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-   -   how much RMS we can expect from OPA549? (http://www.diyaudio.com/forums/chip-amps/218568-how-much-rms-we-can-expect-opa549.html)

rhythmsandy 26th August 2012 05:54 PM

how much RMS we can expect from OPA549?
 
Considering OPA549 single chip with power supplies +/-30v how much power we can expect in 8 ohms? Since its said in the datasheet about 8A continuous at the output so what could be the RMS ratings of this amp is it 30x8 I dont think so... since its not a official audio amp chip like LM3886 how OPA549 be rated?

stratus46 26th August 2012 09:54 PM

Quote:

Originally Posted by rhythmsandy (Post 3140753)
Considering OPA549 single chip with power supplies +/-30v how much power we can expect in 8 ohms? Since its said in the datasheet about 8A continuous at the output so what could be the RMS ratings of this amp is it 30x8 I dont think so... since its not a official audio amp chip like LM3886 how OPA549 be rated?

About 50 Watts. Into a resistive load peak currents of 3.75 Amps but remember a speaker is not at all resistive. RMS power is P=E/R and P=IR and E=IR. Unlike the laws of man, Ohm's law MUST be obeyed. You may not exceed max Voltages of components nor maximum currents. Bottom line is the power in a direct coupled solid state amp is directly related to the power supply. Doubling the power supply will quadruple the power and double the load current. A 10% droop in the supply Voltage will lower the power to 81% of 'non drooped' supply Voltage.

The LM3886 claims lower distortion than the OPA but it would still be worth playing with the OPA. You might even prefer it.

DiGiKey wants $21.60 for the OPA549 vs $6.27 for the LM3886.

HTH

G

rhythmsandy 26th August 2012 10:34 PM

how did u get this value 3.75 Amps peak but the chip itself rated at 8amps of continuous output...

AJT 26th August 2012 10:39 PM

Quote:

Originally Posted by rhythmsandy (Post 3140753)
Considering OPA549 single chip with power supplies +/-30v how much power we can expect in 8 ohms? Since its said in the datasheet about 8A continuous at the output so what could be the RMS ratings of this amp is it 30x8 I dont think so... since its not a official audio amp chip like LM3886 how OPA549 be rated?

with rails of +-30V, ac output is estimated at 60/3 or 20volts ac, this is 50watts into 8ohm resistor load...

the "8A continuous at the output" rating is considered with the device dissipation, rail voltages and loading of the output pin.

nigelwright7557 27th August 2012 12:02 AM

Quote:

Originally Posted by rhythmsandy (Post 3140753)
Considering OPA549 single chip with power supplies +/-30v how much power we can expect in 8 ohms? Since its said in the datasheet about 8A continuous at the output so what could be the RMS ratings of this amp is it 30x8 I dont think so... since its not a official audio amp chip like LM3886 how OPA549 be rated?

If the amp will output 8 amps then it should also run into 4 ohms to get more power.

stratus46 27th August 2012 07:56 AM

Quote:

Originally Posted by rhythmsandy (Post 3141080)
how did u get this value 3.75 Amps peak but the chip itself rated at 8amps of continuous output...

I said Ohms law must be obeyed. When you put 30 Volts into an 8 Ohm load it will take 3.75 Amps regardless of the capabilities. If you use a 3.75 Ohm load you will run 8 Amps. A lower resistive load will exceed the output rating of the amp at 30 Volts. Draw it on a graph with maximum current and Voltage as the boundaries. You need to stay inside that box. It gets even more complicated when you take into account the power dissipation of the chip. For example, take a 1 Ohm load and run it to 8 Volts which will be 8 Amps into the load. E=IR However the chip will have 22 Volts across it (30-8) at the 8 amps for 64 Watts into the load (P=IR) BUT the chip will be dissipating 22 Volts times 8 Amps or 176 Watts of pure heat for a total of 240 Watts from your transformer. And it gets more difficult when you factor in that the transistors (whether internal to a chip amp or discrete devices) have to be calculated for safe operating area Voltage vs Amperes vs temperature.

HTH

G

AndrewT 27th August 2012 12:40 PM

Then add in reactive loading to Stratus' examples.

Have you read anything about reactive loading? Or anything about the chip dissipation when reactive loads are driven?
Have you read any of ESP's site?

macboy 27th August 2012 01:39 PM

Quote:

Originally Posted by nigelwright7557 (Post 3141144)
If the amp will output 8 amps then it should also run into 4 ohms to get more power.

Yes, but don't forget about power dissipation in the chip. There is a maximum rating for that too. You will likely exceed that before exceeding the maximum current.

AndrewT 27th August 2012 06:17 PM

Using the maximum specified 30Vdc, the maximum peak output voltage on sinewave into a 4r0 resistive load will be ~25.2Vpk.
The maximum Power is Vpk * Vpk / Rload / 2 ~ 80W.

The chipamp cannot be run at+-30Vdc. That is the maximum rating.
Expect the normal supply voltage to be ~ 4volts below that when mains voltage is @ nominal.
That reduces the maximum power to ~ 56W into 4r0. and requires ~ 5.3Apk into the resistive load.
The peak current on fast transients into speakers can approach 3times that, i.e the OPA549 cannot properly drive a 4ohms speaker when on maximum supply rails.

If yopu want to retain that margin of 3times for reactive loads then the peak sinewave output current must be restricted to ~3.3A.
The maximum output power would be ~ 22W

If you double the speaker impedance to 8ohms then that maximum becomes 44W and that is achievable with a sinewave output of 26.6Vpk and running the amp almost at the maximum supply specified.
In practice this chipamp is like all others. It is current crippled and cannot properly drive reactive speaker loads.


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