Ground loop problem?

[Someone]

Everything you have said is consistent with mains leakage coming from the speakers and/or laptop power supply (could well have been both together and at different times). These are know issues with known solutions that you have failed to implement.

Since you have been unable to control the problem with your efforts it is clear you need to obtain the services of someone who knows what they are doing and can test for safety, then rectify if needed. Sounds like the manufacturer of the speakers has offered this to you for free. The step beyond that of reducing the current below "safe" levels gets into specialists but they do exist.

[themadhippy]

I’d used it fine for six days, however I unplug and move it every day, I also by pure chance realised if the fig 8 cable was plugged in one way the problem was there, however if I flipped the cable over, the problem disappeared.

I find that to be very odd and don’t think the reasoning for this happening has been explained

The thing inside the speaker  thats causing the leakage is connected between one side of the mains supply and signal earth,with the mains lead connected one way around that part is connected to live  ,rotate the mains lead and its now connected to neutral.
Neutral and  electrical earth normally are connected together somewhere on your incoming electrical supply  so with the power lead that way around theirs no difference  between the signal ground and electrical ground so nothing to raise a tingle.However with the power lead the other way around your signal earth is connected to live,and thats at a different potential to the electrical ground causing the tingle when you complete the circuit between signal earth and electrical ground

[CaptainBucko]

The evil class Y capacitor leaks a tiny bit of dangerous mains across to the safe side, into your laptop and then into you when you touch its case.  It's enough to feel but not enough to harm you.   (Technically it could harm you if you attach it to subdermal metal probes in your body, especially if they're close to your heart or in your brain.  Please don't do that.)

Great write up - one of the best (and funniest too) I have read. It can (and has) blown up sensitive test equipment, I know of someone (not me) who blew up the front end of an extensive spectrum analyser which cost $30k AUD at the time to repair. After that, every laptop PSU adapter was modified to properly ground the laptop virtual ground. It was the RS-232 serial port that did the damage in the above example.

[Ian.M]

You've made progress by discovering the polarity of the figure 8 connector matters. Mark the lead end and the back of the speakers so you can always set up for minimum touch current.  Using an Apple grounded mains lead may also help as it gives the Apple PSU's AC leakage current a path to ground other than via the Mac and you, even though the USB C shell, and probably the 0V (Gnd) pin, aren't DC grounded to the stud and thus to mains PE.  You can test this without the Apple grounded lead by using an ordinary figure 8 lead for the Apple PSU, and connecting a temporary PE ground to the stud.

Some measurements of the touch current would help determine if its cause for concern.  Use a DMM on AC V, with a 10K resistor in parallel, connected between the metal part you want to test and a known good ground.  That will read 1V per 100uA of current.  Don't simply use the DMM's AC mA range as it may not have enough sensitivity,  and if there is a real fault, 10K resistors are much cheaper than multimeter fuses!

[Nominal Animal]

Reply 10 Nominalanimal suggests making my own isolator and goes on to say a Di box with earth lift will solve it.

I purchased a sterio DI box with earth lift, while I had problems with my audio being so quiet, using the earth lift did not stop the tingle

Is that a typo, or did you really still get a tingle with the DI box with ground lift?

I thought you wrote the tingle was gone but the audio too quiet to use.

It matters a lot.  If the tingle was still there with the DI box (with ground lift enabled), it means the leakage current path is not through the audio ground –– common signal in unbalanced stereo, at the sleeve of the 3.5mm TRRS connector ––, because the ground lift breaks that.  It would have to be something else then.  As audio signal lines have DC blocking capacitors, the leakage current must be mains-frequency AC; perhaps it gets coupled via the unbalanced signal lines?  That would mean a serious fault in the speakers, as it is not supposed to try and inject AC signals on its inputs!

[soldar]

Did you know there are class I and class II products and the only difference between them is the inlet/cord ? It is true a class II appliance cannot have a protective ground path, but that does not mean it is illegal or against the rules to add one, because then it becomes a class I appliance and that rule no longer applies. Cheap and easy to go in that direction, I'm unaware of any class II construction that would fail class I after bonding a protective earth. This is aside from adding a functional or technical or earth which that quote backs up as a-ok and not prohibited as you first claimed.

I agree with you 100% on this and I have had this argument before on this and other sites. And the problem is that the misunderstanding is so widespread that I have given up. I remember leaving a thread here on account of this.

The simplest I can explain it:

Devices need to comply with this rule and they can comply by meeting A or by meeting B. Any of the two is good.

If a device meets both A and B it will be considered as B. In other words, B takes priority.

