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Old 26th May 2005, 08:37 PM   #1
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Join Date: Jan 2005
Location: Tel Aviv
Default Question about OPA549

I want to design a power amplifier for my bass guitar. If I incorporate the OPA459 how much output power should I expect ?

my goal is a pretty strong amp : I need 300-400 Watts RMS into 4Ohms cabinet (or maybe 8Ohms, haven't decided yet)
Of course I intend to use bridged or paralleld mode. How much devices I need for that kind of output?
I know this device can output massive 8 ampers in steady state.
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Old 27th May 2005, 01:44 PM   #2
macboy is offline macboy  Canada
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Join Date: Oct 2003
Location: Ottawa, Canada
So let's take a look at your requirements. 400 W(RMS) into 4 ohms will require a RMS voltage of 40 V ( P = V^2 / R ). The peak voltage of each half-wave is then about 56.5 V ( Vrms = Vpeak / sqrt(2) ), so the peak-to-peak voltage will be 113 V. The maximum voltage for the OPA549 is 60 V (i.e. dual 30 V supplies), so a single chip clearly can't do it. You will need a bridged design to do this, and even then you won't quite get 56.5 V of swing out of each '549. You might get 55 to 56 V, and that assumes that your power supply rails do not sag under load (which they will do). That will reduces the power to about 375 W(RMS), if you use the absolute maximum +-30 V supplies, and as I said, they do not sage.

Forget about doing this for 8 ohms. You will get only half that power, since there is no way to increase the output voltage.

Bridging two 549s into 4 ohms would exceed their current limit and/or safe operating area. So you will need at minimum, two parallel 549s per side of the bridge for 4 total. A bridge-parallel amp with a 4 ohm load will present a 4 ohm load to each chip in the amp. But can the 549 handle a 4 ohm load at 60 V? The LM3886 datasheet includes some great charts to help in designing their amps and some are applicable to other designs as well. See in particular the top of page 14, this shows Power dissipation and heatsinking requirements for various load impedances and supply voltages. According to this, at 60 V supply and 4 ohm load will result in 45 W of power dissipation in the chip. Hmm. It says you need a heatsink with a thermal resistance of 1.6 deg/W. That's a bigass heatsink. But that's for the LM3886. It so happens that the OPA549 has a higher juntion-to-case thermal resistance than the LM3886 by 0.4 deg/W. So your heatsink needs to be 1.2 deg/W maximum (or the devices will overheat). And that's at 25 degrees C (77 deg F) ambient temperature. That's bordering on completely unreasonable. Methinks you will need to add another pair of 549s to keep things sane. Now each chip sees a 6 ohm load (ah...much better). Looking at that chart again, the heatsinking requirements are now 3.0 deg/W (LM3886) or for the OPA549, 2.6 deg/W. That is perfectly reasonable, and you will no longer have to constantly worry about your chips overheating. You will still need some serious heatsinking, but it will be sane.

So that's 6 OPA549s in a bridge-parallel arrangement, with the maximum 60 V supplies, delivering (absolute best case) ~375 Wrms. And don't forgot about the big heatsinks and the monterous power supply that you will need to keep those voltage rails up to spec under load.

For some solid theory on building a bridge-parallel amp, refer to National Semi's application note # 1192. Keep in mind there will be subtle differences, like ignoring the connection to the LM3886's mute pin, but adding a connection for the 549's current limit pin.
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