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I need help to figure out why this amp doesnt work
I need help to figure out why this amp doesnt work
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Old 22nd February 2019, 11:07 AM   #11
Hextejas is offline Hextejas
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I need help to figure out why this amp doesnt work
Quote:
Originally Posted by tomchr View Post
Pin 8 is MUTE. Looks like the "no good" amp is muted when signal is applied. I'd try to figure out why.

I'd also investigate why you have DC on the output (Pin 3) of the "good" amp.

Tom
Thank you Tom, but why would I investigate this since the amp seems to be working ?
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Old 22nd February 2019, 11:08 AM   #12
Hextejas is offline Hextejas
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I need help to figure out why this amp doesnt work
Quote:
Originally Posted by john65b View Post
Resolder everything connected with pin 8. Your amp is likely in mute.

Thank you John, I will do that.
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Old 22nd February 2019, 11:33 AM   #13
FauxFrench is online now FauxFrench  France
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Quote:
Originally Posted by Hextejas View Post
Thank you Tom, but why would I investigate this since the amp seems to be working ?

Tom is right. You may have an output signal from an amplifier that is wrongly connected. Such may mean you have a less good performance or even that the wrong connection may leave the amplifier to break down after short time of operation.
A bit like if the motor in your car turns but only with a thick smoke from the exhaust pipe. It is likely not to last for long.
The rest position at the positive voltage rail is a serious flaw and the missing connection between pins 1 and 5 perhaps (a part of) the reason.
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Old 23rd February 2019, 12:46 PM   #14
Hextejas is offline Hextejas
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I need help to figure out why this amp doesnt work
Re pin 8, this is what the data sheet says.
Pin8 Openorat0V,Mute:OnCurrentoutofPin8 > 0.5mA,11580dB(min)Mute:Off


I am trying to make sense of that and this is how I interpret it so please correct my interpretation.
1) If the pin is open or at 0V, it means mute, ie the chip turns off.

I dont know what open means. Can the chip tell if there is no signal vs 0V ?
2) If the current out is greater than 0.5ma, then the chip is switched on, if something is greater than 11580dB (Huh ?).


I am measuring voltages not current nor the mysterious dB. I had thought dB had to do with audio listening levels. IE, 90dB is front row orchestra,
Total confusion. Help !
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Old 23rd February 2019, 02:14 PM   #15
FauxFrench is online now FauxFrench  France
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MUTE pin 8 is un-muted (chip active) if more than 0.5mA leaves from the pin. In such case, you have typically 115dB damping and at least 80dB. The datasheet page 7 shows what is inside the chip for pin 8. One basis-emitter drop, a 1K resistor and two series-coupled diodes. When a minimum of 0.5mA runs through that you have a voltage drop of more than two volts. This you see for the "GOOD" chip where you have a voltage of -2.9V. Your "NO GOOD" chip has 0V with a signal. This is why we conclude you have no current running and the chip is muted.

If you please measure the voltages on both sides of R5 (10K), as requested in posting #10 , I can estimate the current.
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Old 23rd February 2019, 03:10 PM   #16
Hextejas is offline Hextejas
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I need help to figure out why this amp doesnt work
Quote:
Originally Posted by FauxFrench View Post
MUTE pin 8 is un-muted (chip active) if more than 0.5mA leaves from the pin. In such case, you have typically 115dB damping and at least 80dB. The datasheet page 7 shows what is inside the chip for pin 8. One basis-emitter drop, a 1K resistor and two series-coupled diodes. When a minimum of 0.5mA runs through that you have a voltage drop of more than two volts. This you see for the "GOOD" chip where you have a voltage of -2.9V. Your "NO GOOD" chip has 0V with a signal. This is why we conclude you have no current running and the chip is muted.

If you please measure the voltages on both sides of R5 (10K), as requested in posting #10 , I can estimate the current.
Well, ok have done some re measuring as I had an assistant recording the readings and she got a few wrong.
Resistor 5 shows 25.67 and 18.76. This is the no good board.
Additionally pin 3 shows 0v DC
Pin 8 shows -2.24 for the no good board, with and without a signal in.
Pin 9 shows -.07 with no signal and -.21 with a signal.

Comparing the good vs bad, both with a signal, the only pin that is different is pin 9.
Good=0, bad= .21--
So what does pin 9 control ?
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Old 23rd February 2019, 10:08 PM   #17
FauxFrench is online now FauxFrench  France
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Resistor 5 shows 25.67 and 18.76. This is the no good board.
OK, close to 7V across R5 (10K) leaves a current of 0.7mA that should be sufficient to un-mute the chip.

Additionally pin 3 shows 0v DC
That is the right DC value.

Pin 8 shows -2.24 for the no good board, with and without a signal in.
Now it is a stable value and we assume the chip is un-muted.

Pin 9 shows -.07 with no signal and -.21 with a signal.
We will need to look at this pin with an oscilloscope.

