?  Are any of them high current enough to be used in amp bridges?

[aurelius]

This is purely a distraction...

Its been at least 10 years since I did solid state physics, but shadows of memories are telling me that the term "Majority Carriers" refers to electrons in N-doped material and holes in P-doped material? If one were to place a potential across a homogenous chunk of doped silicon, majority carriers play the major role in current conduction, however, across many PN junction configurations, minority carriers play a large role.  

What does this have to do with fast recovery diodes?  I haven't the faintest idea.

[ginger]

Aurelius has it correct. To try to explain things a bit.

Skip to the last 3 paragraphs if you don't want to wade thru' any of the physics.

Whenever you have a P and N type semiconductor material the P type has an affinity for electrons (called holes - somewhere for electrons to go) and the N type wants to give electrons up for each of them to attain their most stable states. This has to do with the properties of the doping elements.

When you bung em together to form a PN Junction in a diode (Anode and Cathode respectively) or a transistor junction electrons flow from the N to the P creating a "depletion region" around the junction  - that is its depleted of "holes" and electrons. The electron flow is terminated when the electric potential created by this flow of electrons (charge) is sufficient to resist any more flow. The depletion region will have a certain width

The junction (depletion region) will  therefore has an electric field across it and hence has capacitance.

This junction capacitance has a Reverse Recovery charge (Qrr). That is, before a diode can turn off this stored charge has to be removed (by recombination of majority carriers). Current has to flow to remove this charge. Therefore as the AC voltage reverses on a rectifier diode there will be a pulse of current in the wrong direction before the diode actually switches off. This puts a noise spike on the power supply rail which has very high current rise time (di/dt). Filter capacitors are not very good at removing (shunting to ground) this noise spike because they have a small amount of inductance and inductors resist current change. Therefore you end up with a noise "SPLAT" on the power supply rail.

The differences in Silicon, Ultrafast Silicon, Schottky and Silicon Carbide Shottky of interest when using them as rectifiers is how fast they can turn off and how large this Current "SPLAT" is before turning off. This is obviously directly related to their Reverse Recovery Charge (Qrr). Small capacitors across each diode can help to absorb this splat and for some time they were in fact required by Electromagnetic Interference (EMI) standards for all commercial products (at least in Europe - I don't know if this is still true).

SKIP To HERE:
So here is the useful (representative ONLY) Qrr data:
Standard Silicon Diodes Qrr approx 500 nC (nano-Coulombs)
Ultrafast Silicon Diodes Qrr can be down to 100 nC
Schottky Diodes Qrr 50 to 70 nC
Silicon Carbide Schottky Diodes Qrr <20nC

Oh - To answer the original question - the other thing you need to know is that the noise spike associated with the current "SLAT" makes your amp sound seriously crap and that its edge speed is often fast enough to radiate into anything that looks like remotely like an arial (resistor leads, connecting wire etc). It can make the sound very "harsh" and is typically noticed mostly in the smoothness and detail of the high frequency "top end".

I rabitted on a bit here because the concept of junction capacitance is very important not just in rectifier diodes but also in transistors and is something amplifier designers need to deal with. If you want to understand things like EARLY Effect in transistors its a concept you'll need to know.

Cheers,
Ginger

[Jens]

Some of this is still way over my head (in spite of Ginger's fine explanation) - but I think I understand the essence.

From what Ginger says it seems that Schottkys are slightly better than ultrafast - have I got that right?

Obviously, the reason why I am asking is that I am considering trying out Schottkys in the AKSAs (and also in another small upgrade project that I'm currently doing with a friend), so before I go out and buy components, I would like to know a bit about this. I'm sure Hugh will kick in something here ....  

There is not necessarily a big price difference between Schottkys and good ultrafast diodes - so that is not the issue.

[Jens]

C'mon, guys ....

I seem to remember someone having exchanged the supplied diodes in the AKSA amp for Schottkys - any input on differences in sound? Improvements?

[AKSA]

Jens,

Based on the argument that power diodes inject switching transients into the filter caps which they are unable to entirely purge, it makes sense that the power supply can be improved - in terms of silence - either by lowering the ESR of the filter caps (we all know about this one!!) or by reducing the switching speed of the diode.  

Since the Schottkys exhibit around 50-70% of switching times of the UFSR diodes, it's a fair bet that there would be improvement.  The silicon carbide diodes should be even better;  they sit at 20% switching period.

So, in theory, and I've not tried them YET, I'd say go for it!

Cheers,

Hugh

[mb]

C'mon, guys ....

I seem to remember someone having exchanged the supplied diodes in the AKSA amp for Schottkys - any input on differences in sound? Improvements?

I guess I be one of them... trouble is I never installed Hugh's supplied soft recovery diodes

In the instances I've used schottky instead of soft recovery, the perceived result is a "darker" background and smoother, even slightly rolled-off top. This is very noticeable with power supplies for cd players, where obviously these are pretty positive factors (esp. lack of "digital" signature). Haven't tried the latest technology schottkys, but I expect they will be good too.

BTW, too many DIYers are choosing ultrafast diodes that aren't neccessarily soft recovery. Hugh supplies diodes that have nice recovery behaviour, but I know the diodes often used by gainclone builders are fast, but not very soft in recovery characteristics.

YMMV, etc, etc..

[NealH]

Remember we're dealing with sinusoidal 60Hz (or 50 as applicable) which is anything but fast, and typical line impedance of around 150 milliohms or so at the outlet on a 120V, 60Hz home distribution system - not to mention the slight added impedance by the front end power transformer.  I suspect you have already reached the point of diminishing returns in regards to the  switching speed of even general purpose power diodes.

[Jens]

I'm doing some cabling upgrades on my 55N anyway, so I might try some Schottkys there, as they are quite easy to get to.

Any such thing as 'Schottky soft recovery diodes', or are they just inherently soft recovery?

[ginger]

Confused by what is a fast recovery diode and what is a soft recovery diode.

Recommended bedtime reading - if this doesn't put you to sleep I don't know what will.

Just kidding - actually a useful paper, not too much physics.
http://www.microsemi.com/micnotes/302.pdf

Cheers,
Ginger

[RonR]

So,

Did anyone try the Silicon Carbide Diodes?

If there's a gain to be made with (relatively) little effort and outlay, I think we should be told!

Cheers,

Ron.

[AKSA]

Hmmm, SiC diodes, I ordered some from Cree in the US about a month ago, but still nothing has turned up.

I'll let you know the instant I have something, sorry about delays....

Cheers,

Hugh

[PSP]

I spent about 20 spare minuutes one day looking for a US supplier of the Cree diodes... no luck.  Can someone help me out?

Several years ago, I replaced the stock rectifier bridge diodes in an old NAD 7020 reciever with FRED soft recovery diodes.  This lead to a significant loss of high end grit and grunge.

In the AKSAs, I have not messed with Hugh's diodes (I did put 100v Schottkys at D1 in my 55Ns, but did not compare vs. stock).  I'd be very interested in hearing about any experiments people have done with the power supply diodes.

Peter

[Occam]

PSP- the only source I know of is PartsConnexion, silly money -
http://www.partsconnexion.com/catalog/semiconductors.html
click 'diodes' on the horizontal menue on the bottom

Ignoring his abberrant taste for cilanto, Bob Crump seems to like the Fairchild Hyperfast and Stealth diodes.

FWIW

[PSP]

In case anyone missed it, there is a recent diode thread on AA:  
http://www.audioasylum.com/audio/tweaks/messages/123567.html

Peter