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Class D Switching Power Amplifiers and Power D/A conversion

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Old 4th October 2004, 12:57 PM   #21
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Aha, here is a picture inside from the manufacturers website. I will drop them a note and see if they have any ideas.
http://www.kempelektroniks.nl/index.php?item=1
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Old 4th October 2004, 01:24 PM   #22
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All of these Class D amps generate large amounts of EMI. Doesn't matter who makes them. You have to be aware of that when you connect anything to them.

I have friends who build products with similar units. All by themselves, they pass the emissions test. Connect anything to them, especially the speaker, and they fail. Consider any wire connected to them to be an effective antenna.

Jocko
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Old 4th October 2004, 06:29 PM   #23
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Quote:
Originally posted by Jocko Homo
All of these Class D amps generate large amounts of EMI. Doesn't matter who makes them. You have to be aware of that when you connect anything to them.

I have friends who build products with similar units. All by themselves, they pass the emissions test. Connect anything to them, especially the speaker, and they fail. Consider any wire connected to them to be an effective antenna.

Jocko
I can't vouch for my esteemed competitors, but you should definitely try UcD then. Not only do we measure the amplifiers with all cables installed (3metres, and fanning out in all directions), we target (and manage) EMI performance better than 20dB below agency limits.

The modules are also used in audio sets containing tuners. The acid test there is quite different, and much worse.
Such sets have only a piece of wire hanging out the back for an FM antenna, running in parallel with the speaker cables at a few inches distance. So we drape the cables straight down to the floor, along with the FM antennq.

The spec: FM reception on a weak station should not worsen noticeably when the amp is operating. More specifically, we check the field strength required to have 28dB SNR from the FM tuner (=very bad reception). Then we turn on the amp. If the signal strength needs to be upped more than 3dB the amp is back to the drawing board.

It is very easy to make a circuit that complies with agency limits when it has no cables coming in or out. How could it get any emissions out otherwise? Cables are an integral part of any valid EMC test setup.
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Old 4th October 2004, 07:04 PM   #24
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Some of your competitors design their product for use in powered loudspeakers, where the speaker leads are fairly short.

And then someone gets the idea to make a stand-alone amp with them..............and looooong speaker cables.

Jocko
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Old 4th October 2004, 09:53 PM   #25
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Yeah I know who you mean. They use a full bridge with only a differential-mode filter. The common mode voltage potentially spikes from rail to rail. In reality it's a bit better luckily and they're only 10dB over the limit (at least the 250W model I tested).
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Old 4th October 2004, 11:47 PM   #26
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You are positive that yours is better...............?

How high does your spectrum analyser go??

(Mine goes to 18 GHz, no need to go that high. 80 MHz or so will work.)

Jocko
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Old 5th October 2004, 07:03 AM   #27
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Quote:
Originally posted by Jocko Homo
You are positive that yours is better...............?

How high does your spectrum analyser go??

(Mine goes to 18 GHz, no need to go that high. 80 MHz or so will work.)

Jocko
I measure up to 1GHz but that's pro forma because really there's nothing coming out of the noise floor above 150MHz.
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Old 5th October 2004, 07:57 AM   #28
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I do only know one specific 250 Watt model !?!

Regards

Charles
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Old 5th October 2004, 08:02 AM   #29
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Quote:
Originally posted by phase_accurate
I do only know one specific 250 Watt model !?!
Shh!
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Old 5th October 2004, 08:31 AM   #30
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I have spent some time reading through all of this thread, and it's links to the outside world.

The Kemp line conditioner does indeed seem to be fully passive, and thus does not have an internal switching freq that can interfere with ZAPpulse (Like also mentioned by Charles).

Every ZAPpulse module is noise tested before being shipped, by
amplifying the output signal 50 times, and listening to the
noise floor. Any whine, crackle etc. would turn up rather clear in this test.

A single ZAPpulse module by itself can not make a whining noise because of the modulator design, so an interference signal must come from the outside. This is also confirmed by Ed's taking out the interconnects and finding the noise gone.

So what can be the problem? Either a RF noise is coming in on the interconnect signal wires or on the interconnect GND wires.
To find out, just use a bit of systematics:

To remove the latter connect the GND wire directly to the center of the PSU (goes for any kind of power amp BTW). Not to ZAPpulse's input GND.
We have also seen a (disturbingly large) number of people who connect pin 1 and GND of an RCA receptor to the - (MINUS) input of ZAPpulse. The is WRONG ....! It should of course hook up to GND. Of course this is not the case here.
So hook up pin 1 to the center of the power supply (GND).
Pin 2 to + input and
Pin 3 to - input.

And also make sure that your intercoonect cable
is the IEC standard with the above signal standards (not all are).

Futher to see if the noise is induced on the cable or coming from inside the preamp, simply disconnect the OTHER end of the interconnect cable. If the whine is still there, it's induced into the cable from an outside source.

Bruno: Yes we have had that exact same test here for years. With an FM tuner and a ZAPpulse module. We also call it our workshop radio

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