Firewire isolation

by **JulienG** » Fri May 18, 2007 12:30 am

I've just picked up a MOTU 828mk2, and it works great.

Except...

I can't use it on my laptop if it's plugged in, I believe due to a ground current, so does anyone know of a device to isolate firewire signals?

And yes, the laptop is a 4-pin device so it's not that.

by **heathen** » Fri May 18, 2007 8:22 am

Does it still hum when both units are connected to the same power board? Or are you using 2 different wall sockets? If you are that could explain it.
Using 1 power board should fix it if its an earth loop. Otherwise I dunno.

by **Martin** » Fri May 18, 2007 8:55 am

could be totally unrelated but anything is worht a try...

we get a ton of noise through our firewire audio interface when a usb hard drive is plugged in and mounted.......

by **JulienG** » Fri May 18, 2007 4:46 pm

heathen - One wall socket, I do need to test with everything on its own powerboard, I think that's how its configured now, but with the amount of crap that's plugged in there I can't tell.

Martin - 'tis just the laptop by itself, nothing connected by USB.

My strong suspicion is that it's my laptop not being properly isolated. Oh well, it is almost time for a new laptop, the new T61 looks nice.

by **rob** » Fri May 18, 2007 6:04 pm

does it do it running on batteries only? I've heard of similar probs caused when running laptops on the charger with firewire audio devices

Rob

by **JulienG** » Sat May 19, 2007 10:16 pm

rob wrote:does it do it running on batteries only? I've heard of similar probs caused when running laptops on the charger with firewire audio devices

Yes, only when on the charger.

I've just tested it after changing the firewire cable, the laptop charger (I have ones from three different manufacturers) and using the same power socket, and it works.

So now it's time to test each item individually, my bet's on the charger, but we'll see.

by **JulienG** » Sat May 19, 2007 11:12 pm

OK, so it does appear to be resolved when everythings on the same outlet, BUT, depending on which charger I use makes it better or worse. Luckily the one that works better is the one I keep in my bag so it's the one I'll have with me at gigs.

by **Howard Jones** » Sun May 20, 2007 8:22 am

Hi Julien,

Reviewing this thread, you never actually told us the exact problem, other than to say that "you can't use it". Was it excessive noise? Or just didn't work at all? I'm presuming not the latter since you have now said "depending on which charger I use makes it better or worse".

This then begs another question: it sounds like even the charger that makes it better still isn't giving perfect results. It may be time to build your self a decent linear power supply with sensible internal grounding and see if you can't improve things even further.

Howard

by **TimS** » Tue May 22, 2007 7:26 pm

Another point to consider is what sort of power supply does your FW require - 240V AC, 9VDC or AC, 12V DC - this can be a common cause of audio hum, particularly if its AC based power.
If its transformer based PSU, shouldnt be too much of a problem..
TimS

by **JulienG** » Tue May 22, 2007 11:32 pm

Been busy.

I'm going to try on my other PC and mac, as I think the dodgyness of my laptop's firewire is causing (more) problems.

Its the MOTU 828mk2, so an internal 120/240v supply. From the big iron inside I'd say it's a linear supply.

Howard, yeah didn't work at all. As I've said I think I'm going to end up just assuming the two boxes don't want to talk and use the G4 or PC for it.

by **JulienG** » Mon May 28, 2007 3:55 pm

Picked up an old Tek skope today.

Found the problem within 20 seconds.

There's a complex wave with about 60 volts peak-peak on anything that claims to be ground on this machine whenever the power supply is plugged in. There is also a fair bit of higher frequency noise albeit only a volt or two peak-peak.

by **rob** » Tue May 29, 2007 6:34 pm

so have you found a fix for the problem ?
Does the PSU have a mains earth pin ?

by **JulienG** » Tue May 29, 2007 10:56 pm

It's all waiting until I can pick up a 6-pin firewire cable to test on another machine and make sure that the MOTU is perfectly ok and not flakey.

by **JulienG** » Fri Jun 01, 2007 10:42 pm

Picked up a 6-pin cable tonight, installed drivers, plugged the MOTU in, and 10 minutes later I'm recording at 96k.

As I already plan to replace the laptop soon I'm not going to bother with more than that. I can just continue to take a mac to gigs until then.

by **hugo** » Mon Jun 18, 2007 6:21 pm

Think Rob was about to get to this?

Quick fix I have used many times in live situations where thinks have to happen now....

Get a double adaptor or powerboard, remove earth pin. no ground left to loop = no noise

Good luck...

by **rob** » Mon Jun 18, 2007 6:32 pm

or indeed visa versa

:) Rob

by **Howard Jones** » Mon Jun 18, 2007 10:57 pm

No-one like myself will ever, ever be seen saying in print that it's okay to remove the earth pin from a plug. It's not okay. It's double not okay to introduce such a thing into a live situation.