Conclusion of those who misunderstand: A device which has A cannot have B.

Well, that's not what the rule says.

[HobGoblyn]

Reply 10 Nominalanimal suggests making my own isolator and goes on to say a Di box with earth lift will solve it.

I purchased a sterio DI box with earth lift, while I had problems with my audio being so quiet, using the earth lift did not stop the tingle

Is that a typo, or did you really still get a tingle with the DI box with ground lift?

I thought you wrote the tingle was gone but the audio too quiet to use.

It matters a lot.  If the tingle was still there with the DI box (with ground lift enabled), it means the leakage current path is not through the audio ground –– common signal in unbalanced stereo, at the sleeve of the 3.5mm TRRS connector ––, because the ground lift breaks that.  It would have to be something else then.  As audio signal lines have DC blocking capacitors, the leakage current must be mains-frequency AC; perhaps it gets coupled via the unbalanced signal lines?  That would mean a serious fault in the speakers, as it is not supposed to try and inject AC signals on its inputs!

Yes, the lift didn’t stop the tingle.  I double checked before returning it, otherwise I would have tried it inserting my mixer into the chain first.

[tooki]

I said in post 11, “ There sadly doesn’t appear to be any continuity from the psu ground stud to the usb-c.  As Ian.M says, Apple do an extension cable that also connects to the ground stud and doing a lot of research today, a fair few people have said it solves their problem, but no continuity from mine so not worth buying

Apple USB-C laptop chargers don’t appear to ground the shield, but they definitely ground the ground pins, as is visible in teardowns of these chargers. I’d be very surprised if using the grounded cord didn’t solve the problem. If it doesn’t, I suspect there are bigger problems here.

[Someone]

Last two posts are sounding  like they’re tired of me still asking questions about this, not sure why, I’ve tried most suggestions.

You said you've tried several different ways to make a path back to mains earth. I believe you have made your best attempt at that. So now it is something along these lines:

a) there is some subtle and non user fixable problem in your overall supply/system which needs a qualified and experienced person to check
or
b) the problem is simple and your attempts didn't work for some reason you cannot see/understand

Either way you're in over your head as the explanations so far have not solved your problem. How much free support do you expect to get? This needs someone with the skills and experience to get hands on with your setup and find out what has gone wrong. Noting that there is still the chance you have a damaged/failed item(s) that are dangerous (according to international standards), something which you ignore and gloss over rather than confirming.

Given things are confusing and apparently not resolved by the normal solutions: step 1, confirm each item alone is of no danger.

[Someone]

I said in post 11, “ There sadly doesn’t appear to be any continuity from the psu ground stud to the usb-c.  As Ian.M says, Apple do an extension cable that also connects to the ground stud and doing a lot of research today, a fair few people have said it solves their problem, but no continuity from mine so not worth buying

Apple USB-C laptop chargers don’t appear to ground the shield, but they definitely ground the ground pins, as is visible in teardowns of these chargers. I’d be very surprised if using the grounded cord didn’t solve the problem. If it doesn’t, I suspect there are bigger problems here.

I've not checked every single adaptor model they make but I have never found protective bonding on Apple branded laptop supplies with the interchangeable mains plugs. When using the earthed plug to mains there is (on all I have measured) a 1k (exact!) impedance to mains earth from the secondary side. Usually enough to get rid of the tingle but not the fraction of an ohm a protective bond would provide.

[HobGoblyn]

How much free support do you expect to get?

Another dig, sorry but I don’t need it.  I appreciate your help but you are entitled to stop helping anytime you wish.

I only continued my post when I discovered about the speaker plug polarity, I found it interesting and Ian.m explained it.

I won’t bother explaining my progress any more and I won’t ask any more questions anywhere incase I’m exceeding my free quota.

[tooki]

I said in post 11, “ There sadly doesn’t appear to be any continuity from the psu ground stud to the usb-c.  As Ian.M says, Apple do an extension cable that also connects to the ground stud and doing a lot of research today, a fair few people have said it solves their problem, but no continuity from mine so not worth buying

Apple USB-C laptop chargers don’t appear to ground the shield, but they definitely ground the ground pins, as is visible in teardowns of these chargers. I’d be very surprised if using the grounded cord didn’t solve the problem. If it doesn’t, I suspect there are bigger problems here.

I've not checked every single adaptor model they make but I have never found protective bonding on Apple branded laptop supplies with the interchangeable mains plugs. When using the earthed plug to mains there is (on all I have measured) a 1k (exact!) impedance to mains earth from the secondary side. Usually enough to get rid of the tingle but not the fraction of an ohm a protective bond would provide.

