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Old 24th July 2001, 12:00 PM   #1
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I recently have been told that by replacing the rectifier bridge in your PSU with ultra fast rectifiers, one could improve the sound quality of one's amplifier - better resolution and more details. I did so and found audible improvements.

Can someone enlighten me with the reasons behind it?

Would one expect the same result for doing the same to a pre-amplifier?

Many thanks,

William
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Old 24th July 2001, 12:34 PM   #2
Geoff is offline Geoff  United Kingdom
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William

Have a look at the the following Application Note on the General Semiconductor site.

'High speed rectifier applications in high end audio'

http://www.gensemi.com/product/AppNotes/appnotes.htm

Geoff
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Old 24th July 2001, 07:37 PM   #3
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I took a look at this reference and was unimpressed.
As far as I can tell, no one has yet to offer a decent
set of measurements or even a good hand waving explanation
as to the audible improvements of fast diodes in a
60 Hz application
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Old 24th July 2001, 08:52 PM   #4
Geoff is offline Geoff  United Kingdom
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Nelson

I don't disagree with you, I merely offered the link as 'further reading' for William, since it contained a possible explanation for his perceived improvement (there are many other reasons why he may believe the sound is better).

My own view is that perhaps manufacturers would like persuade people to buy higher spec'ed devices, at higher prices, and with greater profit margins. If this is the case, then they are going to try to justify the additional expense by using all sorts of claims and explanations. Unfortunately, the audio world is full of such practices (we won't get into the subject of cables etc. here :-).

Geoff
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Old 24th July 2001, 10:01 PM   #5
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I mean no reproach, and actually I appreciate
being able to see the reference. For that matter
I wouldn't stand here and argue that it can't sound
better.

Does anyone out there have a more reasonable theory?
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Old 25th July 2001, 12:01 AM   #6
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From the application note, I can't understand how replacing standard diodes with ultra-fast diodes in a bridge rectifier can change the sound.

On the other hand, there might be an issue with electrical or RF noise being spit out when the diodes switch. I saw this idea in Randy Slone's amplifier book. He suggests that each diode in the bridge have a 0.1uF cap in parallel with it. It might be interesting to compare the results of this method to the results of the 'fast diode' method.

[Edited by thoth on 07-24-2001 at 06:35 PM]
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Old 25th July 2001, 02:51 AM   #7
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My assumption has always been that sufficient capacitance in the main bank would shunt the high frequencies (after all, it's a spike, hence Fourier analysis would indicate scads of high frequencies) to ground. Arguably, film caps (having a better high frequency response) would do a better job, either in conjuction with electrolytics, or by themselves.
A question I've always wanted to play with is whether any residual high frequency hash made it through a regulator--but then you get into a thousand questions about the 'sound' of whatever regulator circuit you might chose.
Personally, I have no opinion about the audibility of various sorts of diodes, as I've never tried them out, one against the other. I have, however, used both kinds (normal & soft recovery, but I've not used Schottky for anything) over the years, not out of any sense of conviction either way, just whatever mood I was in when I was buying parts for what I was building at the time: It's only a couple of bucks...why not? vs. Fooey, probably can't hear the blasted things anyway, put the money into the caps instead. Nelson's position, at least at his day job, would have to lean towards parts cost I'm sure, unless there was a fairly large sonic difference that the eventual customer would definitely notice. Since I'm only answerable to myself for the amount I spend on the rectifier, I have more freedom...but don't always vote the same way from one project to the next. One of these days, I'll take the time to fiddle the rectifier back and forth--at that point I may be able to resolve the question (at least to my own satisfaction). For the moment, that'll have to take a back seat whilst I'm rebuilding my system.
I don't take Audio Amateur, so I didn't see the article referenced in the paper. Did anyone read the original article? Is it, perhaps, available somewhere on the web? That might give us a firmer basis for discussion.
I generally bypass the diodes in rectifiers with smallish (~.1 uF) caps just as a matter of principle--not necessarily because of sonics (I haven't A-B'ed this either), but just as a matter of engineering. It serves as a good way to preserve diodes from high voltage spikes coming in from the AC line. It's another trick I picked up from the ham radio people. If you were to assume that the faster diodes did make a difference, the cap would presumably serve to reduce the spike still further. Yes, you're putting in four more parts, but .01 to .1 uF caps aren't that expensive, at least compared to the cost of these diodes.

Grey
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Old 25th July 2001, 05:06 AM   #8
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John Curl has commented on this on several occasions in the Tweaks section of the Audio Asylum. His position is that standard solid-state power rectifiers exhibit a glitch at turn-off which may find its way into the ground system of the host audio device.

He asserts that this glitch can be seen by using a differential scope probe or RF spectrum analyzer to monitor the current through the diode. He further notes that the cleanest behavior is shown by "soft recovery" diodes, as opposed to "high-speed" types which may form a secondary glitch or induce high-frequency ringing.

A number of these comments apparently are still available for review in the Audio Asylum archives. I found several by going to http://www.audioasylum.com and using the Search feature to do Boolean search in the Tweaker's Asylum archives for John+Curl+Schottky, John+Curl+Hexfred, and John+Curl+Diode.
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Old 25th July 2001, 05:25 AM   #9
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Grey,

Unfortunately electrolytic capacitors have exceptionally poor performance at high frequencies, and will do little to attenuate switching noise. The normal method of reducing this noise is ceramic snubber capacitors immediately across the diodes i.e. as close as possible.

While I am increasingly impressed with the performance of circuit computer simulators, I believe they do tend to immerse us in a terribly artificial environment. The packages do assume, for example, “perfect” performance from components. It is possible that we are tending to assume this same type of performance in the real world. In addition, there are many “components” that simply never appear on a diagram simply due to the fact that we have to place the parts on something and hook the whole lot up together.

While I can’t really offer any valid explanation as to why switching noise should affect audio circuits, factors such as IMD and ground paths should possibly be considered. Ok, this is a fancy way of saying “dunno”, but I am dubious of an environment which mixes RF and any form of non-linearity.

Switching noise is a fact, I come from a HF background, and it is well established there. I’m with Grey, if there is any doubt as to whether less than ideal conditions will affect the circuit, I’ll steer on the safe side; it’s often only a few cents difference in low voltage applications.

I have used Schottky diodes in a CD power supply to good effect. In that case the PS supplied both AF and DAC circuits. DACs are particularly sensitive to line noise, hence I assumed the improvements were due to a reduction in this noise (indeed I still feel this way), however it’s not to say that it didn’t also improve the audio stages.

Cheers,

Pete


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Old 25th July 2001, 03:37 PM   #10
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Many thanks, Geoff. I had read the technical notes and it sounds to be an explanation. I should also thank those who participated with this discussion.

However, I went one step further and changed the rectifiers for my valve pre-amp (the high tension part), again with ultra fast rectifiers. This time, instead of making a bridge, I used only two because I got a centre tap for the secondary coil. The result is appalling. It does not sound like valve at all. It sounds tight and the mid frequencies are less (or should I describe as gone). I then switched back to my IC pre-amp [which had only ordinary bridge rectifier] and it sounded more pleasing (to the changed valve pre-amp). Did I overdo it because I had already changed the ones for my power amplifier?

Technically, two rectifiers and centre tapping result in full-wave rectification as well as a bridge rectifier. So the above phenonemon should not be blamed for not using a bridge rectifier.

Any further comments please?


William
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