old tv audio amplifier

[franticEB]

Hello everyone I'm trying to understand the operation of this circuit



the chip used is the tda7056b **broken link removed**.

From the diagram, I realized that it is powered by 13v and takes as input a sine wave that can be at most 1.5Vpp. The volume control is made ​​through a PWM generated by the microcontroller. The RC filter on pin 5 makes a mean value of the PWM and the voltage sweeps, with the duty cycle of PWM, from 0.5 V to 1.5 V. The output can be 13Vpp max.

On the datasheet of the IC it is written that with the volume control the amplifier can have a gain up to 40dB i.e. the input multiplied by 100. But the ratio Vout / Vin doesn't get 40dB.

How can you explain the relationship between the volume control and the effective gain of the amplifier?

Could you give me your opinion on the functioning of this circuit?

thanks

 

[godfreyl]

the amplifier can have a gain up to 40dB i.e. the input multiplied by 100. But the ratio Vout / Vin doesn't get 40dB.

You're looking at the maximum input voltage, but the actual input voltage may be much lower than that.

Having more gain than you need is better than having less than you need.

 

[Audioguru]

The volume control is a DC voltage. The datasheet says the gain and attenuation in text and shows it on a graph.
When the voltage at pin 5 is +1.2V to +1.6V then the gain is 40dB (100 times).
When the voltage at pin 5 is about 0.65V then there is no gain and no attenuation.
When the voltage at pin 5 is +0.4V or less then the attenuation is about -90dB and the amplifier is muted.