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Class D Switching Power Amplifiers and Power D/A conversion

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Old 26th January 2006, 01:41 AM   #121
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Guess I can't really listen to the AM. Maybe I'll just stream it over the internet, but that always sounds bad. How much shielding do I need if I'm going to try to listen to AM? Can I just tack up some metal mesh or will I need to go with plates?

I'm going to be bi-amping, and my speakers don't share a common ground.

For IMD, it just seemed high, and that effect probably would be accentuated by the one channel only driving the highs. Should I not worry about this?
Thanks for the answers. Right now, my amps and electronic parts are in transit. Hopefully I'll get them up by next week.
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Old 28th January 2006, 07:31 AM   #122
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Has anybody read this little article put out by tripath?
http://www.tripath.com/downloads/an11.pdf
It says to reduce RFI, you can put small 10uH inductors in series with the speaker outputs. Has anyone done this? I'm wondering if it would affect the sound. Also, it says to put a 10uH inductor on the power supply rail. That's pretty much cake, and it probably won't affect the sound, but the inductors on the outputs, that's a little scary. I suppose if they're small enough, they don't make any difference at lower frequencies. Just blocks the RF from making it to the outputs, right? Can someone comment? Thanks.
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Old 28th January 2006, 11:03 AM   #123
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Quote:
Originally posted by philibuster
Has anybody read this little article put out by tripath?
http://www.tripath.com/downloads/an11.pdf
It says to reduce RFI, you can put small 10uH inductors in series with the speaker outputs. Has anyone done this? I'm wondering if it would affect the sound. Also, it says to put a 10uH inductor on the power supply rail. That's pretty much cake, and it probably won't affect the sound, but the inductors on the outputs, that's a little scary. I suppose if they're small enough, they don't make any difference at lower frequencies. Just blocks the RF from making it to the outputs, right? Can someone comment? Thanks.

?..the T already has 10uH inductors on the outputs....L3,L4,L5,L6....Not very good I'll grantyou but they're there Replace them with better toroids....much better!

Inductors on the power input....Hmmm
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Old 28th January 2006, 11:43 AM   #124
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It's a little hard to tell from that paper what exactly they are talking about in terms of the 10uH inductors. As stated in the post above, most Tripath amps already havea 10uH inductor on the output. Do they imply another? Hard to say. It seems to be a general overview of EMI, without thought to specific amp circuit design.

Some good tips there, tho. I have already used the AM radio trick to test for EMI.
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Old 28th January 2006, 04:12 PM   #125
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Some good tips there, tho. I have already used the AM radio trick to test for EMI.
It's sad that AM is so good at picking up interference that you can use it as a pretty good test for other equipment. That's some strong noise rejection.

Since I'm using an SMPS, I think I'll try the 10uH inductor on the power input. Do you think I should put it after the capacitors I'm adding to the line, or before the capacitors? Or, I could put it between the capacitors, such as in a CLC filter, but with a much smaller inductance. Only problem is finding an inductor that will take 5A continuous.

The caps on the output are already implemented in the T-amp, but the 10uH inductor, I think, is in addition to the basic lowpass filter. I haven't seen any of those on a schematic ever. The 10uH inductor probably won't cut down the frequency response very much, as it's so small, but just helps to block the RF, if I know what I'm talking about. What do you think? Have you tried it? Should I be the first?
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Old 28th January 2006, 10:04 PM   #126
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Quote:
Originally posted by philibuster
It's sad that AM is so good at picking up interference
It's not that bad. I meant I use the antenna of the AM radio to sniff out RFI. Works great because the Tripath switching frequencies lie right in the middle of the AM radio range.

As for inductors, you can use something from the JW Miller 2100HT series. I have used them on T-Amp outputs.
There are a bit big, too big to fit well on most boards. But they are rated at 7A. More than enough. You can get them at Digikey.com

The new Super-T amp uses 2 inductors in series, I guess they are 10uH each (I'll measure). That gives a 4th order filter. Sure kills the RF. Just about nothing left. But it does NOT help the sound.

I'd like to see what you come up with in an LC filter for the power, the right filter should work well. Tube amps use them all the time. I stuck a large inductor on one of my SMPS and it didn't like it. But I have not tried any further tests.
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Old 28th January 2006, 11:24 PM   #127
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I just had a look thru my Jameco catalog, they have a good selection of inductors at lower prices than Digikey. You might check the website.

Jameco
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Old 28th January 2006, 11:59 PM   #128
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If you're using a switching supply I would try using an RC filter instead of an LC filter. A resistance will interact less with the switching supply than an inductor would. The downside is that you will get more voltage drop with a resistor, so you have to choose the value wisely.
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Old 29th January 2006, 12:54 AM   #129
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Hmm....
I'm actually powering two T-amps with the one 5A SMPS. I can trim the voltage up to 5% over 12V, so it's about 12.6V. What value resistor do you think would work? <.1ohms would drop that down to ~12V at 5A. So that seems all right. But what I wanted the inductor for was to block RF on the line. I have 20000uF, so .1 ohms would give a low pass corner frequency of ~80hz. I can't find the switching frequency of the power supply, but they're usually in the >20khz region, right? So at 20dB/decade, it would be about -50dB at 20khz. This seems pretty clean enough. Now to find a resistor that can take >5A.

Can someone tell me how the inductor could mess with the control of the power supply? Will it be too reactive for the current spikes? Thanks.
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Old 29th January 2006, 01:39 AM   #130
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0.1ohm sounds like a decent starting value. You don't need to worry about the resistor being able to handle 5A because these amps really don't draw that much current. A 1W 0.1ohm resistor would be just fine for a continuous draw of 2A! A couple of 1ohm 1/4W resistors in parallel would even work fine under normal listening conditions (they might get pretty warm though).

Like you said, the 0.1 ohm resistor with the 20,000uF cap yields an 80Hz cutoff frequency which is certainly low enough to filter out RF. Generally, most SM supplies switch above 100kHz.

Without getting into how switching supplies work, let's just say that they're happier driving a more resistive load.
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