Understanding the output power of an amp from datasheet

[Adhith]

Hello friends...
I'm planning to buy a tda7492p class d amp board with bluetooth module. On the web it is written that it could produce an output of 25w per channel. But what i'm having is a 20w 8 ohm driver. While refering the graph from the datasheet it is seen that it could only produce 25 w with 20v and 8 ohm load and for my project the max voltage will be 16.4 v and 8 ohm load for which the output from datasheet is not more than 17 w . So should I go ahead with buying this board or look for a much lower power board?? Datasheet is attached with this post.

[Zero999]

P = V²/R

In the case of AC, V is the RMS voltage, but with an amplifier, it's easier to use the peak voltage, since that's dependant on the power supply. For a sinewave, the RMS power is half the peak power, therefore:

P = V_P²/(2R)

Unfortunately, no amplifier is perfect. The maximum output voltage is always slightly less than the input voltage, so the actual maximum output power will be slightly less than the value calculated by the above formula. If the amplifier's datasheet says it can output more power than that calculated, using the above formula, then it's either lying or the output waveform is no longer sinusoidal. In short your amplifier can't really drive an 8 Ohm speaker with 25W and a nice sine wave, when operating off 20V, hence why the data sheet specifies a THD of 10%, which is horrible. The output will be clipping with a huge amount of distortion. The useful output power will be much less. You should look at the output power specification with a THD of 1% for guidance.

Another thing to note is the actual output power depends on the, input signal level, volume control setting and amplifier gain. You could use an amplifier capable of driving 100W into an 8 Ohm speaker, with a 10W speaker and it won't blow up, as long as you don't play it too loud.

[Audioguru]

The amplifier board with Bluetooth is probably the cheap Chinese product sold on ebay. Does it use a real TDA7492 or a Chinese copy? Is it built properly? Is it reliable?

How did they rate the "20W" for the speaker? Momentary "music power" or continuous RMS power?
Years ago I bought speakers rated at 40W. When one of them died I opened it and the dead woofer was marked "5W".

[IanMacdonald]

The min supply for this chip is 8v, so yes it will work OK.
 
The reduced output power will be barely noticeable. Perceived sound loudness is logarithmic, and about a halving or doubling of power is needed before you notice any change.

[Adhith]

P = V²/R

In the case of AC, V is the RMS voltage, but with an amplifier, it's easier to use the peak voltage, since that's dependant on the power supply. For a sinewave, the RMS power is half the peak power, therefore:

P = V_P²/(2R)

Unfortunately, no amplifier is perfect. The maximum output voltage is always slightly less than the input voltage, so the actual maximum output power will be slightly less than the value calculated by the above formula. If the amplifier's datasheet says it can output more power than that calculated, using the above formula, then it's either lying or the output waveform is no longer sinusoidal. In short your amplifier can't really drive an 8 Ohm speaker with 25W and a nice sine wave, when operating off 20V, hence why the data sheet specifies a THD of 10%, which is horrible. The output will be clipping with a huge amount of distortion. The useful output power will be much less. You should look at the output power specification with a THD of 1% for guidance.

Another thing to note is the actual output power depends on the, input signal level, volume control setting and amplifier gain. You could use an amplifier capable of driving 100W into an 8 Ohm speaker, with a 10W speaker and it won't blow up, as long as you don't play it too loud.

Thank you very much for the help Hero999. So if I move forward with using the same amp board then is there any thing that I could do to reduce the distortion & to increase better performance?? I'm powering the board with 4 Li ion batteries in series (18650). I got a max voltage of 4.1 from each of these cells thus delivering a max voltage of 16.4 and I believe it won't be less than 12v towards full discharge.
 You said that the actual output power depends on the, input signal level, volume control setting and amplifier gain. there is a switch on the amp board which could set the gain to  21.6dB, 27.1dB, 31.1dB, 33.6dB. So could this be used to limit the output power for the speakers??

[Adhith]

The amplifier board with Bluetooth is probably the cheap Chinese product sold on ebay. Does it use a real TDA7492 or a Chinese copy? Is it built properly? Is it reliable?

How did they rate the "20W" for the speaker? Momentary "music power" or continuous RMS power?
Years ago I bought speakers rated at 40W. When one of them died I opened it and the dead woofer was marked "5W".

Thank you very much Audioguru for your help. I think its a Chinese product built by SANWU, which is sold on banggood. I have attached a pic of it. I think its a good product for this price & it has also good reviews. I brought the speaker for a local speaker building shop and on the speaker it says that minimum 5 w and max 20w and 3 inch size, unfortunately no other specs are labelled on it.

[Zero999]

P = V²/R

In the case of AC, V is the RMS voltage, but with an amplifier, it's easier to use the peak voltage, since that's dependant on the power supply. For a sinewave, the RMS power is half the peak power, therefore:

P = V_P²/(2R)

Unfortunately, no amplifier is perfect. The maximum output voltage is always slightly less than the input voltage, so the actual maximum output power will be slightly less than the value calculated by the above formula. If the amplifier's datasheet says it can output more power than that calculated, using the above formula, then it's either lying or the output waveform is no longer sinusoidal. In short your amplifier can't really drive an 8 Ohm speaker with 25W and a nice sine wave, when operating off 20V, hence why the data sheet specifies a THD of 10%, which is horrible. The output will be clipping with a huge amount of distortion. The useful output power will be much less. You should look at the output power specification with a THD of 1% for guidance.