I was just disputing OP’s claim that the ground stud is not connected at all. The low-power adapters that have detachable plugs but aren’t groundable use a plastic stud instead of the metal one.

[Nominal Animal]

I suppose Presonus is as interested in the particular fault pattern as I am, especially if the fault current bypasses a ground break and is thus coupled to the monitor inputs –– without affecting the actual sound generated.

I think it is more likely that the ground lift on the DI box HobGoblyn used was faulty.  For example, a solder bridge in the switch (assuming it has a ground lift slide switch).  Reason is that if the tingle-causing leakage current was transmitted via the left or right signal line (instead of the unbalanced common line, "unbalanced ground"), I think it should have blown the output drivers in the Audiobox Go.

The last resort is USB isolation.  USB isolation does the same as a DI box ground lift, except between the Audiobox Go (or similar USB ADC/DAC) and the Mac.  Because it is done at the digital signaling, it has zero effect on the data transferred and thus the quality of the audio, and there is no galvanic or even capacitively coupled connection to the host Mac.

One I personally might consider, is the Hifime High-Speed USB Isolator v2, although to me the price is quite high – as much as a new Audiobox Go costs hereabouts.  You'd also need two female USB-C to male USB-A converters, or two USB-C male to USB-A male cables, too.

I haven't torn one down myself, or even seen its circuit board image, but based on their specs, they didn't skimp on the isolated DC-DC converter –– and that is what matters the most.


For USB Audio Class 1, and USB Low speed (LS, 1 Mbit/s) and full speed (FS, 12 Mbit/s), there are essentially two common chips to do USB isolation: Analog Devices ADuM3160 and ADuM4160.  They are very easy to use, as they only need a couple of resistors and a few bypass capacitors, with everything needed included in the datasheet.  The chip does not pass power, though, so the cheap $10 class full-speed isolators vary a lot in which isolated DC-DC converter they use.  Most can only provide 200mA to the device, and require a 40mA or so load for their USB 5V to be within spec.

For USB Audio Class 2 and 3, and USB LS, FS, and High Speed (HS, 480 Mbit/s), there are essentially five common chips to do the USB isolation: Analog Devices ADuM3165 and ADuM3166, ADuM4165 and ADuM4166, and Texas Instruments ISOUSB211.  They are only slightly more complicated, requiring a 24 MHz crystal oscillator, and preferably some voltage regulators –– they have some internal, but using external regulators reduces the thermal load within the chip, and is worth the extra cost.  The Hifime one I linked to above is supposed to use ADuM4166.

There is nothing special in USB Audio Classes, they are just standard USB protocols for audio.  There are similar classes for USB webcams, which is why most don't need any dedicated drivers at all.  The only thing that varies between the audio classes is whether they work at Full Speed (12 Mbit/s) or High Speed (480 Mbit/s); I do believe they are all specifically USB 2.0 (and not SuperSpeed).

When picking an USB isolator, it is a Very Good Idea to check which isolator chip it uses, and how much current it provides downstream.  There is very little that can go wrong with the data isolation – the datasheets cover it all, even suggesting specific PCB layouts! ––, so the isolated DC-DC conversion is what one should focus on when getting one.

Isolated DC-DC converters are typically galvanically isolated, with no EMI-reducing capacitor across the isolation at all; it is just not needed.

There are quite a few DIY projects and design guides to implement one oneself, too.  For example, Mark Harris' full-speed and high-speed isolator projects at resources.altium.com, Stefan Wagner's (wagiminator) full-speed isolator at oshwlab.com, and so on.  My own full-speed one is what you typically get when you buy a <$10 USB isolator off eBay, except with soldered-in USB-A male and female connectors; mine is intended to be soldered in-line in a scavenged cable for minimal cost but sufficient functionality.  Metal enclosure for the isolator helps reduce any electromagnetic interference, but should obviously be connected only to one of the grounds (host/Mac) and at one point only; perhaps even through a parallel resistor-capacitor network for limiting ESD and other spikes.

Apologies for the long post.

[themadhippy]

I think it is more likely that the ground lift on the DI box HobGoblyn used was fault

I dont think so as they also tried physically breaking the signal earth on the cables,something more weird is at foot.

[Nominal Animal]

I think it is more likely that the ground lift on the DI box HobGoblyn used was fault

I dont think so as they also tried physically breaking the signal earth on the cables,something more weird is at foot.

Yep, coupling 50/60Hz mains AC via the signal pin is possible, but tingle currents (1-10mA range) are higher than the current passed in audio signals (less than milliamp); the isolation transformers are typically rated for just a few milliamps.

