0
\$\begingroup\$

I am new to designing audio circuit. Looking at TI TAS6424 class-D amplifier (https://www.ti.com/lit/gpn/tas6424-q1), on section 9.3.3 p.23, it mentioned volume control from -100 to +24 dB.

  1. Given the input comes from I2S, what is the input voltage audio voltage level encoded in the data? Is there a standard on the voltage swing for encoded PCM audio signals?
  2. Should the max +24 dB be limited by the IC supply voltage?
  3. On the same section, it mentioned peak output voltage swing can be set to 29 V, which is higher than the max supply voltage of 26 V. How can it be higher than the supply voltage?
\$\endgroup\$
3
  • 2
    \$\begingroup\$ The term "dB" does not imply voltage; it implies gain and no voltage level. If it were talking about voltages it would use the term "dBV" \$\endgroup\$
    – Andy aka
    Aug 9, 2023 at 19:10
  • \$\begingroup\$ @Andyaka, thanks. My understanding is gain is a ratio. For example, -3dB gain => output voltage = ~0.7 of input. \$\endgroup\$
    – jsmith
    Aug 9, 2023 at 19:36
  • \$\begingroup\$ 1. Has no voltage. Look up the term dBFS (decibel full scale). \$\endgroup\$
    – winny
    Aug 9, 2023 at 20:41

2 Answers 2

1
\$\begingroup\$

Many class D amplifiers have, in reality, zero PSRR. They measure PSRR with no modulation, getting very good numbers since the switches are mostly off, but with signal applied the output varies directly with PVDD.

This device, on the other hand, has PWM-generator feedback from the output to control it. To prove it, it measures PSRR with a 1 kHz modulation.

  1. The class D amp doesn't modify amplitude directly, it modifies duty cycle, which is variable between 0-100%. Therefore, your peak amplitude would be +/-PVDD at the extremes of the digital input at 0 dB gain. Varying the gain will change this relationship accordingly.
  2. The max +24 dB isn't limited by supply voltage, only the max output. If you set the gain to +24 dB and don't want to clip, make sure the 5 most significant bits of the input are identical all the time (assuming 2s complement). (Each bit represents 6 dB of dynamic range.) Setting the gain negative (in dB) may prevent maximum excursions of the output.
  3. The setting of 29 V on the gain is the output voltage corresponding to the maximum input value. It does not imply that you can achieve it; you can achieve +/-PVDD, and if your expected output exceeds PVDD, it will clip.
\$\endgroup\$
4
  • \$\begingroup\$ Thanks Cristobol. I guess I am not clear about gain on class-d amplifier. I got confused with simple op-amp. For PWM, i suppose the gain doesn't affect the output voltage amplitude. What does the gain change? Duty cycle? Also does the gain affect volume? \$\endgroup\$
    – jsmith
    Aug 9, 2023 at 20:11
  • \$\begingroup\$ The gain affects the maximum output you can specify at full range of the I2S inputs. Higher gain will attempt higher output for the same input. The PVDD voltage affects the maximum amount you can actually get. Hence the 29V output range spec...you can specify that with the gain setting, but you can't achieve it operating within the IC spec. \$\endgroup\$ Aug 9, 2023 at 20:17
  • \$\begingroup\$ Thanks. So the gain change the amplitude of the pulses in PWM signal? \$\endgroup\$
    – jsmith
    Aug 9, 2023 at 20:31
  • \$\begingroup\$ No, PVDD controls the amplitude of the pulses. The gain controls the duty cycle. The effective output voltage in any window is the pulse height times the duty cycle. \$\endgroup\$ Aug 9, 2023 at 20:51
1
\$\begingroup\$

The chip has two gain mechanisms, one is digital volume control for adjusting the digital gain, and the second is the selection how many volt the full scale digital value is.

  1. The datasheet is pretty clear on this. If you search the datasheet for the word "gain", you will find the table on page 8 which says how many volts peak the full scale digital is. There are four settings so they are all listed.

I don't think there are any standards for how many volts an amplifier must give out at full scale digital amplitude, that would be pretty useless as anyway different speakers would output different sound pressure level at same voltage. Do correct me if I am wrong.

  1. Likely yes. The problem is, if you are already feeding full scale digital values in via I2S, then you can't apply any positive gain with the digital volume control, as the amplitudes would go beyond full scale. And after the digital volume control, you have to select the output gain so that whatever your full scale value is, you have enough supply voltage to achieve it. Otherwise, you must limit your volume control and output gain selection so that you don't select values that would either clip in the digital or analog domain.

  2. Well as explained already, it obviously can't. If you have selected output gain of 29V/FS, and you only have 26V supply, then you can't play back full scale digital values. If you provide full scale data via I2S, you must use the digital volume control to bring down the digital amplitude so that analog output would not be 29V but 26V. That would be approximately -1 dB of digital volume control.

\$\endgroup\$
5
  • \$\begingroup\$ There is a digital-to-sound pressure level standard that's followed in cinema, and therefore nicer audio equipment (hi-fi). That presumes some default speaker sensitivity (dB SPL for 1 W at 1 m), typically around 85-87 dB SPL at 1 W for home speakers. All my entry- to mid-range speakers, when calibrated in my listening position, seem to be +- 3 dB of whatever default the receiver manufacturer used, and I think I've seen no worse than 6 dB off. \$\endgroup\$
    – user71659
    Aug 9, 2023 at 21:57
  • \$\begingroup\$ For item 3, how does the full scale voltage translate to actual gain in dB? For some class-D amplifier, they have pre-amp before PWM. That can easily be translated to dB. But not sure how raising the pulse amplitude can be done. Thanks again. \$\endgroup\$
    – jsmith
    Aug 10, 2023 at 16:53
  • \$\begingroup\$ @jsmith0910 Sorry I don't follow. If you have some digital level and some analog level, they have no relation that can be compared in dB, unless you know the conversion factor (which is given for the four gain settings that tell you how much in volts the 0dB digital full scale is). \$\endgroup\$
    – Justme
    Aug 10, 2023 at 16:58
  • \$\begingroup\$ Thanks Justme. I am not familiar with what the 29V/FS vs 15/FS vs other settings do. I know the pulse amplitude will change and I guess the duty cycle will change accordingly. But does it affect the loudness of the audio? \$\endgroup\$
    – jsmith
    Aug 10, 2023 at 17:44
  • \$\begingroup\$ @jsmith0910 The settings do exactly what they say. If you feed 100% full scale I2S digital audio in, you get 29V analog output, or 15V, or whatever the setting is. When volume control is set to 0dB that is. \$\endgroup\$
    – Justme
    Aug 10, 2023 at 18:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.