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Class D Switching Power Amplifiers and Power D/A conversion

3eaudio EAUMT-0260 TPA3255 Output power seems low
3eaudio EAUMT-0260 TPA3255 Output power seems low
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Old 5th October 2019, 05:56 PM   #11
Think is online now Think  Netherlands
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3eaudio EAUMT-0260 TPA3255 Output power seems low
@asiadiy: input voltage x gain = output voltage
output voltage is limited by power supply voltage (minus some loses)

@mboxler: Doesn't the 0 volt on the minus not stay 0 volt instead of getting amplified to a negative voltage when a balanced input would be used? :?
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Old 5th October 2019, 07:09 PM   #12
mboxler is offline mboxler  United States
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Quote:
Originally Posted by Think View Post
@@mboxler: Doesn't the 0 volt on the minus not stay 0 volt instead of getting amplified to a negative voltage when a balanced input would be used? :?
Since the amp uses a single rail power supply (0 - 48 volts), the output voltages must also fall into this range. This maximum 48 volt P-P output therefore rides on a DC offset of 1/2 this voltage, or 24 volts. With no voltage on the minus side to amplify, the output will be 24 volts. This will be the same if the plus input has a quiet (no voltage) signal....it too will be 24 volts. Having 24 volts on the positive side of the speaker and 24 volts on the negative side of the speaker is identical to having zero volts on each side.

With differential signals, when the positive speaker terminal swings to 26 volts, the negative side swings to 22 volts. The minus side isn't negative, it's just less than the positive side.
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Old 6th October 2019, 05:37 AM   #13
asiadiy is offline asiadiy
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3eaudio EAUMT-0260 TPA3255 Output power seems low
Thanks Mike, that is a great simple test, really helped me understand gain in a hands-on way. I just gave it a try.

I played a 60 hz tone through my PC sound card at full volume and got: 1.02 vac rms from center pin to ground.

I then connected it to my amp input, and used a 4ohm dummy load across one speaker terminal pair and got: 24.47 vac rms. DC ground to speaker negative 12.27vac to speaker positive 12.27vac.

So if my measurements are good, gain is 24:1. Amp spec says 21:1

* 24.47v / 4ohms = 6amps. 6a * 24.47v = 146watts. Is the calculation correct?

Naturally, I had to play the 60hz tone through my 8 ohm speakers to hear it. At maximum volume it sounded a little distorted, but was not that loud really, During test I measured 24.42 vac across the speaker terminals. Strange.

I took maximum available output voltage divide by gain (48.5/24) = 2.02 vac.

If I'm doing this right a single ended input for my amp needs to to be 2.02 volts to get full power?
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Old 6th October 2019, 01:43 PM   #14
mboxler is offline mboxler  United States
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This is good news. Without the schematic, I guessed wrong on how the board worked with single-ended inputs. The TPA3255 chip contains 4 amplifiers. Your board must be sending the original input to amp 1, and also creating an inverted signal from that and sending that inverted signal to amp 2. The same is happening to the other stereo signal to amps 3 and 4.

The gain of each amplifier is 21.5 dB, which equals a 11.9 voltage gain.

Decibels to Voltage Gain and Loss convert calculation conversion amplification amplifier electronics - sengpielaudio Sengpiel Berlin

1.02 * 11.9 = 12.14, pretty close to what you got. Since the speaker outs are equal to each other but one inverted, you get double that voltage across the load. In other words, a 1.02 input voltage to a 24.47 output voltage equals 27.6 dB amplification...close to the 27.5 I would expect.

The maximum voltage out is 48 volts peak-to-peak, or 16.8 volts RMS. 16.8 / 11.9 = 1.4, which is the maximum input voltage to reach full output.

You might want to do the same test on your other amp, but lower the input voltage to, say, 1/4 volt. It probably has a MUCH higher gain, which is why you notice such a difference in volume.

Your wattage calculation looks correct. I like to use this

Voltage current resistance and electric power general basic electrical formulas mathematical calculations calculator formula for power calculating energy work equation power law watts understandimg general electrical pie chart electricity calculation

Sorry about the confusion. Wasn't sure how the board worked single-ended.
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