Speccing power transformer for several speaker loads

[privateer]

Hi there. I just wanted to check the results of my head-scratching session with some of you guys (I'm a solid-state newbie).

I want my amp (LM3875 kit) to be able to drive different loads, depending on the cab I hook up: 8 or 16 ohms. If I run through the P=V*I (etc) equations, I find that 56W into 16 ohms is 29V at 1.9A and the same into 8 ohms is 21V into 2.6A (in theory, though I'm not sure if the ChipAmp would struggle to deliver all 56W into 16 ohms?).

So if I want the flexibility of being able to use both loads, I guess that I need a power transformer that can deliver at least 2.6A at 29V? (80VA-ish - per channel)

If so, do I need to over-rate these two factors to add a safety margin?

Any other stuff I need to watch out for?

TIA

[mwmkravchenko]

Check out the attachment and it will help.

Mark

[privateer]

Thanks Mark. That's a cool tool - someone put a lot of work in there.

Trying an 8-ohm load and 30V predicts 42W (which would be acceptable to me).

But the spreadsheet's built-in checks don't allow for 16ohm load calculation of the Vpeak, Ipeak and Output power. And it's password-protected, so I can't reverse engineer that maths (which I'm not sure enough of to work out for myself!).

What do I do next?

[mwmkravchenko]

A 16 ohm load is limited by the voltage the amp chip can handle. It doesn't particularily like it. If you look over to the bridged section you will find that you can indeed use this chip to power a 16 ohm load in a two chip or bridged configuration. The bridge allows double the voltage swing and more power into a 16 ohm load.

Mark

[privateer]

I see. So a 16ohm load driven by an unbridged LM3875 GC is limited in output power to a lot less than 56W... hmm - time to do some reading on bridging GCs.

I really appreciate your help, Mark

[yldouright]

privateer
The 3886 really prefers to see a lower ohm load because of its current delivery optimization. An 1875, 3875 or a 4780 might be better suited at those higher rails but I would look at some of the other manufacturers op-amps too.

[mwmkravchenko]

WHen you go higher up in impedance it is not the current that is starved but the voltage. All the chips from NAtional have a similar maximum voltage. THere are chips from ST Microelectronics. THe 100 volt 100 watt amps that are optimised for the higher impedance loads. They have a good app note on them with their own pcb to boot!

The other option is to bridge as has allready been noted. Ther are plenty of PCB designs floating around the forum. SOme of the members are even selling quality de3signs at good prices. BrianGT is one and jackinnj ( sorry jack can't remember your moniker ) Any way check them out. If you ned help you know where I'll be.

Mark