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Author Topic:  Some Tips For Rack Amp System Users
John Floyd

 

From:
R.I.P.
Post  Posted 3 Jan 2004 8:40 pm    
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Do you use a rack amp system?

Chances are if you have hum, that you have ground Loops.

Too much grounding is bad, you need to ensure that only one ground path exists between units and preferably that is in the power cable.

Do you have metal rack rails?
These provide a duplictate ground path and the way to take care of this is by using isolation tabs to isolate the chassis of each unit from the rack rails and from each other. One Neat idea I've found is a kit from Musicians Friend called Humfrees Consists of eight isolation Tabs (enough for two units) for around 5 bucks, well worth it. They also space the adjacent units so they don't make contact with each other.

If possible use XLR or TRS (3 wire) Patch cables between units. This is the Professional way and They work well,(much better than standard Guitar cords) when the ground is lifted on the Male pin end of the cable. The idea is for the sending unit to provide the shield ground in the cable and the shield is cut at pin 1 of an XLR or Shield of a TRS of the receiving unit. In order to take advantage of this your rack equipment must have XLR or TRS inputs and Outputs.

Always buy Preamps or effects processors with built in power supplies and Three wire power cables. Avoid units with Wall Wart or inline (brick) Power supplies.

Get a Good Power Distribution System preferably rack mounted with a good surge protector and Voltage meter. This can reallly be useful when working in clubs with Questionable power in the building.

Also a handy item to have on hand if you are moving from club to club is an electrical outlet tester that plugs into an AC oulet and tests for Ground (most Important) and Reverse polarity or any other combination of miswires in an AC Outlet. This can actually be a Lifesaver and the life might just be yours. These testers usually have 4 neon lamps and give various indications for faulty wiring or connections. The one I use is From ECOS Electronics Corp Called a Ground Check model 7100. Most important is that it has a Ground impedance test, If the Ground is above 2 ohms the ground light will flash.

Hope all of this helps someone who has Hum or electrical problems and just didn't want to ask

John

[This message was edited by John Floyd on 03 January 2004 at 08:53 PM.]

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James Quackenbush

 

From:
Pomona, New York, USA
Post  Posted 4 Jan 2004 4:20 pm    
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John,
Great tips !!....Another thing that I do with my rack gear is that I use electrical tape instead of humfree's ...I run the tape the full length of both rack rails and cut out little holes with and Exacto knife for the screws....I then use white plastic scews with fibre washers on the outside ....No hum problems over here !!... I've also got rack's of keyboard modules that I did the same way..
Very quiet ...The company Rane also has application guides for wiring to best eliminate ground loops and proper wiring up of various connetors !!...Very good information
Thanks for your tips...They are very usefull
Sincerely, Jim
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John Russell

 

From:
Austin, Texas
Post  Posted 4 Jan 2004 8:24 pm    
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Interesting. I recently bought a Behringer 8-channel mixer and its power supply is on a brick with a tightening ring on the plug that goes into the pre-amp and a cord on the other side of the brick to plug into the wall or strip. In their user's manual they say the separate p.s. is a feature that eliminates hum keeping it away from the device. Someone has suggested that it's also a way for the manufacturer to use an off the shelf p.s. that's already been tested so they don't have to pay for the expensive U/L certification.
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Dennis Detweiler


From:
Solon, Iowa, US
Post  Posted 4 Jan 2004 9:07 pm    
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When you isolate the mounts (chassis) of each unit from each other...aren't they still grounded to each other via shielded ground cables patching each unit to the other?
DD
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John Floyd

 

From:
R.I.P.
Post  Posted 5 Jan 2004 3:02 am    
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Dennis
Thats why you should use Balanced line, 3 Wire patch cables when ever possible. I cut the Shield on the receiving unit end of each patch cable. Some very high end Equipment has ground lift switches. Thats the way the Pro's do it and Studios as well. It really makes a difference. But you have to have the matching Cconnectors on the inputs and outputs of your Rack units. If you just have the two wire standard phone jacks, you are stuck with what you have and yes there will be ground loops aand more 60Hz hum.

When you are buying equipment for your rack system, Balanced line connectors need to be one of your requirements.

