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Alistair W

New Chip Amp design - Class-D TPA3251

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Looks like a 1kHz signal (400us per horizontal division? please correct me if I am wrong), what do higher frequencies look like?

Good work BTW.

Regards,

SS

Hey Sub Sonic,

 

Yeah, 1KHz from a sig-generator on the bench at work (I don't have all this gear at home). I'll get a better idea when I can do some more testing on monday.

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I'll tweak it to go into clipping with something like 1V RMS.

 

 

I'm no amp' designer but would you not want it to go into clipping at a higher voltage? Given that an active preamp will be able to provide a lot more than that and even a DAC will output 2volts I would have guessed you'd want the clipping to be somewhere nearer 2V (or am I misunderstanding something?).

 

Mark

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I'm no amp' designer but would you not want it to go into clipping at a higher voltage? Given that an active preamp will be able to provide a lot more than that and even a DAC will output 2volts I would have guessed you'd want the clipping to be somewhere nearer 2V (or am I misunderstanding something?).

 

Mark

Yeah, somewhere between 1 and 2 volts RMS. I have a Nelson Pass B2 pre which doesn't have any gain, and my own design with x4 gain, so it'll depend on what works best..

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Alistair,

 

My Bride of Zen will output 9V before hitting 1% distortion but NP quoted "2.5V at 0.1% THD" so I guess he felt that was an acceptable level. 

 

Mark

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Things are moving along (slowly) here. I've run the amp for a few hours on a dummy load and swept it with the sig gen. Clean output from 20Hz to 20KHz. You can see some peaking of the output around 35-40KHz, due to the output filter, but there is bugger all audio in those freqs so not really an issue. The TPA chip is VERY efficient and the minimal heatsink I'm using hardly gets warm at modest levels.

 

Will be off to the audio show this weekend, so there won't be any DIY-ing for a few days. Hopefully come back with lots of ideas.

 

More to come..

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Alistair,

 

Does the amp have a built in low pass filter?

Mark

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Mark,

 

I put a low-pass at around 80kHz on the input buffers driving the chip amp's inputs, and I can always tweak this if needed.

I couldn't find any published spec for frequency response of this chip amp. Not sure if there is a low pass built into the IC.

 

-Alistair.

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http://www.ti.com.cn/cn/lit/ds/symlink/tpa3251.pdf

 

Table 8. Design Requirements, SE Application

 

SE Application:

 

Output Filters Inductor-Capacitor Low Pass Filter (15 µH + 680 nF)

 

BTL application:

 

Output Filters Inductor-Capacitor Low Pass FIlter (10 µH + 1 µF)

 

Also:

 

Most audio analyzers will not give correct readings of Class-D amplifiers’ performance due to their sensitivity to
out of band noise present at the amplifier output. AES-17 + AUX-0025 pre-analyzer filters are recommended to
use for Class-D amplifier measurements. In absence of such filters, a 30-kHz low-pass filter (10 Ω + 47 nF) can
be used to reduce the out of band noise remaining on the amplifier outputs.

 

Does that help?

 

Mark

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Hi Mark,

 

Sorry, I just didn't click you were talking about the output filter. I've spec'd the output filter to suit a 6-8 ohm load as that's what my speakers are.

 

Here's a white paper from TI explaining how to calculate the output filter values.

 

www.ti.com/lit/pdf/sloa119

 

I'm using 15uH in series and 0.47uF between the two inductors (AD modulation)

 

So the output filter interacts with the speaker load - to get the 'best' response (critical damped or whatever), you need to know your speaker impedance. Of course this can change with frequency!

Or to put it another way, if the amp's output filter is made to suit a 4 ohm load, then with a 8 ohm load it won't neccessarily give a flat response. I have this as an LTspice sim somewhere and may be able to post it later.

 

Cheers,

 

Al.

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Thought I'd better resurrect this thread, the amp is built and sounds great!

I've put it in a diecast box as this one is going to be a workhorse. I'm going to build up a few more channels for my next project.

 

Amp finished #1.jpgAmp finished #2.jpg

 

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I'm very happy with the amp - I can only describe the sound as very natural.

Instruments are clearly defined and positioned, good depth of soundstage, etc etc..

There is no background noise at all, even when I put my ear right up against the speaker cone. Neither are there any thumps or pops at power on/off.

 

Congrats TI, a bloody good chip amp in my opinion! - now come on and bring out the higher power one.

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On 9/28/2016 at 8:59 PM, Alistair W said:

Congrats TI, a bloody good chip amp in my opinion! - now come on and bring out the higher power one.

 

Nice one. TPA3255 should be available soon for more power.

Did you use feedback after or before the filter? 

Wish I was closer to Victoria, I would love to test your board.

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I just got samples of TPA3255 this week  :-)

So I'm building up a 2nd board and can't wait to compare the two!  I designed the PCB from the start to handle the higher power chip, and was lucky the lower power TPA3251 was drop-in compatible.

 

My board uses the TI reference design as it was back in June-July. They had the post-filter feedback in their schematic as an option, but there was no explanation as to what it was or how to configure it for the TPA chip. So I left it out of my PCB. I'm interested if it makes a big difference - if so I may want to revise my PCB and add it in.

 

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I'm interested to see how this new build goes.

What will be the typical output be into 8 ohms on the new amp?

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3 hours ago, Mungbean66 said:

I'm interested to see how this new build goes.

What will be the typical output be into 8 ohms on the new amp?

http://www.ti.com/product/TPA3255

Assuming 1 board per channel and allowing 1% distortion:

    • 315-W Stereo into 4 ? in BTL Configuration
    • 185-W Stereo into 8 ? in BTL Configuration
    • 600-W Mono into 2 ? in PBTL Configuration
  • Total Output Power at 1%THD+N
  • Given that these babies are designed to run down to 2ohm, I think running a higher impedance speaker is a bit of a waste - assuming you have a choice.

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My thoughts are trying to get 600W from this tiny little chip is a bit of a stretch, so I'm setting up the board to run PBTL and will run 2 boards to get a stereo amp capable of 150W/8R, 300W/4R, and it should run cooler and lower distortion, as each channel 'sees' twice the total load - i.e. 2 channels set up as PBTL into a 2 ohm load effectively are driving 4 ohms each channel..

I've been building a regulated linear PSU for the board. TI say the absolute maximum for this chip is 36V (TPA3251) and 52V (TPA3255), so I've come up with a linear reg with capacitance muliplier circuit (inspired by one of Rod Elliott's designs). It removes the ripple from the unregulated supply, running the output about 4V below the input. Should the mains voltage go too high, it limits the output to a safe level, 36V or 52V.

Pictures to follow..

 

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