TDA7266

[gaspode42]

Hi

I am looking for some advice for my first amp. project.

I want to build a micro controlled audio circuit that contains an amplifier and am looking at something like the TDA7266 (TDA7266 7 W + 7 W dual bridge amplifier - STMicroelectronics).

I want to use a digital volume control on the system so I am looking at putting something like this https://www.ti.com/lit/ds/symlink/pga2311.pdf on the amplifier input.

The question I have is that the datasheet for the TDA7266 states that the open loop gain for the chip is 26dB. The output of the circuit feeding the amp is giving 2V RMS centered around ground. If my math is correct that would result in an output of 56V P-P (10^26/20*2/sqrt(2))???? So I would need some sort of additional attenuation on the input.

What is the best way to achieve this? Could I use an additional PGA2311 or is there a better way. Do I also need to provide a bias to the input signal to center it around something other than ground?

Any comments would be much appreciated as although I am fine in the 'digital domain' analog is out of my comfort zone. I will post my finished project with all schematics and code when the project is complete.

Thanks in advance for your help and support.

 

[audioguru]

The datasheet fore the TDA7266 amplifier shows that with an 11V supply the output is clipping severely at 7W and begins clipping at about 4W into 8 ohms. Then the output voltage is 5.7V.
Its gain is 26dB so the input for maximum undistorted output is 285mV.
Your 2V input is 7 times too high (16dB) so simply turn down the volume control. The volume control is supposed to have a logarithmic taper so when it is turned down to half then the output is at -20dB. Do you ever use an amplifier with its volume control set to maximum when the input signal is high?

 

[gaspode42]

Thanks Audioguru,

This has confirmed my thinking, although I think my maths was a bit off!!

Do I need to bias the input signal away from GND as this is a single supply bridged amplifier and should therefore remove the bias by virtue of the way the outputs are driven?

Your advice has been much appreciated.

 

[Nigel Goodwin]

Do I need to bias the input signal away from GND as this is a single supply bridged amplifier and should therefore remove the bias by virtue of the way the outputs are driven?

You simply connect the input via a capacitor, any required biasing arrangements for the power amp are simply part of it's intrinsic design and will be shown in the datasheet - biasing for a bridged amp is not really any different to a single one.

As you have a digital volume control, so probably more likely to be turned right up, I would suggest adding a preset potentiometer before or after the digital one to limit the maximum signal level.

 

[audioguru]

I have never seen a volume control with gain before.
The Texas Instruments digital volume control provides gain up to 30 times that you do not want since the power amplifier has a gain of 20 times which is already 7 times too high for your 2V input level. Then at maximum volume control setting the gain will be 30 x 7= 210 times too high. You should use a digital volume control that attenuates only.

The Texas instruments digital volume control is recommended to be driven with a low impedance of 600 ohms to 2k ohms.

 

[gaspode42]

Again very many thanks to you both. Sorry for all the silly questions and my lack of knowledge in this area.