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Old 20th February 2004, 07:56 PM   #11
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Quote:
Originally posted by runebivrin
I'm more of a "If I sing along, it probaby sounds OK" type.
That is actually a very good approach, and at the end of a day, this is what I go by too
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Old 20th February 2004, 09:14 PM   #12
UrSv is offline UrSv  Sweden
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Quote:
Originally posted by runebivrin
UrSv:
Judging from what I'm seeing as proposed designs, I'm not the only one to have missed those threads, and they must be at least nearly a year old, so maybe there's some point in raising the issue for the 51:st time.
--snip--
but I do consider myself reasonably clever
I'm not that clever but rather a bit on the daft side. However I have found the search button and took the first one I got:

2 Rectifier bridges : Why ?
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Old 20th February 2004, 09:48 PM   #13
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Default Re: Are dual rectifier bridges useful?

Quote:
Originally posted by runebivrin
Can someone please explain to me how a single diode bridge feeding from a center tapped transformer could somehow result in 50 Hz ripple?

I
Well, that won't happen in my house....60 hz maybe...

The ripple component normally is twice line..

However, if there is an imbalance in the bridge, one of the die having a higher than normal VF...or lower, for that matter, then there will indeed be a line freq ripple..

Some bridges, especially higher current potted ones, will have more than one diode in parallel in each leg..but they will normally be matched to within one millivolt to insure sharing.

On occasion, a diffusion furnace "mishap" can cause semi-four layer device operation...one where the chip will not conduct until a threshold has been reached...somewhat like a very leaky thyristor. Then, once on...it looks just like it's mates...

Unfortunately, if it's threshold is more than 10 millivolts or more higher than it's mates, it will not turn on...leaving that leg with a much higher VF. hence, the line freq ripple..

This can be verified in spice if you use a different diode model for one of the legs.

(I noticed nobody actually answered your question...so I chipped in..)

BTW...this is from actual experience in a bridge production line..took a coupla days to analyze it..

Cheers, John
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Old 21st February 2004, 06:56 PM   #14
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Rune, multiple bridges can give a real improvement in audio quality. That is why many designers now use this technique. First, all power transformers have capacitance between the input and the output windings. Yes, you can pay extra for a special isolation tranformer, but generally, the available transformers are fairly lousy, especially toroids. By not connecting a center tap to the ground, you create another layer of isolation between the power line and the internal ground.
About 15 years ago, I was also suspicious of dual rectifiers. However, while I made my Vendetta preamps, I decided to change over. I found that a potential ground loop had been broken by using the dual rectifiers.
This is one of the serious problems with Spice emulation, and little or no hands-on experience. The actual operating conditions are seldom re-created
in the Spice emulation, and potential advantages are overlooked.
There is one warning about using dual bridges. This is because the +/- loads have to be nearly exact, or the DC voltages will drift. A center tap usually drains off any difference.
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Old 21st February 2004, 07:17 PM   #15
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Quote:
Originally posted by peranders
.... "hugget som stucket"(*).
Hugget som stucket ==>> it's six of one and half a dozen of the other...
OR...
it comes to the same thing.......
Source: Norstedts


(of topic)
Per Anders,

har du jobbat på Ericsson EKA/KK?
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Old 21st February 2004, 09:27 PM   #16
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More diodes = more troubles

Each diode switches 50-60 times per second and has his own potential ringing and EMI radiation problems

Using two bridges to isolate ground actually doesn't provide any isolation, even worse, grounds and power rails are switchied between a high and a low capacitance to mains, ie: more ringing and more EMI radiation

The way to get isolation is simple : Common mode filtering [leakage inductance of the transformer already does differential mode filtering]

Everything connected simultaneously to mains and signal sources needs some kind of common mode filtering, no matter if it's powered by a 200Khz SMPS or a 60Hz transformer, common mode potentials are allways here since everything picks up RF from the air

People is just fooled into thinking that 50-60Hz passive switching power supplies are a magic thing that doesn't need filtering nor is capable to generate any EMI ...
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Old 21st February 2004, 09:37 PM   #17
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I'm disappointed that many of you cannot learn from the previous experience of others. No one wants to use more diodes than necessary. Good diodes are expensive and often are not available as bridges. This means more time and effort is necessary to make a proper diode bridge, and even more cost and complication to make a dual diode bridge, BUT we do it, when we want to do it best for preamps, especially.
The basic idea is to make as isolated an AC-DC power supply as possible. This includes a good power transformer with high isolation, high speed diodes dual diode bridge power supply, common mode chokes, and good electrolytic caps. Anything less is a compromise.
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Old 21st February 2004, 10:06 PM   #18
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Oh, high speed diodes for 50-60Hz switching is other of these things I don't understand

Ultrafast diodes also have their reverse recovery period and this period gets longer when the diode is operated by the very low dI/dt of a very inductive transformer in series with huge mains inductance [Trr gets as long as for a normal recovery diode]

In the other hand, having faster reverse recovery only means shifting upwards the spectrum of radiated EMI, and the higher the frequency the easier to radiate it and contaminate other circuits

If people wants to suppress reverse recoveries, why they don't use strings of schottky diodes in series?. Schottkys have truly negligible Trr

Ultrafast diodes tend to be designed for SMPS and are not suitable to be driven from very large inductances and low dI/dt [those 60nS Trr are for 50A/uS or 100A/uS dI/dt, not for 10mA/uS]
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Old 21st February 2004, 11:10 PM   #19
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We tend to use 'soft recovery' high speed diodes. If you use a current probe, you can easily see what slow diodes do.
I resisted this for several years. It, at first, made little or no sense to me as well.
My Vendetta Research power supplies went through many small, but important changes over 20 years. First, I just used pre-packaged +/-15V supplies, just like Mark Levinson did even 10 years earlier.
Then, about 20 years ago, I started to build my own.
First, I used a single full wave rectifier bridge, and a few thousand microfarads load capacitance.
Then, about 5 years later, I tried dual bridges, just for a special project. We found that we could hear the difference between this new power supply and the standard power supply that we replaced. In fact, J Gordon Holt, ordered one from me. Then I was reminded by my associate that I was only using a single bridge, so we changed, and this became our 'A' modification. I found that it was easier to remove any potential ground loops with these new power supplies.
Then, a friend of mine tried high speed diodes, and published his results in 'The Audio Amateur' in the early '90's. Well, I had stopped making Vendetta products, but I had a friend (now business partner) Bob Crump, who replaced the dual diode bridges with discrete high speed devices. Guess what, it improved things, even though the power supply was remotely located. Apparently, stuff gets into the grounding. I then made some measurements with a current probe and was I surprised! Wow! The standard diodes spit out all kinds of stuff. Check it out.
Today, I use high isolation transformers, high speed, soft recovery diodes, with dual bridge supplies in any preamp that I make, as well as quality caps, common mode chokes and triple active filtering.
Remember, we are trying to make the equivalent of a battery, without the problems and cost of using batteries. This is not that easy.
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Old 22nd February 2004, 12:27 AM   #20
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Default Power supplies in the raw.

We had a trhead a while back about High Speed Diodes. The soft recovery characteristic are more important than the switching speed.


http://www.diyaudio.com/forums/showt...277#post146277
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