Passive Filter for Class D Amp Testing

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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 Precision'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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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
 
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.
 
yes, Audio Precision. thanks for catching that and sharing the link.

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

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
 
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. :(
 
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.
 

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I scoured Ebay for these but all my searches included "Class D" text :)

It is unfortunately an active filter with a maximum input of 10V but thanks for the link anyway!

For decades I've built the required attenuators into my power amp dummy loads, which really doesn't take a lot of much of anything because a good 5K or 10K 2 watt linear potentiometer usually does the job.

The dummy loads have an XLR jack for the analytical equipment and a pair of banana jacks for the voltmeter for monitoring power levels.
 
JohnW, It is the input opamp that I am clipping because of the high amplitude of the switching "noise". The clipping issue is no different with an active filter because it also uses input opamps similar to a sound card. With a passive filter network, the switching noise would be reduced and not clip the input opamp of the sound card.

Arnyk, good idea and I will try this also.
 
leadcoma,

I VERY much doubt you have 10V pk switching carrier (unless this is a filterless ClassD design) - if you have over 10Vpk switching carrier then you have other issues to worry about.

Also, are you trying to measure a full bridge or half bridge ClassD design? if a full bridge design then you will need a Diff to single-ended stage to correctly measure the Amplifier with a typical Single-ended input sound card.

I'm sure the linked EBAY AES17 filter will have some form of Passive filter in front of the Opamp Diff stage - as its been designed for this application.

As both Arnyk and I have tried to point out, a simple input attenuator will solve any input overload problems you may have.

The typical PC grade sound card has very low input range - maybe only +/- 1V, the above filter box with 10V input has 20dB greater input headroom.
 
Hi,

The AES-17 filter is intended for measurig DAC output. The filter remove
the out of band noise of the DAC to make the SNR numbers prettier.
The filter is very steep (about -60 dB @ 24 kHz) and are
generally not intended for class-D filtering with very high dV/dt.

Udo
 
Udo,

I believe your confusing the AP's use of the AES17 filter and the general concept of the AES17 filter.

There is NOTHING in the official AES17 spec. to prevent it being applicable to ClassD applications. The AES17 just defines a filter response, it says nothing about its design.

The AP requires the 20KHz / 40KHz "Pre Filter" board on its input stage to prevent the Gain scaling detector getting confused - this extra filter PCB is part of the AES17 filter kit.

In my experience the AUX-0025 is only required with AP's - I've never need to use it with my R&S. The use of the AUX0025 introduces quite high distortion - around 0.0025% IIRC.

If low distortion is important then I do not recommend a L/C filter design as standard inductors struggle to achieve THD below 0.001% due to there core hysteresis - air cores are linear but it requires great care to prevent them coupling with the chassis & each other.

A very simple single stage passive R/C filter will reduce the high dV/dt EMI of a ClassD output stage to a level that will not upset a decent opamp.

The AP is not very linear in the presence of RF on its input - this is typical behavior of the 5534 and AD797 opamps used. The AD797 uses a non degenerated input pair to achieve its low noise at the expense of RF IMD linearity. I avoid the AD797 for this reason.

The filter I linked to is apparently specifically designed to measure low power ClassD amplifiers:-

AES17 1998 Switching Amplifier LowPass (low-pass) Filter | eBay

AES17 1998 Switching Amplifier LowPass(low-pass) Filter

AES17-1998,2CH,4-Poles,30KHz,true differential input

Besides, with any filtered ClassD amplifier you will not be exposed to the high dV/dt of the output stage - if the residual RF energy after the amplifiers LC filter upsets a decent opamp, then you have a much bigger problem with EMC to worry about.
 
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Hi John,

Yes, I referred to the Audio Precision usage of the AES-17 filter. A simple R-C filter gives about 20 dB attenuation and should be ok for "normal" Class-D amps.
Interesting note you made about the AD797. Did'nt think of the degeneration resistors. AD claim that the AD797 holds its superb noise performance up to very high frequencies due to the single stage design.
Can you post a graph of the R&S THD+N over frequency if you find time? Would be
interesing to see the comparison to AP Sys2.
We have at work a lot of UPV and UPD but none with the low distortion generator option :-/
But i can post a graph of the newest AP 555 machine :)

Greetings,
Udo
 
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