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Class D Switching Power Amplifiers and Power D/A conversion

TAS2770 20W Digital Input Mono Class-D Amp, Speaker I/V Sense, I2S input all digital
TAS2770 20W Digital Input Mono Class-D Amp, Speaker I/V Sense, I2S input all digital
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Old 30th December 2017, 07:03 PM   #1
AudioLapDance is offline AudioLapDance  Canada
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Default TAS2770 20W Digital Input Mono Class-D Amp, Speaker I/V Sense, I2S input all digital

In the D amp thread, Windforce85 noted:

Quote:
Originally Posted by Windforce85 View Post
http://www.ti.com/lit/ds/symlink/tas2770.pdf

This is not for 'real audiophiles' becouse:
- It lacks analogue inputs (you cannot put your fancy stuff between source and amp anymore) .
- Power is limited to circa 10-20W (you will not be able use it as a heater in winter conditions or to weld speaker cables with 1kW RMS power).
- It's dirt cheap, too small, to little external components (you have no place to put WIMA's and Coilcraft).
- You cannot solder it at your workshop.

But...
- It uses PFFB loop with I/V kelvin sensing at the outputs.
- Declares 32 uV A-weighted output noise.
- Low and clean THD+N versus power .
- Finally beautifully consistent THD+N versus frequency.
- 90 dB worst-case PSRR.
- I2S input so you can do what you want with DSP before it.



Audio performance is on pair with NCxxxMP modules at least not the power. I am curious what about the sound?

Lovely solution for portable battery-powered or compact, low-power systems.

Windforce's tongue-in-cheek is perfect:

no a in -less D/A A/D conversions is better
low power -I only need 10 - 20W for tweets and hi eff mids
cheap -Cheap, small and low ext comp is GOOD
unsolderable -If you can find a good eval or diy ...


Great addition to an all digital system.

Perfect for tweeters and high eff mids!

"Description

The TAS2770 is a mono, digital input Class-D audio
amplifier optimized for efficiently driving high peak
power into small loudspeakers. The Class-D amplifier
is capable of delivering 15.4 W of peak power into a
4-Ω load while sustaining 11.6 W continuously with
less than 0.03 % THD+N at a battery voltage of 12.6
V.


Integrated speaker voltage and current sense
provides for real time monitoring of loudspeaker
behavior
. A battery tracking peak voltage limiter with
brown out prevention optimizes amplifier headroom
over the entire charge cycle of 2S or 3S battery
systems.

Up to eight devices can share a common bus via
either I2S/TDM + I2C."

from

http://www.ti.com/lit/ds/symlink/tas2770.pdf

TAS2770 typ app.png


Stereo EVM Layout Diagram
TAS2770 eval.jpg

Stereo EVM Layout Reference Design - 01
EVM Layout Reference Design - 01.jpg



Cheers,
Jeff

Last edited by AudioLapDance; 30th December 2017 at 07:46 PM.
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Old 31st December 2017, 10:23 AM   #2
Windforce85 is offline Windforce85  Poland
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Yes I love how it looks in datasheet and overal its tiny size and great capabilities. Looking forward someone will manufacture it soon!
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Old 31st December 2017, 11:09 AM   #3
TNT is offline TNT  Sweden
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Yes - it measures well indeed. Interesting.

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Old 31st December 2017, 09:56 PM   #4
Windforce85 is offline Windforce85  Poland
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This is the first class D amplifier (after NCore) which is free from THD rise at high frequency. What is more, first monolithic and first one with digital inputs. This, paired with PFFB promises really interesting amplifier especially for high efficiency mids and tweeters. In modern multi-way DSP-powered systems you can freely add any of powerful multi-Watt Class D amp for your woofers without bothering about trebles performance.

I have negative experience with amplifiers showing significant THD rise at high frequencies. They could have record-low 1 kHz distortion number but every time there was something wrong with them while focusing attention on trebles. Class D has ability to control low-frequency transducers very well so we all tend to distract our attentions from weak highs. Lack of long analog input path which is prone to many problems is, for me, another plus.