Ah, but wait: the output on the Audiobox Go is a *headphone* output, which are typically able to drive somewhat higher currents, say 25mA, so if the tingle current is around 5mA or so, it could be capacitively conducted via the signal line instead of the ground/common line, with the headphone output drivers somehow sinking that to ground.

(Also, transformer saturation/reaching its limits due to the back-fed AC signal might also explain the much higher than expected attenuation.  DI boxes shouldn't do more than about halve the effective volume.)

I would connect the Eris monitors to mains, with the IEC C8 plug in the tingle-causing orientation, but without any inputs connected.  Then, I'd use a multimeter to measure the DC and AC voltage and current on the female RCA connector *inputs* on the Eris monitors.  I would measure between left and right signals (centers), between left and ground (center and shell), right and ground (center and shell), and also from left, right, or ground, to a true ground (ground pin on a grounded mains socket).  Make sure it is a true safe ground, not live or neutral, because mains live or neutral will blow the Eris inputs.  If nothing else, this would satisfy my own curiosity.

[Someone]

I think it is more likely that the ground lift on the DI box HobGoblyn used was fault

I dont think so as they also tried physically breaking the signal earth on the cables,something more weird is at foot.

Yep, coupling 50/60Hz mains AC via the signal pin is possible, but tingle currents (1-10mA range)

That sort of current is up in the range of "involuntary muscular contractions likely", perception starts under 500uA and is highly variable across conditions and individuals. IEC 60479-1 is a good starting point.

[Siwastaja]

Conclusion of those who misunderstand: A device which has A cannot have B.

Well, that's not what the rule says.

Yeah. Given the fact that in the sockets of many (most?) countries, protective earth is glaringly user-accessible; AND that stuff like water piping, radiators, sometimes even kitchen sinks etc. can be directly bonded to the same protective earth, AND the fact that SELV power supplies bond their GND net directly into connectors such as outside metal barrel of a DC plug, or USB connector's metal case which you can't avoid touching, it seems pretty interesting that some people think connecting these two by the end user must be somehow dangerous and illegal. If it was, we would be all dead or in jail by now by doing mundane things like putting your laptop near the kitchen sink, so that once you unplug power or USB, the connector touches the kitchen sink (earthed by pipework) by accident.

[soldar]

Conclusion of those who misunderstand: A device which has A cannot have B.

Well, that's not what the rule says.

Yeah. Given the fact that in the sockets of many (most?) countries, protective earth is glaringly user-accessible; AND that stuff like water piping, radiators, sometimes even kitchen sinks etc. can be directly bonded to the same protective earth, AND the fact that SELV power supplies bond their GND net directly into connectors such as outside metal barrel of a DC plug, or USB connector's metal case which you can't avoid touching, it seems pretty interesting that some people think connecting these two by the end user must be somehow dangerous and illegal. If it was, we would be all dead or in jail by now by doing mundane things like putting your laptop near the kitchen sink, so that once you unplug power or USB, the connector touches the kitchen sink (earthed by pipework) by accident.

I remember leaving a thread in frustration over this. I mean the proposition makes no sense whatsoever from any point of view. Why would they enact such an absurd rule?   But, furthermore, you would need two separate universes of devices. You could not use a drill of one type on a device of the other type. You could not use an audio amplifier of one type with a receiver of the other type because their gnds would be connected. It is just absurd.

There is always again in safety by grounding any exposed metal parts. Always.

[HobGoblyn]

Ah, but wait: the output on the Audiobox Go is a *headphone* output, which are typically able to drive somewhat higher currents, say 25mA, so if the tingle current is around 5mA or so, it could be capacitively conducted via the signal line instead of the ground/common line, with the headphone output drivers somehow sinking that to ground.

It has a stereo headphone output on the front and both RCA and 6.35mm jack inputs on the back, I'm using the 6.35 jacks.

My initial very first try before buying the Audiobox go was a cable going from the MacBook headphone socket to the aux in on the front of the Eris speakers, that's when I first noticed the tingle and decided to get an audio interface and do things properly.

[Nominal Animal]

As you said it will be a few weeks before the Eris monitors can be checked out, have you considered using an USB isolator with the Audiobox Go?

With the likelihood that there really is something wrong in the Eris monitors, I think a high-speed USB isolator would be a good idea, to protect both yourself and your Mac.  For one, it absolutely should get rid of the tingle, if the leakage source is the Eris monitors.

[HobGoblyn]

Thanks, I’m considering it but it’s a little pricey for temp use.