Health and safety are equally important in studios and live gigs and yet years of experience show that protection seems to be more important at live venues because more incidents occur at them.

Have you read the latest column from the person who hosts this forum?

I have never had to solve an earth loop problem by placing my client at risk and I suggest that you don't endanger yourself and others by this practice either.

Sorry for the heavy lines, but it's high time that this was stamped on...

by **TimS** » Mon Jun 18, 2007 11:50 pm

Howard Jones wrote:No-one like myself will ever, ever be seen saying in print that it's okay to remove the earth pin from a plug. It's not okay. It's double not okay to introduce such a thing into a live situation.

Health and safety are equally important in studios and live gigs and yet years of experience show that protection seems to be more important at live venues because more incidents occur at them.

Have you read the latest column from the person who hosts this forum?

I have never had to solve an earth loop problem by placing my client at risk and I suggest that you don't endanger yourself and others by this practice either.

Sorry for the heavy lines, but it's high time that this was stamped on...

I must say I agree.
I have designed guitar rack systems for some time, including my own, and I must say that I have never had to "lift the earth" on any power leads/rack units to resolve ground loop hum.
Good troubleshooting and resolving of audio loops is essential in anyones practice - its a good habit to get into and its not dangerous (to you or anyone else, including punters at gigs).
Sure its tedious and quite frustrating at times, but when it works, at least you know you have a safe system in place..
My 2c's worth..
TimS

by **rob** » Tue Jun 19, 2007 8:34 am

indeed!

my earlier post asking does it have an earth pin was less a suggestion for removing it if did have one, than enquiring if it did have one in the first place!
Some PSU bricks don't have an earth pin and legally aren't required to as they are double insulated. Not having this mains ground can cause as many issues as removing an exisiting earth pin may seem to solve.
and ditto to Howards comments, i've been building equipment, installing and wiring studios for 25 years and never had to "lift the ground" to fix an earth loop problem. Although, i do carry a piggyback mains plug in my toolkit that has the earth pin removed and i may use it to "prove a point", but at my risk and with my experience of knowing exactly what i'm doing. It never stays in an installation. The solution lies elsewhere.

by **Martin** » Tue Jun 19, 2007 9:40 am

[quote="JulienG"]
I can't use it on my laptop if it's plugged in, [/quote]

i think this may be a common problem, i can't record to a laptop via its own sound card or mbox etc with the thing plugged in... at the moment it just records sets then gets plugged back in during breaks, not a solution, but when you find one i'd be curious to hear it!

by **chris p** » Tue Jun 19, 2007 9:41 am

Rob and Howard

Can we be more specific about the solutions? That may help stop people getting fried as they remove power earth pins.

Something like:

Source of hum - Diagnosis - Solution

Faulty wiring of power outlet - Dick Smith Power tester - Get electrician to fix

Faulty wiring of powerboard - ditto - Ditch powerboard

Different signal gound levels - Unplug and replug signal cables starting from the top until the hum disappears - ground lift the identified cable

etc

I'll host a PDF for downloading if someone makes one up.

by **Howard Jones** » Wed Jun 20, 2007 2:33 pm

It's a topic about which reams have been written. In fact, I'm working on a presentation for my own website. I'll try and give some info here later this week unless Rob manages to do it before me.

In the meantime, for those interested in understanding the audio side of this question, it's hard to go past this: http://www.rane.com/note151.html The Rane library is a great resource and recommended for many sujects.

by **JulienG** » Fri Jun 22, 2007 2:51 am

chris p wrote:Different signal gound levels - Unplug and replug signal cables starting from the top until the hum disappears - ground lift the identified cable

That's basically what happend, except you can't ground lift firewire.

The laptop is indeed double insulated so it seems to just choose a random potential WRT the incoming 240. I need to get my scope calibrated (hey, it's a 30 year old portable unit, I'm not complaining).

A linear supply for the laptop would be interesting, and I doubt it would be any hotter then my current power supply is (at least on 240, runs ice cold on 110), but as I've said upthread this machine is due to die soon (and would be dead if only F@#$ing Lenovo would let me buy a custom T61 here)

by **TimS** » Fri Jun 22, 2007 4:27 pm

There are excellent articles online
AUDIO NOISE AND AC SYSTEMS REVISITED
RETHINKING STUDIO ELECTRICAL SYSTEMS.
By Martin Glasband

The Origin of Balanced Power
By Martin Glasband

Lifting the Grounding Enigma
By Martin Glasband

Google and do a search - some very interesting reading.
I would post the articles online but I dont have FTP access from work and have no way to upload.
I can email or post online next week if anyone is interested (I'm away for the w/e)

TimS

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