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Passive Filter for Class D Amp Testing
Passive Filter for Class D Amp Testing
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Old 6th November 2017, 04:50 AM   #61
lholland is offline lholland  United States
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Passive Filter for Class D Amp Testing
Quote:
Originally Posted by bnae38 View Post
Isn't it local pickup only? :o
It was, but the seller told me it was an error and he'd be glad to ship, but fussed that he didn't know how to set a reserve price on an auction. Apparently he got concerned I'd win it at a much lower price than the $600 buy it now price and magically I get an eBay email stating the auction had been cancelled.

The tools Duke has given me are invaluable nevertheless, particularly in regard to measurement techniques for performance verification. I've been fooling around with a circuit design for a month or so and should have it completed in another month. It includes ideas from Chris Strahm's LF280 as well as the AP unit. At least now I have solid methods to figure out how bad I fail at it. :)
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Old 19th November 2017, 11:36 PM   #62
bnae38 is offline bnae38  United States
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Have a classD in my hands finally (doing some mods for somebody else), happy to report the filter works as expected.

Seeing minimal phase shift, otherwise not much else.. just a touch of insertion loss..

Amplifier noise floor without and with filter below.

Click the image to open in full size.
Click the image to open in full size.
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Old 20th November 2017, 12:45 AM   #63
Audio1Man is offline Audio1Man  United States
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Passive Filter for Class D Amp Testing
Hi Bane
It look like you have a good start on the filter. Can you run an ANALOG frequency response with and without the filter only using the Dscope? One of the HF spikes is not attenuated. What about checking the differential mode? What is the distortion of the filter? How much change in the distortion of the amplifier with & without the filter?

Send me a message or call as I can help you set up some tests. You may be able to make some improvements.
Duke
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Old 20th November 2017, 09:27 PM   #64
bnae38 is offline bnae38  United States
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Quote:
Originally Posted by Audio1Man View Post
Hi Bane
It look like you have a good start on the filter. Can you run an ANALOG frequency response with and without the filter only using the Dscope? One of the HF spikes is not attenuated. What about checking the differential mode? What is the distortion of the filter? How much change in the distortion of the amplifier with & without the filter?

Send me a message or call as I can help you set up some tests. You may be able to make some improvements.
Duke
Hi Duke, appreciate the help. Pics below. Freq response, common mode response, and thdn with two different values for lpf 22k and 40k (hpf was 22hz). Last is full shot of freq response.

I can't remember how large of a thdn difference it made with the classD amp, it was fairly significant.

Thanks,
Ben

Click the image to open in full size.
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Old 20th November 2017, 10:19 PM   #65
bnae38 is offline bnae38  United States
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Keep in mind the pics in 62 were the noise floor of the amp, not necessarily clean .

Very happy to see the shifty large spikes in the 5-10k range gone, seen that many times when looking at a class d amp. Obviously aliasing crap..
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Old 21st November 2017, 06:24 PM   #66
QAMAtt is offline QAMAtt
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Some brief notes on measuring class D: The big class D amps with hefty output filters can usually be measured as-is directly into the analyzer. They are required to have really good LC output filters because these amps must be designed to drive various types of tweeters, and tweeters don't usually have enough inductance to filter the switching. And so, if you don't have a solid LC filter and drive a tweeter, the tweeter is probably ruined.

Note these big amps generate a 0V signal by "pushing" the +V rail and the "pulling" the -V rail. Even though the output is zero, the amp is still slamming the signal rail to rail on each switching cycle. That is heavily filtered, resulting in a zero.

These big class D amps also have to meet EMI, which means they cannot have high slew rate signals existing on a wire going to a speaker. Again, this means great LC output filter.

In the end, these types of amps can usually go straight into an audio analyzer (differential, of course).

There's another type of class D amplifier called "filterless". These are usually small, few watt amps where the amp is married to the speaker by the equipment maker (think of cell phone) and the speaker is very inductive by design.

These amps generate a 0V signal across by pushing the same switching signal (no phase shift) to both terminals of the speaker. They do it this way because these are usually supply amps. To generate a non-zero signal, they simply slide one of the output switching waveforms in time. Very clever. The fidelity on these amps isn't great. But the other wins are huge.

If you put the output of these amps directly into an analyzer, the slew rates cause big problems. But it's easy to fix: just run the amp output through the TI-recommended first-order 30 KHz RC filter posted earlier in the thread. That it! That filter smears the fast edges of the switching signal just enough so that the CMRR of the analyzer can take care of the rest.

