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Old 14th March 2005, 01:57 AM   #1
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Default OPA549 x 8, bridge/parallel

I am planning on building this PCB I've designed. It will use 8 opa549, 4 in parallel for each channel. It can be used as a stereo amp, or, another pcb I am going to make will allow bridge operation through a DRV134.

If I am right, I should be able to run stable into 1 ohm when bridged, or 1/2 ohm when in stereo. What I want to do is have it easily power a 4-ohm woofer, and when I upgrade to a dual voice coil woofer (either dual 2 or dual 4 ohm, so a 2ohm or 1ohm load to the amp), it's gotta be able to handle it.

Would this power such a speaker? I care more about power with this project than sound quality, so would this circuit provide plenty of power into a 2 ohm speaker?

Second, does the PCB look good? It is a quick design and may need more work. The circuit is based on 8 LM3875 amp schematics, but with a resistor on each output to parallel them. The two resistors (input and the feedback resistor from - input to ground) are on the board, and the main feedback resistors will be mounted right on the chips.

Finally, how big of a heatsink do you think this thing will need? Would both heatsinks (4 chips per heatsink), from a logitech z-680 amplifier be enough, or will I need even bigger than that?

Thanks for all of your help on this project.
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Old 14th March 2005, 05:54 AM   #2
PRR is offline PRR  United States
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What supply voltage and power goal are you thinking?

80V supply and 8A peak give, for one chip, 40V pk 8A pk or 5 ohms, 150W RMS. Actually less because the chip has voltage drop and because you can't safely run AT the voltage rating: utility voltage variation will kill chips. Also the 150W output is uncomfortably close to the chip 90W dissipation rating on an infinite heatsink.

+/-30V and 75W in 4 ohms looks possible. You may lose 20% power in the paralleling resistors, 60W per chip. Times four is 240W in 1 ohm, or times 4+4 in Bridge about 480W in 2 ohms.

> does the PCB look good?

At a glance: NO, because....

> how big of a heatsink do you think this thing will need?

BIG.. MUCH bigger than this PCB. You can't put eight hot devices in a few inches: heat does not spread THAT well through even thick aluminum. (If you consider liquid-cooling....)

> Would both heatsinks (4 chips per heatsink), from a logitech z-680 amplifier be enough

Hmmm... that does claim 505 Watts, but looks like a 200W job to me. Probably they use switching amps to hump the subwoofer; or let the transistors run awful hot and trust in thermal shutdown. Your chips will also shut-down, so that might work. Looks awful skimpy to my old eyes.
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Old 14th March 2005, 09:50 PM   #3
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The max supply is +/-30VDC, so I was thinking of using a 40VCT torrid, probably somewere around 600-800VA.

How about these heatsinks?

they are halfway down this page:

No, the amps from those speakers are not switching subwoofer amp. They are TDA chips, I can't remember which ones, but there are 7 of them, one for each satellite and two bridged for the sub.
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