laptop power supply noise [Archive] - SAWStudio / SAC User Group

demodoc

11-14-2009, 06:55 PM

I have what is, obviously, a common problem and I want to be sure I can come up with the right solution.

I do a lot of remote recording with Sawstudio and an ACER laptop, using a Focusrite Pro 40 Firewire interface. All works very well except for sometimes getting the high pitched noise induced into the audio from the switching power supply for the laptop. I can often get rid of the noise by putting the computer power supply on a different AC circuit than everything else, though there have been times that didn't work. This high pitched whine will end up very slightly on the tracks or maybe not at all, depending on where I am recording. The more troublesome thing is that it gets into the audio that's going out into my hardware console that feeds the PA or a live 2 track mix. I use high end mic preamps with two balanced outputs on each channel. One output goes directly to my interface and the other to the console. As far as I can tell, these outputs are simply paralelled inside the preamp...no internal isolation between the two. I know it comes from the power supply because if I run on battery power all is quiet.

So, question is, simply, will a good AC isolation transformer plugged in between the wall outlet and the laptop power supply solve the problem? Or plugged in between the wall and my Furman power conditioner, which powers the entire audio chain.

I realize, I have to keep the high frequency switching noise from the power supply from entering the audio gear at all, through mic lines, or even more likely, high-impedance unbalanced lines like those from a keyboard or bass before it goes through a DI box etc.

Bob, whad'ya think? This has to be a very common problem. Iso-transformer or simply a better power supply. If so, what supply?

Thanks!:)

Bill Park

11-14-2009, 07:05 PM

First, Laptop supplies are often a problem.

But you should try a balanced isolation transformer with a ground lift switch between the mic pres and the Focusrite. (Or elsewhere in that chain... I'm trying to isolate you from the console power supply and therefore, from the main PA.)

Bob L

11-14-2009, 07:05 PM

No one likes my solution but it always seems to work... a $.79 AC ground plug adaptor... simple and efficient... plug the laptop supply into one of these little grey or orange ground adaptor plug... lifting the ground.

Please... spare me all the hype about electrocution issues... this is not a power tool we are talking about where motor windings might fly off the armature and short the tool casing to the hot line... you will not be in any danger... in fact... you may notice that many laptop power supplies already only use a two wire connection with no ground.

So... if your power supply has a ground pin on the AC cord... lift it... and problem be gone.

Bob L

Bill Park

11-14-2009, 07:08 PM

No one likes my solution but it always seems to work... a $.79 AC ground plug adaptor...

Bob L

Of my three laptops, only one has the third pin.

905shmick

11-14-2009, 07:09 PM

Bob is 100% correct.

Get rid of the ground or run off battery power.

Tim Miskimon

11-14-2009, 07:24 PM

Bob's suggestion works 95 percent of the time.
The other thing that causes the high pitch whine sometimes is the power supply being too close to the computer/other gear.
Try moving it away or putting it in an ungrounded metal box.
Desk top computers don't generally have the power supply noise problem because of all the metal the case is made out of provides a good shield.
Unless you want to put your AM radio too close to it...:)
Otherwise you should be okay.

demodoc

11-15-2009, 01:09 AM

My power supply has a 3 prong plug...I'll try a simple ground lift first...It is a CHEAP fix, if it works. I'll let you know.

PC

Bill Park

11-15-2009, 08:02 AM

My power supply has a 3 prong plug...I'll try a simple ground lift first...It is a CHEAP fix, if it works. I'll let you know.

PC

Breaking multiple paths to ground will stop the 60 cycle thing. The high pitched whine, which as you say seems to be dependent upon trade winds for how it is going to appear and what it is going to affect, can sometimes be solved by running on battery, but not always.

As Tim says, it can often be cured by moving things around. We had a bad buzz on a Larry King Live remote and spent hours chasing possibilities in the theater, only to come out to the truck and find that someone had hung a cheap fluorescent light on the patch panel. Power supplies too close to audio lines, there are all sorts of places ti induce noise into the system.

I still find it a very wise idea to be electronically isolated from the house A/V.

demodoc

11-15-2009, 11:11 AM

Thanks, Bill.

I take it, I would have to put an iso transformer on every output of the preamp...could get expensive! And this solution would keep the noise out of the PA console but nothing about the noise that sometimes ends up on my tracks, admittedly at a very low level.?

Recently I recorded a concert in our local small theater. When I had the laptop plugged into the same outlet as my audio chain (which, as usual, was all running off a Furman), I got the whine everywhere...no good. I then plugged only the laptop into an outlet downstairs, via an extension cord and no whine at all, problem solved. On the other hand, I recorded a singer-songwriter in someone's home (a house concert) just the other night. I started by plugging the laptop into what the owner and builder of the house said was a separate circuit in the house....BAD whine!. I then plugged it into the same outlet as everything else and got much LESS whine...still there, but low enough that, at the level I fed the PA, you couldn't hear the noise. The tracks sound good but, in headphones, I can hear a very low level of whine on the tracks...not enough to cause a real problem but it shouldn't be there at all. This is how unpredictable this problem can be. In my most common use of this system (same preamps etc) I just plug the laptop only into an outlet on the other side of the room and no noise at all. I always have the laptop on a table, often on top of the Focusrite Pro 40 and the power supply on the floor away from everything. I suppose merely having the computer near the interface could be part of the problem too. In fact, at first, I thought the whine was Firewire induced noise. But when I ran on battery power and the noise vanished, I knew where it was coming from for sure.

