Why am I measuring a voltage on the output of an audio amplifier with no input signal?

I am working on a small audio amplifier using an NXP 1W BTL mono amplifier (TDA7052).

I've made a quick test circuit on a breadboard using the typical application schematic provided by the datasheet:

I'm currently providing 5V from a regulated supply and have an input signal from the line out (left channel) of a typical stereo receiver.

The sound quality is good and I've used the oscilloscope to look for clipping and set the gain, etc. The 100nF cap is close to the IC as a decoupling cap and all other values are as shown in the schematic.

If I reference ground and connect the scope probe to pin 8 (speaker negative) I measure 2.5 volts DC when no audio signal is present. The flat line turns into a waveform when I introduce a signal, and it works.

I expected the output voltage to be 0 volts when no audio signal is present, from speaker to ground. If I measure across the speaker, from pin 5 to pin 8, I measure between 7 and 80 mV from min to max volume, respectively (as controlled by the pot on pin 4).

Is this correct operation, or do I have a problem to fix?

Answer

According to the schematic you included, it is an amplifier with differential output. You have to connect the loudspeaker between pins 5 and 8, not to ground. Both outputs will probably carry a signal half way between 0 and Vcc, with one output going down and the other going up in the rythm of the music. Output 5 and 8 are each others mirror.

As with the speaker, to measure output voltage, you again have to measure between pins 5 and 8, and not with reference to ground. Now with a multimeter, this is pretty easy to accomplish, but with a scope this is probably a different story.

The problem with a scope is that the scope ground may very well be attached to the circuit ground for safety. Either through its own power supply, or the attached music player('s power supply) or any other attached device.

If this is the case, then attaching scope ground to one of the pins6 or 8 will short circuit the amplifier which is ... bad. Most oscilloscopes with at least two channels have the option to add or subtract the two input channels. Use the subtract mode to measure the voltage difference on both outputs of this amp.