Advice on power filtering - Getting car alternator noise! - Page 1
Forum section: electronics / projects

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 13, 2025, 07:50:30 pm
content: So I made a small board that fools the head unit pretending to be a CD changer.Everything works, except at high volumes, I get plenty of alternator noise, mostly 500Hz-2Khz.AFAIK there're no ground loops, the wiring goes from the head unit and returns to it, doesn't connect to anything else.I went a bit mental with the filtering, placing chokes everywhere with low ESR solid electrolytics+tantalums.L6 /7 / 8 areCBW321609U252T, rated for 2.5A, providing 2.5K@100MHz.Current is low, at most 200mA, and the board has 4 layers with heavy ground planes.Maybe biasing the op-amp inputs at VDD/2 was a bad idea? Should I use a voltage reference instead?Maybe I did it worse by adding too much filtering to the regulator outputs, killing PSRR response?I might add an isolated DD/DC converter if I can't solve it.

author: incf
author_rank: Frequent Contributor
timestamp: April 13, 2025, 08:33:09 pm
content: In a professional settings, one might do some more measurements and diagnostics.Off the cuff, I'd suggest trying to insert a linear regulator between your switch-mode supply and the DAC/etc. (Cut a trace, solder some wires, etc.)I'm dubious about the performance of supply bypassing/filtering at low frequencies.SMPS supplies by their nature are not ideal for rejecting external noise and having perfectly stable ripple-free supply rails.In general all the bias (and supply) voltages should not be allowed to "wave around." ***I'd blame R28 - assuming your DAC is using a well-regulated 3.3V or 5V *** (but I can't see the rest of the circuit so I am probably wrong). Do you bias R28 from the same supply as the DAC?Also DACs (and ADCs) are not known for being particularly robust against power supply ripple.Disclaimer: I don't know how your supply rails are configured or what the voltages are.

author: Konkedout
author_rank: Frequent Contributor
timestamp: April 13, 2025, 08:38:44 pm
content: I have a couple of thoughts:1) Very often the "secret sauce" is pcb layout. Can you show us that?2) Is your output destination connected to vehicle ground somewhere? That could be an issue. Regardless; perhaps you need a common mode choke connected between (Ground and +12 input) and (your circuitry.)

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 12:08:50 am
content: 1 - Sure, it's very similar tothis, I enhanced few things... The bunch of traces at the top are low speed digital signals, placed as far as posible from the analog section.More pics attached.2 - Nope. Only to the head end unit connnector meant for this purpose.Takes ground and power there. Not grounded or connected anywhere else, not even the housing or other cables.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 14, 2025, 12:19:14 am
content: Try bypassing R20 to reduce the impedance at the input of the opamp and thus make it more robust against noise pickup.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 12:20:54 am
content: R20 function is to divide the input /2 so the output doesn't clip...@incf So what? Use a tl431 and connect the bias there?What about this version?

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 14, 2025, 12:27:12 am
content: Use a rail to rail opamp so that it won't clip? As drawn, the DAC is running from the same supply voltage as the amp.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 12:36:23 am
content: The DAC has its own LDOs for 3.3VA and 3.3V feeding from the filtered 5V_DAC.Seems like I made a bad choice:https://www.analog.com/en/resources/analog-dialogue/articles/avoiding-op-amp-instability-problems.htmlThe opamp is RR, but the DAC outputs 6Vpp, even if I increased VDD to avoid clipping, the level would make the head unit very unhappy, that's why I'm cutting it in half in first place.I'll try increasing he DC module output to 6-7V and placing a 5V LDO in between, also modifying the biasing circuit to the filtered version.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 14, 2025, 01:55:26 am
content: What DAC is it? Sounds like it's the kind with a built in headphone driver if it can pull below ground, in which case you shouldn't need the separate amplifier.

author: Smokey
author_rank: Super Contributor
timestamp: April 14, 2025, 02:00:01 am
content: Use regulators with good PSRR (power supply rejection ratio).