Today I will do some experimenting. First, I'll just try Bob's groundlift idea...easy and cheap. Then I will play with positioning of things. I don't even know if I will get the whine here at home...we'll see. I have no audio iso transformers at the moment, nor an AC iso transformer...only a couple of Furman PL PLus II units. I never record anything without powering everything (except the laptop) with one of these. I sincerely hope I can solve this once and for all without having to buy multiple channels of transformers.

Will keep you posted.

Thanks,

Peter aka "Demodoc"

Atonepoet

11-15-2009, 11:24 AM

This is a good source of info on the subject.
http://www.ground1.com/news.htm

MMP

11-15-2009, 11:52 AM

I have found LCD monitors and laptops both add noise when grounded and I lift all of them. Another issue I often run into is ground loops through USB connectors and I have yet to find an affordable means to isolate those issues.

Regards,

MM

Bill Park

11-15-2009, 03:00 PM

" I always have the laptop on a table, often on top of the Focusrite Pro 40 ..."

Gave you tried separating these? That does put the drive motors over the pres.

demodoc

11-15-2009, 04:08 PM

Yes I have separated the computer and interface. In fact, I just did that a few minutes ago, when I tried Bob's idea of simply lifting the computer power supply's ground and lo and behold, the noise is GONE, completely. Interestingly enough, I do not get any noise when the computer is still grounded, if I plug mics etc directly into the Focusrite interface. It's only when I use my preamps in front of the interface, which is how I always record, since they are much better than what's in the Focusrite. As soon as the outboard preamps are in the chain, the noise follows. Now I know, all I have to do is ground lift the computer and, apparently, it can be powered from the same AC outlet, no problem. I can even place the computer directly on top of the outboard preamps with no noise, now that I have lifted its ground.

For what it may be worth, the preamps are all vacuum tube "Vac Rac"s from Inward Connections. My friend, the late Steven Barker designed these years ago, originally for his own use, then sold them under the Vac Rac name. The ones I have are extremely rare. Each box contains 10 channels of preamp...I have 2 such boxes. The ones that were available to the public were boxes that would hold up to six modules which could be either the mic preamp, limiter or EQ...a "lunchbox", basically. As far as I know, there were only about 5 or 6 of these 10 channel boxes ever built! They contain not one chip or transistor anywhere. Even the power supply, which is a separate box and connects with a multi-pin cable, is all tube in design. So something about the design of the preamps and/or their power supply requires that the computer be ground lifted, whereas the Focusrite doesn't care either way!

And, by the way, it doesn't seem to matter if the output of the preamps is plugged into any other analog device, such as a PA console as I described. I can get the noise simply plugging the output of the Vac Rac preamps into the Focusrite. (all BALANCED, by the way, though I have tried it unbabanced as well...results are the same). If the noise is heard in my headphones plugged into the Focusrite than it surely will also make it out to the PA console etc.

So, it would appear that Bob had the right answer once again! Hooray and THANK YOU, Bob.

On that score, I must, once again, say to Bob and all of us on this forum, what amazing and effective service Bob Lentini offers us. I have called him, even over a weekend, when I've had problems I just couldn't solve, even problems that had nothing to do with SAW at all, and he has cheerfully and patiently worked with me over the phone until I was straightened out. I just can't tell you how valuable that has been to me over the years. Try that with Digidesign!! I started with SAWPlus back in '96 and have never looked back or considered another application for my work. I know SAW isn't as well known or respected as it should be...no one at Guitar Center has ever heard of it, of course, but DAMN, I'm satisfied!:D

Thanks, Bob and all who contributed. I hope this may help any others out there who have been having this problem.

Peter Cutler
aka "Demodoc"
"Folkscene" radio program recording engineer

DominicPerry

11-15-2009, 04:39 PM

It's not a very constructive comment, but, the Focusrite stuff I've used (computer interface stuff) has been pretty poor. Maybe it's just not up to the job. All the RME stuff is fantastic at rejecting ambient noise (EMF) and port noise (PSU, firewire, USB etc).

Dominic

demodoc

11-15-2009, 05:52 PM

Actually, it isn't the Focusrite that has been picking up the noise, it has been my preamps. No matter, the problem has been solved

And for the record, my experience with the Focusrite Pro 40 has been nothing but good. It works perfectly every time, sounds excellent to my ears and I like the driver software. No complaints at all.

D.Chada

11-18-2009, 02:10 AM

jazzboxmaker

11-18-2009, 03:10 AM

If you mean the connector in the laptop it depends on which type it is. Older ones are soldered on the motherboard while the newer ones plug in. The new ones can be had on Ebay for 4 or 5 bucks from China- takes a couple of extra days to get here otherwise pay $40 for one stocked in the US and save 2 days.

