LM3886 problem...

[alexisk10]

" what's the voltage on the INPUT GROUND compared to the DC ground? "

What do you mean?

 

[unclejed613]

put the negative meter probe on the power supply ground, put the positive probe on the input ground next to the input connection. you showed that on the board layout as having been isolated from the power ground by a 2.7 ohm resistor.

 

[ronv]

Jed, None of the board layouts are his. This is kind of a double hijacked thread.

 

[unclejed613]

that's ok, since there are serious problems with both layouts, and neither of the people posting them seem to have a clue what the schematic of their board is, or how they work. i'd like to help them out, but if they had the basics of op amp troubleshooting down, they should be able to figure it out themselves. there are a few aspects of an LM3886 that are a little bit different (such as the zobel network at the output, and the need for carefully planned ground and power traces). one of the things that makes the board layouts shown in this thread problematic, is the fact that power and ground traces have been laid out poorly, and can cause distortion or instability through magnetic coupling, and neither board used a true star ground. the second board with the isolated input ground (through a resistor) also has the wrong component values in the zobel network. a rule i have with power amp designs, is don't do anything in the design without knowing why. that includes the schematic design, the types of components used (metal film vs carbon film resistors, etc...) and the board layout. if you don't know the WHY, then when something goes wrong, you won't know WHAT or HOW. i was trying to help alexisk10 troubleshoot an offset problem. offset problems are super simple to figure out, if you know the usual causes. in this case, i think he either got the wrong isolation resistor (too high a value) or forgot the isolation resistor from the input ground to the power ground. so the input stage has no ground reference, and you get it latched to a rail. if it were me building that board, i would replace that resistor with a jumper, first of all because National's reference designs don't include it, and the WHY of that resistor isn't made clear. the WHY of National's reference design is simple and clear. the data sheet, and any application notes for the chip should be read carefully to understand the proper way to design an amp with this chip.

i saw where one poster said you don't need a ground at all.... yeah, good luck with that... let me know when you get the output unstuck from the rail...

 

[alexisk10]

I had fix the mistake it was a cold solder at the R2 resistor which goes to ground at the input.

Now, I'm measuring at Pin 9 & 10 -4mV and at the output less than -1mV

 

[unclejed613]

glad to know you found it... i've seen commercial amplifiers use a resistor for isolating the input ground like that, but never seen a good reason why. a lot of these amplifiers for whatever reason end up with the resistor going open, leading to offset. to me it seems just something else that can go wrong. if it goes wrong with a speaker connected, then the speaker gets fried in the process.

 

[LM_gabriel]

I am sorry but i have not followed up on this post in years.
its an actual design and i can upload pictures of my boards if u would like.
i dont steal other peoples work and LM3876 and LM3886 books do not show you the WHOLE truth about the ground system and how its used correctly. go to NATIONAL SEMI CONDUCTOR and they will ask u to BUY a demo board for a hellof a price. and you have to understand that the LM series has a DC and ADC ground on each and every chip.

INPUT GROUND IS THE ACTUAL GROUND" ANALOG GROUND " from your ANALOG CHIP output ground to the LM3886 input ADC which is isolated from LM3886 DC supply ground.

ANALOG GROUND IS THE SOUND GROUND AND SUPPLY GROUND IS YOUR POWER SUPPLY GROUND AND THEY ARE NOT THE SAME VOLTAGE. EVER.

The Resistor from the Lm3886 DC ground is meant for Audio Ground feedback to your Analog output chip and it needs to be separated from the power supply which is +/- 34 DC and your little analog output to the amp is probably 5V or less, can you imagine the bad feed back you would get from your amp's power supply ???

And it should be bridged with a 2.7ohm resistor to the main supply ground THUS isolating DC noise from suppy to ANALOG GROUND as there are 2 ground systems in all audio forms, there is AUDIO GROUND AND POWER SUPPLY GROUND.
there is absolutely no need for a star actually if the Resistor is used from LM3886 Audio ground to the supply Ground.

 

[LM_gabriel]

WELL I HAD THE SAME PROBLEM WITH THE LM3886.
if it does not RUN 100% first time you will either have 0 volts on the output meanning the MUTE is somehow engaged in the chip or the output is shorted internally.
i had endless hasles with lm3886. but i belive that alot of people now make them in china and not in EU. alot of bad chips out there belive me.
you will need to buy the 2 samples of both LM3886 and LM3876 and see which one works FIRST then decide what is wrong with the chip !!! not your board.
but remember that the LM3886 has an extra power pin which i think its pin 5, which the LM3876 has it as EMPTY. you need to add that second power of LM3886 to the +24DC
Hope this helps.
one thing i can say is that i take out the crap of LM3886 and put in the old LM3876 and it RUNS like a DREAM. can anyone explain that one to me !?!

 

[unclejed613]

if you look here https://www.electro-tech-online.com/custompdfs/2012/08/snaa021a.pdf and read the app note, several functional amplifiers are described, and nowhere does it mention isolating the input ground. with the amp designs shown, the noide level is below -100db. with a properly laid out star ground, no isolation resistor is necessary. the "feedback" through the ground you are talking about is from a poorly designed ground, which will have voltage drops across it, causing instability, noise and, distortion. the isolation resistor is a "band-aid" for a much more serious problem (an improperly laid out ground system).

an app note that shows as well as describes proper board layout for an amplifier is here: https://www.electro-tech-online.com/custompdfs/2012/08/snaa031.pdf

 

[LM_gabriel]

i agree with you entirely. but the ground on the amp is actually analog input ground and not the ground of the supply. and they should be separated.

