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.
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.
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.
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...
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.
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.
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.).
NO !
the Safety Earth wire must be connected permanently to the chassis. Nothing else is allowed to interfere with this permanent mechanical fixing.
The Centre Tap is connected to the PSU Zero Volts.
zero volts
Ok understood for the safety earth wire.
I've just read Elliot's guide on hard wiring the PSU.
It seems to me that the zero volts is also connected to the chassis on a separate point than the earth point.
thanks for your advice. I'm using a PSU on a PCB (from a kit) and a EI transformer. So all the arrangement described are often different to mine and I have to transpose as best as I can.
Ok understood for the safety earth wire.
I've just read Elliot's guide on hard wiring the PSU.
It seems to me that the zero volts is also connected to the chassis on a separate point than the earth point.
thanks for your advice. I'm using a PSU on a PCB (from a kit) and a EI transformer. So all the arrangement described are often different to mine and I have to transpose as best as I can.
The forum has a good article on grounding. This section applies to you question:
http://www.diyaudio.com/forums/diya...ponent-grounding-interconnection.html?garpg=5
"Basic" PSU -vs- switched
A lot of information on this thread - so I may be mixed up on what I am about to ask.
I am in the process of building a PSU for a stereo LM3875 kit (Peter Daniel's). I originally planned on having only one supply power both amps. But I bought a 160VA transformer, as that was the highest VA I could get a hold of with 22-0-22 secondaries, which appears to be slightly under rated for both amps at max power. So I will buy a second 160VA transformer and more MUR860 for dual mono. I plan on powering an Alpair 12.2 full-range driver on each channel. This load is rated at 7 ohms.
The question I have is this - and it has been asked before - and not answered: Is there any sonic benefit to building a regulated switched PSU for this application? I gather from some posts there is no benefit, just more work. But surely there is some kind of "diference". Or is this another one of those highly contriversal topics like the whole snubberized -vs- not bit!?! Feel free to direct me to other threads.
Thanks in advance
Allen
A lot of information on this thread - so I may be mixed up on what I am about to ask.
I am in the process of building a PSU for a stereo LM3875 kit (Peter Daniel's). I originally planned on having only one supply power both amps. But I bought a 160VA transformer, as that was the highest VA I could get a hold of with 22-0-22 secondaries, which appears to be slightly under rated for both amps at max power. So I will buy a second 160VA transformer and more MUR860 for dual mono. I plan on powering an Alpair 12.2 full-range driver on each channel. This load is rated at 7 ohms.
The question I have is this - and it has been asked before - and not answered: Is there any sonic benefit to building a regulated switched PSU for this application? I gather from some posts there is no benefit, just more work. But surely there is some kind of "diference". Or is this another one of those highly contriversal topics like the whole snubberized -vs- not bit!?! Feel free to direct me to other threads.
Thanks in advance
Allen
What does the term "single supply" mean?
Excuse my lack of knowledge, but what does the term "single supply" mean. I'm working on a gainclone and I intend to power it using only one power supply supplying B+ to both channels. Is this known as a "single supply"? If so, what do I need to do to ensure success?
Thanks in advance,
Nicholas
Excuse my lack of knowledge, but what does the term "single supply" mean. I'm working on a gainclone and I intend to power it using only one power supply supplying B+ to both channels. Is this known as a "single supply"? If so, what do I need to do to ensure success?
Thanks in advance,
Nicholas
Yes, a single supply needs two rails, one positive and one negative, where the negative is usually at the same time the ground. A split supply has three rails, positive, negative and ground, where the ground potential is in between positive and negative.
The basic differences are usually that you
- need a virtual ground between the positive rail and the real ground, to which the input signal is referenced. That means usually three more resistors and two more capacitors. An additional transistor or even op amp is sometimes used to improve the stability of that virtual ground.
- need a (big) DC blocking capacitor between the amp output and the speaker. The virtual ground is present at the amp output and the resulting DC signal at half the rail voltage could destroy the connected speakers without that capacitor.