The plug in ones require some surgery to do properly as the motherboard may have to come out. I just did one on my son's Thinkpad R51 but Thinkpads are comparatively easy to work on if you're careful. Lenovo has step by step directions for all work in their hardware maintenance manuals. Can't help with other brands.

Just a thought- your charging problem may be a motherboard problem as well.
Get some good screwdrivers and good luck

Cary B. Cornett

11-18-2009, 06:31 AM

My laptop has stopped charging at times. I think the connector is loose. The power supply seems ok because the led light is on. Anyone ever replace theconnector. It seems pretty easy. I checked online and a new connector is40 bucks. That seems like a lot for a little piece. One of my sons works fixing computers, and he has replaced the power inlet connectors on many laptops. It is quite common for these replacement parts to be expensive.

It is very important to make sure that there is close to zero mechanical stress ever placed on the power inlet connector. It should never have to support the weight of the cable. It is generally also best for the computer to be sitting on a stable and stationary surface whenever the AC supply is connected (use battery power when holding the computer on your lap). Another important safety tip is to always have the external power supply ON THE FLOOR (when is the last time you ever saw anything fall FROM the floor??).

HTH

jazzboxmaker

11-18-2009, 12:13 PM

I checked online and a new connector is40 bucks. That seems like a lot for a little piece.

Yeah, when you get the part and see it's a 2 or 3 pin jack about 1/4" square & probably cost 5cents to make you'll realize you've been ripped off- Um I mean paid for the convenience of fast shipping :rolleyes:. It's not as if the part in the US is any better cause they aren't made in Kansas anymore Toto;)

studio-c

11-24-2009, 04:09 PM

DaveS

11-24-2009, 11:19 PM

I just ran into a similar problem with laptop p/s noise getting into the system. I have an MSI laptop connected to a a M-audio FW1814 and I have an ADA8000 connected for the extra inputs. I'm playing with some old band mates Wed. evening and said I would provide a mix and record the gig....

So I got everything together and tested all of the mic inputs - worked fine. Hooked up my bass preamps direct out to the line level balanced input on the FW1814 - and the noise reared it's ugly head. After trying different cables and such I finally used the 'gozinta' (a nickname we have for the ground lift adapters) and the supply noise went away. What I was finding strange was that the mic inputs were all fine - just the line level inputs on the FW1814 were the problem. I suspect that since it was picking up a ground thru the firewire cable to the laptop and the power supply had a grounding pin as well...not sure why that would cause the problem...but it did. The ADA8000 is isolated from the FW1814 by the light-pipe cables...of course all of them are bolted into an SKB case..

Looking forward to running the system on Wed.

Cary B. Cornett

11-25-2009, 06:26 AM

After trying different cables and such I finally used the 'gozinta' (a nickname we have for the ground lift adapters) and the supply noise went away. What I was finding strange was that the mic inputs were all fine - just the line level inputs on the FW1814 were the problem. All "ground noise" problems are caused by differences in potential between the grounds on different pieces of equipment. You would not have trouble with the built-in mic inputs on the FW1814 because the microphones get their "ground" only through the cable to the FW1814. On the ADA8000, the optical connection effectively isolates the signal path from ground problems. Once you hook up your bass preamp, though, you have two things in the system, each with its own grounded supply, and then the fun begins.

How is your bass preamp connected to the FW1814? Does the FW1814 have balanced line inputs? If so, the problem may be your cable between the bass preamp and the FW1814. If the bass preamp has a balanced output, and the FW1814 has a balanced line input, you need to use the proper balanced cable between the two. If you have an unbalanced output connected to a balanced input, using the ordinary "guitar cord" will defeat the balanced input.

I have several cables that I made up for connecting unbalanced outputs to balanced inputs. I use 2-conductor shielded cable (the kind used for microphones) and put a TRS ("stereo") phone plug at one end and a TS ("guitar") phone plug on the other. The TRS end is wired the usual way, tip hot, ring cold, sleeve shield. On the TS end, tip is hot and both cold and shield tie to the sleeve connection. The TS end plugs into the unbalanced output, the TRS end goes to the balanced input.

Usually using such a cable will eliminate ground noise by causing it to cancel out at that balanced input.

The use of "ground lifters" on power cables should only be a last resort. The ground pin is there for SAFETY reasons. I do keep a couple "lifters" handy, but I can't remember the last time I used one. A properly wired system usually will not "buzz" unless some of the inputs are unbalanced. For that, you should use isolation transformers in the audio signal path. Granted that the really good ones are expensive, the cheapest solution I have found is the little inline isolation transformer that Radio Shack sells. It is a black cylinder with stereo RCA cables coming out of both ends, and last I knew it cost somewhere around $20.

MMP

11-25-2009, 06:37 AM

Ebtech sells some cheap iso transformers that most GC stores have in stock.

I have found some ground loops caused by USB & Firewire connections and not audio connections & I have not yet found an inexpensive solution to those issues.

