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Old 23rd June 2013, 04:39 PM   #1
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Default Increased noise with multiple modules

As you all saw, my class d modules do not intefer with a FM tuner, but they do seem to interfer with each other.

each module on its own has little noise but as soon as i bring the second module into operation the noise level increases quite a bit, and it also seem to go up and down, sometimes with a little bit of crackle and hum in it, and sometimes barely even audible from a meter away.

This is why i put on the ferrite thingies, but they had little to no effect, separating the gate drive to a isolated supply for each module reduced the hiss alittle.

It turns out these modules are a perfect full signal AM station!! PCB layout FAIL!

Last edited by Tekko; 23rd June 2013 at 04:55 PM.
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Old 23rd June 2013, 05:42 PM   #2
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This is it, this is the end. I am leaving class d... for now.

After discovering my class d modules make for perfect full signal strenght AM transmitters, i am pulling the plug on my class d and related work until i have learned more abounf proper layouting above 20kHz.

I have already gone through my computer and image hosting and deleted all pictures of board layouts, leaving only the spice simulations and pics of the built modules.

The remaining bare boards from this project will be gathered up and destroyed, the modules already built i will keep for now and will eventually be stripped for usable parts.

I urge moderators on this forum to delete all my uploaded picturs of my board layouts to further prevent lesser knowing ppl from copying my work and also end up with unintentional AM transmitters.

Last edited by Tekko; 23rd June 2013 at 05:44 PM.
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Old 23rd June 2013, 05:57 PM   #3
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The problem you're describing sounds exactly like switch frequency locking between channels and is both very common and very hard to control. Before you bash your head into the piano keys and destroy everything I'd recommend to fully, individually shield the boards you already have and lead the grounds and supplies all to central points and see what happens. If you don't do that first you'll lose a lot of work and experimental knowledge. There is probably no way to build such a circuit that creates zero radiation, so shielding is the only option. Use output chokes with pot type cores or evenly wound toroids, and keep those under the shields as well, maybe even make sub-shields for them. Interference by conduction then is a matter of proper lead wire layout and dressing. If your boards operate correctly individually, I'd worry less at first about the per-channel circuit layout. Even manufacturer demo boards come with a warning about this trouble.

Last edited by Andrew Eckhardt; 23rd June 2013 at 06:02 PM.
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Old 23rd June 2013, 06:02 PM   #4
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Well the UcD modules from hypex can and do operate properly right next to each other with none of the issues i have with no shielding what so ever when ran from the same psu just like my setup.

This confirms that my issue is board layout alone, hence my decision.

I still wanna thank everyone for the interest in my adventures in class d, but this is the end... until i have learned more about proper layouting to prevent this issue i discovered.
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Old 23rd June 2013, 06:06 PM   #5
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Where oh where will you start fixing if you don't find out exactly where your "transmit and receive" nodes are in the boards you already have?

Also, "none" is an impossibly small amount. If I were serious about housing multiple switching amps in one box for audiophile purposes I'd shield them all no matter what.

Last edited by Andrew Eckhardt; 23rd June 2013 at 06:10 PM.
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Old 23rd June 2013, 06:19 PM   #6
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Thats the thing, i dont know anywhere near enough enough about layouting to know where to look for the node acting transmitter, but i wouldent be the least suprised if its actually the speaker wires, but then i wonder how the hell do the Hypex UcD, Anaview and all others pass EMC testing if just 5 meters of speaker wire is all it takes to make a class d into a AM transmitter ? Because having a AM receiver pick up the class d all around 600kHz -2MHz(switching fundamental and its 10 harmonics) i'd say would be unacceptable in EMC testing.

Atleast its not ringing based interference as this would be in the 20+MHz region = it wouldent affect AM, but be all over the FM broandcast band, so that part of the layout i seem to have gotten right, only one FM station was affected by the class d, but that station was very weak and near unlistenable to begin with regardless of the class d beeing on or off.

The transmitting note is on the board itself, removing the speaker wires and shorting the audio input near the modules did nothing other than shift the AM reception up from 670kHz to 800kHz.

Last edited by Tekko; 23rd June 2013 at 06:24 PM.
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Old 23rd June 2013, 06:23 PM   #7
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Class D is critical about layout and decoupling.
It took me 3 attempts to get a pcb that didnt reset all the time.

The class d IC needs decoupling very closely.
The output mosfets need decoupling closely too , from b+/b- b+/gnd and b-/gnd.

PCB tracks should be as short and as thick as possible.
Components should be kept as close together as possible.


I sold a couple of my amplifiers. I had one customer who said the amp kept resetting all the time.
Turned out he had removed the output transistors and mounted them on an external heatsink on 4 inches or wire !!
Of course putting them back on the pcb sorted out his problems.

I have had very little problem with hiss and hum on my pcbs.

Star grounding is also important (as with any amp class) but more important in class d.
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Last edited by nigelwright7557; 23rd June 2013 at 06:26 PM.
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Old 23rd June 2013, 06:26 PM   #8
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That is exactly how i laid this out. I see commercial class d with parts spread abount over alot bigger board area than my modules and still pass emc testing and cause little to no reception on AM.
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Old 23rd June 2013, 06:29 PM   #9
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It sounds like you know just about everything you need to know to complete your work, although it might take a couple weeks. The only thing I know for sure is that scrapping what you have and just starting over would be a bum plan. Step by step troubleshooting technique will get you where you need to go.
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Old 23rd June 2013, 06:35 PM   #10
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You need to experiment with things to see what improves things or makes them worse.

You could try a ground sheet of aluminium between the pcb's to see if it is a radiated or conducted emissions problem.

I use to try extra decoupling.

Also the separate pcbs need star grounding from each other if using the same supply.

I have just finished a high power valve amplifier and I found that good earthing was vital to low noise.
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