Comparing the good vs bad, both with a signal, the only pin that is different is pin 9.
Good=0, bad= .21--
So what does pin 9 control ?
Pin 9 is the inverting input. This pin receives the output signal as feedback. Due to C4 (470uF), the amplifier is only AC coupled meaning that the gain at DC level is 1 (voltage follower) and the voltage at pin 9 should be close to that at the output (pin 3). For an amplifier with negative feedback that works, the signal at the inverting input (pin 9) should follow the signal at the non inverting input (pin 10). Please put your 1KHz signal generator between "SNG_GND" and "AUDIO IN+" and check (with the oscilloscope) that the signal at the "AUDIO IN+" terminal is very similar to the signal at pin 10 of the LM3886. If so, is the signal at pin 9 of the LM3886 also very similar to the signal at pin 10?

Last edited by FauxFrench; 23rd February 2019 at 10:12 PM.
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Old 24th February 2019, 09:32 PM   #18
Hextejas is offline Hextejas
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I need help to figure out why this amp doesnt work
Quote:
Originally Posted by FauxFrench View Post
Resistor 5 shows 25.67 and 18.76. This is the no good board.
OK, close to 7V across R5 (10K) leaves a current of 0.7mA that should be sufficient to un-mute the chip.

Additionally pin 3 shows 0v DC
That is the right DC value.

Pin 8 shows -2.24 for the no good board, with and without a signal in.
Now it is a stable value and we assume the chip is un-muted.

Pin 9 shows -.07 with no signal and -.21 with a signal.
We will need to look at this pin with an oscilloscope.

Comparing the good vs bad, both with a signal, the only pin that is different is pin 9.
Good=0, bad= .21--
So what does pin 9 control ?
Pin 9 is the inverting input. This pin receives the output signal as feedback. Due to C4 (470uF), the amplifier is only AC coupled meaning that the gain at DC level is 1 (voltage follower) and the voltage at pin 9 should be close to that at the output (pin 3). For an amplifier with negative feedback that works, the signal at the inverting input (pin 9) should follow the signal at the non inverting input (pin 10). Please put your 1KHz signal generator between "SNG_GND" and "AUDIO IN+" and check (with the oscilloscope) that the signal at the "AUDIO IN+" terminal is very similar to the signal at pin 10 of the LM3886. If so, is the signal at pin 9 of the LM3886 also very similar to the signal at pin 10?
Ok,here we go.
Picture is of pin 10 and AUDIO IN +.
I don't know what you mean by " put your 1Khz signal generator between SIG_GND and AUDIO IN+". From the signal generator, I have the red cable connected to IN+ and the black cable connected to SIG_GND.
Pin 9 is a flat line.

Question: Is there a way to check that the LM3886 is OK?

Thanks
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Old 25th February 2019, 12:17 AM   #19
FauxFrench is online now FauxFrench  France
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Picture is of pin 10 and AUDIO IN +.
I don't know what you mean by " put your 1Khz signal generator between SIG_GND and AUDIO IN+". From the signal generator, I have the red cable connected to IN+ and the black cable connected to SIG_GND.
Exactly what I wanted. We can see the reference signal moving up and down as a sine-wave. As an amplifier is nothing but a high precision servo system, the output voltage should move such that the feedback pin (9) follows the reference pin (10). It doesn't.
Though it is not very likely, the output could be "hanging" (shorted to) on the ground line. This is checked without supply voltage and without load (disconnected). Check with an Ohm-meter from pin 3 to ground that the resistance is more than 100 Ohm.
I have some doubts about the MUTE pin because it does not have a voltage very close to that of the "GOOD" amplifier though the current should be sufficient. I suggest you try the "NO GOOD" amplifier with a resistor of 10KOhm connected directly between pin 8 (MUTE) and the negative supply voltage (-25V). With such a resistor, pin 8 is forced down with good 2mA. If that cannot "wake up" the amplifier, the mute pin is not the problem.

Pin 9 is a flat line.
Yes, pin 9 is actually controlled by the output through the feedback network. Pin 9 should have very close to the same value as pin 3. Without reaction at the output, the chip may be defect.
Does it become hot?

Question: Is there a way to check that the LM3886 is OK?
Yes, but only in a rather trivial sense. Class AB amplifiers do not have diagnosis output pins such as you may find on class D chips. You test if a class AB amplifier is working by putting a voltage or a signal within what is specified in the datasheet on all essential pins and see if the chip does as expected. Hopefully the chips are made with intelligence but they are themselves not intelligent and cannot tell us what is wrong.

Last edited by FauxFrench; 25th February 2019 at 12:24 AM.
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Old 25th February 2019, 01:09 AM   #20
JMFahey is offline JMFahey  Argentina
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Quote:
Question: Is there a way to check that the LM3886 is OK?
That´s exactly what we are doing.
We apply supply voltages and measure all píns, to check whether we find expected values or not.

Once happy with that, we apply signal and check whether we find expected values or not.

The first one is called Static testing; the second one is Dynamic testing.

We do not know or care what´s inside that literally "black box", we can only check at outside pins.
Even IF we found a bad part "inside", we have no way to replace it.

*Some* packaged amplifiers can be salvaged that way (sometimes) but only because they are real discrete parts amps including a PCB, and a somewhat removable (by hacking) cover (STK or Sanken modules) ..... not the case here.
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