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Class D Switching Power Amplifiers and Power D/A conversion 

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5th October 2019, 05:56 PM  #11 
diyAudio Member
Join Date: Nov 2017
Location: Den Haag, Holland

@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? :? 
5th October 2019, 07:09 PM  #12  
diyAudio Member
Join Date: Nov 2011

Quote:
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. 

6th October 2019, 05:37 AM  #13 
diyAudio Member

Thanks Mike, that is a great simple test, really helped me understand gain in a handson 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? 
6th October 2019, 01:43 PM  #14 
diyAudio Member
Join Date: Nov 2011

This is good news. Without the schematic, I guessed wrong on how the board worked with singleended 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 peaktopeak, 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 singleended. 
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