at least my research of years of working with it has led me to this effort. just about everyone designs differently and if your AMP power supply is close to the amp due to the fact that its in an isolated metal box with all the other digital components , you will get a massive interference from the DC side and from transformer, unfortunately my design is not meant to be for ALL but it was designed for an INDUSTRIAL purpose not personal entertainment.
but once you cross this bridge of ideas you will come to the same result. i have seen 100's of different schematics and ideas about sound distortion correction, and have tried most of them only to find that they were meant for a different purpose rather than cutting to what the amp is really about. and semiconductor firm did not say in the book that its AGND and that was a fault.
have you ever studied the reason for a sound output ground buffer ? that is a ground that comes directly from the sound chip or DAC and that needs isolation enless it comes and goes directly to the GND of the amp FIRST without interference from other ic's
i had to study AGND and GND and they have to be separated. but if in your case this was not needed, then so bee it. but i had to manufacture a jukebox and it needed to meet this criteria or no noise could be canceled out.
i guess we can seat and argue till we are blue in the face. but this is my last post. i am actually out of the electronic industry. hope my idea will help someone else out there and hope that you all can understand that AGND and GND have to be separated regardless.

 

[unclejed613]

ok, i'm going to post this just for information then.... in case anybody comes back later through a forum search. the PCB layout in post #7 has a poorly laid out ground system, and the person who laid out the board has chosen to isolate the input ground from the power supply ground through a resistor. this isolation is only a "band-aid" and really doesn't help much, as it will become clear. shown below is a rough model of the ground system showing the arrangement of resistances within the copper. the ground trace carrying almost all of the ground current begins at <spkr_gnd? and ends at <vcc_gnd>. since this is a buss ground and not a star ground, currents travel linearly through the copper, creating a voltage divider between the various points along the buss. the <zobel_gnd> is next in line, followed by the <input_gnd>. with current returning through the <spkr_gnd>, there will be a voltage developed across these copper resistances, in phase with the input signal. a portion of this voltage will appear on the <input_gnd>, in phase with the input signal, resulting in positive feedback to the amplifier. positive feedback reduces the stability of the amplifier, often causing oscillation. oscillation will be snubbed by the zobel network, but since it's ground return is attached between the speaker ground and input ground, will also add positive feedback to the amp input, continuing the oscillation. another problem is that <+vcc_gnd> is also on this side of <vcc_gnd>, and any ripple current through the capacitor will develop a voltage that will also appear on <input_gnd> and add a 60hz sawtooth voltage at the input of the amp. granted these voltages are very small, but they are there and they do add up, and are reinforced by the speaker current when they appear at the amp's output. adding a resistor to isolate the input ground doesn't help much, because the resistor floats at the same potential as <input_gnd>. to eliminate this voltage divider within the copper, a star ground is required, terminating at the <vcc_gnd> wire. the ground system on the board layout in post #1 is better, because it is a star ground.

 

[alexisk10]

I still have a problem with this amplifier.

I had build the following circuit with a +-27v power supply

View attachment 68384

and i had connect an input of a 100Hz with Vp-p = 2v as the following oscilloscope image

View attachment 68385

and at the output i have 100Hz with 40Vp-p

View attachment 68386

so i had conclude that by using the formula of P=V^2 / R = 200W with 8Ω dummy load and this is wrong because the lm3886 @ 8Ω with power supply 28V has an output of 38watts

whats is happening here?

 

[JimB]

so i had conclude that by using the formula of P=V^2 / R = 200W with 8Ω dummy load and this is wrong because the lm3886 @ 8Ω with power supply 28V has an output of 38watts

whats is happening here?

You are doing the power calculation using the peak to peak voltage.
You should be using the RMS voltage.
Vpp = Vrms/(2*√2). on edit What a load of nonsense!

Calculating your power output I get 50 watts. on edit and this is not much better! See later post


JimB

 

[alexisk10]

so to calculate the watt i will use the RMS values so the Vrms = 15.20V , that oscilloscope shows, and i apply the folmula of Pavg = Vrms^2/R, R = 8Ω/60watts, Pavg = 28.88W.

Right?

 

[JimB]

What I should have said is:

Vrms = Vpp/(2 x √2)
So in this case Vpp is 40 volts, and Vrms is 14.14 volts.

Using power = V^2 /R where R = 8 ohm
Power = 200/8 = 25 watts.

Sorry for the confusion.
That is what happens when you receive an e-mail which requires urgent attention while playing at electronics here on ETO.

JimB

 

[audioguru]

The 1k to 22k input divider reduces the 2V p-p to 1.91V p-p.
The 22k to 1k negative feedback makes the amplifier have a gain of 23 times so its output is 1.91V x 23= 44V p-p, not 40V p-p.
Then its output power is 30.3W into 8 ohms.

 

[surya0562]

Hello friend can you check my circuit and pcb image is this circuit is o.k
when i measure my output from pin 3 and power ground it showing -32volts what's wrong with the circuit. plz plz help me.

 

[audioguru]

View attachment 70606View attachment 70607View attachment 70608
Hello friend can you check my circuit and pcb image is this circuit is o.k
when i measure my output from pin 3 and power ground it showing -32volts what's wrong with the circuit. plz plz help me.

Your schematic is correct.
HOW can the output be -32V when the negative supply is only -24V??
Your meter is wrong.

 

[unclejed613]

you probabbly don't have the jumper between the power ground and input ground connected. and AG is right, how can you have -32V at the output with +/-24V supplies?.... maybe you're measuring from the wrong point? or if you are measuring from the input ground, and it's isolated from the power ground, the input ground is a few volts positive, and your output is at the -24V rail, and that could add up to more than one half of the supply voltage.