Regards,

MM

Bill Park

11-25-2009, 07:42 AM

I have an MSI laptop connected to a a M-audio FW1814 and I have an ADA8000 connected for the extra inputs. ...

So I...tested all of the mic inputs - worked fine. Hooked up my bass preamps direct out to the line level balanced input on the FW1814 - and the noise reared it's ugly head. ... the mic inputs were all fine - just the line level inputs on the FW1814 were the problem. ... it was picking up a ground thru the firewire cable to the laptop and the power supply had a grounding pin as well... The ADA8000 is isolated from the FW1814 by the light-pipe cables... all of them are bolted into an SKB case..
.

gozintas and comzoutas are old time names for inputs and outputs of any stripe.

Plugging a single-ended device like a bass into one piece in a string of gear and having it cause problems does not indicate that the "problem" is with the piece. XLRs are balanced, the bass is not.

It used to be common, when connecting XLR outputs between devices, to lift the ground of the XLR on one end. This held to prevent the multiple paths to ground that is usually the cause of the noise we are talking about.

Many pieces of equipment (most if not all newer pieces) do not have a separate technical ground from the safety ground, so racking them up on standard rack rails connects all of the pieces together.

Bill Park

11-25-2009, 07:56 AM

I have one on all my laptops, and just leave 'em on.

That is great, so long as you are working in places where nobody ever checks.
But in many facilities in which I would work, there are either City/County electrical inspectors, fire inspectors, or internal safety inspectors whos job it is is to see that outside contractors adhere to local regulations. Not a single one will allow ground lifters on power cables of any type. When a large scale production comes to a halt while you try to talk the guy into letting you get by (which will never work) then try to solve the problem, it will likely be your last gig with that employer. Better to solve the problem in a way that makes electrical inspectors and technical directors happy. This could be for you as simple as cutting off the grounded plug and replacing it with a non-grounded plug.... very cheap and oddly something that I have never seen them check for.

But this is a solution with a smaller subset of users, as not all laptops have grounded power supplies. As I said, the majority of mine don't.

Bob L

11-25-2009, 08:21 AM

What will those inspectors do with devices that only have a two wire connection? Make you add a ground wire?

I have been all over the world on many many tours and gigs and have NEVER EVER run into a problem where someone forced me to NOT USE LIFTERS... and virtually every case where I had any kind of grounding noise problem was easily solved by chasing down the offending equipment with grounding plugs and lifting them.

The single point grounding system is virtually impossible to implement in the real world environment... especially on live stages.... and especially with a mixture of balanced and unbalanced equipment.

Bob L

DaveS

11-25-2009, 08:46 AM

How is your bass preamp connected to the FW1814? Does the FW1814 have balanced line inputs? If so, the problem may be your cable between the bass preamp and the FW1814. If the bass preamp has a balanced output, and the FW1814 has a balanced line input, you need to use the proper balanced cable between the two. If you have an unbalanced output connected to a balanced input, using the ordinary "guitar cord" will defeat the balanced input.

The bass preamp has a balanced direct out...however it is not a transformer balanced output - it is an op-amp based system. The line inputs to the FW1814 are also balanced and the jacks are the plastic shell kind that are not attached to chassis (makes them weaker then I would like). I thought that this should have worked fine - at least as well as the FW1814s MIC inputs, but it didn't. Neither piece has a ground lift button, however, I have a multitude of specialty cables I have made over the years including a ground-lifted XLR to XLR cable. That didn't help either. I eventually decided to just pull the plug out of the back of the laptop (also as a failure test to see how the laptop would behave) and the noise went away. Put the ground lift plug in place and problem solved. The bass preamp - an Ampeg SVP-PRO - does have a grounded power cable as well. I didn't put a ground lift adapter on it however.

Bill Park

11-25-2009, 11:19 AM

What will those inspectors do with devices that only have a two wire connection? Make you add a ground wire?

Bob L

They usually do not allow cords with pulled ground pins, nor lifters, or power cables wherein the outside insulation does not extend into the cable clamp of the connector, or a handful of other things that they can see just by walking around. Oh, and anything that has been obviously modified and not mechanically wired to code. And it depends upon who is looking...city or county electrical, building, fire, or safety inspectors, or a site-provided person who insures that incoming productions adhere to local codes.

I've experienced these inspections at the Wintergarden, PPG Place, Byham Theater, C-MU, University of Pittsburgh, the arena at Hershey PA, the David Lawrence Convention Center, and other places around the country. Is it a daily occurrence? No. Does it happen? Yes.

I do agree that it often eliminates the hum when the laptop has a grounded power supply. But I said that along time ago.

Bill Park

11-25-2009, 11:33 AM

...That didn't help either. I eventually decided to just pull the plug out of the back of the laptop... and problem solved. ..
I think the point that some of us are trying to make is that when a symptom goes away, we have not always solved the problem.

When Bob made his first statement about lifting the power ground and tried to short circuit the inevitable cries about safety, I agreed with his original premise.... low current draw, not much chance of something bad happening, why not take the easy way out? Fair enough.

