FWIW, I run one LM4780 off of two MUR860 bridges in series. That means about 16A continuous (?)
Without a heatsink, the diodes get mighty warm at higher levels. In contrast, a KPBC composite bridge stays very cool when driving a similar load, but that is rated to >2x the current capacity of the MUR860 bridges, and I can't detect any sonic degradation with it in place.
I would advise staying within a 50-60% average output current range, so you have sufficient headroom for output peaks. So if you're looking at a 16A bridge, the output current should be about 8A, which is about the current limit of a single 3886. You could push it to 2, if you mostly play at low levels or have efficient speakers. This is what the LM4780 kits for PD do (the ones I have, and a 4780 is ~ 2x3886 in a single capsule) and that's why they run hot at high levels.
Capacitance on the rails has an effect on this also, higher capacitance may allow you to get a little more juice out of the system even if the rectifiers aren't fully up to the task. This is not recommended as a cure, it's only first aid.
Without a heatsink, the diodes get mighty warm at higher levels. In contrast, a KPBC composite bridge stays very cool when driving a similar load, but that is rated to >2x the current capacity of the MUR860 bridges, and I can't detect any sonic degradation with it in place.
I would advise staying within a 50-60% average output current range, so you have sufficient headroom for output peaks. So if you're looking at a 16A bridge, the output current should be about 8A, which is about the current limit of a single 3886. You could push it to 2, if you mostly play at low levels or have efficient speakers. This is what the LM4780 kits for PD do (the ones I have, and a 4780 is ~ 2x3886 in a single capsule) and that's why they run hot at high levels.
Capacitance on the rails has an effect on this also, higher capacitance may allow you to get a little more juice out of the system even if the rectifiers aren't fully up to the task. This is not recommended as a cure, it's only first aid.
Two rectifier bridges in series do not provide double the current. That would be the case for parallel bridges.
Two rectifier bridges in series provide double the voltage rating at the cost of double the voltage drop.
MUR860 should be refitted with heatsinks for 8 A. If you look at the datasheet, they can only stand 3 A without heatsink and on the condition of 25 °C ambient temperature. 8 A means something around 10 W power dissipation.
You get away without a heatsink, because 8 A is the maximum DC current, that gives 5,6 A sinusoid AC peak, and because the average current of a music signal is much lower.
Two rectifier bridges in series provide double the voltage rating at the cost of double the voltage drop.
MUR860 should be refitted with heatsinks for 8 A. If you look at the datasheet, they can only stand 3 A without heatsink and on the condition of 25 °C ambient temperature. 8 A means something around 10 W power dissipation.
You get away without a heatsink, because 8 A is the maximum DC current, that gives 5,6 A sinusoid AC peak, and because the average current of a music signal is much lower.
My understanding was that each bridge has 2X the continuous current capacity of a single diode used in it, at least theoretically, as at any point in time on an AC cycle two diodes are conducting.
I do know that connecting the bridges in series would result in the same current capacity as a single bridge, maybe I should have been a little clearer in my text.
The OP is planning to run two LM4780 amps off a single rectifier. I think that's pushing it, it's about two times the load I'm running (and worrying about the heat), and the current requirements seem really steep on a single MUR860 bridge. That was really my point, inspite of post number 2 which indicates that he has no problems with something like that.
I do know that connecting the bridges in series would result in the same current capacity as a single bridge, maybe I should have been a little clearer in my text.
The OP is planning to run two LM4780 amps off a single rectifier. I think that's pushing it, it's about two times the load I'm running (and worrying about the heat), and the current requirements seem really steep on a single MUR860 bridge. That was really my point, inspite of post number 2 which indicates that he has no problems with something like that.
depends a lot how hard you push your amplifiers
to be safe you should count for worst case scenario (hypothetic, theoretical)
Say you have bridge that can deliver 16 A average, continuously.
Say you have 4 amplifiers that are limited to max 8 A output current.
For worst case situation, all four running at max, would need 32 A.
You need at last two rectifier bridges. Each one feeding two amplifiers.
Now to play with margin, I would not use one rectifier more than upto 50% of max rated current.
This means, that for delivering 32 A, I would rectifier for 64 A.
And so I would use four (4) bridges, each rated max 16 A
to feed each one of the four (4) ampifiers. (max output 8 A)
The probability that worst case will ever happen is very small.
But rectifiers are comparably cheap stuff (if you make them yourself using 4 diodes).
So is no problem .. for me.
to be safe you should count for worst case scenario (hypothetic, theoretical)
Say you have bridge that can deliver 16 A average, continuously.
Say you have 4 amplifiers that are limited to max 8 A output current.
For worst case situation, all four running at max, would need 32 A.
You need at last two rectifier bridges. Each one feeding two amplifiers.
Now to play with margin, I would not use one rectifier more than upto 50% of max rated current.
This means, that for delivering 32 A, I would rectifier for 64 A.
And so I would use four (4) bridges, each rated max 16 A
to feed each one of the four (4) ampifiers. (max output 8 A)
The probability that worst case will ever happen is very small.
But rectifiers are comparably cheap stuff (if you make them yourself using 4 diodes).
So is no problem .. for me.
In my research, I haven't been able to come up with any explanations involving two bridge rectifiers, one for positive and one for negative. Is this a series arrangement? I would expect for audio this would (almost) double the current capability, as only the positive or negative would be used depending on the polarity of the audio signal at any given moment.
However, I discovered that with a bridge rectifier, each diode must be rated for the full current. Despite two diodes conducting at any given time, they are in series and both pass the full current.
However, I discovered that with a bridge rectifier, each diode must be rated for the full current. Despite two diodes conducting at any given time, they are in series and both pass the full current.
Redshift187 said:
It is not a series arrangement. It is more like two separate circuits, where the positive rail of one is connected to the negative rail of the other. The use of a single rectifier for a split power supply is kind of an economic version.
The advantages of separate rectifiers for positive and negative rails are less losses in the transformer, better power distribution across the secondary windings, better regulation and better suppression of noise (hum) during unbalanced load situations. Disadvantages are higher component count = higher price and double the voltage drop. And of course two separate bridges only make sense with double secondaries, not with center tapped transformers.
Redshift187 said:
Yes to both.
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