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Old 14th July 2015, 12:18 AM   #1
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Default Passive Filter for Class D Amp Testing

I'm testing a Class D amp with my computer sound card and I've been told I need a passive 20Hz-20KHz filter to limit the high frequency output of the amp. Audio Control's AUX-0025 is an obvious choice but beyond my budget.

Does anyone know where I can buy a low cost passive filter for testing Class D amps?
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Old 14th July 2015, 06:13 AM   #2
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You probably mean Audio Precision, not Audio Control?

If the amplifier is well designed (in particular has low conducted EMI), you could very likely use a DIY active low-pass filter, or perhaps even get away without any filter (particularly if your sound card has good anti-image filters).

See also the post by Putzeys here: Stanford Research Systems - SR1 - Dual-domain audio analyzer - Experiences?

Samuel
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Old 14th July 2015, 06:31 AM   #3
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Construct a high pass 20HZ and low pass 20kHz active filter. This can easily be done.
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Old 14th July 2015, 07:21 AM   #4
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Quote:
Originally Posted by leadcoma View Post
I'm testing a Class D amp with my computer sound card and I've been told I need a passive 20Hz-20KHz filter to limit the high frequency output of the amp. Audio Control's AUX-0025 is an obvious choice but beyond my budget.

Does anyone know where I can buy a low cost passive filter for testing Class D amps?
If the ultrasonics at the amp output aren't large enough to cause appreciable distortion in the measuring gear, you can do the low pass filtering in the digital domain, after you collect the signal and before you apply your measurements.

Or you can just identify the high frequency components you want to ignore and remove their influence from your analysis of the output signal.
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Old 14th July 2015, 07:21 AM   #5
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Good paper, looking mostly at their "filter free" class D amps. http://www.ti.com/lit/an/sloa068/sloa068.pdf
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Old 14th July 2015, 09:46 PM   #6
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yes, Audio Precision. thanks for catching that and sharing the link.

DIY is an option but I'd rather purchase a low cost filter.

Quote:
Originally Posted by Samuel Groner View Post
You probably mean Audio Precision, not Audio Control?

If the amplifier is well designed (in particular has low conducted EMI), you could very likely use a DIY active low-pass filter, or perhaps even get away without any filter (particularly if your sound card has good anti-image filters).

See also the post by Putzeys here: Stanford Research Systems - SR1 - Dual-domain audio analyzer - Experiences?

Samuel
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Old 14th July 2015, 09:57 PM   #7
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Quote:
Originally Posted by arnyk View Post
If the ultrasonics at the amp output aren't large enough to cause appreciable distortion in the measuring gear, you can do the low pass filtering in the digital domain, after you collect the signal and before you apply your measurements.

Or you can just identify the high frequency components you want to ignore and remove their influence from your analysis of the output signal.
These are 250W Class D amps with significant 480KHz output that clips the sound card input buffers.
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Old 15th July 2015, 08:27 AM   #8
udok is online now udok  Austria
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Here is a simple passive filter which does what you need. Beware that
the 10 Ohm source impedance is important and that the values should be better than 2%.

Use Polypropylen capacitors and good wire wound inductors, ideally with an air core.

The second filter has a -3dB of 50 kHz and imperfections of L and C do not effect the audio band as much as the first version.
Attached Images
File Type: png PassiveFilt1.png (34.2 KB, 100 views)
File Type: png PassiveFilt2.png (32.7 KB, 96 views)

Last edited by udok; 15th July 2015 at 08:56 AM. Reason: added 2nd filter
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Old 15th July 2015, 08:58 AM   #9
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Quote:
Originally Posted by leadcoma View Post
These are 250W Class D amps with significant 480KHz output that clips the sound card input buffers.
One word: attenuator.

I build mine into the dummy loads.
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Old 15th July 2015, 11:17 PM   #10
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Udok, thank you for the filter designs! It looks like I may just need to build my own.

Arnyk, good suggestion. I will give it a try.
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