Audio power amp has large DC offset in output - SOLVED! - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 05:11:23 pm
content: I built this power amp design that I found online and it has been working very well for a few months. Recently it developed a - 18 VDC bias on the output, as well as 1.36 Vppk, 120 Hz ripple. The rectifier diodes in the power supply get hot quickly, and the current through the 8 ohm resistive load is 1.7 Amps DC. So a ton of power in the DC offset.My first thought was to look at the power supply output since the issue is in both channels, because the ripple is 120 Hz, and because of the sawtooth shape of the ripple. All that seems like a filter cap problem. But the power supply is putting out a clean -27 VDC with virtual no ripple, which is to spec for the design.So it would seem the problem is in the power amp boards. Since it's in both sides I have to wonder if I shorted something while working on it that caused a spike in current to the LM 3886 chips. I've seen op amps fail in this way...where the output gets pegged to a rail or something similar. Or perhaps I should look at the caps on the power amp boards? But if it's not the power supply it's seems really odd that the ripple would be at 120 Hz.I'm working on attaching a schematic for one of the power amp channels, a schematic of the power supply and a screen shot from my scope taken with the output attached to an 8 ohm resistive load. I keep getting an error message when I attach the files even though the format is okay and the total size is less than 1MB.

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 05:12:32 pm
content: Here's the schematic for the power supply.

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2021, 05:21:50 pm
content: Quote from: hummusdude on June 03, 2021, 05:11:23 pmIs this tested with the load connected and putting out the fault current? What does the positive supply measure under the fault condition?

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 05:24:50 pm
content: Here's the scope shot of the output waveform.

author: fcb
author_rank: Super Contributor
timestamp: June 03, 2021, 05:31:49 pm
content: The zener in the MUTE circuit? Can you try it without, in-case you are putting it in some weird half-way state. Schematic on p7 of the datasheet looks like you could end up in some middle bias area?How does it perform without a load attached? What if you put a very light loadf on the o/p?

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 06:02:17 pm
content: Yes, I tested with 8 ohm resistive load. The positive supply measures 25.2 VDC during fault condition. Negative supply is at -22.2 VDC. Would that be sag due to the high current issue?

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2021, 06:10:49 pm
content: Yes, the sag and the sawtooth ripple are probably simply a result of the supply being overloaded. Something has biased your amplifiers all the way negative. Are the inputs (audio and signal ground both) completely disconnected?EDIT: Can you scope the power supply rails with the load connected (fault condition)?

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 06:15:16 pm
content: With no load attached output is -27VDC and +27VDC.I would have to de-solder the zener to get it out of the circuit. I'd like to rule out everything else before I pull the boards and do that.about the zener...I just noticed that it is forward biased in the schematic, but I thought zener's had to be reverse biased to work as a voltage regulator. The datasheet says pin 8 needs 0.5mA to turn off the mute function. Not sure why the designer thought a zener would be useful here. With R5 at 10k it seems like V- would have to sag to - 5VDC before the current to pin 8 would be insufficient to keep the chip turned on.

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 06:16:04 pm
content: yes, both are disconnected.

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2021, 06:27:51 pm
content: Quote from: hummusdude on June 03, 2021, 06:15:16 pmYou can just bypass the zeners for test purposes. I'm wondering if you've had a rectifier fail open (or even short) in your power supply. Perhaps check those next.

author: madires
author_rank: Super Contributor
timestamp: June 03, 2021, 06:30:43 pm
content: Looking at the schematics I don't see any common ground for the positive and negative rail in the power supply section. Same for signal ground and power ground in the amp section???

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 06:49:01 pm
content: Quote from: bdunham7 on June 03, 2021, 06:27:51 pmOh of course. I can just jump the leads of the zener to remove it from the circuit and see if that changes anything.As for the rectifier diodes, not sure how that might produce the symptom. Can you explain?

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 06:50:13 pm
content: Quote from: fcb on June 03, 2021, 05:31:49 pmBy load I assume you mean the load on the output, yes?EDIT: Just for clarity, I measured the output of the power supply with no load attached to the output of the amp. I guessed it would be okay to do this since there is no signal on the input. The output of the power supply was a clean +- 27 VDC. The symptom shows up when I connect an 8 ohm resistive load to the output of the amp, also with 6 ohm speakers (that's how I discovered the problem). I could try 4 ohms on one channel but I don't have any other high-wattage resistors handy to go lower.