But someone else made the correct point that the rule exists for a safety reason. Also a very fair point of view.

Then I present a practical example of a reason to follow the rule... not to save anyones life, just to save hassle... and I get called a liar. (sigh...)

Do what works best for you. It is my suggestion that the more technical stuff you chase down and understand, the more that knowledge can serve you in the future.

Bob L

11-25-2009, 12:02 PM

Oh, and anything that has been obviously modified and not mechanically wired to code.

I am talking about equipment that comes from the factory with only a two pronged plug... if there was such danger involved... why would these be legal?

Many laptops and other audio amps and processing equipment ship from the factory with a two pronged plug... so if its ok to use that gear onstage... why is a ground lifter so dangerous?

why not take the easy way out?

I rather see it as a very good solution that gets the job done instantly so I can move on to other things of importance to get evrything sounding good by showtime.

Bob L

Bill Park

11-25-2009, 02:01 PM

I am talking about equipment that comes from the factory with only a two pronged plug... if there was such danger involved... why would these be legal?

why is a ground lifter so dangerous?

Bob L

as I said, unmodified.

As for your ground lifter, that is not what they are supposed to do. It is the misuse of a part. The 'ground lifters' are adapters, made to convert 3 prong AC cables to allow them to be plugged into two prong AC supplies; and YOU ARE SUPPOSED TO SCREW THE LITTLE TAB TO THE OUTLET COVER, THEREBY GROUNDING THE SAFETY GROUND INTO THE AC SYSTEM. However we have chosen to misuse them, that is their intended usage. Like using a screwdriver as a chisel.

Tim Miskimon

11-25-2009, 03:50 PM

as I said, unmodified.

YOU ARE SUPPOSED TO SCREW THE LITTLE TAB TO THE OUTLET COVER, THEREBY GROUNDING THE SAFETY GROUND INTO THE AC SYSTEM.

That's counting on the fact that there is actually a ground wire hooked to the outlet and/or that there isn't any paint on the screw...:)

When hooking up audio equipment lots of times you have to float the ground on the preamp, mixer, etc. to keep from getting hum.
As long as the stuff is interconnected to a power amp that is grounded the system will be safe.
The shock problem pops up when the polarity on one of the pieces of gear is out of phase.
That's why it's always best to use the same circuit (same phase) for all the audio gear.

Craig Allen

11-25-2009, 10:25 PM

The shock problem pops up when the polarity on one of the pieces of gear is out of phase.
That's why it's always best to use the same circuit (same phase) for all the audio gear.
It has nothing to do with what phase your on, but the path the ground wire takes. All grounds go to the same bus bar in the panel, but the lengths of the wires are different, which will cause a resistance, and a difference of potiential to ground. The trick is to make all grounds the same potiential regardless of what circuit or phase they're on. I do this using outlet strips I modified (this is against all code, so use at your own risk). I put two 1/4" connectors in each outlet strip the members of my band use. Both the tip and sleeve are connected to ground in the strip. I then run 16ga speaker cables from strip to strip until it gets to FOH. All grounds are at the same potiential, and no one gets shocked. I've done this for 10 years and it's been 100% effective. And you don't have to plug into the same circuit (which can cause tripped breakers). I also check outlets with a tester to make sure they're wired correctly before plugging into anything.

Bill Park

11-26-2009, 04:20 AM

jcgriggs

11-26-2009, 07:47 AM

I remember being at a lecture from Dan Healy (the Grateful Dead's FOH guy) at the Institute of Audio Research in the 1980's where someone asked him what it was like to be at Woodstock. His reply was something like:

"Imagine being dosed on acid, stepping up to check a microphone and getting 300 volts through your lips - that was Woodstock"

Every time I see someone get a shock from microphone-guitar ground potential differences, I think of that line...

- John

Leadfoot

11-26-2009, 08:19 AM

I carried a wiggy in my guitar case. Touch it to your strings and the mic, to see if it lights up. So that you don't.

A wiggy is a probe type (direct contact)voltage tester. A non-contact voltage tester, sometimes called a tic tracer, which lights up when in very close proximity to ac voltage, is what I think you mean. I have been shocked between my guitar and the mic, and it wasn't just a tingle. Some of these clubs and buildings we work in are electrical nightmares, outlets with reversed polarity, open neutrals, no ground. Most premisis that are run in pipe(conduit), use the pipe for the ground connection, (unless someone specifically asked for a dedicated/isolated ground wire) if you lose the neutral, the circuit could use the pipe and everything connected to it for return path. All it takes is for the neutral wire to be loose, and then the ground could become hot when a device is plugged in.
So many ways to get nailed, just can't be careful enough. Happy Thanksgiving :)

Bill Park

11-26-2009, 08:31 AM

A wiggy is a probe type (direct contact)voltage tester. A non-contact voltage tester, sometimes called a tic tracer, ... :)

I carry a tic tracer in my toolbag. I carry a wiggy in my guitar case. Connecting the amp, though the guitar strings to the PA though the mic to see if the potential lights the wiggy. (I have a couple of different wiggys, both AC and DC and one with lights for different voltage readouts.)

