If you are using a single 12VDC supply to power the amplifier, then of course the output will measure 6VDC with respect to the -ve supply line. This does not represent the RMS AC power output.

For a single-ended output to a loudspeaker you need to use a DC blocking capacitor, 1000-4700μF in series with the loudspeaker.

You can eliminate the coupling capacitor if you configure the 2-channel amplifier for bridged output.

 

1) Get rid of the breadboard that causes amplifiers to pick up electricity hum and oscillate. The contacts on a breadboard cannot pass the high current to a speaker.
2) With your 12V supply, feed an input of 1000Hz. Turn up the volume so that the output is barely clipping then then the output will be 6W and the AC voltage will be 6.9VAC RMS.

The amplifiers are not single-ended, instead they are bridging then a series output capacitor is not needed.

 

Audio amplifiers are measured with a signal generator with a continuous sinewave feeding the amplifier input and an oscilloscope across the speaker to show the peak-to-peak signal across the speaker.
A VOM cannot measure normal audio frequencies and cannot measure peak voltages of music or voices.
A current meter increases the load resistance that reduces the voltage and current at the load.

 

That's what I thought you were measuring. You need to measure the AC voltage across the speaker using a sine wave signal.
Go on line and find "Test Tone Generator". Set the frequency for a 1000hz sine wave and use this for the signal input.
Since you don't have a scope all you can do is crank up the volume until the level is just below any noticeable distortion and measure the AC voltage across the speaker.

I give it a try. I searched for a tone generator online and set it 1000Hz, then cranked up the volume from small to loud. However, I noticed that the AC voltages are only around 0.08V to 0.66V. But for the input signal, it's from 0.06V to 0.80V. What do those numbers tell?

 

1) Get rid of the breadboard that causes amplifiers to pick up electricity hum and oscillate. The contacts on a breadboard cannot pass the high current to a speaker.
2) With your 12V supply, feed an input of 1000Hz. Turn up the volume so that the output is barely clipping then then the output will be 6W and the AC voltage will be 6.9VAC RMS.

The amplifiers are not single-ended, instead they are bridging then a series output capacitor is not needed.

Well, I give it a try and the AC voltage is only around 0.08 to 0.66V...may be there's a chance that my VOM break.
P/s when I use DC voltage to measure it still above 6V

 

Well, I give it a try and the AC voltage is only around 0.08 to 0.66V...may be there's a chance that my VOM break.
P/s when I use DC voltage to measure it still above 6V

Where are you measuring this AC voltage? The probes should be across the speaker.

The DC measurement is meaningless. If it were averaged correctly, it would read 0 across the speakers and 6V between either speaker lead and ground, regardless of the output level.

 

Where are you measuring this AC voltage? The probes should be across the speaker.

The DC measurement is meaningless. If it were averaged correctly, it would read 0 across the speakers and 6V between either speaker lead and ground, regardless of the output level.

I put the probe to one of the speaker's lead, the other probe to the ground, and read the AC voltage. I have attached the video I measured it.

 

I put the probe to one of the speaker's lead, the other probe to the ground, and read the AC voltage. I have attached the video I measured it.

That is not the right way to measure it, as you have been told before. Is it really that difficult to do the right measurement?

 

A You Tube video says that your multimeter is The Worlds Worst and says it is cheap and crappy. Its instructions and specs are in Chinese! It is probably accurate measuring 50Hz and 60Hz voltages but probably measures higher frequencies like your 1kHz wrongly showing levels much too low.

The amplifier IC has bridged outputs with no ground since there is a signal on both speaker wires to double the speaker AC voltage.

 

That is not the right way to measure it, as you have been told before. Is it really that difficult to do the right measurement?

Well, can you please show me the correct way to do it please

 

That is not the right way to measure it, as you have been told before. Is it really that difficult to do the right measurement?

I just read your comment above again, do you mean that I should put the two probes to the two polarities of the speaker? If I misunderstand, hope that you can tell me.

 

The datasheet for the old TDA7297 shows that each channel's output has two terminals. Each channel has two amplifiers inside it, each amplifier drives each or the two speaker wires. Then connect your multimeter probes to each of the channel's outputs that also connect to each of the speakers terminals.

 

I just read your comment above again, do you mean that I should put the two probes to the two polarities of the speaker? If I misunderstand, hope that you can tell me.

Yes, exactly. That actually will read the voltage the speaker is seeing.

But it still might not be accurate at 1000Hz. Can you set your source to 60Hz? It should be accurate there.

 

We do not know if you connected the speaker correctly as shown on the datasheet of the amplifier, or if your multimeter wrongly probed only one speaker wire and ground.
We also do not know if your amplifier plays frequencies as low as 60Hz.

Set your signal source to one AC voltage at 60Hz then at 1kHz and see if the same AC voltage is measured by your multimeter at both frequencies at the signal source and at the amplifier output.

 

We do not know if you connected the speaker correctly as shown on the datasheet of the amplifier, or if your multimeter wrongly probed only one speaker wire and ground.
We also do not know if your amplifier plays frequencies as low as 60Hz.

Set your signal source to one AC voltage at 60Hz then at 1kHz and see if the same AC voltage is measured by your multimeter at both frequencies at the signal source and at the amplifier output.

Yes, exactly. That actually will read the voltage the speaker is seeing.

But it still might not be accurate at 1000Hz. Can you set your source to 60Hz? It should be accurate there.

I give it a try, at 60Hz, the AC voltage across the speaker is 0.08VAC, at 1000Hz I check it again and it says 1,34VAC. I see that the AC voltage tends to increase when the frequency increases.
For the input, the AC voltage across them is 0.06VAC at both 60Hz and 1000Hz. I suspected that the VOM had broken so I measured the Voltage of the power outlet, and it says 223VAC. So may be the problem is not the VOM.

 

I give it a try, at 60Hz, the AC voltage across the speaker is 0.08VAC, at 1000Hz I check it again and it says 1,34VAC.

Probably because of the value of the input capacitors. At lower frequencies the impedance of the input capacitor is higher causing the 60hz signal to be attenuated.
What value caps are you using for the inputs?

 

Probably because of the value of the input capacitors. At lower frequencies the impedance of the input capacitor is higher causing the 60hz signal to be attenuated.
What value caps are you using for the inputs?

I use the default schematic, so the value of capacitors are both 0.22uf

 

I use the default schematic, so the value of capacitors are both 0.22uf

That explains the lower output at 60hz but the signal out should be higher. Are you sure they are 0.22 uf they look small on the breadboard?
How is the speaker connected, across which two pins?

 

That explains the lower output at 60hz but the signal out should be higher. Are you sure they are 0.22 uf they look small on the breadboard?
How is the speaker connected, across which two pins?

Pin 14 of the IC for the positive side of the speaker, and pin 15 for the other. But can you explain to me why the capacitor makes the output lower please?

 

The impedance of the input capacitor varies with frequency and is higher at lower frequencies. This forms a voltage divider with the input impedance of the chip which is typically 30K ohms according to the data sheet.
A 0.22 uf cap has an impedance of appx 12K at 60 hz. Without going into the math a 0.06 signal will be reduced to 0.042 volts hence the output voltage is lower. What doesn't add up, from your measurements, is the output voltage vs the gain of the chip which is typically 32db or appx a gain of 40.
Multiplying 40 X 0.042 volts = 1.68 volts not 0.08 volts, if the caps are actually 0.22uf as you say.