I would like help in determining why this amp doesnt work - Page 1
Forum section: electronics / projects

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 20, 2019, 05:20:39 pm
content: It was working and apparently something came loose when I moved it from the bench to the audio room.This is the right channel and the left channel works.I dont know where to take voltage readings or to see what the signal looks like with a scope.I put a 1khz signal in and tried to trace it through to the output and I got lost. Ditto reading voltages.I tried comparing the voltages and waveform to the left channel but I dont know what to look for. For example, what the waveform should look like and where to look for it,

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 20, 2019, 05:28:33 pm
content: You could start by connecting the black lead of your meter to ground.With the load (speaker) connected but no audio connected to the output.And the power turned on. COnfirm that the + power and - power are going into each channel properly.Then go to each pin of the integrated circuits and record the voltage on each pin. On BOTH channels.Are the voltages the same on the functioning channel as on the failed channel?Be careful to not short anything when you are probing around with the circuit powered up.A tiny slip of the probe could short out adjacent pins/pads which could destroy the IC.Since there is only one active component (the integrated circuit), there isn't much "signal tracing" opportunity here.Since you have a functioning channel, you could establish exactly what the scope sees when the circuit is working properly.

author: Audioguru
author_rank: Super Contributor
timestamp: February 20, 2019, 09:09:44 pm
content: Maybe the output voltage of the non-working channel suddenly went to a high voltage positive or negative and blew out the speaker because the other supply voltage came loose.Without a speaker connected and with no input signal, measure the DC voltage at the output. It should be very close to 0VDC. Also try its speaker on the channel that works.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 21, 2019, 04:25:57 pm
content: I took a bunch of readings at the pins of the LM3886 and some are different. I tried interpreting them vs the datasheet, but that is beyond me.Here are the readings.

author: Audioguru
author_rank: Super Contributor
timestamp: February 21, 2019, 06:50:21 pm
content: Inputs pin 9 and pin 10 and output pin 3 shouldalwaysbe about 0VDC but yours go to a high positive voltage when the GOOD channel has no signal. That will destroy the GOOD speaker.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 21, 2019, 09:08:53 pm
content: As @Audioguru said, the input pins 9 and 10, and the output pin 3 should always be very very close to zero.When there is no signal present and you are measuring with a meter (not an oscilloscope).Of course, with an oscilloscope, with no signal present they should also measure essentially zero.And WITH a signal, you should see some AC audio signal floating around zero.If they are more than a few milliVolts away from zero, then there is some significant problem.I question whether your measurements (or your table of measurement numbers) is accurate?The voltages in the "Good/No Signal" indicate a serious problem and they are what I would expect in the "No Good" columns.There is no connection to pins 2, 6, and 11. So the voltages on those pins shouldn't matter.The supply voltages on pins 1, 4, 5 and 7 look good.(Although the slightly lower voltage on "Good/NoSignal pin 1" may be an indication of trouble.It should be exactly the same as pin 5 (as in all your other measurements.)The reference circuit form TI shows that the mute pin (pin 8 ) should be either open circuit (=mute)or it should be connected to V- through RM(= un-mute)I don't see anything in the TI data sheet that suggests anything like that zener diode D1.Why did you put that zener diode there? That just seems bizarre and problematic.But the main issue is where pins 9, 10 and 3 are up around 24 volts.That is an indication of a catastrophic problem.With no signal they should always be very close to zero.And even with signal, they should average out to zero.A voltage close to the positive or negative power rail is a huge red flag.In particular, it is not at all clear how pin 10 could EVER get to 24V.It should be completely DC-blocked by C1.The only two possibilities that I can think of are:1) There is a wiring error which shorts pin 10 to the +25 V power bus.2) The chip has been severely damaged ("blown" or "toasted")The "top" of R2 is connected (through R1) to the +Input (pin 10)Can you confirm that the "bottom" of R2 is truly connected to ground 0 volts?The voltage (relative to ground in all cases) at BOTH ends of R2 should be 0.000 VIt is easy to understand how pin 9 could get to +24V if there was +24V coming out of pin 3With the LM3886 completely removed from the circuit the voltages on pin 3 and pin 9 should be zero.And pin 10 should absolutely be 0.000 VIf pin 10 (with the LM3886 removed) is at zero, then that suggests that the LM3886 is blown up.If pin 10 (with the LM3886 removed) is at some non-zero voltage, that indicates you have a wiring error.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 22, 2019, 12:30:23 pm
content: Quote from: Richard Crowley on February 21, 2019, 09:08:53 pmThank you Richard but this is not my design. It came from here.http://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/#Required-Transformer-PowerI will get back to you re your other comments after I return to my shack.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 22, 2019, 12:40:22 pm
content: Quote from: Audioguru on February 21, 2019, 06:50:21 pmThank you Guru.I will recheck pins 9, 10, and 3.Question: On the good amp, at pin 3, I see that you are saying that pin 3 should be at or near 0V DC. If the amp is working properly should there be any voltage there ?DC = no, AC =When I put my scope probe on it, it shows what I think to be a highly amplified 1khz signal. I will see if I can get a picture of it.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 22, 2019, 04:05:39 pm
content: Quote from: Hextejas on February 22, 2019, 12:40:22 pmYes, that is what you should see.But the signal should becentered around 0 volts. It should peak in the positive direction exactly the same voltage as it peaks in the negative direction.If you use the DC-coupled mode of your oscilloscope, and you properly adjust the oscilloscope trace to show zero when the probe is grounded, then when you probe pin 3 you should see the audio signal going symmetrically above and below zero volts.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 23, 2019, 12:18:44 pm
content: Quote from: Richard Crowley link=topic=170688.msg2217945#msg2217945 date=1550851539But the signal should be [bThank you Richard for this excellent oscilloscope tutorial. I was looking all over trying to find how to do this.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 23, 2019, 04:42:45 pm
content: But if you adjust the trace on your oscilloscope along the center-line on the screen, and then it jumps up or down when you touch pin 3, that means there is some DC offset coming out and something is wrong.