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2021, 07:04:14 pm
content: Quote from: hummusdude on June 03, 2021, 06:49:01 pmUnder no load you will get a nice clean 27VDC even with a blown (open) rectifier, since you'll still have half-wave rectification. Under load you'll see problems. That should be easy to check. And by 'light load" I think fcp means something like 100 ohms--see what you get for an output that way with no inputs connected. It seems very strange that both channels would do this if it isn't power supply related. You may have a strange combination of issues, like a negative power supply failure causing the amps to go in and out of mute mode.

author: floobydust
author_rank: Super Contributor
timestamp: June 03, 2021, 08:16:06 pm
content: I would suspect an open ground, there's at least four per channel that have to be done right. I include reverse diodes across the PSU rails, in case one of the two fuses blows or one rail goes reverse polarity (on power up or down) so it doesn't damage the IC's.A long-term big DC offset will damage the NFB capacitor C4, you can put a diode(s) across it to protect it or use a bi-polar part.

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 08:45:29 pm
content: Quote from: bdunham7 on June 03, 2021, 07:04:14 pmThat makes sense. If the output were half-wave rectification would that give a 60 Hz ripple?I tested the ouput of the amps with a 250 ohm load and got some strange behavioiur...see the attached images. Interesting that the right and left channels both have high frequency oscillations but amplitude and freq vary. With this load seeing some negative offset but not as much as under higher load.

author: fcb
author_rank: Super Contributor
timestamp: June 03, 2021, 09:16:38 pm
content: You’ve got a Zobel network on the o/p and nothing looks wildly out on the feedback network etc.The HPF cap on the feedback network looks a bit big, but probably not enough to be an issue.The HF oscillation could be down to layout OR could be your mute circuit bouncing around - did you try linking it straight to -ve?You keep talking about your PSU as a possible source - you are overthinking things. If you can get a +v, -v and 0V from it and you have a bit of local capacitance on the LM3886 you’ll be fine at this stage.

author: hummusdude
author_rank: Regular Contributor
timestamp: June 03, 2021, 10:49:00 pm
content: Quote from: fcb on June 03, 2021, 09:16:38 pmWell, there were several things pointing to that in the beginning. Mostly I'm just responding to others asking about the power supply.Here's a scope shot with the zener jumped. I left the 10k resistor in series to limit current to 2.7 mA on the mute pin. To me it looks less like oscillation and more like a cap being charged and discharged. The time constant for the soft mute circuit (R5 and C5) is 1 sec so if it's related to that it's not jumping out at me.I don't think the layout is the issue because it has been working fine for a few months and I have not changed the layout.

author: hummusdude
author_rank: Regular Contributor
timestamp: June 04, 2021, 04:01:37 am
content: Quote from: madires on June 03, 2021, 06:30:43 pmHere's the layout of the power supply. I think this addresses your question.

author: madires
author_rank: Super Contributor
timestamp: June 04, 2021, 09:47:10 am
content: Yes, it does!

author: hummusdude
author_rank: Regular Contributor
timestamp: June 04, 2021, 10:33:47 pm
content: This is just odd enough that I wanted to run this by the forum. In order to eliminate the power supply as a possible source of the problem once and for all, I connected V- directly to an 8 ohm load and turned it on long enough to get a scope shot. With the horizontal scale = 10 ns, I get 800 mV of ripple in a square wave?? I'm about 99% certain that this must be an artifact of my scope...siglent SDS 1104X-E (100 MHz, 1GSa/s). Perhaps it is quantizing the ripple because the frequency is too high for the front end?

author: bdunham7
author_rank: Super Contributor
timestamp: June 04, 2021, 11:10:34 pm
content: I've no idea what you are trying to see in that scope shot at 10ns/div. Everything you are seeing is an artifact of how you have set it up and it doesn't indicate anything useful.The amplifier section should reject almost any conceivable noise in the power supply (OK--not at 100MHz, but....), soif all of the grounds are indeed securely connected togetherand you have anything reasonable on V+ and V-, the amplifier should not be going all the way negative nor oscillating.The 120Hz ripple you saw is what you would always see if you overload a full-wave linear power supply like this. The only reason any of us were suggesting looking at the power supply was in case the amplifier mute circuit or some other part was not properly designed or constructed or had failed and was unstable enough that it might be affected by a power supply deviation that would ordinarily not affect a properly set up amplifier. In my case, the only reason to go down that road at all was because you stated that the amplifier had worked properly for a while and then failed in both channels for some unknown reason.If you have relatively clean and balanced power under no load and small load conditions, but you still have that oscillation or beating, the problem is not the power supply. I would start checking all of the components, but really I think the most likely cause here is that you have somehow blown up your LM3886 chips. They're supposedly well protected and you don't have a lot of voltage, so I'm not sure exactly how that would happen. The big mystery would seem to be why the amplifier feedback circuit does not respond to it going full negative. Since it doesn't do this under no load, it probably isn't a shorted output.You might start by connecting more channels of your scope right to the LM3886--to both inputs, the output and the mute pin. Seeing those simultaneously (at a slower timebase please!) would be very helpful. Also, if you can, measure R1, R2, R3 and R4 in circuit and check their related capacitors, especially C4.

