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Old 25th November 2016, 02:29 AM   #1
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Default Noise Floor PAM8403

Hello to all,

I've made a pair of class - D laptop speakers, which are IMHO very amazing.
Full Range Speaker Photo Gallery

Their clarity and stage and richness are satisfying, but they have one annoying problem: Heavy noise floor. It's like this:

My speakers + Laptop = noise
My speakers + Other source = OK
Other amp + Laptop = OK

The noise I hear is like the noise of my laptop. I can hear the hard disk working, USB ports exchanging data, and even my mouse moving - and I can distinguish between them because they have different tones. That's intolerable!

Power is supplied by the laptop USB and no, it's not the supply. I've done some heavy LC filters there.

It's so annoying and loud I had to put a potentiometer at the input, unlike what I like. With the potentiometer at its middle state and Laptop level at 100% though, the noise is quite inherent. The side effect to the low - quality pot is that, now I have a near 3dB level difference between the 2 channels.

But what's the problem? is it the impedance matching issue? Even other laptops are OK with the speakers!

Amp: PAM8403 - USB powered
Drivers: 4 ohms

Regards, Ali
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Old 25th November 2016, 04:52 AM   #2
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The PAM8403 has a single-ended input meaning its amplifying the voltage between its input pin and its GND. Your laptop's DAC has a single ended output - producing a signal between its output pin and its local GND. Trouble is your amp's GND and your laptop's DAC's GND are not the same potential due to current flowing into your amp - that difference gets amplified by your PAM8403.

One solution is an audio isolation transformer. Another is a TDA8575.
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Last edited by abraxalito; 25th November 2016 at 04:56 AM.
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Old 25th November 2016, 06:19 AM   #3
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Hello abraxalito,
Quote:
Originally Posted by abraxalito View Post
... not the same potential due to current flowing into your amp
I don't think so - If that was the problem, the amp wouldn't have worked with other laptops.

Also, I've tried supplying the amp with other power sources like mobile chargers. It still makes my laptop noises.

PS: Also, it makes noises even when there's no current draw except for the idle current.

Regards, Ali.

Last edited by 4392Viper; 25th November 2016 at 06:21 AM.
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Old 25th November 2016, 07:29 AM   #4
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Perhaps you could sketch out an interconnection diagram for the situations when there's noise and when there's no noise. From reading your original post I got the impression you only got noise when the power and the signal came from the same laptop.

So in post 3 you're saying that only one laptop (signal and power supplied by it) gives the noise?
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Old 25th November 2016, 07:38 AM   #5
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Quote:
Originally Posted by abraxalito View Post
So in post 3 you're saying that only one laptop (signal and power supplied by it) gives the noise?
Yes, my laptop, and it's only about the input - no matter I supply the board via laptop or anything else.

Also, I can hear the switching noise when my laptop is in charge.
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Old 25th November 2016, 08:41 AM   #6
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So then if the noise only comes from one laptop's output, sounds like that laptop's sub-par. Not much you can do other than get a USB DAC for it.
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Old 25th November 2016, 09:04 AM   #7
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Quote:
Originally Posted by abraxalito View Post
... sounds like that laptop's sub-par.
I guess so, as my former PC's motherboard sound chip used to make these kind of noises since I accidentally shorted some of its pins.

But the wired part about my laptop is that is sounds crystal clear with my bigger amp which uses TDA7560.

For years I've been connecting different sound sources(MP3 players, USB players, mobiles, PCs etc.) to different amps( Car head unit, car amps, Hi-Fi amps, portable amps etc.) and I've faced this situation several times, where where source x doesn't match to amp y, but I've never understood why. And in this project, the floor noise is too high and intolerable.

Regards, Ali.
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Old 27th November 2016, 09:50 PM   #8
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Anyone got same experience?
I just want to know where the problem originates from.
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Old 29th November 2016, 12:15 AM   #9
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I have yet to come across a PC or Laptop that doesn't have this issue in some form or another.

Its the same issue that affects every audio interconnect, its an issue with common shared grounds. (Try not to confuse ground returns with Earthing)

Internally Laptop and PC motherboards will use a simple common ground, to which everything is connected wherever it can. Its usually a single complete layer of the board. No star grounding nonsense here.
Then think of the processor using 50 Watts at 1.8 Volts. That's a shed load of amps through the ground plane; and that's the very same ground that your audio signal is using.

There is also an issue with switchmode power supplies, in that the output tends to float at around 300V (for 230V supply)
So this has to be controlled some way, often by caps to Earth, or to 1/2 the mains AC if no earth. Of course this generates a current to earth, which makes noise.

Lookup 'pin1 problem' for lots more info.
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Old 29th November 2016, 11:02 AM   #10
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Hi whizgeek,
Quote:
Originally Posted by whizgeek View Post
Of course this generates a current to earth, which makes noise.
That's true and understandable in general, but gets me pretty messed up when it comes to actual cases. Let me explain more:

My portable speakers connect to my laptop via USB (Power) and 3.5 mm (signal). USB and audio voltages are both internally measured precisely to the laptop common ground. So, no matter how much noise there is in the ground plane, there should be no ripples in the signal and USB voltage. Unluckily my portables play a whole lot of noises!

On the other side, my DIY Hi-Fi amp( Which is basically a car amp chip with no special tricks) that is powered via 220v mains, is crystal clear with the laptop. And the only thing that the two circuits share ground via, is the 3.5mm cable's shielding/ground wire. That wire is much more vulnerable to noises due to potential difference between the amp circuit and the laptop motherboard, being applied across the wire.

I'm totally confused! Could you explain more?

Regards, Ali.
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