There are too many rack systems out there that are just thrown together with inexpensive components and people are usually dis satisfied with them. I suspect this is the case when you hear complaints against rack mounted systems. Just because it is Rack mounted doesn't mean that it is the best way to go, but when you have a rack system that is properly assembled and good choices in equipment, it is like the comparison between a high End Component Stereo and a Cheap boom box.

I don't use the Effects loop of my Walker Stereo Steel Preamp because it uses Standard 2 wire Phone patch cables. I am talking to G.D. about building me a preamp with all balanced line Outputs and Effects loop connectors. He builds probably the best preamp out there, best clean tone of any and a wide range of controls. you can do almost anything with this unit, but there is a little room for improvement in the Choices of Connectors in the effects loop and main outputs. I'm sure that this will mean more bucks for the new preamp, but I am more than willing to pay for these features.

[This message was edited by John Floyd on 05 January 2004 at 03:06 AM.]

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Stephen Gambrell

 

From:
Over there
Post  Posted 5 Jan 2004 5:38 am    
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I've always wondered about the use of power conditioners with meters on the front. If the voltage is low all over the building, how do you fix it? Do you refuse to play?
Not trying to start something, just curious.
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John Floyd

 

From:
R.I.P.
Post  Posted 5 Jan 2004 5:53 am    
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That is up to you, not a lot you can do, but refuse to plug in. Shouldn't hurt anything if it is less than say 10% out, but after that I would have some concerns. I would definitely have a tester plugged in like the one I described to see if there is a miswire in the house wiring, then I would definitely refuse to plug in my system if there was
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Lee Baucum


From:
McAllen, Texas (Extreme South) The Final Frontier
Post  Posted 5 Jan 2004 6:14 am    
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The electrical outlet tester is an inexpensive insurance policy against a potential tragedy. A few years ago a local band performed in a recently remodeled hall that had a brand new stage. All of their equipment was fried when they started turning things on. They later found out that the electrician had screwed up the wiring. After a lot of fighting the owners of the hall finally coughed up the thousands of dollars it took to repair/replace the equipment that was fried. One little test would have revealed the problem.

------------------
Lee, from South Texas
Down On The Rio Grande

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Joe E

 

From:
Houston Texas
Post  Posted 5 Jan 2004 10:39 am    
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As far as external power supplies....In the studio I usually look for an external supplies that is well build. Not really talking about a wall wart. But the units with a AC cord on one end a box in the middle and a power cord to the unit on the other. This does reduce hum, and provides a quiter unit. But thier are also some great units made were the power supply is built in, which are quite too. It really comes down to design.

As far as grounding, this is part of design too. If signal ground and chasis ground is tied together, lifting one end won't really help. Unless you know which unit has them tied and which don't.

Joe
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John Floyd

 

From:
R.I.P.
Post  Posted 5 Jan 2004 11:10 am    
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Joe
I don't claim to know it all. Good point on the ground lifting. Glad to see your comments here,

I'm basing my comments on inexpensive (sometimes not so inexpensive) preamps and effects units that either have a brick power supply or a wall wart. I had an Alesis quadraverb that drove me Crazy with the Hum, had a Brick power Supply. I had to be especially careful where I placed the brick or the hum was terrible.

I know Some Mixing Consoles have an External Power Supply That is Rack mounted and as Joe so ably points out its all in the Design and engineering.
Thanks
John

Keep Em Coming, we all can learn Something here.
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Joe E

 

From:
Houston Texas
Post  Posted 5 Jan 2004 11:27 am    
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John,

I still learning about the design side of this. I just racked up some old API mic preamps and had to determine to tie signal and chasis ground together.

You are right, on the cheaper (and not so cheap) gear there are tons of problems with ground loops. Often times we see guys break off the third prong or use the 3 prong adapter. I assume that these units have the signal ground and chasis ground together.

This was deffinetly more pronounce on unballanced units. If you recall on them, the 1/4" jack is screwed to a metal panel and is grounded via the chasis. The xlr are not tied to the case. This is also why some manufacturers are using the plastic jacks for 1/4". The can maintain a seperate ground.

Joe
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Donny Hinson

 

From:
Glen Burnie, Md. U.S.A.
Post  Posted 7 Jan 2004 3:11 pm    
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Quote:
...its all in the Design and engineering.


How true! Manufacturers today often take the "cheap way out" with power supply design, or they use someone else's power supply (an unregulated, unshielded, plastic-encapsulated "wall wart"), and as John said, that's what causes most of these hum problems.
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