3eaudio EAUMT-0260 TPA3255 Output power seems low


2019-05-06 11:57 am
I finally got my 3eaudio amplifier wired up, everything works, except the output power seems quite under the spec for the amp. :confused:

My old amp is a tiny Nobsound TPA3116 2x100. It is significantly louder than the new 260w TPA3255 unit. Same input source, same speakers.

I'm quite new to all this and don't have much in the way of test gear. Fluke 87V, and another Fluke clamp-on amp meter. No scope available, no signal generator. I can get a resistive load if that helps.

Any ideas for how to get a rough idea on what the actual output in watts is with what instruments I have?

Input selector jumper is on SE
Input pins used: in+ and agnd-
Input used is a mobile phone 3.5mm headphone jack, and output from my PC sound card
Input power is 48.2V from the 3e power supply

I am not using the 3E pre-amp board yet, so just a selector switch and volume pot. I tried bypassing the pot and selector, same maximum output.

Thanks for any help.

The effect that the 3116 is "louder" has nothing to do with the output power.
The GAIN of the both amps is different. as i have some 3116 amps i can imagine that the gain is set to 30 or 33db and normally the tpa3255 chip is about 26dB. so about 6-3db less voltage gain. so you have to push more volume in for the same output voltage --> at the same load (speaker) you sould have the some power.
so practical that means that you have to move the volume knob at the tpa 3255 amp e.g. more in the direction 12 - 1 o´clock. so set the output level of your source CD, DAC, or other source up.

the tpa3116 can handle 24v max but the tpa3255 can handle more voltage at the supply 50V and more so therefore it has more power.

Last edited:
here you can find an overview what is gain. left sine is the input -right the output.
and you can see what happened if the amp is clipping --> means he "run out" of the voltage supply...:D



  • what is clipping.gif
    what is clipping.gif
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2019-05-06 11:57 am
Thanks for the comments! I am working on the pre-amp configuration now.

TurboWatch- I'm confused, does symmetrical input mean balanced (differential)? My amp has a jumper select for input type (Differential or Single Ended).

If I understand this correctly, I must use the unbalanced configuration if the source is through a 3.5mm jack (single negative for both channels)

So far I have this understanding:
Input types
Balanced Input-----------------Unbalanced Input
Negatives not grounded------ Negatives grounded
"Differential"-------------------"Single Ended"
Less noise-----------------------More noise

Levels compared
"Pro level input" +4dBu = 1.228v rms-------"Consumer level input" 10dBu = 316mv rms
TPA3225 Amp input----------------------------TPA3116 Amp input
Pre-amp output-----------------------------------Mobile phone output

Corrections and comments appreciated!


2019-05-06 11:57 am
Thanks chermann
Is the clipping shown in your graph caused by under voltage of the input source?

It seems I need to study the relationships involved carefully; gain, input voltage, output voltage, power supply voltage. Can you recommend a reference that a non EE can understand?


2011-11-14 3:35 pm
Like Turbowatch2 says, it looks like your amp requires differential input signals to reach full output. You say your source is a PC? You can use this website to generate a sine wave.

Online Tone Generator - generate pure tones of any frequency

While generating a 60hz signal, measure the AC voltage between the signal and ground rings on the 3.5mm plug.

Now, with a dummy load, plug the 3.5mm into the amp jack and play the same 60hz tone. Measure the voltage across the dummy load. That is you voltage gain. I have a feeling it will be 11 - 12 times the input voltage.

Because you are using a se signal, the 0 volt signal on the minus becomes a steady 24 volts on the output minus. The output plus is varying between 0 and 48 volts, so you end up with a max 16.8 volts RMS across the load. If the load is 8 ohms, that's 38 watts. If the input minus signal were the inverted plus signal, the output minus will now vary between 48 volts and 0 volts, doubling the voltage across the load, and quadrupling the wattage.

Hope that helps.



2011-11-14 3:35 pm
@@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.


2019-05-06 11:57 am
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?


2011-11-14 3:35 pm
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.