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 04:03:24 am
content: It's a PCM5102A, but again, 2Vrms is too much for my head unit, that's why the amp.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 01:23:20 pm
content: So this is v2 (Not the German rocket, if it explodes it's pure coincidence!) .The 5v rail has max. consumption of 100mA, it also supplies the STM32 3.3V LDO, but this is not a problem, there's no digital noise at all, just the alternator whine changing with the engine revs.I simulated it with a noisy 5V rail, injecting 1-100KHz 500mVpp, the output was completely clean.Now, from simulation to real life things can change a lot...

author: bostonman
author_rank: Super Contributor
timestamp: April 14, 2025, 02:16:41 pm
content: I'm just curious, you stated this is to trick the head unit to think it has a CD changer. What exactly is your application/need?Long shot, is this an older car? Way back I installed a stereo system and it had engine whine. The entire summer was spent attempting various things including buying line filters without the problem getting resolved.One day towards the end of the summer the car died while driving and turned out to the coil for the distributor cap. Replaced it, drove home, and didn't hear any engine whine.From that point on I never had engine whine again coming through the stereo.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 03:04:41 pm
content: Yep this is an old Alfa Romeo car. The head unit is basically the same as the Porsche PCM1.I'm mainly making it for myself, as well as way to refresh my knowledge as it's been ages since I last played with analog.This is not a commercial product, more like a hobby/DIY thing, though I'll probably sell a dozen at Whatsapp "card nerds" groups and forums.I'm making the FW open source here, though the board will not, I consider it to be a lot of work and testing to just give it for free.https://github.com/deividAlfa/Alfa-166-CD-Changer-Unilink-EmulatorIn the end the mod didn't got as ugly as I though it would be!

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 08:25:57 pm
content: No avail, it's still there.perhabs the ac component getting filtetered so heavily filtered that it causes a small ac voltage drop in the ground wire itself.I will remove the large part caps filtering the 12V part...

author: incf
author_rank: Frequent Contributor
timestamp: April 14, 2025, 09:37:32 pm
content: Quote from: DavidAlfa on April 14, 2025, 08:25:57 pmAny possibility of magnetic or capacitive interference?

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 10:46:41 pm
content: Nah, I can switch this board to aux mode, leaving the audio wires floating, the whine goes away.What I suspect:Let's say I try to filter a noisy 12V supply with 200mvpp ac in it.The caps will filter it out, but only locally (In my board), but this filtering will induce certain ac currents across the ground return wire.It might only be 50mA, but in long wires it'll translate to a drop of few AC mV... that will be added to your audio signal.So, I need to avoid filtering this noise right at the input.I'll reduce the caps C11 and C12, let the dc/dc module and the 5V LDO clean the ac component, THEN filter.

author: incf
author_rank: Frequent Contributor
timestamp: April 14, 2025, 10:49:46 pm
content: Quote from: DavidAlfa on April 14, 2025, 10:46:41 pmSo you've proved that it is conducted EMI via the power supply connection? (affecting so portion of the board but not another?)

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 14, 2025, 10:58:36 pm
content: There's a relay switching the audio source betwern the amp or a 3.5mm jack, everything is the same board, which is pretty small anyway, about 100x70mm.In aux mode the audio lines are connected to nothing.But I'm pretty sure that I'll get the whine if I short them to gnd in the board.Almost for sure, this is a ground loop issue, not EMI.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 15, 2025, 04:24:30 am
content: Quote from: DavidAlfa on April 14, 2025, 10:58:36 pmTry a common mode choke on the power input.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 15, 2025, 08:13:07 am
content: Hmm, I think an isolated dc/dc converter will be more effective, as this way the audio ground will effectively become isolated and unaffected by the power consumption.I have a B0505s lying around, should be enough for testing, if working then I'll get a 1W 15Vin/9Vout module, filter the output and feed it to a 5V linear reg.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 15, 2025, 01:20:14 pm
content: The B0505S looks promising.I expected an extremely noisy output, I've seen reports of 100-300mVpp noise at the output in these modules, but maybe they weren't filtering them properly.The output was surprisingly clean with a 10uF MLCC alone, and became almost perfect with the small 1206 inductor and a a 820uF low-ESR cap.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 16, 2025, 04:54:30 pm
content: I made a quick contraption, the 1m extension cable already drops 15mV, so go figure how much will it be when mounted in the car, the existing wiring goes all the way from the trunk to the head unit!My DMM can only go down to 0.1mV, but It looks good, the potential between the board ground and the headunit reads zero.(I have a 2nd head unit at home testing and debugging the firmware)Now the power and audio grounds are completely isolated, only joined at the connector of the head unit.This taught me to not overlook low currents, 70mA ain't zero, the drop will always be there, even if it's only 1mV, our extremely picky senses will detect itWill try in a while, I have high expectations...!

