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Old 27th October 2013, 02:29 PM   #1
kentxon is offline kentxon  Hong Kong
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Default lm3886, advice before soldering

i've brought lm3886 pcbs, just right start the soldering job, but before that, i wish to double check if this schematic has missing something and any flaws on it. May some experts give any ideas before i start soldering?

P1100121_nEO_IMG.jpg

clover3886_schematic.jpg

btw, is that ok to use a CFM60 1watt resistor? (i saw others mostly use 1/4w)

Any advice/comments are apperciate.
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Old 27th October 2013, 02:34 PM   #2
Mooly is offline Mooly  United Kingdom
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1 watt resistors are fine as long as not to bulky.

I would question the omission of a zobel network, a 0.1uf and 10 ohm connected in series and fitted from amplifier output (pin 3) to the speaker ground return point. For most amps that is a must for stability.

When you power the amp up I would suggest a bulb tester (search the forums for that )
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Old 27th October 2013, 03:52 PM   #3
kentxon is offline kentxon  Hong Kong
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Quote:
Originally Posted by Mooly View Post
1 watt resistors are fine as long as not to bulky.

I would question the omission of a zobel network, a 0.1uf and 10 ohm connected in series and fitted from amplifier output (pin 3) to the speaker ground return point. For most amps that is a must for stability.

When you power the amp up I would suggest a bulb tester (search the forums for that )
the resistor is too bulky, decide to solder on the back side of the board.

sorry to bring up two beginner questions, just googled:

1) zobel network with a 0.1uf or 0.1uh? i saw others have a copper coil pack at the output.

2) i had a ready made powersupply board, comes with recifitier and DC output protection using upc1237ha chip. Do i still need the zobel network?
Click the image to open in full size.
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Old 27th October 2013, 05:04 PM   #4
Mooly is offline Mooly  United Kingdom
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It's no problem

The zobel is a cap plus resistor. 0.1uf is typical. The zobel network gives a defined "termination" to the amplifier output. Many amplifiers would just oscillate and self destruct without.

Sometimes a coil is used too and this always goes after the zobel network. The circuit you have looks an absolute basic bare bones implementation.

Also a small (say 22 pico farad) might also be used across the feedback resistor.

Have you looked at the data sheet for the IC ? There is a "single rail" circuit in there that shows all these features.
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Old 27th October 2013, 05:07 PM   #5
Mooly is offline Mooly  United Kingdom
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Here is the data sheet, look at fig 2. In fact the zobel recommended there is 2.7 ohm and 0.1 uf and 50pf cap in series with a resistor over the feedback resistor. It also includes an output coil (which is just half a dozen turns of copper wire, air spaced, no former).
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File Type: pdf LM3886.pdf (852.4 KB, 76 views)
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Old 3rd November 2013, 11:33 AM   #6
kentxon is offline kentxon  Hong Kong
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thanks mooly mate,

re-read the figure2 schematics u've provide, to my dumpiest understanding, have the schematic redraw, have i miss anything about it?

any comments , good or bad, are appreciate

lm3886-advice-before-soldering-clover-modify1..jpg
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Old 3rd November 2013, 11:42 AM   #7
AndrewT is offline AndrewT  Scotland
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No RF filtering.
HF decoupling in the wrong position.
Output Zobel in the wrong position.
The added feedback (in blue) may result in oscillation, ringing, or lack of stability.
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Old 3rd November 2013, 12:13 PM   #8
gootee is offline gootee  United States
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Yeah, the output is still wrong. You have 10R in parallel with 0.1uF, in series with the output. For that part, the cap should be an inductor (coil). But completely missing is the RC from output to ground; 2.7 Ohms in series with 0.1 uF from ouput pin to load ground return point.

Also, the 100 uF caps from the power pins to ground should be at least 470 uF, according to the text in the datasheet. And in parallel with those caps, you don't show the mandatory 0.1 uF from each power pin to ground, which should be connected right at each power pin, with the shortest-possible path to ground.

And many people make CD1 much larger, saying they otherwise are unhappy with the bass output.
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Last edited by gootee; 3rd November 2013 at 12:22 PM.
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Old 3rd November 2013, 12:49 PM   #9
gootee is offline gootee  United States
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I would also add 470 pF in parallel with the 22k R from pin 10 (input) to ground, to form a low-pass RF (radio frequency) filter (with cutoff frequency somewhere above 300 kHz). If the 1k input R changes, or more is added, the 470 pF would need to be lowered.
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Old 3rd November 2013, 01:19 PM   #10
kentxon is offline kentxon  Hong Kong
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RF filtering, I google around found something similiar.
Is that mean a 220pf connect between pin9 and pin10?

Added Feedback(in blue), removed.

Power pins to ground: i can change to 470uf or larger.

Output Zobel: i saw others here , said is a copper coil windings?

As a middle-age bloke, which has fail his physics exams 20 years ago, I've tried very much to understand the technical terms on learning this chip amp. (i've never build something like this.)

May please check, if i have done it right?
lm3886-advice-before-soldering-clover-modify2..jpg
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