Another thing to note is the actual output power depends on the, input signal level, volume control setting and amplifier gain. You could use an amplifier capable of driving 100W into an 8 Ohm speaker, with a 10W speaker and it won't blow up, as long as you don't play it too loud.

Thank you very much for the help Hero999. So if I move forward with using the same amp board then is there any thing that I could do to reduce the distortion & to increase better performance?? I'm powering the board with 4 Li ion batteries in series (18650). I got a max voltage of 4.1 from each of these cells thus delivering a max voltage of 16.4 and I believe it won't be less than 12v towards full discharge.
 You said that the actual output power depends on the, input signal level, volume control setting and amplifier gain. there is a switch on the amp board which could set the gain to  21.6dB, 27.1dB, 31.1dB, 33.6dB. So could this be used to limit the output power for the speakers??

The easiest way to minimise distortion is don't play it too loud. The distortion caused by too higher output (clipping) occurs very suddenly, as the volume is increased and is more noticeable on the loud bass, part of the music.

Yes, the gain switch can be used to help control the power output.

What's the audio signal source for this amplifier?

[Audioguru]

Hee, hee. A little 3 inch speaker is used on a crappy sounding clock radio. But maybe you have the fairly good sounding and fairly expensive 3 inch speaker that people talked about on DIY Audio forum.

[Zero999]

Hee, hee. A little 3 inch speaker is used on a crappy sounding clock radio. But maybe you have the fairly good sounding and fairly expensive 3 inch speaker that people talked about on DIY Audio forum.

That's a point. The physical size of the speaker its construction makes more of a difference to the sound quality, than its power rating or the amplifier. You're better off spending more on the speakers, than the amplifier.

[Adhith]

Hee, hee. A little 3 inch speaker is used on a crappy sounding clock radio. But maybe you have the fairly good sounding and fairly expensive 3 inch speaker that people talked about on DIY Audio forum.

The driver is not a costlier one but seems like it is working fine without much distortion when connected to a la4440 amp circuit.   

[Adhith]

P = V²/R

In the case of AC, V is the RMS voltage, but with an amplifier, it's easier to use the peak voltage, since that's dependant on the power supply. For a sinewave, the RMS power is half the peak power, therefore:

P = V_P²/(2R)

Unfortunately, no amplifier is perfect. The maximum output voltage is always slightly less than the input voltage, so the actual maximum output power will be slightly less than the value calculated by the above formula. If the amplifier's datasheet says it can output more power than that calculated, using the above formula, then it's either lying or the output waveform is no longer sinusoidal. In short your amplifier can't really drive an 8 Ohm speaker with 25W and a nice sine wave, when operating off 20V, hence why the data sheet specifies a THD of 10%, which is horrible. The output will be clipping with a huge amount of distortion. The useful output power will be much less. You should look at the output power specification with a THD of 1% for guidance.

Another thing to note is the actual output power depends on the, input signal level, volume control setting and amplifier gain. You could use an amplifier capable of driving 100W into an 8 Ohm speaker, with a 10W speaker and it won't blow up, as long as you don't play it too loud.

Thank you very much for the help Hero999. So if I move forward with using the same amp board then is there any thing that I could do to reduce the distortion & to increase better performance?? I'm powering the board with 4 Li ion batteries in series (18650). I got a max voltage of 4.1 from each of these cells thus delivering a max voltage of 16.4 and I believe it won't be less than 12v towards full discharge.
 You said that the actual output power depends on the, input signal level, volume control setting and amplifier gain. there is a switch on the amp board which could set the gain to  21.6dB, 27.1dB, 31.1dB, 33.6dB. So could this be used to limit the output power for the speakers??

The easiest way to minimise distortion is don't play it too loud. The distortion caused by too higher output (clipping) occurs very suddenly, as the volume is increased and is more noticeable on the loud bass, part of the music.

Yes, the gain switch can be used to help control the power output.

What's the audio signal source for this amplifier?

Can you give me some hint on calculating the output power according to the different gain settings?? The audio source is from my phone via the Bluetooth module.

[Audioguru]

The recording level, the phone level and the Bluetooth level are all unknown so use an oscilloscope to look at the output signal from the amplifier and reduce the level until it shows no clipping.
Music in India is very different to music in the West. It might not have bass sounds then the cheap little speaker sounds fine. The old low power LA4440 amplifier has poor low and high frequency response.

[Zero999]

The output level depends on the type of music played and the volume setting. Lots of Indian music is very bass heavy, just as much as RAP, funk and dance.

The voltage gain (A_V) can be calculated from gain in dB.

dB = 20log₁₀(A_V)

Just rearrange to make A_V the subject of the formula.

[vealmike]

When people ask about amplifier wattage, what they are often asking about is volume.

If you take a speaker and drive it with different amps, then yes the higher wattage amp will be louder (until the speaker goes pop.)
But, this relationship is not linear. To be twice as loud, you need ten times the power. So the difference between 18W & 25W isn't as great as you might think.

The other factor that gets largely ignored is that two different speakers on the same amplifier can have completely different volumes. Look for speaker sensitivity to compare speakers, it will be quoted in Decibels per watt at 1 meter.
Decibels CAN be compared linearly, +10dB is twice as loud. +20dB four times etc.

I appreciate that this doesn't answer the question and I apologise if I'm teaching you to suck eggs. However, many moons ago I worked in a hi-fi store and " 'ow many Watts 'as it got?" was usually asked when what the customer really wanted to know was "is it loud?" 
Well, that and the fact that the original question has been thoroughly answered.