I am testing a transformer for use with a LM3886 chip. It has a centre tap and is putting out 27.3-0-27.3 ac. I built a single bridge rectifier and measure +26Vdc and -25.34Vdc after the bridge. Is ok to have a voltage difference on the dc rails. I have not started with the rest of the power supply yet. I apologise in advance if this has been answered already in other posts.
The absolute max voltage rating given in the specification/datasheet is 84Vdc when signal is present and 94Vdc, when no signal is present.
+-38Vdc equals 76Vdc and is well below the 84Vdc limit.
But only use 8ohms or higher impedance speakers when voltages are this high.
Q.
How high will the supply voltage go when mains voltage rises to maximum tolerance?
+-38Vdc equals 76Vdc and is well below the 84Vdc limit.
But only use 8ohms or higher impedance speakers when voltages are this high.
Q.
How high will the supply voltage go when mains voltage rises to maximum tolerance?
Hello Andrew, thank you for your response. I will be using my 8ohm speakers with this amp (assuming I come right that is) I was hoping to use the transformer as it is a Frost Magnetics designed for audio use. I am however concerned about my ac voltage supply as it is 238V most of the time. I also ordered another toroidal tranny with dual 24v windings so I will have to compare them when I get the parts to build the power supply. I am also a bit confused as to which type of power supply to build. There seem to be so many options.
Go to the TNT website and/or the ESP website.
Both have comprehensive guidance on building simple transformer/rectifier/smoothing capacitor power supplies.
Solid State Power Amplifier Supply Part 1
DIY Audio Articles
go down to section "Power Supply Design".
Both have comprehensive guidance on building simple transformer/rectifier/smoothing capacitor power supplies.
Solid State Power Amplifier Supply Part 1
DIY Audio Articles
go down to section "Power Supply Design".
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you measured
this was probably a 25Vac transformer which when fed with 238Vac instead of 230Vac would give 25.9Vac when fully loaded.
When open circuit the voltage rises by the regulation for that transfomer.
Assuming it was 7%, then the 25.9 becomes {1+0.07}*25.9Vac = 27.7Vac
The UK has a normal maximum of 253Vac. Feeding this into a 7% regulation 230:0-25, 0-25Vac transformer will give an open circuit voltage of 29.4Vac
rectify and smooth and the maximum DC voltage will be ~40,9Vdc
This is without any load. The voltage will drop a bit when a chipamp is added as a load.
This virtually guarantees that a 230:dual 25Vac transformer will not take the LM3886 beyond the 84Vdc limit.
And is safe for 8ohms driving during normal domestic duty, not PA and not drunken party nights.
I suspect you measured the open circuit voltage with an unknown mains voltage fed to the primary.
this was probably a 25Vac transformer which when fed with 238Vac instead of 230Vac would give 25.9Vac when fully loaded.
When open circuit the voltage rises by the regulation for that transfomer.
Assuming it was 7%, then the 25.9 becomes {1+0.07}*25.9Vac = 27.7Vac
The UK has a normal maximum of 253Vac. Feeding this into a 7% regulation 230:0-25, 0-25Vac transformer will give an open circuit voltage of 29.4Vac
rectify and smooth and the maximum DC voltage will be ~40,9Vdc
This is without any load. The voltage will drop a bit when a chipamp is added as a load.
This virtually guarantees that a 230:dual 25Vac transformer will not take the LM3886 beyond the 84Vdc limit.
And is safe for 8ohms driving during normal domestic duty, not PA and not drunken party nights.
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I will try that route then, as I have already made up two small boards with just the rectifiers and two terminal blocks on them. I was going to try Mick Feuerbachers minimalist point to point design but I like the look of the small APEX boards posted on this forum. I have already etched one board as an excercise, It really is small.
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