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Old 2nd May 2011, 04:13 PM   #641
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And I am still convinced that a simple fuse on the secondary side is easier and cheaper than a soft-start circuit.
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Old 2nd May 2011, 05:23 PM   #642
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soft start for the mains fuse and primary circuit have nothing to do with secondary fusing.

The secondary fusing is to stop anything downstream catching fire or setting the house on fire.

The only thing in common with the primary fusing is that it too is to prevent anything downstream of the primary fuse from catching fire or setting the house on fire.
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Old 3rd May 2011, 06:06 PM   #643
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The soft-start allows you to size the primary fuse in a way that it can protect the transformer against shorts and overload. Without the soft-start, the primary fuse must be rated higher, so that is does not blow from the transformer's inrush current. In that case the primary fuse can only provide short-circuit protection and you need a fuse on the secondary side to provide the overload protection.
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Old 31st May 2011, 10:02 PM   #644
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How do you go about choosing a transformer power rating wise? If I'm using an LM4780, which is rated at 60W*2, I suppose 120VA would be nowhere near enough if you factor in inefficiency?
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Old 2nd June 2011, 11:21 PM   #645
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120VA is about the minimum that can work adequately for two channels producing a total of 120W.
240VA is about the maximum that would be used.
Outside that range, you either reduce performance significantly or increase the cost significantly for a small performance gain.
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Old 2nd June 2011, 11:54 PM   #646
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Quote:
Originally Posted by Ben B View Post
How do you go about choosing a transformer power rating wise? If I'm using an LM4780, which is rated at 60W*2, I suppose 120VA would be nowhere near enough if you factor in inefficiency?
It depends on what you are trying to build. If it's an expensive, *nice* looking amp, you wouldn't want to skimp on the transformer, but if you just want something to drive a couple bookshelf speakers at moderate levels with a small chassis and heatsink at low cost, you could use something pretty small like 48VA transformer, by simply accepting it won't be getting the maximum potential from the chips. I like a bit of margin anyway, from components whether it be an IC or a diode, resistor, capacitor, whatever...
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Old 3rd June 2011, 09:07 AM   #647
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Determine the transformer rating according to the actual amplifier output power, not according to datasheet's best case assumption.
To achieve 2x60 W into 8 Ohm with an LM4780 you need a power supply that is stable enough to deliver 35 V at full blast and enormous heatsinks. You also need speakers that don't demand more current than an 8 Ohm resistor anywhere in the amplifier's working frequency range.

If you factor in efficiency and losses, the transformer will need a power rating of around three times the amplifier's output power. In real life your amplifier will not run at continuous maximum output power and the smoothing caps buffer peak power demands. The range specified by Andrew is reasonable. You can even use less than that, e.g. Douglas Self stated that 70 % of the amplifier's output power is a sufficient power rating for the transformer. Another example for a smaller transformer is Siegfried Linkwitz's Pluto speaker. One LM3886 feeds the 8 Ohm tweeter, two in BTL configuration feed the 8 Ohm woofer from 30 V. In theory that means over 200 W of output power, but it only has a single 50 VA tranformer.
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Old 3rd June 2011, 04:08 PM   #648
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Quote:
Originally Posted by AndrewT View Post
120VA is about the minimum that can work adequately for two channels producing a total of 120W.
240VA is about the maximum that would be used.
Outside that range, you either reduce performance significantly or increase the cost significantly for a small performance gain.
What he said ... Always spot on advice from an experienced, well qualified DIY builder.

It might be noted that using the minimum sized (120 VA) xformer may add a bit of heat to your box.

My advise would be to error on the higher side, a bit more VA will usually generate less heat ... up to the "overkill" maximum (240 VA.).
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Old 3rd June 2011, 06:42 PM   #649
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That depends on the actual output power. At nominal output the copper losses tip the scales in favour of the bigger transformer. At usual home listening levels the core losses give the smaller transformer the edge.
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Old 3rd June 2011, 08:21 PM   #650
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Quote:
Originally Posted by pacificblue View Post
That depends on the actual output power. At nominal output the copper losses tip the scales in favour of the bigger transformer. At usual home listening levels the core losses give the smaller transformer the edge.
OK ... all granted, witnessed and understood.
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