John Ludlow

11-26-2009, 08:47 AM

A wiggy is a probe type (direct contact)voltage tester. A non-contact voltage tester, sometimes called a tic tracer, which lights up when in very close proximity to ac voltage, is what I think you mean. I have been shocked between my guitar and the mic, and it wasn't just a tingle. Some of these clubs and buildings we work in are electrical nightmares, outlets with reversed polarity, open neutrals, no ground. Most premisis that are run in pipe(conduit), use the pipe for the ground connection, (unless someone specifically asked for a dedicated/isolated ground wire) if you lose the neutral, the circuit could use the pipe and everything connected to it for return path. All it takes is for the neutral wire to be loose, and then the ground could become hot when a device is plugged in.
So many ways to get nailed, just can't be careful enough. Happy Thanksgiving :)

Amen to that brother. Nothing takes concentration away from performance like having "they're-trying-to-take-it-away" voltage burn your lips every time you inadvertantly touch both your guitar and your mic. That was my experience with the bar circuit too. It was as consistant as the rickety metal stairs in the back that the B3 had to go up and down. They setup a stage and, as an afterthought seemingly, somebodies drunk uncle wired it for free booze.

Wink0r

11-26-2009, 09:07 AM

I once had a guitar player in a little dive that accidentally touched the mike with a guitar string and burned the guitar string in two just before the band was going to start playing. The outlet that he plugged his amp into was reverse wired. We used another outlet for his amp and all was well.

If you don't have a tool always touch the mike with the back of your fingers first. If there is a shock your fingers will close (nerve reaction). If you use the back of your fingers they will pull away from the mike. If you were to grip the mike your hand will tend to close on the mike and continue the shock.

Cary B. Cornett

11-26-2009, 09:11 AM

The bass preamp has a balanced direct out...however it is not a transformer balanced output - it is an op-amp based system. That should not matter. In fact, for the intended noise cancellation purpose, all that matters is that the impedance to ground of both "hot" and "cold" pins is the same.
The line inputs to the FW1814 are also balanced and the jacks are the plastic shell kind that are not attached to chassis (makes them weaker then I would like). I have seen more then one of those plastic jacks stop working when a cable plugged into it was jerked sideways. If they don't get stressed they are usually OK... except that the sleeve terminal on the jack should go directly to chassis ground (AT THE JACK), and some are not wired that way. The Jensen transformers website has a couple of really good articles about grounding and balancing that should be required reading for all techs and sound engineers.
I thought that this should have worked fine - at least as well as the FW1814s MIC inputs, but it didn't. Neither piece has a ground lift button, however, I have a multitude of specialty cables I have made over the years including a ground-lifted XLR to XLR cable. That didn't help either. I would have first tried a proper balanced cable that had the shield connected at both ends. Did you do that? In particular, if you used a regular "guitar cord" (TS instead of TRS) you would have defeated the function of the balanced input.

Craig Allen

11-26-2009, 10:46 AM

"...the path the ground wire takes. All grounds go to the same bus bar in the panel, but the lengths of the wires are different, which will cause a resistance, and a difference of potiential to ground."

Which has killed people, and shocked many guitarists.
Absolutely - I've been shocked many times myself. That's why I did the outlet strip thing - bring everything to the same potiential, and no shocks whatsoever.

When I was playing out I carried a wiggy in my guitar case. Touch it to your strings and the mic, to see if it lights up. So that you don't.
Nice idea - I'll have to remember that.

demodoc

11-26-2009, 01:31 PM

All I can say is, Bob's solution WORKED perfectly, the noise is gone and all is well, end of story. I have never experienced any problems with using "ground lifters" in well over 30 years of recording. Maybe I'm just lucky. This really saves my butt because I do a lot of live concert recording in several venues, a home studio, a coffee house, a small theater and more and I have to just set up and punch record. I don't have time to be pulling what little hair I have left over a power supply noise injected into the system for all to hear:eek: So, thanks again, Bob, for the quick and easy fix.:D

Ian Alexander

11-28-2009, 10:17 AM

"...the path the ground wire takes. All grounds go to the same bus bar in the panel, but the lengths of the wires are different, which will cause a resistance, and a difference of potiential to ground."

Which has killed people, and shocked many guitarists. When I was playing out I carried a wiggy in my guitar case. Touch it to your strings and the mic, to see if it lights up. So that you don't.
OK, ignorant question from someone who doesn't play guitar.:o Are the strings part of the circuit? I've looked at several wiring diagrams for electric guitars and I don't see the strings or the bridge getting connected. It is simply that, duh, everyone knows that the bridge is grounded, so none of the diagrams show that?

Mark Stebbeds

11-28-2009, 10:39 AM

All I can say is, Bob's solution WORKED perfectly, the noise is gone and all is well, end of story. I have never experienced any problems with using "ground lifters" in well over 30 years of recording.

This is not the argument. The AC ground lifter works most of the time to eliminate a ground loop and we all know that ...but it is not safe.