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 17, 2025, 06:16:09 am
content: So I wanted to know how Sony did it without any isolation (CDX-T62 SM)...Just a capacitor between audio gnd and vref !I don't see how this would work.Sure, the cap will block DC Vdrop from the main ground and prevent current flowing across the audio ground but it won't block ac ripple reaching the audio signal.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 03, 2025, 07:43:28 pm
content: The secret sauce is careful routing of grounds an power rail.The magic touch is to avoid having power supply currents flowing in signal grounds.Power grounds and signal grounds need to converge to single point where power enters the PCB. "Star gounding"Note that as far as small signals are concerned, power rails are also "grounds" and they need to be routed using similar principles.Know where your signals are going!

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 08, 2025, 12:34:25 pm
content: Terry, I don't think it's a board layout problem, but the long wiring.See, power and audio grounds at the bulkhead are common, so if they're also common in the device at the other end, the return current will be shared between the audio and power grounds.Correct me if wrong, but to me there's nothing you can do except isolating the ground, that's why I added an isolated DC/DC module.The ripple did still pass through, so I added a capacitance multiplier circuit before the DC/DC to drop few volts and hopefully remove the ripple, and also a LDO after it.Still getting noise, though way lower...Maybe I shouldn't connect the audio ground directly and do like the original CD changer, through a big cap.However I can't see how any current would return through the audio ground...

author: ejeffrey
author_rank: Super Contributor
timestamp: June 08, 2025, 01:30:58 pm
content: You might try removing C1 and possibly adding some series resistance to L1 if you have enough headroom. C2 provides a food low impedance input for the DC DC but if there is a lot of noise on the input power C1 will shunt that to your ground return which is shared with the audio ground. Adding series impedance in front of C2 will reduce the ripple current of the supply.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 08, 2025, 01:52:21 pm
content: Note that I'm not sharing the audio ground anymore

author: Sorama
author_rank: Frequent Contributor
timestamp: June 08, 2025, 02:31:07 pm
content: I remember working as a consultant for BMW in the early nineties when the then new 850 coupé came out which had the same whining when accelerating.The higher the rpm of the alternator, the higher the whining in the audio system of the car.AS far as I remember, it was due to the batterie being placed in the back and the alternator in the front so the powerline to it was a bit long and every cable in the neighbourhood picked it up.I did create a LC filter but don't remember the specs of it anymore.So:where did you took power from for the headunit?try to make the power cable to this unit as short as possible and have a good ground.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 08, 2025, 03:48:04 pm
content: Yeah this car also has the battery there.I'm using the original connection from the cd changer at the trunk, but the wiring goes straight to the head unit, inclusing the ground, there's no other connection anywhere else.

author: DaJMasta
author_rank: Super Contributor
timestamp: June 08, 2025, 04:23:22 pm
content: Is there any chance the noise is being coupled in after the output stage, in the car wiring? I wouldn't expect so as stock solutions would run into the same issue, but maybe something to verify, at least. Also probably worth driving the board from a battery supply not coming from the car and checking the effect on the noise. Would also be worth scoping the car rails themselves - I was under the impression that car power noise could be a lot more than 500mVpp.That said, you've got a substantial LC filter on the input, so that should be plenty to filter the power rail - have you tried a common mode choke?

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 08, 2025, 06:23:43 pm
content: No, switching the aux-in relay doesn't make any noise. This relay is in the same board..

author: ejeffrey
author_rank: Super Contributor
timestamp: June 08, 2025, 06:36:27 pm
content: Quote from: DavidAlfa on June 08, 2025, 01:52:21 pmYes and the fact that this made a dramatic improvement but did not eliminate the problem entirely could mean you had two problems or it could mean it was just so bad you didn't eliminate it completely.Reducing ripple current through your input filter capacitor is still worth trying.