author: xavier60
author_rank: Super Contributor
timestamp: June 04, 2021, 11:27:11 pm
content: Also use only one ground reference point for all measurement. The 0V junction of the large filter capacitors is a good point.Voltage readings of all IC pins while in the fault condition might be useful.

author: floobydust
author_rank: Super Contributor
timestamp: June 05, 2021, 05:52:43 pm
content: LM3886 amplifiers kits do fall into audiophool territory, some ripoff parts out there like the filter capacitors being hollow and tiny crap parts inside, as fakes.If there is unexplained mains ripple on the rails, or the amplifier is oscillating, make sure the rail capacitors are what they claim to be. Next to check is the layout and grounding.Otherwise, If OP's outputs are stuck at the rail, then both IC's are dead. Looking at nsec sweeps will not bring them back and I've seen Rigols add nonsense to their traces especially at the noise floor, so don't bother with the microscope.Measure the DC voltages at each IC pin and post them here.

author: andy3055
author_rank: Super Contributor
timestamp: June 05, 2021, 08:17:17 pm
content: Check the voltages directly on the pins 1,5 for +ve and pin4 for -ve. They should be identical in the number, if you know what I mean. If so, you might be having a failed LM3886.In case you need, here is the link to the data sheet:https://www.ti.com/lit/ds/symlink/lm3886.pdf

author: hummusdude
author_rank: Regular Contributor
timestamp: June 05, 2021, 11:39:36 pm
content: QuoteThe resistors all check out okay. I checked the caps with my ohm meter and there were no shorts. They all had a slowly increasing resistance which I interpret as the meter charging the caps and the meter getting confused by the way the caps are absorbing the current.While testing the pins of the LM3886s for DC voltage some even more strange behaviour is manifesting. I haven't touched the amp since yesterday when I was seeing the negative DC offset at the output. Today when picked it up again to take voltage measurements on the pins of the LM3886s I kept the scope on the output for good measure. The first time I powered on the amp I heard a high pitched whistle-like noise that lasted about 3 seconds. When that was happening I saw a positive offset on one channel, and both channels had some oscillations. After about 3 seconds the behaviour stopped and the signals were relatively flat. I don't recall if there was still some offset...the episode came and went rather quickly and I didn't get all the details. I was unable to recreate the symptom. I figured it was an artifact of some cap charging so I waited about 5 minutes and tried again but the symptom did not reappear. But later as I was taking voltage measurements on the pins a similar but shorter lived thing happened a couple of times. But there was less than a minute between powering on for a reading and powering on again for the next reading.So I continued on with taking voltage measurements on the pins. I turned on the amp for only a few seconds to get a reading and then I turned it off again. For the first 4 pins, the left channel continued to show about +20VDC offset. When I went to take a reading on the 5th pin the offset on the left channel had changed to -16VDC. The offset has remained at -16VDC with 2.8 vpk ripple at around 140 Hz. The chip on the left channel does not appear to be heating up despite the large negative current at the output. The 8 ohm resistive load is definitely heating up (2 16 ohm, 25 watt resistors in parallel).The right channel also has some very erratic behaviour. On power up there is a lot oscillation with about -2VDC offset at the output (with 8 ohm resistive load) that changes rapidly while the offset slowly decreases to 0V. The chip on the right channel gets quite warm within a few seconds although unlike the left channel, the resistive load on the output is not heating up at all.I'm attaching the data for the pin measurements as well.It sure looks like the LM3886s are toast. The only thing I can think of as a possible source of the damage could be the power to the preamp board that I'm working on. I don't know exactly when the problem started...I was focused on the preamp when I powered it all up together. There was no signal from the preamp to the power amp. But the preamp is powered by the same power supply so it's plausible that something cause a spike to the LM3886s. It uses some linear voltage regulators to step down the voltage from +-27 VDC from the power supply. I'm attaching a schematic of the preamp for reference in case it helps.

