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Passive Filter for Class D Amp Testing
Passive Filter for Class D Amp Testing
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Old 16th July 2015, 07:42 AM   #11
JohnW is offline JohnW  Czech Republic
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What about this guy?

AES17 1998 Switching Amplifier Lowpass Low Pass Filter | eBay

I have no idea if its any good... but it seems perfect for your needs wit balanced inputs for testing of BTL ClassD output stages...
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Old 16th July 2015, 10:34 PM   #12
leadcoma is offline leadcoma  Canada
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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!

Last edited by leadcoma; 16th July 2015 at 10:41 PM.
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Old 17th July 2015, 07:42 AM   #13
JohnW is offline JohnW  Czech Republic
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Maybe You could try a X10 probe - I'm not sure about the filters input impedance.
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Old 17th July 2015, 08:02 AM   #14
arnyk is offline arnyk  United States
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Quote:
Originally Posted by leadcoma View Post
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.
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Old 17th July 2015, 02:28 PM   #15
leadcoma is offline leadcoma  Canada
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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.
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Old 17th July 2015, 02:50 PM   #16
JohnW is offline JohnW  Czech Republic
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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.
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Old 17th July 2015, 09:07 PM   #17
udok is offline udok  Austria
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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
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Old 17th July 2015, 09:52 PM   #18
JohnW is offline JohnW  Czech Republic
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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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Last edited by JohnW; 17th July 2015 at 09:58 PM.
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Old 18th July 2015, 08:28 AM   #19
udok is offline udok  Austria
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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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Old 18th July 2015, 03:14 PM   #20
LukasLouw is offline LukasLouw  United States
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Passive Filter for Class D Amp Testing
Elektor published a design for I believe an 8th order passive low pass filter for testing Class D amps.
https://www.elektormagazine.com/magazine/201107/19647
I see they offer a PCB:
Elektor Measurement Filter for Class D PCB Elektor

LinearX also offers what seem to be a pretty decent 8th order passive filter:
LF280 - Precision Amplifier Filter & Attenuator - 1
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