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Old 15th June 2013, 10:40 AM   #1
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Default TDA2009 input noise?

Hello,

This is my second shot on a chip amp, the first one was TDA2050 which worked really well. I have some simple pc speakers with a dead amp lying around and thought about doing this 2x10W simple amp. So i downloaded the datasheet, etched my pcb, soldered all the components and it came out pretty good. Definitely not one of my first diy projects, i've built a dozen guitar effect pedals

However, i noticed this strange behavior:
1. when i use an old short cable stripped from the dead amp, from the PC output directly soldered on the PCB i get amazing sound, no noise at all.
2. when i use a common 3.5mm audio cable (1.5m long) and solder it directly on the board, i get noticeable noise
3. when i use a jack to connect the audo cable, and then solder the jack to the PCB, i get some huge buzz/clapping sound, and as you can guess, no music at all. The TDA also overhears very fast, in about 2-3 seconds!
(There was no big heating at all in the other 2 cases)

So what could it be, is there anyway i can fix this?
Thank you for this great forum
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Old 15th June 2013, 12:03 PM   #2
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Vagos21,
Is your circuit (schematic and PCB) the same as page 4 of the datasheet?

Can you attach a picture of each cable with your PCB? It may be no problem for someone else, but I cannot visualize noise sources from words only.

I had similar issues with a recent chip amp build. My issues were all due to grounding on the prototype stripboard, which were fixed when I laid out final version. But just because the symptoms are similar, I cannot be sure that the cause is the same.
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Old 15th June 2013, 12:49 PM   #3
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Thank you for your prompr quick answer!

Here are some images of the 3 cases i listed above. the schematic and pcb is taken from here, which i guess is taken from the datasheet (matced it)

Power Amp 20w With IC TDA2009 - HQEW.net

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

maybe i need some kind of preamp buffer in the input to straighten things out?
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Old 15th June 2013, 03:20 PM   #4
Mihkus is offline Mihkus  Estonia
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Worst PCB ive ever seen.

NO MOTHERSUCKING preamp, The cables are fu...ed

These amps have pretty low input resistance, add like 10k resistor between input and gnd.
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Old 15th June 2013, 05:36 PM   #5
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thanks for your kind answer, yes ofcourse there's no preamp in there, the pcb is just a trace of a pdf image in photoshop, it's really a prototype, that's why it looks like that?
Do you have a pcb to suggest? i'm in need of some more constructive criticism here... it's my first time in amps i'm no pro or anything
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Old 16th June 2013, 02:56 AM   #6
gootee is offline gootee  United States
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If your audio jack's ground is not insulated from the chassis, that would not be good.

And what are the other jacks etc, there? Some have only one wire, indicating that you are using the chassis. That is usually not a good thing.

ALL pairs of conductors should always be kept as close together as possible. Otherwise, they make ANTENNAS, and have currents induced in their loops by any time-varying magnetic fields there, in proportion to how much geometric area they enclose.

The main one to worry about is the input signal and input signal ground. Those two must be tightly (4 turns per inch) twisted together, ALL the way from the jack to the PCB, and the input signal ground must not connect to the chassis, at the jack.

Also, on the PCB, they should stay as close together as possible, and ideally you should use a ground plane in the signal input area (but separate from the power grounds).

Also keep all other pairs tightly twisted. If the AC transformer is anywhere nearby, make sure that both the primary and secondary pairs are tightly twisted, everywhere.
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Old 16th June 2013, 09:03 AM   #7
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Thank you very much gootee, will try all these.
But do i need a preamp? could the problem be one of impedance?
and how about the pcb? it's exactly taken from the pdf, should i make changes to it and bring the input pads as close as possible to the TDA?
thanks again!
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Old 16th June 2013, 01:52 PM   #8
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I followed your advice gootee and also added 10K resistors at the input like Mihkus suggested and the noise really went down (not very silent but quite good) and no huge buzz when using the input jack. But when connecting the external wire to the PC the noise comes back at a considerable level again...
(i tried various power supplies and the result is the same)

maybe i do need a preamp after all? is there a preamp with positive only (12V) power requirement? thank you for your suggestions
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Old 16th June 2013, 03:50 PM   #9
gootee is offline gootee  United States
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If you need a way to control the volume, you could use either a passive or active preamp. Lowering the input level somewhat might make it sound better. You could try it by making a temporary two-resistor divider for each channel. For example, If you put 3k in series with each input, before the 10k to ground that you added, that would cut the input level to 10k/(10k +3k) or 10/13ths of what it is now, roughly.

I don't really understand why it was suggested to put the 10k to ground. If the input impedance was too low, putting 10k in parallel with it would lower it further. But if it made a noticable improvement then you should keep it that way.

If you want to see if buffering would help you could try it with a single dual opamp, with outputs tied directly to negative inputs, and 10 uF in parallel with 0.1uF to ground from each power pin. That would give you two buffers for the two channels. You could put another set of buffers ahead of those and put a dual volume pot between them and have an ideal preamp.

But it sounds like your main problem is grounding or cables, between the two systems. Is there some other source you can try? Maybe a battery-operated source?
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Old 16th June 2013, 04:00 PM   #10
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The first issue that I see are the connections to pin 7 and pin 11. These are clearly "NC" - no connection - in the datasheet. I do not know if this relates to your issues, but it is not a good thing.

Gootee gave good advise to avoid noise.

Mihkus gave partial advice, I think. TDA2009 has a minimum input impedance of 70kohm (datasheet again). Something like a 10kohm potentiometer volume control would be the simplest "preamp". It looks like there is one mounted on your enclosure, but not connected in the picture.
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