Here's a capture of a TI TPA2038 filterless class D. It's rated for 1% at 2.5W into 4 ohms from 5V, and the measurement confirms that. This was using TI's prescribed first order RC filter.
Attached Images
File Type: png image1.png (51.0 KB, 28 views)
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Old 22nd November 2017, 01:40 PM   #67
bnae38 is offline bnae38  United States
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Quote:
Originally Posted by QAMAtt View Post
Some brief notes on measuring class D: The big class D amps with hefty output filters can usually be measured as-is directly into the analyzer. They are required to have really good LC output filters because these amps must be designed to drive various types of tweeters, and tweeters don't usually have enough inductance to filter the switching. And so, if you don't have a solid LC filter and drive a tweeter, the tweeter is probably ruined.

Note these big amps generate a 0V signal by "pushing" the +V rail and the "pulling" the -V rail. Even though the output is zero, the amp is still slamming the signal rail to rail on each switching cycle. That is heavily filtered, resulting in a zero.

These big class D amps also have to meet EMI, which means they cannot have high slew rate signals existing on a wire going to a speaker. Again, this means great LC output filter.

In the end, these types of amps can usually go straight into an audio analyzer (differential, of course).

There's another type of class D amplifier called "filterless". These are usually small, few watt amps where the amp is married to the speaker by the equipment maker (think of cell phone) and the speaker is very inductive by design.

These amps generate a 0V signal across by pushing the same switching signal (no phase shift) to both terminals of the speaker. They do it this way because these are usually supply amps. To generate a non-zero signal, they simply slide one of the output switching waveforms in time. Very clever. The fidelity on these amps isn't great. But the other wins are huge.

If you put the output of these amps directly into an analyzer, the slew rates cause big problems. But it's easy to fix: just run the amp output through the TI-recommended first-order 30 KHz RC filter posted earlier in the thread. That it! That filter smears the fast edges of the switching signal just enough so that the CMRR of the analyzer can take care of the rest.

Here's a capture of a TI TPA2038 filterless class D. It's rated for 1% at 2.5W into 4 ohms from 5V, and the measurement confirms that. This was using TI's prescribed first order RC filter.
Not sure I'm totally on board. Every mobile audio Class-D i've run on a dscope has had decent output filters built in. See post 62. All have had problems with aliasing crap into the 20-20khz range from higher up.

Begs the question, is rolling off the top with an aux-0025 (or the like) a fair way to measure thd... In my mind, that stuff isn't really there and is analyzer error but.... hmm

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Old 22nd November 2017, 04:08 PM   #68
QAMAtt is offline QAMAtt
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Quote:
Originally Posted by bnae38 View Post
Not sure I'm totally on board. Every mobile audio Class-D i've run on a dscope has had decent output filters built in. See post 62. All have had problems with aliasing crap into the 20-20khz range from higher up.

Begs the question, is rolling off the top with an aux-0025 (or the like) a fair way to measure thd... In my mind, that stuff isn't really there and is analyzer error but.... hmm

It's easy to check: Just make the measurement with and without filters as you did. If there's a difference, then that's your answer! I guess my point was more along the lines of there's 2 types of class D, one type (filterless) definitely needs an external first-order filter always and the other type (filtered) often not.

Here's a TI 300W class amp (no input) running at 50V rails straight in with no filters. 91 uV of noise A weighted, 20 to 20 KHz. TI typical spec is 85uV.

Yes, I think AUX-0025 is fair for measuring THD. TI specs their THD measurements with 80 to 80 KHz bandwidth specified on the analyzer BUT with an AUX-0025 and an AES17. So, effectively a brick-wall beyond 20 KHz.
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File Type: png TPE3255EVM Noise Floor (A Weighting).png (40.3 KB, 22 views)
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Old 22nd November 2017, 06:24 PM   #69
Audio1Man is offline Audio1Man  United States
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Passive Filter for Class D Amp Testing
Note: The AUX-0025 filter is not a BRICK WALL FILTER
Duke
Attached Files
File Type: pdf AUX0025-FREQ.pdf (11.9 KB, 41 views)
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Old 22nd November 2017, 07:16 PM   #70
QAMAtt is offline QAMAtt
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Quote:
Originally Posted by Audio1Man View Post
Note: The AUX-0025 filter is not a BRICK WALL FILTER
Duke
The AES17 gives 60 dB of rolloff from 20 to 24KHz. That's certainly seems like a brick wall filter.
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