The DBRB | dual bridge rectifier board

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rjm

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I did up a little 50x80 mm board (40x70 mm M3 mounting holes) to conveniently hold two bridge rectifier's worth of BYV27 diodes to use with low power audio circuits.

I call it the DBRB, because it's cute. Not interesting or exciting, just cute.
 

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rjm

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Joined 2004
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I know. The choice was deliberate.

The extra four diodes (Mouser | 78-BYV27-100) will set you back all of about about $2.

In return, four benefits.

1. Each diode sees just half the applied voltage.
2. 60 Hz hum from any winding imbalance is eliminated. Since 60 Hz is harder to filter than 120 Hz, this is not insignificant.
3. The output can be alternatively configured as two positive rails, or two negative rails, as well as split rails.
4. It is not necessary to correctly identify the phase of the secondary windings when making the connection to the rectifier.

The advantage of the dual bridge connection resultant from point 4 is game-changing... just, um, trust me on this.

**

P.S. I recently contacted Triad Magnetics to get confirmation that the configuration did not go against the warning not to connect the secondaries independently, and was told it was not an issue here.
 
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I know. The choice was deliberate.

The extra four diodes (Mouser | 78-BYV27-100) will set you back all of about about $2.

In return, four benefits.

1. Each diode sees just half the applied voltage.
2. 60 Hz hum from any winding imbalance is eliminated. Since 60 Hz is harder to filter than 120 Hz, this is not insignificant.
3. The output can be alternatively configured as two positive rails, or two negative rails, as well as split rails.
4. It is not necessary to correctly identify the phase of the secondary windings when making the connection to the rectifier.

The advantage of the dual bridge connection resultant from point 4 is game-changing... just, um, trust me on this.

**

P.S. I recently contacted Triad Magnetics to get confirmation that the configuration did not go against the warning not to connect the secondaries independently, and was told it was not an issue here.
3. & 4. above are valid and I agree they are good reasons for adopting the dual secondary/dual bridge rectifier/dual isolated outputs.

1. & 2. are irrelevant and do not affect the outcomes.
 

rjm

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Broadly agreed. 1. is of no concern except at high voltage, while 2. ... well back in the day I did have a measurable 60 Hz hum in a phono stage output I could remove changing to the double bridge. I wouldn't to admit to it being an audible change though. In a power amplifier maybe...
 
4. It is not necessary to correctly identify the phase of the secondary windings when making the connection to the rectifier.


You actually need to be very careful with these connections when doing a dual supply shared transformer amp. If the secondary of the transformers aren't connected the same way on both supplies, there will be a measurable voltage difference between the supply grounds. This will show up on your input grounds and can cause hum issues when connected to a source that has a common input ground connection due to the voltage differences.
 
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I'm still trying to figure out the exact reason for this but I think it has to do with the charging pulse timing. The ground reference will move slightly as each rail charges. If these charges are happening in opposing directions in the supplies the ground will move in opposite directions. I've been helping a fellow building a VHex amplifier and we saw this in his build, and the result was quite dramatic. I haven't had a chance to try to recreate this effect myself yet.
 

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@jwilhelm

How is it possible to connect the secondaries "in the same way" when there is no physical distinction between the two "~" terminals of the bridge they connect to?

I still need to recreate this on my own test bench. I've worked through this in my head several times and figured out why it was happening, then I review my findings and can't figure it out again, so I need to study this properly with test equipment to verify my findings. The way we build our supplies, the ground is floating, so the only way the grounds of both supplies can reference each other is through our earth ground safety loop breaker or audio input ground. This can allow for quite a discrepancy between the two supply grounds. Any discrepancy will show up as current flow through the audio ground connections of out RCA cabling through a common audio ground source.

In the application where we were seeing this, the bridge rectifiers are built onto the power supply boards and connected through a common ground plane. Two power supplies share a common main supply transformer. Possibly the build configuration is aggravating the situation. In my own builds my wiring OCD caused me to connect everything symmetrically so I never witnessed this first hand. Voltage discrepancy between my two supply grounds is always zero volts. In the case where we were seeing the large voltage discrepancy, the builder had just plugged the AC windings in without paying attention to which way the windings were connected, and saw a large amount of ripple between the two supply grounds. He cured the issue by making the exact same wire connections from the transformer to the bridge rectifiers.
 

rjm

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Perhaps the primaries of the two transformers are connected in opposite phase? Depending on the internal construction, the leakage can be worse when the AC neutral is connected to one end of the primary or the other. I'm not super-expert on transformer design, but if one transformer was connected one way, and the other transformer the other, it could cause more leakage current flow along the common ground path linking them through the signal wiring since the potential differences would likely be greater.

As I suggested earlier, there is only one way to wire the secondaries, so I'm pretty sure you can rule that side out.
 
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