I’ll contact Presonus on Monday to see if they have any idea how long it will be before I get my new ones, then decide.

[HobGoblyn]

You've made progress by discovering the polarity of the figure 8 connector matters. Mark the lead end and the back of the speakers so you can always set up for minimum touch current.  Using an Apple grounded mains lead may also help as it gives the Apple PSU's AC leakage current a path to ground other than via the Mac and you, even though the USB C shell, and probably the 0V (Gnd) pin, aren't DC grounded to the stud and thus to mains PE.  You can test this without the Apple grounded lead by using an ordinary figure 8 lead for the Apple PSU, and connecting a temporary PE ground to the stud.

I've not checked every single adaptor model they make but I have never found protective bonding on Apple branded laptop supplies with the interchangeable mains plugs. When using the earthed plug to mains there is (on all I have measured) a 1k (exact!) impedance to mains earth from the secondary side. Usually enough to get rid of the tingle but not the fraction of an ohm a protective bond would provide.

I was just disputing OP’s claim that the ground stud is not connected at all. The low-power adapters that have detachable plugs but aren’t groundable use a plastic stud instead of the metal one.

I’ve measured resistance from metal stud to the USB-C  and it measures 1.02K

Have made a cable to go from ground stud to mains earth as shown at end of post.

When I plugged it all in, the tingling was gone on initial touching of MacBook case, but if I lightly rubbed a finger over the case (very lightly, like if you rubbed your finger down your arm so lightly that you’re just touching your skin) and after about 4 seconds of doing this, a very very very faint tingle could be felt. When I say faint, I mean faint, I wouldn’t have bothered starting this thread if it was that faint.


Which isn’t really a worry, however I then turned the speakers off, MacBook still earthed, I got a tingle that felt at least twice as strong as the tingle that caused me to start this thread, causing my reflexes to instantly remove my finger, then no tingle at all. 

I tried various combinations of things,  including repeating the above precisely, and never got that big tingle when turning off the speakers again, so not sure what was going on.

So I repeated:

When I plugged it all in, the tingling was gone on initial touching of MacBook case, but if I lightly rubbed a finger over the case (very lightly, like if you rubbed your finger down your arm so lightly that you’re just touching your skin) and after about 4 seconds of doing this, a very very very faint tingle could be felt.

But now when I turn the speaker off, no tingle at all, no initial large tingle like what happened when I did exactly this a few mins ago, and no very very very slight tingle when I gently rub my finger on the case for about 4 seconds.

Turn the speakers on, again no tingle but if I rub my finger very very lightly, after about 4 seconds I can feel  that very very very slight tingle.

So it looks like buying the cable will fix it, although I didn’t really want the MacBook plugged in all the time, still it’s no big deal, now all I’ve got to do is find one, extension leads on Apple UK have plastic for the ground lug to slot into.

Some measurements of the touch current would help determine if its cause for concern.  Use a DMM on AC V, with a 10K resistor in parallel, connected between the metal part you want to test and a known good ground.  That will read 1V per 100uA of current.  Don't simply use the DMM's AC mA range as it may not have enough sensitivity,  and if there is a real fault, 10K resistors are much cheaper than multimeter fuses!

Oddly tonight, I get a tingle whichever way round I plug the fig 8 cable into the speakers although it’s still a lot worse one way, the other day I had no tingle at all one way.  Same cables, same sockets,  nothing different (this was before carrying out the above tests)

I can’t find a way of accurately measuring the touch current, tried two DMM, used resistor in parallel and I simply couldn’t get a reading from the case.  This is with negative attached to mains ground and positive probe touching the metal case,

I’ve had to stop for the night, will continue my testing tomorrow including the speaker ones Nominal Animal suggested a few days ago in post 64

[tooki]

So it looks like buying the cable will fix it, although I didn’t really want the MacBook plugged in all the time, still it’s no big deal, now all I’ve got to do is find one, extension leads on Apple UK have plastic for the ground lug to slot into.

And what’s inside that slot is two metal spring contacts that firmly contact the ground stud. That’s how this system works. Apple has been using this system for over 20 years, not sure why you seem so skeptical of it.

[HobGoblyn]

I'm not meaning to sound skeptical, from the pics I could see online, it didn't look like there was any metal in the slot.

I also didn't want my MacBook plugged in all the time.

Anyway, just ordered it from Apple, will be here Tuesday.

Many thanks

[HobGoblyn]

I also didn't want my MacBook plugged in all the time.

I'm being stupid as usual, plug socket switch does nothing to the ground, its always there, So I can simply leave the lead connected to my MacBook and only turn on when battery getting low.