Rectifying bridge.

Bear, Thanks for the info about the fast recovery type adding noise. What about Hexfred's vs Schottky's for an SS preamp or power amp ? Sean
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The diode bridge converts the AC voltage out of the transformer (after being converted to a lower voltage). The only ways it can offer improvement are:
1) little more reliable
2) Because it is operating at a lower percentage of its rated voltage, it may have lower reverse voltage leakage currents that may somehow improve the sound.
3) Being higher voltage may mean that it has less capacitance input to output which would help keep noise from passing thru.
That all I've got.

Sean, sounds like a digital RF amp ur talking about...maybe even a class E? Stil hard to see how 1% suply voltage change alters the output 18%... sounds like these are switching diodes, not rectifying power diodes.

Anyhow you want soft recovery diodes, like HexFreds, fast recovery and turn on will usually add to the noise...

sometimes changing the diodes will really help, sometimes not. Try it.

Sorry about that Bear. A "6A4" is a 6 amp 400 volt diode.

In the particular situation that i'm using these in, the design calls for a "one way valve" in series with the modulated rail voltage feeding an rf output section. These voltages run up somewhere between 25 - 35 volts with an average running at about 30 volts. Obviously, this is WAY above the turn-on threshold of the diode itself. Believe it or not, the difference between one of these diodes with a .4 and a .7 voltage drop is VERY visible in terms of total output and gain characteristics. That is, if you have the right test equipment and know what to look for.

In effect, the differences between the two diodes (.3 volts out of an average of 30 ) is equivalent to 1/100th of the total power supplied to the output section. If we were talking about an audio component, that 1% difference would be equivalent to the line voltage falling from 120 volts down to 118.8, which is negligible to say the least. That same 1% gain in rectification efficiency results in an increase at output of somewhere around 15% - 18% on modulated peaks. I know that this does not sound "logical", but the rest of the circuit is so far "tweaked" that ANY change is highly noticeable. I don't know if we have audio designs that are this far "tweaked out" or running on the ragged edge.

Having seen the difference that such a small change can make and knowing that diodes produce TONS of "hash" and switching distortion, i would think that this could make a difference in a bridge. Using hand selected diodes with a quick turn on rate and fast recovery characteristics could further clean up / reduce the amount of "grundge" fed into the rest of the component and increase efficiency. Not only would the diodes turn on faster and stay on easier ( reduced switching due to the lower turn-on threshold ), ringing should also be minimized due to their quick recovery. Adding a "snubber" would be the icing on the cake.

While i've never tried doing this, i may have the opportunity to do such a thing very shortly. I've to 7 identical preamps that i'm "tweaking", so this may be a great opportunity to try something like this. I'll have to make note to order the necessary parts.

Obviously, the benefits of such a modification would strictly be limited by how well the rest of the component was designed and how revealing the rest of the system was. As such, i'd love to hear your thoughts or those of Ralph, Bob Crump or anyone else that could throw a wrench into the gears i've got turning in my head : ) Sean
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Sean, dunno what a 6A4 is... a tube? But the turn on voltage of a diode in a bridge that is rectifying lots of volts is in essence meaningless. There are some applications where the forward bias voltage of a diode makes a difference, this isn't one of them, imho.

If you're using them to bias something, then it makes a difference, but then you have to ask what's the variation in the part you are trying to bias with the diodes? :- )

Bear, diodes from the same batch with the same part number can show measurably different "turn on" voltages. I have had some 6A4's ( use them at work all the time ) measure from anywhere between .4 up to slightly over .7 or so. That is a variance of 43% !!! How much of a difference this would make in a bridge, i don't know : ) Sean
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There is absoloutely no need to "match" diodes - they do not have gain like transistors. All you have to do is get 4 of the same part number. Of course, you have to wire them correctly too. :- )

As far as Hexfred bridges, there may be some, just check around the parts houses and IR's website.

