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In Reply to: RE: Yes, they have a higher forward voltage drop. posted by Al Sekela on March 15, 2009 at 15:50:34
thanks guys,
i am using the DSEE type and pretty sure i've got them wired correctly to form a brige rectifier .
And as u guys have mentioned the measured DC across the main caps does seem to be a little low then before. The schematics says +/-49VDC but measured voltage in stock condition was usually around 51V . But now I'm gettin something around 48.5V . Is it the nature of this HyperDyns to have higher loss in the diodes ?
Al , is the less surge capacity a worry in my case and regarding failure in the rectifier , if the fuse blew , that would kindda isolate the preamp/amp stage from further damage wouldnt it ?
Follow Ups:
When you throw the power switch on your amp (to turn it on from being off for a long time), the power transformer and rectifier bridge see the filter caps as a dead short. The peak current that flows into them depends on exactly when in the AC power wave the switch closes, and is limited only by the series resistance in the transformer, rectifier bridge, and associated wiring.
The caps charge up rapidly and the current drops to the normal idle current once the caps reach full charge. The turn-on pulse is what applies over-current stress to the rectifiers. It is difficult to predict what level this stress reaches. Big amps, with big aggregate filter capacity and large transformers, may generate very high surge current levels upon start-up. Some amps include "soft-start" provisions that limit the surge current at start-up, then get out of the way for normal operation.
Conventional silicon junction rectifiers have large surge capacities, but the refined fast-recovery devices have little or no surge capacity beyond the rated current. Improving the heat sinks will not do much to increase their surge capacity, as it is controlled by the internal resistances of the active regions in the diodes. Improving the heat sinks will allow the diodes to run cooler when the amp demands a lot of current during operation.
Is your amp a big one, with a heavy power transformer? Does it have a soft-start feature, or do the room lights dim briefly when you turn it on? If it seems to have a large surge at start-up, I would seriously think about a soft-start circuit to protect the rectifiers.
Hi Al,
thanks for the lengthy explanation. The amp is a rotel 60watter integrated and the transformer is just a decent toroidal, definitely not the huge ones and never had light dimming issue before.
The stock rectifier had a surge rating of 125A whereas the new Dyn that i swapped in had a surge rating of 200A. SO this part should be fine shouldnt it ?
As u've mentioned earlier the Dyn does seem to have a higher forward voltage as the overall DC on each main cap drops around 2V and the datasheet confirms this. ( DYN has max forward of 2.5v vs 1.1v of the stock rectifier ).
The increased forward voltage is a consequence of how the rectifiers are made. They have to minimize the electrical charge stored in the junctions during forward conduction, and it takes more voltage drop across the minimized regions to push the current through.
Don't push the amp to the point where you hear some distortion. The voltage margin has been reduced by the lower rail voltage, and you don't want to harm your speakers by driving the amp into clipping.
Al, strangely , when i first heard the distortion , the amp was in an idle condition with vol all the way down ? That was the first i noticed it and it became louder as i increased the volume and even any movement on the volume pot cuased it to distort a bit.
And all these was after a while of playing music.
If you get about normal rail voltages with your new rectifiers, and can check that the rail voltages stay the same when you get the distortion, then it is not the rectifiers but something else. It may be collateral damage from your rectifier replacement project.
hi Al, i tried replacing it with the stock 4 pin bridge, and there're no distortion at all now. it's looking more and more like heat sinking issue on the DYNs to me... what u reckon ?
something related to the wiring of the fancy rectifiers.
i'm sure i got it rite the first time as i spend considerable amount of time actualy sketcing the connections just to avoid any mistakes...but i think i'll spend more time on it tomm just to make sure....
basiclly , i had pin 1( of DYN 1 ) soldered to pin 1 ( DYN 2 ) , pin 3 soldered to pin 3. These were then the +/- out. For +AC supply , it was pin 2 of DYN 1 and -AC , pin 2 of DYN 2. I had these drawn up correctly and compared it against the stock 4 pin case and it was identical and i worked fine when i turned it on. But i could be missing something somewhere....
I don't know about the IXYS Isoplus-220 package. If it is all plastic then you are OK. If there is a metal plate on the back, the metal may be connected to one of the pins inside the package. This (or the heat sink) has to be insulated from any other metal parts.
It really depends on the robustness of the design, how fast the fuse is, (you haven't put in a slow-blow "audiophile" fuse?) the alighnment of the planets... etc.
Since solid state rectifiers tend to fail short. This will dump AC directly onto the main DC buss. Worst case this takes out the outputs and predrivers in the power amp stage and your filter caps blow shredded paper and aluminum soaked in sticky, stinky goo all over the inside of your amp.
Possibly other lower voltage sections could be effected depending on their regulators and over all power supply design.
this is just not good....i'll try using a bigger heatsink and see how it goes, if not i guess i've got to opt for something else....
maybe the IXYS FRED full bridge rectifier ( in the 4 pin square case ) ?
by da way, what is slow-blow fuse ? does it offer much better electrical isolation in event of failure... ?
A slow blow fuse is pretty much what it sounds like.
A fuse made to withstand a short peak of current higher than the rated blow amperage without blowing.
These are commonly used on motor starters and other devices with large start up currents.
I just mentioned it because the majority of "audiophile tuning fuses" I see reviewed on here are of the slow blow types.
Replacing a fast blow fuse with a slow blow is (IMHO) never a good idea.
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