# Volume control in class D amplifiers

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#### potstip

It is easy to make volume control if you using build in chip amps. But for discrete class d amplifiers, how we can make volume control after PDM modulation? I saw some sample schematics use volume control before PWM modulation but in my case i use PDM instead of PWM also output voltage of PDM modulation is constant. Simple resistor or r-2r resistor network work on PDM signal?

#### FauxFrench

It is easy to make volume control if you using build in chip amps. But for discrete class d amplifiers, how we can make volume control after PDM modulation? I saw some sample schematics use volume control before PWM modulation but in my case i use PDM instead of PWM also output voltage of PDM modulation is constant. Simple resistor or r-2r resistor network work on PDM signal?

A volume control on the modulated (PWM or PDM) side is unusual. If you really insist, vary the supply voltage to the output stage but it will only be a limited solution with a narrow regulation range.

#### potstip

A volume control on the modulated (PWM or PDM) side is unusual. If you really insist, vary the supply voltage to the output stage but it will only be a limited solution with a narrow regulation range.

So do you suggest before pdm mofulation?

#### nigelwright7557

I have never seen volume control on digital side on class d.
Its always done on the front end in analogue.

#### FauxFrench

So do you suggest before pdm mofulation?

"before pdm mofulation" I find is not clear?

If you have an analog signal, before converting it into PWM or PDM as Nigel mentions, you regulate the volume the usual way.
If you have to do it after the signal is converted into PWM or DPM, you change the Pulse Width or the Pulse Density to vary the volume but that is very difficult. You can achieve a limited variation of the volume through variation of the gain (for PWM) or the energy in each pulse (for DPM) by varying the supply voltage to the output stage. But, upward the maximum supply voltage becomes a limit and downward a change of the switching shape becomes a limit. So, it is far from a good solution.
What is the last point in the signal chain where you have the analog signal?

#### Mark Tillotson

All power amps are used after volume controls, as they have fixed gain and handle power, which is very difficult to work with for any signal processing. Class D is no different.

#### potstip

My idea was use pcm4202 ADC as PDM modulator but no matter what input voltage is output voltage of digital outputs are constant. So thats why i looking for another solution to make volume control. What i do not underatand is dsp based pwm modulators need to have same issues but dont. I wonder how they solve this issue.

But i guess i have to find a great pwm modulator instead of PDM.

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#### FauxFrench

You mention "dsp based pwm modulators". I guess that means that the dsp is changing the pwm-modulation and thereby achieving volume control. A dsp can also change a pulse density to achieve volume control. What I understood was that you want a kind of simple circuit element, like a potentiometer for analog signals, to perform volume control?

#### Eva

In the digital domain, provided unlimited resources, anything can be done. For example, a PDM signal can be modulated with another higher frequency PWM signal... Or the PDM signal could be redone at a different volume.

But what is practical? That's a different question. The DAC conversion, the analog or digital potentiometer, and the class D IC or self oscillating class D circuit, is still among the most practical, for audio listening requirements.

For a paging system or industrial controls it would be different.

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