We have all done this, and most of us have been very lucky as you have, but using an AC ground lifter COULD eliminate someone's life when their body becomes the path to ground. It really is that simple.

The audio ground lifter or iso transformer is safe. This is a question of being too cheap to spend a couple of bucks, or too lazy to do it safely.

The risk of accident increases dramatically when you are moving from venue to venue with totally different and unknown power distribution and other equipment connected to it.

OBC 6/10

Mark

Mark Stebbeds

11-28-2009, 10:42 AM

That's counting on the fact that there is actually a ground wire hooked to the outlet and/or that there isn't any paint on the screw...:)

There is almost always a path to ground via the electrical outlet to the conduit back to the breaker box. This has been minimal electrical code all over for decades. I don't think explaining away the danger because of someone else's potential oversight (like power distribution) is an excuse for us not doing our jobs as safely as possible.

Mark

Mark Stebbeds

11-28-2009, 10:58 AM

OK, ignorant question from someone who doesn't play guitar.:o Are the strings part of the circuit?

Most guitar players can't play guitar either, they just think they can.:eek:

I've looked at several wiring diagrams for electric guitars and I don't see the strings or the bridge getting connected. It is simply that, duh, everyone knows that the bridge is grounded, so none of the diagrams show that?

Good question. I never thought of that.

Mark

EDIT: I was surfing on the 'net for articles regarding guitar players getting shocked and found a reference to bridges on Les Paul's being connected to ground, but unconfirmed. Interesting that this info isn't easier to find.

jazzboxmaker

11-28-2009, 12:08 PM

Cary B. Cornett

11-28-2009, 01:20 PM

OK, ignorant question from someone who doesn't play guitar.:o Are the strings part of the circuit? ... It is simply that, duh, everyone knows that the bridge is grounded, so none of the diagrams show that? Yes, the bridge is grounded (Haven't met one yet that wasn't). This has an odd consequence. Ever notice that when the guitar player completely lets go of the strings, there is a buzz? There is a sneaky studio trick for that (probably NOT recommended for on stage in a "strange" venue): a clip lead on the bridge, other end always contacting the guitar player's skin. Result: no more "let go buzz". Again, do this ONLY if the wiring setup is known to be safe, as when the guitar amp is properly grounded.

jazzboxmaker

11-28-2009, 01:50 PM

Yes, the bridge is grounded (Haven't met one yet that wasn't). .
Well, not always. There are several methods of grounding. Archtop guitars, for example, do not take their ground from the bridge as it is a wood base free to float on the top. The strings would be grounded at the tailpiece. For example a trapeze style tailpiece could be grounded by contacting the jack which goes through at the end block, or with a trapped wire or wire through the end block.Some electric instruments have the ground wire trapped under the tailpiece thence to the jack. Many bridges provide ground for sure. Tremelo units provide easy access for a ground wire soldered to hardware. Many ways to skin the cat

Brent Evans

11-28-2009, 02:26 PM

Yes, the bridge is grounded (Haven't met one yet that wasn't). This has an odd consequence. Ever notice that when the guitar player completely lets go of the strings, there is a buzz? There is a sneaky studio trick for that (probably NOT recommended for on stage in a "strange" venue): a clip lead on the bridge, other end always contacting the guitar player's skin. Result: no more "let go buzz". Again, do this ONLY if the wiring setup is known to be safe, as when the guitar amp is properly grounded.

Talk about an electrifying performance....

http://lc.fdots.com/cc/lc/d1/d1d0423803bab645dd361e8ea06b62ea.gif

Bill Park

11-28-2009, 04:26 PM

OK, ignorant question from someone who doesn't play guitar.:o Are the strings part of the circuit? I've looked at several wiring diagrams for electric guitars and I don't see the strings or the bridge getting connected. It is simply that, duh, everyone knows that the bridge is grounded, so none of the diagrams show that?

On most guitars but not all, the strings are tied to the ground. I have an early Nashville Les Paul Deluxe (circa 1979) where there is no string grounding. I'm sure that there are other examples.

In terms of grounding to the bridge being 'unconfirmed', let me confirm it for you now, no doubt, clear and concise, a standard electric guitar has its strings tied to the ground. In the case of most guitars, it is through the bridge, some through the tailpiece.
Open any guitar cavity. Soldered to the case of one of the pots there is usually a small wire that disappears into a hole in the body headed generally in the direction of the bridge. Guess what it is?

Tim Miskimon

11-29-2009, 04:12 PM

There is almost always a path to ground via the electrical outlet to the conduit back to the breaker box. This has been minimal electrical code all over for decades. I don't think explaining away the danger because of someone else's potential oversight (like power distribution) is an excuse for us not doing our jobs as safely as possible.

Mark

Well Mark many of the boxes used today are made of plastic, most use romex wire with a plastic cover - not exactly what I call a good ground path.
The older wiring used 2 conductor wire inside of metal casing.
Code doesn't mean anything.
I can't count the times I've been in a school, church, club, etc. where every other recepticle was wired backwards, lack of any ground, etc.

