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Class D Switching Power Amplifiers and Power D/A conversion

Analog input filter on class D amplifier
Analog input filter on class D amplifier
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Old 24th June 2017, 09:33 PM   #1
David1998 is offline David1998
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Default Analog input filter on class D amplifier

Hello,

Is it supposed to be possible to use an analog filter (RC filter, with/without OpAmp) to filter out certain frequencies before amplifying the signal with a class D amplifier? It seems like with everything I try, the audio quality goes down dramatically. Even if I use a unity gain OpAmp buffer, the audio sounds like crap.

I do use cheap Ebay class D amplifier modules, I've tried the PAM8610 and the TPA3110, but they both sound just fine without the filtering.

When I search about this on Google, it seems like everyone uses either a DSP or an output crossover of some sort.

Thanks in advance.
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Old 24th June 2017, 11:06 PM   #2
vacuphile is offline vacuphile  Netherlands
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If they sound just fine, why change a winning team? Perhaps you could explain in a bit more detail what you want to accomplish and how.
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Old 25th June 2017, 05:04 AM   #3
Eva is offline Eva  Spain
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Grounding problem resulting in increased noise floor? HF interference? The input signal to a class D amplifier should be as free as possible from >20khz components such as produced by digital circuits.
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Old 25th June 2017, 06:13 AM   #4
abraxalito is offline abraxalito  United Kingdom
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When an opamp follower stage sounds lousy pay attention to input RF filtering and power supply purity.
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Old 25th June 2017, 10:32 AM   #5
David1998 is offline David1998
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Thank you all for the replies so far. I will try your suggestions so far.

Quote:
Originally Posted by vacuphile View Post
If they sound just fine, why change a winning team? Perhaps you could explain in a bit more detail what you want to accomplish and how.
What I'm trying to do is boost the lower frequencies a little, since I want more bass. Eventually I want to make my own Bluetooth portable speaker. Since class D amplifiers offer a high power output with a relatively low battery voltage, and are super efficient, this would be a great option for the amplifier.

I'm thinking of using this kind of filter:

Click the image to open in full size.

Which should have a frequency response like this:
Click the image to open in full size.

I tried it with a TDA2030 class AB amplifier and I finetuned the R and C values. This works good but now I want this to work with a class D amplifier
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Old 25th June 2017, 11:33 AM   #6
AndrewT is offline AndrewT  Scotland
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That looks terrible.

You have a step function with +46.8dB in the low bass region and -13.2dB in the high treble region.
The band through the remaining audio band is NOT FLAT. It slopes from +46dB to -13dB over six and a half octaves. That's ~ 9dB per octave.
No wonder it sounds crap.

Just use a normal RF input filter. Set it somewhere around 30kHz to 200kHz.
It can be a passive RC using a 1k resistor and 1nF capacitor.

If you think you need a more severe filter, then adopt an active S&K unity gain low pass. Set this to somewhere between 25kHz and 100kHz.

Do the interference filtering first.

Then read up on doing a bass boost, but apply that separately. Maybe make it defeatable/switchable.
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Old 25th June 2017, 11:44 AM   #7
vacuphile is offline vacuphile  Netherlands
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Hi David,

If you want tone control, it might be worthwhile for you to look at a Baxandall schematic and build one if you like it.
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Old 25th June 2017, 12:14 PM   #8
AndrewT is offline AndrewT  Scotland
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Here's a nice explanation of RC filtering:
https://www.youtube.com/watch?v=OBM5T5_kgdI
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Old 25th June 2017, 01:14 PM   #9
David1998 is offline David1998
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Thanks for the replies.

Quote:
Originally Posted by AndrewT View Post
That looks terrible.

You have a step function with +46.8dB in the low bass region and -13.2dB in the high treble region.
The band through the remaining audio band is NOT FLAT. It slopes from +46dB to -13dB over six and a half octaves. That's ~ 9dB per octave.
No wonder it sounds crap.

Just use a normal RF input filter. Set it somewhere around 30kHz to 200kHz.
It can be a passive RC using a 1k resistor and 1nF capacitor.

If you think you need a more severe filter, then adopt an active S&K unity gain low pass. Set this to somewhere between 25kHz and 100kHz.

Do the interference filtering first.

Then read up on doing a bass boost, but apply that separately. Maybe make it defeatable/switchable.
Well I did finetune the R and C values, according to simulations and calculations.
By the way, I got the filter configuration from this document I found the other day. http://labkit.ru/userfiles/file/docu...st_Filters.pdf

I tested the filter with these component values:
Click the image to open in full size.

Which should give this kind of response: (Input voltage 1V sine wave)
Click the image to open in full size.

But thank you, I will try the interference filtering.

Quote:
Originally Posted by AndrewT View Post
Here's a nice explanation of RC filtering:
https://www.youtube.com/watch?v=OBM5T5_kgdI
I do study Electrical Engineering, so I'm familiar with these simple RC filters. I've also used the Sallen-Key filter before.

But I wanted to boost the bass a little, while not completely removing the higher frequencies.

Quote:
Originally Posted by vacuphile View Post
Hi David,

If you want tone control, it might be worthwhile for you to look at a Baxandall schematic and build one if you like it.
Thank you sir, I will look into that type of filter too.
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