Analog input filter on class D amplifier

[David1998]

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.

 

[vacuphile]

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.

 

[Eva]

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.

 

[abraxalito]

When an opamp follower stage sounds lousy pay attention to input RF filtering and power supply purity.

 

[David1998]

Thank you all for the replies so far. I will try your suggestions so far.

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:

Which should have a frequency response like this:

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

 

[AndrewT]

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.

 

[vacuphile]

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.

 

[AndrewT]

Here's a nice explanation of RC filtering:
https://www.youtube.com/watch?v=OBM5T5_kgdI

 

[David1998]

Thanks for the replies.

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/doc...r/The_Bass_Boost_and_Treble_Boost_Filters.pdf

I tested the filter with these component values:

Which should give this kind of response: (Input voltage 1V sine wave)

But thank you, I will try the interference filtering.

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.

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.