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Old 19th March 2011, 06:35 PM   #11
Cobra2 is offline Cobra2  Norway
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If one of the internal chips don't work properly...the boards/construction is wasted.
(board will not be good after desoldering...) Strange that 2 boards act the same whithin 95%
Maybe I should go back to good old discrete amps.

Arne K
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Old 20th March 2011, 09:20 AM   #12
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Quote:
Originally Posted by Cobra2 View Post
I hear "scratching"
That sounds like the SPiKe system trying to protect the IC. You will have to doublecheck your amp for shorts, broken leads, correct component values and orientation, correct voltages everywhere, etc. Since the schematic is an exact copy of the datasheet application, it should work when correctly built.
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Old 20th March 2011, 10:28 AM   #13
Cobra2 is offline Cobra2  Norway
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I also guessed it is some SPiKe issue...but why? Oscillation?
I have checked all component values. And all soldering-flux cleaned. Voltages...? I checked the mute-voltage, found it somewhat low to the mute-pins, tried a more direct route thru a 4,7k resistor, it is still low at ~-4,xV

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Old 20th March 2011, 10:43 AM   #14
AndrewT is offline AndrewT  Scotland
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I checked the mute-voltage, found it somewhat low to the mute-pins, tried a more direct route thru a 4,7k resistor, it is still low at ~-4,xV
The mute pin requires current. Testing voltage does not answer the wrong question.
Using voltage drop and resistor value you can calculate the mute pin current, then check against the datasheet. Are there two mute pins?
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Old 20th March 2011, 11:40 AM   #15
Cobra2 is offline Cobra2  Norway
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Should only require 1mA /pin. So mute function is shot. Well, I can salvage some caps, and trow the rest into the dustbin.

Arne K
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Old 22nd March 2011, 04:18 PM   #16
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I have those exact same boards, and they worked great when I last used them. 4V at the mute pin is probably fine, I have 3.6V at mine.

36V - 4V = 32V
32V / 4700R = 6.8mA

What impedance are your speakers?

+/-36V is REALLY high for bridged.
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Old 23rd March 2011, 10:05 AM   #17
AndrewT is offline AndrewT  Scotland
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Hi Red,
4k7 seems very low for mute pin resistor using +-36Vdc supply.

National state the chip needs <0.2mA to turn fully on.
Any resistor value between 24k and 68k will satisfy this current requirement.

They also state that the amp starts to turn off when mute current ~<=0.15mA.
Your 4k7 will start to turn off when the supply voltage is down to 4V+ 4k7*0.15mA i.e. ~ 4.7Vdc.
The 9V lower limit has already shut down the power amp. The mute comes in far too late on loss of power.
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Old 24th March 2011, 04:36 AM   #18
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I was just referring to the value he said he switched to.
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Old 2nd November 2012, 06:46 AM   #19
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Cobra2 make sure the LM4780's are mounted on the heatsink properly and that you are using insulating pads between the chips and the heatsinks. Also make sure all of the power wires +V, -V-, GND are connected properly. Also I would recommend keeping all of the wires short as possible and keeping the high current wires (such as power wires and speaker wires) away from the small signal wires and components as much as you can. Return all grounds to a single point seperately i.e. use star grounding scheme.
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Old 2nd November 2012, 04:17 PM   #20
Cobra2 is offline Cobra2  Norway
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That was abandoned a year ago, second try worked perfectly.
(Another PCB/layout)

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