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Class D Switching Power Amplifiers and Power D/A conversion

Class D Design Issues
Class D Design Issues
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Old 14th May 2017, 01:27 PM   #101
Ouroboros is offline Ouroboros  England
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Default Output filter perhaps?

I would guess the Behringer uses pre-filter NFB, so the amplitude and phase characteristics of the output low-pass filter directly affect the measured response. The switching frequency isn't likely to be the culprit, except that a higher switching frequency would allow the use of a higher cut-off frequency for the output filter, minimising the effect.
Class-D amps with post filter NFB, such as those from Hypex, greatly reduce the effect, although they can suffer adverse effects if the load is extremely reactive.
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Old 14th May 2017, 01:43 PM   #102
JonasKarud is offline JonasKarud  Sweden
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Quote:
Originally Posted by Ouroboros View Post
I would guess the Behringer uses pre-filter NFB, so the amplitude and phase characteristics of the output low-pass filter directly affect the measured response. The switching frequency isn't likely to be the culprit, except that a higher switching frequency would allow the use of a higher cut-off frequency for the output filter, minimising the effect.
Class-D amps with post filter NFB, such as those from Hypex, greatly reduce the effect, although they can suffer adverse effects if the load is extremely reactive.
Yes, it's a pre-filter NFB design using the IRS20957s driver and the switching frequency is one MHz, checked it with oscilloscope..

What happens if I change the output inductor from 21 uH to, let's say 10 uH?
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Old 14th May 2017, 01:46 PM   #103
Dimonis is offline Dimonis  Russian Federation
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Quote:
Originally Posted by JonasKarud View Post
the switching frequency is one MHz
Joking Class D Design Issues Class D Design Issues ?
It must be 320-350kHz.Not more.....
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Old 14th May 2017, 01:54 PM   #104
Ouroboros is offline Ouroboros  England
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1MHz is a very high switching frequency for a class-D amplifier, normally it would be between 300kHz and 400kHz.

You cannot simply change the L or the C in the output filter on its own. The filter needs to be (roughly) matched to the load.
It might be useful for you to download the free version of TINA-spice from the Texas Instruments website and perform a few frequency response tests using a simple sinewave source. You could start with L=22uH, C= 1uF and R_load = 6 Ohms.

TINA is very easy to use and gives you a good insight into how component changes affect things.
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Old 14th May 2017, 04:19 PM   #105
JonasKarud is offline JonasKarud  Sweden
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Sorry, I misread the oscilloscopes 1 uS/div scale, 400 KHz is a more correct value!

Still high, apparently. Can be the uncalibrated old scope.
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Old 15th May 2017, 06:50 AM   #106
basreflex is offline basreflex  Spain
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the high noise content in the 10-20khz range could be the smeared difference freq of the two L/R modulators interfering . the idling freqs of the 2 channels should be either synced or at least 30khz apart.
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Old 15th May 2017, 08:44 AM   #107
JonasKarud is offline JonasKarud  Sweden
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On the output board there's a ferro core with 4 separate windings. See picture and diagram. In my opinion, this will connect left and right channels with each other inductively. I'm familiar with Tina and P-spice, but I think this one will be difficult to analyze/simulate, involving two channels.

I shortened out the inductor with 4 jumpers(see picture), and measured again.

The noise is the exactly the same, but phase went from -180 deg to +180 in the same freq region. The sound now is improved, with more open/airy treble.(but not as good as my beloved Nad M3)
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Old 15th May 2017, 09:02 AM   #108
JonasKarud is offline JonasKarud  Sweden
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After some more listening, I can tell the sound is very much better!
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Old 15th May 2017, 09:28 AM   #109
JonasKarud is offline JonasKarud  Sweden
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Time for some serious A/B listening, with or without the inductor shorted!
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Old 15th May 2017, 09:34 AM   #110
basreflex is offline basreflex  Spain
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can you measure the Ch1 and Ch2 idle freqs accurately ?
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