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Old 15th June 2011, 10:51 AM   #11
AndrewT is online now AndrewT  Scotland
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You have omitted the decoupling specified by National for the chipamp.
Or do you intend soldering direct to the power pins and the ground pin?
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Old 15th June 2011, 11:13 AM   #12
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Quote:
Originally Posted by hurtz View Post
So as I understand it the decoupling capacitors are there to provide current to chip really fast right?
Never actually thought a cable could be too slow for this :-/ but, OK.
No not really, The purpose of the decoupling capacitors is to shunt any residual ac noise on the DC power supply rails off to earth. By putting them as close to the chip as possible it will maximise their effectiveness.

from page 19 of the datasheet.

Quote:
It is advisable to keep the output compensation
components and the 0.1 μF supply decoupling capacitors as
close as possible to the LM3886 to reduce the effects of PCB
trace resistance and inductance.
Note that the 0.1uF caps deal with high frequencies, the larger (assuming you have them) decoupling caps should also be close to the chip.

Tony.
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Last edited by wintermute; 15th June 2011 at 11:16 AM.
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Old 15th June 2011, 11:56 AM   #13
marce is online now marce  United Kingdom
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Download this:
PCB Via Current | PCB Trace Width | Differential Pair Calculator | PCB Impedance
it covers all the calculations such as current capacity you will ever require for a PCB.
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Old 15th June 2011, 12:32 PM   #14
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Chip amp power supply decoupling needs 2 caps per side.
A small value cap is for high freq and the other is needed to keep output modulation ie feedback from creating stability or oscillation problems. I think the NS app sheets give some values. I would use a 0.1 uF small ceramic or film (closest to IC pins) the other larger cap could be polar electro or tantalum from the high 10's to several 100's of uF, also close by on the same PCB. Some folks use larger values here, up to 2K uF , claiming bass sound is improved.
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Old 15th June 2011, 01:37 PM   #15
AndrewT is online now AndrewT  Scotland
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F..K, the automatic log out just lost a long reply.
I don't feel like repeating it.
When will the Forum properly investigate why the system logs out "logged in" Members?
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Old 15th June 2011, 06:47 PM   #16
hurtz is offline hurtz  Germany
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@discrete, (thanks for your reply) I see now what you mean, but why would I want my PSU to behave like this? A quick number juggling tells me that this RC-filter gives me a cutoff around 7Hz... (23500uF, 1 Ohm <- wild guess). Also I want to avoid a ground-star to avoid jumpers... may seem silly but I like it to be as simple as possible.

Thanks for the program link, this is quite awesome!

I also looked through the data-sheet again, the tip concerning the decoupling was very helpful. as per NS the small one should be about 0.1uF, and the large one above 10uF. In my design I chose 22uF for the large one.
I saw many designs which use around 1000uF for this, is there any significant change in sound going from the advised 10uF to 100 times the value?

So new updated values are:

Cs1: 22uF
Cs2: 0.1uF

Cs3: 22uF
Cs4: 0.1uF

The updated AMP:
Click the image to open in full size.


P.s. I use 10 4700uF caps for the PSU since I have about 35 of them lying around...

P.p.s
@AndrewT I feel with you... since this happend to me two times in a row I almost never forget to copy the whole text just before hitting the "post" button.

Last edited by hurtz; 15th June 2011 at 07:00 PM.
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Old 15th June 2011, 06:57 PM   #17
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Quote:
Originally Posted by AndrewT View Post
...When will the Forum properly investigate why the system logs out "logged in" Members?
Sorry Andrew, but I suspect the problem is at your end. If it was something the S/W was doing at this end I'd expect many more complaints than we see.


Quote:
Originally Posted by hurtz View Post
Also I want to avoid a ground-star to avoid jumpers... may seem silly but I like it to be as simple as possible.
So you're set on having it smaller even though it might significantly degrade performance? Seems a shame to me.
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Old 15th June 2011, 07:47 PM   #18
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Quote:
Originally Posted by pinkmouse View Post
Sorry Andrew, but I suspect the problem is at your end. If it was something the S/W was doing at this end I'd expect many more complaints than we see.
It's not just Andrew, I've had it in the past. If you take longer than about 20 minutes creating your post then when you come to press submit it looses it all. Pressing back just gives you an empty reply. Very frustrating.

Now if I know I am going to take a long time to write a reply I copy the reply before hitting submit.
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Old 15th June 2011, 08:33 PM   #19
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Test.

Right, I left that reply open for 28 minutes whilst I had my tea.

edit: seems fine here.

Sorry for threadjacking Hurtz
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Old 15th June 2011, 11:31 PM   #20
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Quote:
Originally Posted by hurtz View Post
@discrete, (thanks for your reply) I see now what you mean, but why would I want my PSU to behave like this? A quick number juggling tells me that this RC-filter gives me a cutoff around 7Hz... (23500uF, 1 Ohm <- wild guess). Also I want to avoid a ground-star to avoid jumpers... may seem silly but I like it to be as simple as possible.
In the simulation below, do you want to feed your amplifier with V1, V2 or V3?

Using V1 is a good way to have mains hum on your speakers.
The PSRR of the amp chip may (or may not) bring this down to inaudible levels, but why not do it the correct way?

Using a star ground is left as an exercise for the reader .
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