But... who can DIY manufacture this hardcore PCB arrangement and chip package?
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Old 31st December 2017, 10:09 PM   #5
AudioLapDance is offline AudioLapDance  Canada
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Quote:
Originally Posted by Windforce85 View Post
This is the first class D amplifier (after NCore) which is free from THD rise at high frequency. What is more, first monolithic and first one with digital inputs. This, paired with PFFB promises really interesting amplifier especially for high efficiency mids and tweeters. In modern multi-way DSP-powered systems you can freely add any of powerful multi-Watt Class D amp for your woofers without bothering about trebles performance.

I have negative experience with amplifiers showing significant THD rise at high frequencies. They could have record-low 1 kHz distortion number but every time there was something wrong with them while focusing attention on trebles. Class D has ability to control low-frequency transducers very well so we all tend to distract our attentions from weak highs. Lack of long analog input path which is prone to many problems is, for me, another plus.

But... who can DIY manufacture this hardcore PCB arrangement and chip package?

66803546.jpg

THD vs f flat at 0.1 W, 1W, and 5W!

thd vs f.png

thd vs f 2.png

Last edited by AudioLapDance; 31st December 2017 at 10:26 PM.
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Old 31st December 2017, 10:23 PM   #6
Windforce85 is offline Windforce85  Poland
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It dissipates maximum 1W per channel and is super-small. Designed for battery operation right from the start. Modern lo-impedance, high current Li-Ion chemistry opens the doors of new generation, clean and silent power supply. Why even bother about this while having well over 90 dB PSRR???

It makes of really interesting conception self-contained digital transducer: You can put it EVERYWHERE. Micro-cubic liter sealed cabinet. Lithium battery pack inside. The only connection with outside world - microUSB socket for charging and hi-res USB-audio (384 kHz capable!) data transfer. For lazy men you can add BT or Wi-Fi... The future is here.

Last edited by Windforce85; 31st December 2017 at 10:25 PM.
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Old 1st January 2018, 12:44 AM   #7
abraxalito is offline abraxalito  United Kingdom
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That PSRR plot needs to be taken with a huge pinch of salt - TI's way of measuring PSRR isn't real-world, even by their own admission.
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Old 1st January 2018, 01:12 AM   #8
Windforce85 is offline Windforce85  Poland
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Yeah they look like cannot be true. But imagine it is powered from low impedance Li-Ion battery pack. Low noise, low impedance voltage source which is close to ideal PSU considering its price and performance when someone knows how to design and use it. Why bother is it 60 or 90 dB PSRR then?

Finally we are getting rid of over-built analogue stages, too many conversion stages, and we can focus on front-end performance which is mostly related to transducer itself. The transducer has greatest influence on overall audio performance. If not using this new amp like full-band power-hungry device, you can imagine it being like part of linear-array DSP loudspeaker or like med-hi band amplification using modern neodymium strong and efficient drives. In contrary, low to mid frequencies where stroke matters, can be covered by more-common class D amplification this is where sheer power matters.

Last edited by Windforce85; 1st January 2018 at 01:16 AM.
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Old 1st January 2018, 06:33 AM   #9
5th element is offline 5th element  United Kingdom
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Quote:
Originally Posted by Windforce85 View Post
This is the first class D amplifier (after NCore) which is free from THD rise at high frequency.
The Ncore does not do this, it does exhibit a high frequency rise if measured in a way that can display it. Bandwidth limiting is used for hypex's datasheet measurements. Of course TI do the same thing, but when the measurement bandwidth goes up, and the third harmonic beyond ~8kHz can actually be measured properly, the THD shoots up.

Quote:
Originally Posted by Windforce85 View Post
What is more, first monolithic and first one with digital inputs.
Do not confuse this for anything special. The chip simply contains on on-board DAC, there's no difference to using a separate DAC + comparable, analogue input, class D amplifier. Of course it simplifies some things but that's about it.