author: xavier60
author_rank: Super Contributor
timestamp: June 06, 2021, 12:12:48 am
content: It looks like DC is being externally applied to the inputs.

author: hummusdude
author_rank: Regular Contributor
timestamp: June 06, 2021, 12:27:25 am
content: Quote from: xavier60 on June 06, 2021, 12:12:48 amI agree it looks that way but I'm at a loss to see how it could be coming from outside the chip.

author: hummusdude
author_rank: Regular Contributor
timestamp: June 06, 2021, 12:30:00 am
content: QuoteParts were sourced from Mouser so I assume they are genuine.

author: xavier60
author_rank: Super Contributor
timestamp: June 06, 2021, 12:32:04 am
content: Have you checked for DC at the left side of the input caps?Is signal ground tied to power ground?

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2021, 12:59:06 am
content: So look at that circuit and tell me how it is even remotely plausible that you have +20 volts on the output (pin 3) and -19.5 volts on both inputs (9 & 10). Something is not connected correctly. You need to put your negative lead of your DMM on the first power ground point and then test every other point in the circuit, including the ground points. Can you post a good photo?

author: hummusdude
author_rank: Regular Contributor
timestamp: June 06, 2021, 01:11:06 am
content: Quote from: xavier60 on June 06, 2021, 12:32:04 amI haven't. For the right channel I see - 22.5 VDC on the chip side of C1 that is slowly increasing toward 0 VDC and - 14 VDC on the input side. For the left channel, I get - 20.5 VDC steady on the chip side and -16ish VDC on the left that is slowing increasing toward 0 VDC. And just for fun the offset on the left channel is now + 20 VDC.Neither signal ground nor input are connected to anything for both channels.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2021, 01:18:10 am
content: Quote from: hummusdude on June 06, 2021, 01:11:06 amThe signal ground should be connected to the power ground at all times! Didn't you respond tomadiresand show a diagram of them connected that way?And both of your C4 capacitors may have issues, they really need to be bipolar. I suspect they leak at working voltages.

author: floobydust
author_rank: Super Contributor
timestamp: June 06, 2021, 01:18:49 am
content: In your schematic, I see missing resistors for the pre-amp section. U4B, U4C (+) inputs need them for input bias current of the op-amp and capacitor leakage current. C10, C11 33uF is kinda huge there.This could make mystery DC offset going into the tone controls as well, and getting out to the power amp.

author: xavier60
author_rank: Super Contributor
timestamp: June 06, 2021, 01:24:01 am
content: Solved?

author: hummusdude
author_rank: Regular Contributor
timestamp: June 06, 2021, 01:24:37 am
content: Quote from: xavier60 on June 06, 2021, 12:32:04 amThe problem was that I didn't have the signal ground tied to the star ground point. I had the input disconnected because I was testing a new preamp board design and I didn't want to involve the power amp in the signal chain. But when I disconnected the signal ground it isolated he signal side of the power amp board and so it was left floating.I'm pretty new to this but it seems like it makes sense that the op amps were acting like comparators and as someone else commented there didn't seem to be any functioning feedback. Also makes sense that sometimes the offset was positive and sometimes it was negative since the feedback circuit is referenced to ground. Since it was floating that would seem to explain the very erratic behaviour. With the signal grounds actually grounded everything is quiet, cool, and sitting stable at O V at the output.Apologies to everyone for the wild goose chase! But I have to say not sure how long it would have taken to figure this one out without all the great comments and feedback. With the pandemic shutting so many things down, forums are one of my only resources for help with this project so much gratitude for everyone's input!!

author: hummusdude
author_rank: Regular Contributor
timestamp: June 06, 2021, 01:32:37 am
content: Quote from: bdunham7 on June 06, 2021, 01:18:10 amyes, I did. But the design uses a star ground at the power supply board and separate ground planes for the power and signal sections of the amp boards. The power section was connected but I made a classic noob mistake and failed to register that the signal ground was isolated from the power ground on the amp board.As for the caps, I've seen bipolar input caps in different designs as well. I built this one from a design I found here:https://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/. I'm just recently graduated with an EET degree but I'm really new to this and I know enough to know how much I don't know. The power amp has worked really well as is and is very quiet so I'm inclined to let it be without a compelling reason to change the input caps. I hope that makes sense. I appreciate your input!