All I'm saying is "don't count on a good ground coming off a screw in the face plate".
Corrosion of any kind will kill the ground and the electricity will find it's own ground - whatever or whoever that may be...:eek:

Tim Miskimon

11-29-2009, 04:21 PM

Aren't we talking about 12-18 volts that the laptop is running off of?
The wall wart does the conversion from 110v AC to 12 volts DC so I really don't see why a ground pin on the wall wart is even needed.
Most wall warts I have that came with my gear are only 2 pins so I question why laptops would be any differant.
The plug going into the side of the computer is only 2 conductor so where's the ground?
I think Bob's 3 to 2 adapter solution is perfectly safe.

Bill Park

11-29-2009, 06:14 PM

Well Mark many of the boxes used today are made of plastic, most use romex wire with a plastic cover - not exactly what I call a good ground path.
The older wiring used 2 conductor wire inside of metal casing.
Code doesn't mean anything.

The ground wire (the uninsulated third wire in the 14/2, 12/2, etc) is connected to that ground screw on an outlet. That screw is in the same chunk of metal that holds the faceplate screw.

Indeed, I've found many miswired setups.

Bill Park

11-29-2009, 06:18 PM

The wall wart does the conversion from 110v AC to 12 volts DC so I really don't see why a ground pin on the wall wart is even needed.
Most wall warts I have that came with my gear are only 2 pins so I question why laptops would be any differant.
The plug going into the side of the computer is only 2 conductor so where's the ground?
I think Bob's 3 to 2 adapter solution is perfectly safe.

Which takes us around the the beginning again, where I originally accepted Bobs premise (based on similar logic) until it was pointed out that this is a safety issue. What you or I think or don't see is irrelevant to the code, and the use of a grounding adapter as a ground lifter is not really kosher, no matter how many times we've all done it. And what you said:"Most wall warts I have that came with my gear are only 2 pins...", yeah, me too.

Mark Stebbeds

11-29-2009, 06:41 PM

All I'm saying is "don't count on a good ground coming off a screw in the face plate".
Corrosion of any kind will kill the ground and the electricity will find it's own ground - whatever or whoever that may be...:eek:

And I'm saying it's not good engineering practice to "explain away" the need to use proper safety precautions, and instead use AC ground adapters as "ground lifters", because the condition of the power D is unknown, and maybe somebody somewhere used Romex without a ground wire in a church ...or not. There is nothing to lose by fixing audio "humm" safely.

Mark

demodoc

11-30-2009, 01:07 AM

Wow, look what I started!:eek:

Ian Alexander

11-30-2009, 05:58 AM

Wow, look what I started!:eek:
Yes, but even with a little attytood now and then, there's a lot of good information in this thread.:)

studio-c

12-01-2009, 11:36 AM

Agreeing with Mark's point on the many earlier discussions we've had on this topic, I definitely wouldn't lift a ground on anything where the power is getting larger, as on the power amp end of a PA system. On a laptop, you're at the beginning, small end of the food chain. And you are actually grounded through your soundcard audio cable ground to the mixer. The mixer is most likely grounded. Which is why you're getting ground loop hum, because you've double-grounded the laptop, at the power source, and at the audio card.

Nowadays for live gigs, which for me is mainly hotel blalrooms, I'm taking a long power cord from the power drop back stage (front of house) even though I'm working back of house in the sound booth. This power comes from a power drop supplied by the AV guys, and it comes from the 220-240 volt giant cables in the bowels of the building somewhere. It takes its feed before the kitchen with its electric mixers, dishwashers, etc, which downstream would be adding their noise to the line.

I have two powerstrips into which everything plugs, and the one ground connects directly to the power drop, sharing a ground with the lights, projectors, etc. No problems.

I would never just plug into the wall outlets at the back of the room. Being 100 feet away from the stage, there's a very good chance they're on a separate circuit, opening the possibility of them being out of phase, etc. Just plug your gear into a miswired outlet one time, watch your stuff fry, and you'll see the extension cord as cheap insurance :) Basically, try and get your power near the power amp end of things, then everything is grounded in one place.

It would be tempting to use those back of room outlets and lift a ground there, but it's just too risky.

Long answer short, the laptop should be okay, because it really is grounded elsewhere (audio card).

Cary B. Cornett

12-01-2009, 01:05 PM

Nowadays for live gigs, which for me is mainly hotel ballrooms, I'm taking a long power cord from the power drop back stage (front of house) even though I'm working back of house in the sound booth. This power comes from a power drop supplied by the AV guys, ... How you get your power is important. In a theater I worked at, we did something similar. I was providing everything that went between the stage snake and the power amps. The amp rack was up in the former projection booth. I ran my heaviest extension cable from the outlet the amp rack was plugged into to my mix position at back of house. Any sound gear that I had up at the stage end got its power from an extension cable that I ran from my power feed. Thus, no worries about which leg of power, etc., and of course grounding was consistent. No ground lifts were used in this setup, except for one show where, IIRC, a laptop computer in the pit that was used for music playbacks had to be "lifted" from ground.