Isolating ground loops - PeachParts Mercedes-Benz Forum

Shern · #1 01-21-2021, 11:51 PM

Not exactly diesel, but it's in my diesel.

Question for the elec engineers...

Sound comes from my iPhone connected via headphone jack (rip) straight into an EQ in my glove box.
I have 2 amps, and an EQ. I've also wired in an additional 12v socket so I can charge phone below seat without using the ashtray.

iPhone -> EQ -> amps ->speakers.

Everything sounds great until I plug in my phone to charge. Then I get a little buzzing. I believe this is referred to as a ground loop... though could be other things too.

In my setup, everything but one of the amps shares the same ground point (including the 12v charger).
The amp that does not share a ground is located away from the others under the passenger seat so I have that grounded directly to the frame.

Disconnect charge cable from phone -silence.
Connect charge cable, static/buzz.

What am I doing wrong here?

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Felching · #2 01-22-2021, 12:23 AM

Cable EMI issue. Try different charging and audio cables. Or use bluetooth for audio.

Shern · #3 01-22-2021, 01:07 AM

Is there a way to confirm this isn’t due to ground?
Blue tooth is not an option.

From what I understand, Emi is often due to grounding issues.

jay_bob · #4 01-22-2021, 06:07 AM

The noise comes from the fact that the phone charger dc- (out the USB) and the audio common are both connected to the vehicle chassis. The charger uses a buck converter to drop the 12 V dc to 5 V dc for the USB and this involves a high frequency oscillator.

I thought about a ground loop isolator but not sure that would solve the problem as I am not sure it would block the noise from the phone charger.

They do make Bluetooth to Aux-out adapters. They are everywhere on Amazon and eBay. I found one on eBay for $12 that had left/right/ground on a 3 pin plug for some vehicle application. I just cut that plug off and wired it to the aux input on my head unit on my 2008 E320. You do have to give it 12 V dc so just tie it into the switched 12 V dc lead on your stereo or lighter socket.

I was going to do a Bluetooth input on my 87 300TD before I sold it. It has the original 780 Becker that I had modified to add the aux input. I was going to add a relay that was driven off the antenna switch to change it between radio and Bluetooth. The Becker mod intercepts the audio path when the radio is in 'FM' mode and puts it out to a 3.5 mm jack with switch contacts. So if the antenna is sent to the down position I was going to set it up so that it switches the relay so the Bluetooth receiver is in the audio path. And then when the antenna is put back up the audio path switches back to the FM tuner.

The antenna system on the 240D is similar with the dash mounted switch so you should be able to do this here as well.

Diseasel300 · #5 01-22-2021, 10:03 AM

Crappy charger. The output from it is "noisy" and is transmitted as both EMI and power supply noise through the phone itself. Try another charger. Since nothing is affected until the charger is connected, it isn't a ground loop, it's the charger.

Shern · #6 01-22-2021, 11:20 AM

Yeah that makes sense...
I guess a way to test this would be to use headphones while charging?

Jaybob- For the time being, I’m doing my absolute best to avoid Bluetooth. I know about those adapters but I really don’t like the protocol as it is now.

Diseasel300 · #7 01-22-2021, 11:42 AM

Headphones will not amplify the noisy signal generated by the phone. The amplifier is picking up the buzzing/squealing from the noisy signal from the phone and amplifying it. You need a better charger or to run the phone without charging while you're powering the amp. Not much else you can do to work around it.

Shern · #8 01-22-2021, 02:00 PM

Quote:

Originally Posted by Diseasel300

Headphones will not amplify the noisy signal generated by the phone. The amplifier is picking up the buzzing/squealing from the noisy signal from the phone and amplifying it. You need a better charger or to run the phone without charging while you're powering the amp. Not much else you can do to work around it.

Bear with me, I want to make sure I follow the logic here.

What is is that is different about the external amp, versus the internal DAC in the iPhone. I can experience the same decibel level (relative to distance) with both.

And here's another. The way I test for this is by playing no music. I plug the aux cable in, dead silent (not playing music or anything). I plug the usb in and I can hear the sound floor rise. If what you're saying is correct, would it not reason that this sound would be overpowered and replaced by actual voltage coming through the aux cable when I'm playing music? Or is the hum always there... if that doesn't make sense, imagine sitting in a quiet room for a long time and experiencing "deafening silence" until someone speaks and your ears attenuate.

Diseasel300 · #9 01-22-2021, 02:28 PM

The noise is present because the USB charger creates a "noisy" power supply to the phone. Noisy power supply to the phone makes a noisy signal out of the DAC feeding the amplifier. You hear the noisy "sound floor" because the amplifier amplifies everything that comes into it, whether it be noise or actual audio signal. The noise is still present on your headphones, but since the audio signal is not being amplified a 2nd time, you don't really hear it since the DAC output is being loaded by the impedance of the headphones.

There is such a thing as a ground loop isolator, it's essentially just an isolation transformer. You can try something like that to see if it helps, but if the issue is a noisy signal from the charger, the issue will still be present. A new/better phone charger is also *MUCH* cheaper than an isolator. The same noise problem was present on PC's and laptops in the early 2000s due to crappy filtering on the switching power supplies they had onboard, so it's not an unknown/uncommon phenomenon. An actual ground loop would typically cause "hum" (think open mic cord) or alternator whine through the sound system.

250 Coupe · #10 01-22-2021, 03:14 PM

Grab a ground loop isolator from Amazon or eBay or your favorite shop. They are around $10.
The last Bluetooth adapter I bought came with one and it did clear up the noise in my car.
The boy uses one between his mixer and the PC to get rid of noise.
There are noisy usb chargers out there, a friend has one that will trip the TPMS in his ‘08 Superduty if a phone is plugged into it. Causes the TPMSlight to come on, it goes off as soon as it’s unplugged.
Michael

Shern · #11 01-22-2021, 10:47 PM

Solid, I appreciate it.

I was originally tapping into the factory becker's brown wire ground. I've now installed fresh grounds to the chassis for each component as good practice.
I doesn't appear I have any ground loops. Also, from what I've read about ground loop isolators, they do a number on the low end part of the signal.

Definitely appears to be the charger itself. Have ordered a variety from amazon to see if one does the trick...

Shern · #12 01-27-2021, 08:07 PM

Still haven't solved this one...

Tried a variety of different chargers, all created the same buzz.

Under my own steam, the only thing I've left to try are Ferrite chokes, either on the USB cable, the aux cable, or both.
I've read they can be snapped on to cables or one may strip the insulation back and wrap the bare wire around an exposed toroidal core.

If the issue is "dirty power" or a saw-toothed wave, which it seems to be, is there an electrical fix? Installing capacitors? Some sort of DC isolation?
Not my area.

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