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Part Number: TAS5727
I am using one channel of a TAS5727 amplifier as a mono output. I designed the filter based on App note SLOA119B and using components that would fit in the space we had on the board. The schematic for the output filter is as follows:
The PVDD inputs of the TAS5727 are being supplied by a 17.5V power supply.
We decided to try switching to BD Modulation to test power consumption and ripple differences between the modulation schemes. To do this, I removed C43. Registers 0x11 through 0x14 were set according to table 16 on page 46 of the datasheet. Register 0x20 was also set accordingly.
The result of doing this was that the sound quality at the output is greatly improved with BD modulation. In AD mode, there is a constant white noise component when the amplifier is turned on. In BD mode, this white noise is gone and the audio is better.
My understanding is that changing the modulation scheme should not effect the sound quality of the output, but could have other effects on the power supply and emissions. One thing I noticed when looking at the output ripple on the 17.5V power supply is that the ripple increased when using BD modulation.
Can anyone provide an explanation as to why changing the modulation scheme would have such a great effect on the audio quality? Are there other register settings that might need to be changed or adjusted to improve audio quality for the AD modulation scheme?
Any feedback on this would be appreciated.
In reply to Ben Missele:
In reply to Alex Bhandari-Young:
Thanks for the reply. I've been trying to figure out what is going on here in our lab and I still do not have an answer.
We are using an 8 ohm speaker. We do not have an EVM to test against. The design was originally tested with a TAS5717 EVM that I do not have access to.
I have now tested across several boards and found varying results. I do know that with AD Modulation, we get white noise across the spectrum. Using BD modulation, I see varying noise across the spectrum.
Here's an example of the noise using AD Modulation:
Using BD modulation I see varying noise across the spectrum, but here is one example:
Looking at my power up sequence, we have PVDD set to 17.5V. The 17.5V supply is an input to the 3.3V supply, so looking at the ramp, the 3.3V supply comes up before the 17.5V supply and both Reset and PDN are tied to 3.3V through 10K resistors. The scope plot below shows the 17.5V trace coming up and the 3.3V and reset traces ramping at the same time.
This one shows the voltage level of the 17.5V supply when 3.3V is at full voltage. When 3.3V is fully on, the 17.5V (PVDD) supply is at about 9V. It then takes an additional 9 ms to get up to the full 17.5V
Looking at the software side of things, the oscillator trim is done first, then we wait 50us per the datasheet. Volume level is set, and then other register values are set. We are using most of the default settings, so DRC is turned off.
I'm just wondering if any of this could be having an effect on the audio output of the amplifier or could contribute to the differences I am seeing in the audio output between different boards and modulation schemes. Any help would be appreciated.
Thanks again for the reply.
The noise is noticeable. It does not dominate sound coming out of the speaker, but it is noticeable, especially when we have volume turned down to lower levels.
I understand that C43 needs to be placed for AD modulation and needs to be removed for BD modulation.
With C43 placed and the board in AD modulation, I see the noise spectrum shown with what sounds like barely audible white noise.
With C43 removed and the board placed in BD modulation, that white noise is gone, but it replaced by spikes in other areas of the range. On some boards, it falls out of the audible range, on others, it's right around 8khz and is noticeable.
My understanding of the modulation scheme is that changing modulation schemes should not change the output to the speaker, yet when I switch between modulation schemes, I see a difference in the audio output spectrum and I am trying to understand why that might be the case. I'm concerned that this could point to something else that may be wrong in the design.
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