Quote:
Originally Posted by Windforce85 View Post
This, paired with PFFB promises really interesting amplifier especially for high efficiency mids and tweeters.
How are you going to add PFFB to this amplifier? The built in I/V sensing is to add limiting to protect the loudspeaker. If you look at the performance plots you can see that employing the sensing system degrades the amplifiers performance rather significantly.

Proper PFFB is supposed to linearise the amplifier, not de-linearise!


Quote:
Originally Posted by Windforce85 View Post
I have negative experience with amplifiers showing significant THD rise at high frequencies.
I guess you don't like virtually all class A/B amplifiers either then!

Quote:
Originally Posted by Windforce85 View Post
But... who can DIY manufacture this hardcore PCB arrangement and chip package?
I could work with the QFN package (build and solder my own PCBs) if I had to. They aren't the easiest of things to solder but it can be done.

Still the performance of this low power class D chip amp is a significant improvement over pretty much every other similar product that TI have ever released. Noise and THD are a decent improvement.

It will need a proper output filter for a hifi application, where speaker cables will get longer than around 1 meter, otherwise EMI is going to be an issue. Although EMI worsens at high output power levels and when the device is operated from high voltage rails.

What intrigues me though is the overall performance of the class D amplifier module inside the chip. This seems to have many things going for it over what you'll find inside TIs TPA311x and their next generation replacements. The flat THD (measurement limitations notwithstanding) does appear to do something that the others do not. It would be nice to see an analogue input version with only the class D unit inside.

From a performance point of view though you'd be far better off combining a PCM5242 and TPA3244. Two chips of course, but with the TPA3244 you really are free to include post filter feedback for the proper gains that it brings. The PCM5242 is a balanced stereo DAC with DSP processing (if you want it) and can input directly into the 3244 keeping analogue signals very short.

The 3244 (and the rest of that series) might show rising THD with frequency but TIs graphs don't show they entire story.

The image I've attached shows the performance of a PFFB TPA3251 amplifier that I've built. This is for roughly 1 watt into 9.4 ohms. As you can see the midband distortion is far lower than TIs datasheet would lead you to believe. These are distortion measurements not distortion plus noise. Clearly TIs measurements are noise dominated at lower power levels, but my measurements show you the HD spectra, rather than just the THD.

As you can see the distortion is extremely low but shows the typical odd order rise with frequency. The BTL configuration helps to cancel out the even order harmonics, leaving the odd orders to shine through. These amps are really 3rd and 5th order dominant but when 20kHz bandwidth limiting is imposed you don't get to see how much they rise, especially if noise is dominating the measurements at low power levels.

What's impressive about the TAS2770 is that it does appear to keep it's distortion lower at high frequencies in a way that the Purepath UltraHD parts do not. Of course I am referring to the diagrams labelled figures 10 and 12. Where I/V sensing is used (figures 27 and 31) the distortion + noise has risen quite considerably, yes it's flat, but the level is high. I can't imagine this is because of gross noise levels, so it must just be high distortion from some mechanic integrated into the protection scheme.

Figure 32 is what interests me because here we are seeing the typical rising distortion that quickly falls away at 7kHz due to the third harmonic falling out of the measurements bandwidth. This is the 1 watt with 4.5V supply right? Why is it missing elsewhere with the 4.5V supply? The typical distortion rise is due to the amount of internal feedback being reduced for stability, so linearity decreases. This affects all amplifiers, be they class A, AB or D, just to different degrees.

Analogue Devices has an interesting part called the SSM3582 that you might be interesting in looking at too. It's very similar to the TAS2770.
Attached Images
File Type: gif 3.gif (97.8 KB, 246 views)
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Old 1st January 2018, 06:25 PM   #10
doctormord is offline doctormord  Germany
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Thanks for the post mate. Cheers and a happy new year.
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