author: hummusdude
author_rank: Regular Contributor
timestamp: June 06, 2021, 01:43:21 am
content: Quote from: floobydust on June 06, 2021, 01:18:49 amWow...thanks for the observation. In fact I do have an issue with the preamp that is next on my list of problems to tackle. This is v2 and I was just getting started on troubleshooting when the rather severe power amp issue arose. So now that that is resolved I'm getting back to the preamp and will make a new post about that. But the first major issue is that the 2.5 VDC input offset is not being filtered by C10 and C11. I breadboarded the input offset circuit ( I found this configuration in Maxim app note) and it worked perfectly with a unity gain buffer so not sure what is missing. It sounds like you have an idea about what's not right with my design. I should probably continue the conversation in a new posting but if you wouldn't mind could explain further what you mean by "missing resistors"?

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2021, 02:11:55 am
content: Quote from: hummusdude on June 06, 2021, 01:32:37 amOK, so let me explain what the problem is in detail and maybe you'll change your mind. This design is one where you can 'get away with' the error in the cap selection under most circumstances. However, it looks like the differing responses to the lack of signal ground were likely at least in part due to leakage in C4. In particular, there appears to be no other way to get that -19.5 volts on the + input without C4 leaking.Audio blocking capacitors usually are biased if they are polar--that is there is a constant DC voltage on them that exceeds the p-p audio signal. In this case, it is not biased and is subjected to whatever the amplifier output is, less what is attenuated by the feedback voltage divider resistors (R3 + R4). Polar electrolytics can withstand a volt or so in reverse bias without any immediate damage or gross nonlinearity, so at normal audio frequencies where the capacitor is relatively low impedance, the voltage across it is low even at higher output levels. At low frequencies--very low in this case with such a huge capacitor, perhaps at the level of rumble from a phonograph--the capacitor's impedance starts to go up and the voltage across it increases. At DC, it is exposed to the full output voltage, which if it happens to be the wrong way is damaging. The only reason it won't pop is because of R3+R4 limiting the current. As I said, you might get away with it for a while, but any leakage will increase the DC gain, which isn't a good thing.The size of the capacitor seems audiophoolish to me. A 47uF capacitor should give you a -3dB point of about 3 Hz.

author: floobydust
author_rank: Super Contributor
timestamp: June 06, 2021, 02:18:20 am
content: Quote from: hummusdude on June 06, 2021, 01:43:21 amOK that would look like a resistor to GND at each of those pre-amp op-amp's (+) input. Just check textbooks and such about op-amp non-inverting configuration, they will show those resistors because otherwise the JFET op-amp's input is floating (for DC) there is no return path for the input JFET's gate current there, and it will cause offset trouble.So U4C pin 3, 10k-100k resistor to GND, and U4B pin 5, 10k-100k resistor to GND.C10, C11 do not need to be so big and are best a film cap for fidelity instead of electrolytic. I would use 1uF with 47k there. For now, just try add the two resistors and that should fix up the 2.5V issue.

author: floobydust
author_rank: Super Contributor
timestamp: June 06, 2021, 03:06:15 am
content: Quote from: bdunham7 on June 06, 2021, 02:11:55 amIt's been proven that a much larger capacitor there (NFD divider) gives slightly lower distortion. It was in Silicon Chip magazine, I can dig for it but they went huge I think 1,000uF.I thought because it's part of the NFB divider network, non-linearities and dielectric absorption get magnified.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2021, 03:14:21 am
content: Quote from: floobydust on June 06, 2021, 03:06:15 amI don't doubt that non-linearities in polar elcaps would be reduced by lowering the peak reverse voltage across them, but did they compare to something like a 22uF polypropylene film cap?

author: hummusdude
author_rank: Regular Contributor
timestamp: June 06, 2021, 05:35:21 am
content: Quote from: floobydust on June 06, 2021, 02:18:20 amI added a 10k resistor as suggested and it fixed the offset as expected. One channel is working very well at the gain stage (U4B) but the other has a 481 kHz oscillation at the output (TP3). I've disconnected the tone stage pots to try to isolate the issue. Whatever is causing the oscillation (maybe some interference from a power rail?) is only on one channel. I will start a new thread for the preamp problem and add details and scope shots but wanted to confirm your solution and say thanks for all your help!