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Combining class D in one audio application: beat tones
Combining class D in one audio application: beat tones
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Old 18th February 2018, 09:48 AM   #1
taita is offline taita  Netherlands
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Default Combining class D in one audio application: beat tones

I am working on an active multichannel system controlled by a DSP. Currently I do some experiments with a two way system using two TDA7498 amplifiers: one for the woofer and one for the tweeter. When I examined other class D amplifier options I found an alarming phrase in the documentation of both TDA 8953 and TDA 8932 (both NXP)

If two or more Class D amplifiers are used in the same audio application, an external clockcircuit must be used to synchronize all amplifiers. This will ensure that they operate at the same switching frequency, thus avoiding beat tones (if the switching frequencies are different, audible interference known as ‘beat tones’ can be generated).

This sounds like a generic issue that occurs when combining class D amplifiers in general. The text is not clear what is meant with 'the same audio application'. I can imagine that this occurs when amplifiers are used in bridged mode, but does this also count for an audio application where different amplifiers control different frequency ranges like with my DSP based system?
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Old 18th February 2018, 01:30 PM   #2
Khron is offline Khron  Finland
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Class-D amps radiate plenty of RF hash. The phrase "the same audio application" most likely means "within the same enclosure" and/or "in close proximity of each other".

Intermodulation - Wikipedia

Say one amp is switching at 400kHz, while the one next to it works at 398kHz. Odds are good you'll get a 2kHz "extra" tone at the output of both of them. Hence the need to synchronize them, whenever possible. I'm sure they all have some master/slave modes configurable, where they can either output or accept a clock / sync signal on one pin.
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Old 18th February 2018, 10:12 PM   #3
taita is offline taita  Netherlands
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Originally Posted by Khron View Post
Class-D amps radiate plenty of RF hash. The phrase "the same audio application" most likely means "within the same enclosure" and/or "in close proximity of each other".

Intermodulation - Wikipedia

Say one amp is switching at 400kHz, while the one next to it works at 398kHz. Odds are good you'll get a 2kHz "extra" tone at the output of both of them. Hence the need to synchronize them, whenever possible. I'm sure they all have some master/slave modes configurable, where they can either output or accept a clock / sync signal on one pin.
Ok, so this has to do with the high frequency radiation, not with sounds caused by interference between loudspeakers (where the HF component is filtered). Besides the hassle of synchronization, should a faraday box help to avoid beat tones (whatever it may be)?
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Old 19th February 2018, 12:25 AM   #4
Khron is offline Khron  Finland
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In theory, yes (or maybe), but is syncing up the amps really a total no-go?

TDA7498 datasheet, page 19:
http://www.st.com/content/ccc/resour...CD00244535.pdf

TDA8953 datasheet, page 24:
https://www.nxp.com/docs/en/data-sheet/TDA8953.pdf

Even if you were to mix these two chips in your system, it looks like the sync signal should be compatible (in both cases it's double the switching frequency of the half-bridges).

Maybe i'm weird, but running a wire seems to me like a liiiittle bit less hassle than experimenting with Farady cages and whatnot...
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Old 19th February 2018, 01:06 AM   #5
5th element is offline 5th element  United Kingdom
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As far as I am aware this is only when two amplifiers are connected to the same power supply. Otherwise it really doesn't matter.
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Old 19th February 2018, 01:25 AM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
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Its to do with intermodulation between two frequencies which generate the sum and difference frequencies too.
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Old 19th February 2018, 06:14 AM   #7
5th element is offline 5th element  United Kingdom
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Surely once appropriate low pass filtering is applied though the issue is removed, or rather contained and limited to the area before the filters.

Filterless operation could be a problem.
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Old 19th February 2018, 06:42 AM   #8
jan.didden is offline jan.didden  Europe
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It is pretty much impossible to filter and/or screen out, the switching is everywhere. The design is such that it is cancelled at the output but it is there on the supplies, on the grounds, other wiring, as well as radiated. Do the synchronization, it has been incorporated for the purpose to get clean music. Which is what most people would like.

Jan
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Old 19th February 2018, 12:23 PM   #9
taita is offline taita  Netherlands
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Originally Posted by jan.didden View Post
It is pretty much impossible to filter and/or screen out, the switching is everywhere. The design is such that it is cancelled at the output but it is there on the supplies, on the grounds, other wiring, as well as radiated. Do the synchronization, it has been incorporated for the purpose to get clean music. Which is what most people would like.
Jan
Ok, I get your point. I use the same power supply so the intermodulation might be there. I read the TDA7498 datasheet and its not complicated to synchronize both clocks. But how to do this practically? First I have to remove the heatsink. Then locate the right pins. Then I have to disable a pin and make connections with my big soldering device on the tiny IC pins. I am afraid this will lead to failure.

Another possibility is to combine two different amplifiers (class D and class AB)

Last edited by taita; 19th February 2018 at 12:33 PM.
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