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Class D Switching Power Amplifiers and Power D/A conversion

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Old 1st December 2012, 12:53 PM   #11
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I'm afraid the EMC rules aren't enforced by anyone, ever. Those of us who are involved in HF radio know all about the noise generated by switching power supplies in wall-warts, low-voltage halogen supplies, variable speed furnace fan controllers, etc. But, I don't expect to have to deal with the problem in high-end audio devices.
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Old 1st December 2012, 01:31 PM   #12
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All class D amps leak the free-running frequency at disturbing levels - hundreds of millivolts seems to be the norm. Actively driven speakers with ultrashort speaker connections are probably a good solution but 5m cables are bound to radiate quite a bit. I have noticed that my phono setup sounds significantly worse with a class D amp operating near by.
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Old 1st December 2012, 11:12 PM   #13
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Quote:
Originally Posted by analog_sa View Post
All class D amps leak the free-running frequency at disturbing levels - hundreds of millivolts seems to be the norm. Actively driven speakers with ultrashort speaker connections are probably a good solution but 5m cables are bound to radiate quite a bit. I have noticed that my phono setup sounds significantly worse with a class D amp operating near by.
Thanks,
cd, minidisc and internet radio are absolutely quiet, it's just an analog fm signal that is unlistenable.
After reading your reply, I connected the fm radio to a pair of self powered D-Box speakers, these look to be Minimus 7's with a D amp in one box. The tuner sounded great, even though there is a speaker cable running from one box to the unpowered box.
It must be either the chip or the design, the chip in the D-Box is not a 2020
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Old 2nd December 2012, 09:05 PM   #14
dmills is online now dmills  United Kingdom
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Its mostly the design of the output filter, together with managing the switching current in the input supply wiring appropriately (Something that is all too often missed).

Really the common single stage L/C lowpass does not cut it, especially as the cap is usually so large as to have unfortunate series resonances at RF. 4th or 6th order is better, and in all cases you really need an additional filter designed to catch the VHF harmonics (The main reconstruction filter inductor(s) always looks capacitive up there).

Current and voltage probes hooked up to a halfway decent SA can be very revealing.

Regards, Dan.
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Old 3rd December 2012, 05:30 PM   #15
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I have just connected a small CV transformer before my dc power supply and the RF noise has cleared up. There is a slight hiss but this is just a reception problem with the telescopic antenna.
After listening to 128k internet radio for a few months, FM does not sound that good anymore.
The cvt is only .127 amp which gives me only about 1 amp @ 14v dc yet it drives my big old speakers, no problem!
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Old 21st June 2013, 02:14 PM   #16
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Quote:
Originally Posted by radiosmuck View Post
I have just connected a small CV transformer before my dc power supply and the RF noise has cleared up. There is a slight hiss but this is just a reception problem with the telescopic antenna.
After listening to 128k internet radio for a few months, FM does not sound that good anymore.
The cvt is only .127 amp which gives me only about 1 amp @ 14v dc yet it drives my big old speakers, no problem!
I know this is an old thread and pertains to something other than car audio but man we're having fits with EMI and FM interference with these Class D car amps.. Radiosmuck, can you elaborate a little more sir on the CV transformer you're talking about here (a possible link to one) and do you think it will help with the interference we're experiencing with these car audio Class D amps?

Thanks,
UN
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Old 23rd June 2013, 03:37 PM   #17
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Quote:
Originally Posted by UltraNutZ View Post
I know this is an old thread and pertains to something other than car audio but man we're having fits with EMI and FM interference with these Class D car amps.. Radiosmuck, can you elaborate a little more sir on the CV transformer you're talking about here (a possible link to one) and do you think it will help with the interference we're experiencing with these car audio Class D amps?

Thanks,
UN
Sorry, it did not really work for me, the enthusiasm was premature.
I'm now using a very inexpensive TDA7297, it sounds, for me, better than the 2020 and has no FM issues.
Being an enthusiastic embiber of the sparkling beverage I just felt the need. Very hypnotic, I find.
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Old 26th June 2013, 12:47 AM   #18
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SMPS designers shield RF by putting an earthed screen around the PCB.

A class D amp is similar with high power inductors.
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