Alternately, you can get very close to the same results by adding a "snubber" circuit around that bridge. Using a higher PIV bridge will only buy you some immunity from wierd transients. A higher AMPERAGE bridge will likely be slightly larger and run cooler. Doing both will up the margin against failure.

The main difference between low PIV and high PIV rectifiers is the way they came out of the wafer when they were made. So in theory the higher PIV ones of a given type designation are "better." In practice, no difference if they are properly rated for the applicaton.

Thanks for all the reponces.
The upgrade I ment is to bring another bridge assembled as whole without getting into changing diodes(a real pain to match).
The Albert's idea seems to be great if I can acquire the assembled hexfred rectifier.

Clueless link appears to be a photograph of an installed hexfred bridge.

Below is the IR technical sheet at their web site. (Note, Adobe Acrobat is needed.)

http://www.irf.com/product-info/datasheets/data/hfa06pb120.pdf

I have installed these in a number of audio components with great success. These devices are less tolerant of turn on transients and therefore should be rated approximately 20% over the original specification.

A picture of the hexford upgrade in an atmosphere. Cute ain't they?

Mention a bit more Albert. Enlighten us.

No one has mentioned Hexfred rectifiers in place of the standard device. This is a mod I did to my Atmasphere and the results were very worthwhile.

No problem Herman. Feel free to kick me in the seat of the pants ( only as needed though ) : ) Sean
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Ok, my point was that the bridge itself does not determine the amplitude of the DC voltage so they are not rated in this respect. Diodes do have a voltage rating, the PRV (Peak Reverse Voltage). This is the maximum that they can handle when reversed biased. An important rating for sure when designing a power supply. But it does not affect the output so I didn't bring it up.

Thanks for keeping me on my toes Sean.

Going from a half-wave to full-wave rectifier you will get a better wave form. Less ripple. You still only get half the voltage of the secondary winding. The negative part of the AC current is just blocked by the diodes and not used. You use a center tap on the T. This is a waste and means you generally need to wind the transformer for twice the voltage you need. More copper more core materials more inductance scattering around.

Going to a bridge full-wave you will get more efficiency. 2 Pairs of diodes alternate into and out of conduction. This uses the neg current too. Because you have pairs of diodes working together you get a voltage equal to the transformers peek to peek voltage.= effeciency

You don't want to just make this change alone as an upgrade if that's what you meant. It would through your voltage/current off right down the line. Sometimes you can swap out the caps used for filteres following the rectifier, with good results, as a stand alone modification.

Too much to say in to little space maybe.

For a little theoretical background on-line lookhere

If someone has a better site on-line please let me know.

Cheers,

I agree with everything that Herman had to say other than bridges ( which are four diodes in a specific arrangement ) ARE rated for various voltage and current levels. Like anything else, one should always allow a reasonable amount of headroom if you want the parts to last.

I would think that you would gain more from going to a fast recovery type diode than you would by staying with the same type and simply increasing the voltage rating. If you are going to poke around in there, BE CAREFUL. Sean
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The peak DC output voltage of a bridge rectifier is proportional to the amplitude of the AC voltage it is fed. The AC voltage is determined by the winding ratio of the power transformer. To increase the DC voltage would require replacing the power transformer.

The DC voltage is equal to the peak of the AC voltage minus about .7 volts that is dropped across the rectifying diode. The peak of the AC voltage is the RMS value divided by .707, which is about 15% more than the RMS as TWL stated.

Unless you really, really, really know what you are doing, it would most likely result in a lot of smoke. With all due respect, by the sound of your question I don't think it is something you would want to try without doing a lot more studying about electronic theory.

Bridge rectifiers are rated for the amount of current they can handle, not the voltage.

...confounded bridge...

While I am not an expert in this field, I don't know of any advantage to such a modification, unless you plan on increasing the rail voltages for some reason. Perhaps someone else can help you that knows more about it than I do. Generally, a diode bridge will yield about a 15% higher DC voltage out, than the AC voltage in. A fully regulated circuit in typically preferred over a rectifier. That is about all the theory I know on the subject. I am assuming you are talking about its use in some kind of amplifier.