Bridged chipamp for 16Ohm speakers

[blue eagle]

Hi all, I'm just new to the forums here

I was considering building an amp for a nice set of speakers which I already have. A chip based amp seems like the easiest way but my speakers are 16 Ohm which appears to make things a bit more difficult. A common chip people here seem to use is the LM3875 which has a brilliant datasheet, unfortunately it contains no information about 16Ohm applications.

I read about bridging two chips together and wikipedia says an amp rated '100W into 4Ohms will appear as 200W into 8Ohms" so in the case of the LM3875 56W 8R -> 112W 16R which would be great. But have you seen this done before? I couldn't find much about how to do it. I'd like to take as much guess work out of it as possible and be sure about what i'm doing by keeping things reasonably simple, though I am good with electronics in general, just don't know much about audio.

Perhaps I need a chip that already supports bridging (like the ST TDA7296) or a different chip which is already suited to 16Ohm loads?

Also, how much might a project like this cost? i'll need L/R amps rated to about 80W per channel, but not sure about how much 'quality' i want.

I'll stop there now! It is already an essay but i'm sure i'll come up with more questions, cheers guys

[stratus46]

16 ohms is easier than 8 because it takes less current. So bridge your amp and whatever the claimed power into 8 ohms is, divide it by 2 (because you have 16 ohm loads) and you'll have your answer and the amp will run cooler than with 8 ohms. Don't fret over this.

G²

[DigitalJunkie]

And you can use higher rail voltage than a bridged amp with an 8ohm load.

[blue eagle]

Originally Posted by stratus46

16 ohms is easier than 8 because it takes less current. So bridge your amp and whatever the claimed power into 8 ohms is, divide it by 2 (because you have 16 ohm loads) and you'll have your answer and the amp will run cooler than with 8 ohms. Don't fret over this.

G²

So the new power rating would be 56x2 (because of the two amps) divided by 2 (because of 16Ohms) bringing me back to a maximum power of 56W per channel?

Oh and I found a bridged example here. Are the LM38xx the chips of choice then? what about the ST TDA range?

[blue eagle]

Oh yea and as for power supplies, is there any preference between + - 25V and just +25V supplies? (single supply gives a more complicated circuit but would remove the need for two supplies) And is it ok to get -v by connecting the ground and +v rails together on the two supplies.

[stratus46]

Originally Posted by blue eagle

So the new power rating would be 56x2 (because of the two amps) divided by 2 (because of 16Ohms) bringing me back to a maximum power of 56W per channel?

No. Bridging the amp doubles the Voltage and current into the load so the power goes up by 4 and your power goes to 220 Watts into 8 ohms. Changing to 16 ohms will drop you back to 110 Watts. Unless the amplifier channels are rated for 2 (two) ohm use (few are) do NOT run a bridge amp with 4 ohm loads. It lets the smoke out.

G²

[abraxalito]

For a bridged amp into a 16R load, a single LM1876 makes a lot of sense - if 40W is enough for you?

[picowallspeaker]

There are CT ( center tap ) trasformers for this duty . Or just double the trasformers ( same exact type ... to be precise ) to get the specified split supply voltage .
What's wrong with dual/split supply ? there's no need of an output capacitor ... you can put one in the circuit ,either ,if you want to fell safe that any DC current would not pass to the speakers ,burning the coil , then remove it after you're sure the amplifier works .

Oh yea and as for power supplies, is there any preference between + - 25V and just +25V supplies? (single supply gives a more complicated circuit but would remove the need for two supplies) And is it ok to get -v by connecting the ground and +v rails together on the two supplies.

[stratus46]

Originally Posted by DigitalJunkie

And you can use higher rail voltage than a bridged amp with an 8ohm load.

I don't think enough to justify the added stress from bridging. Just run it where National recommends. If you want a bigger amp, buy a bigger amp.

G²

[blue eagle]

Hmm... my last post didn't go through for some reason, I try and say more or less the same thing again.

Originally Posted by abraxalito

For a bridged amp into a 16R load, a single LM1876 makes a lot of sense - if 40W is enough for you?

Problem is I dont really know. I have triple drive bookshelf speakers from a hifi on which one of the channels has died. They don't have any rating on them apart from the impedance so I've been looking at smaller dual driver speakers which are rated for 40W and can only assume mine will take more. I thought I could just be careful to start a new amp at a low level and work up, then I could possibly fix the volume control in hardware so that the max is whatever I determine it to be.

I thought it would be wise to have a bit of headroom in the amp, rather than blasting the amp to get what I remember as maximum volume. So 110W from two bridged LM3875s would be good. One simple question, does the ground from the line in signal go to the ground of the psu, because these might not be the same? Diagrams I see only label '+Vin'.

Two other areas I need to look at are volume control and power supply, but I think the design should follow the order amp->psu->volume.

I've attached a diagram for bridged LM3875s and was wondering if this is a good starting point. The power is labeled 80W @ 8Ohms which contradicts the 220W we decided it should be here? Probably going to be a faff finding or modifying pcbs for this...

And finally... are there types of components I should be ordering to maintain as much of the quality this amp is capable of? I've seen carbon resistors talked about and capacitor brands might be important.