TASCAM 48 grounding issue

  • Thread starter Thread starter lo.fi.love
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Ok, some updates.

I grounded the racks and the PS-520 together, and the ground is discharged into an open receptacle on my power conditioner.

I have been probing around my setup with a multimeter and I found something disturbing. There appears to be a short somewhere.

I have my 48 connected to four dbx 150x noise reduction units with RCA - TS snakes. I unplugged the RCA ins/outs from the back of the 48 to do some testing. I have a patchbay on the back of the rack which connects to a snake that goes to the "main" patchbay. So imagine that a line coming from the console, going to a tape input, connects like this:

Line -> "Bridge" patchbay (RCA) -> dbx 150x (TS) -> TASCAM 48

Now, I disconnected the RCA end of a channel from the bridge patchbay. When I check for continuity with my multimeter, probing the sleeve and the tip of the RCA connector, I find continuity. When I remove the TS connector for that line from the dbx 150x and probe the same RCA connector, there is no continuity.

Does this make sense? Is it clear what I am trying to convey? Basically, it appears as though the dbx 150x units are creating a short between hot and cold. This is troubling!

Later, I unplugged the 48 entirely. No power, no cables going in/out... nothing. I plugged a single RCA cable into an input on the 48 and found continuity between the tip and the shield of that RCA cable. Is this supposed to happen??

I'm having a really hard time trying to figure out what this all means. And I also wonder if I'm looking in the wrong place for this hum. Can someone guide me here, and possibly explain what I found?

Thanks!!!
 
I think this could be normal. Does it do the same for all inputs and outputs? Take a resistance reading to see what the actual ohms value is. It probably isnt 0 Ohms. Does everything seem to work otherwise? Grounding is a very complicated subject. I had to do all kinds of tricks to make my studio absolutely quiet. Even when you hook up perfectly good gear all together you can still get some loud ground loop hums. Trial and error experimentation can eliminate them. Good luck.
VP
 
Right...its normal in the gear for there to be continuity otherwise nothing would happen when you turn it on...the electricity accomplishes work because there is a path to ground. No path to ground (i.e. no connection from + to -) and the electricity can't go anywhere...open circuit, nada happens.

Did you follow the wiring guide for your 150X's in the manual?
 
sweetbeats said:
Right...its normal in the gear for there to be continuity otherwise nothing would happen when you turn it on...the electricity accomplishes work because there is a path to ground. No path to ground (i.e. no connection from + to -) and the electricity can't go anywhere...open circuit, nada happens.
Click to expand...

It seems odd to me, though, that there would be continuity between the pin and the sleeve on the line that feeds in to the tape input. Granted, I have only a rudimentary understanding of audio component interconnection, but it seems like that having the two connected would cause a lot of interference and would also produce hums.

By the way, I'm working under the assumption that ground loops are caused partially by the ground signal getting into the "hot" signal. Is this true, or do I have only part of the whole picture?

Did you follow the wiring guide for your 150X's in the manual?
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Yes, I did. I'm pretty familiar with these units.

I'm going to try this again when I get home from work tonight. Granted that everything is now sharing the same ground path, is there anything that I should consider when I come back to this? The cables check out - there's no loose or broken connections. And it's also odd to me that I'm getting hum on 5 of the 8 channels coming from the tape deck, instead of a hum all across them.

Gosh, this was so much easier when I had less equipment! :)
 
Are the 8 channels to/from the 48 handled on a snake or snakes?

Think of a car. The ground strap connects to the engine and frame, the +terminal goes to all sorts of places that eventually also connect to the frame. What happens if you disconnect the ground strap from the battery? Nothing works. This would be the same scenario if you were to set your DMM to continuity and probe the shield and pin on a jack on the 48 and find no continuity. Or a light bulb...how do you find out a bulb is dead? Check for continuity between the terminals of the bulb and if there is none it is dead...there is no way for energy to pass through the device and thus power it. Think of the input or output jack on the 48 is the "power source" for that side of that circuit and like the car the shield is connected to the chassis and the pin goes all sorts of places that ultimately lead to the chassis. I'm putting "power source" in quotes because of course there is power from the mains that energizes the active components of that same circuit, but my analogy is focusing on the passive circuit entered at the jack. Now in a car with the power source being a battery, the battery would die if the path to ground was connected all the time so the car is designed to have the path to ground broken in all places when the vehicle is not in use...but open a door (dome light turns on) and voila! There is continuity between the positive and negative terminals of the battery. The circuit doesn't work unless there is a way for the electrons to move from the positive potential to ground. In an unbalanced system the shield is providing RF shielding as well as the path to ground for its circuit. The car is setup so that the battery doesn't die when the vehicle is not in use...all paths to ground are broken (unless you are my daughters and you leave your map lights on...then the battery in the van dies). The circuit you are probing at the jack is not powered by a battery. No reason to have a mechanism to break the path to ground at any point so it is good that there is continuity because if there was no continuity it would be like the broken bulb.
 
  1. I'm wondering if there is a way to isolate the 48 and see if the source of the hum is the 48 or if it is a system problem (I apologize if I am missing something addressed above).
  2. You still have your M-30 and can you connect that up to your 48 and connect the 48 and the M-30 directly to each other and directly to the mains to totally bypass the rest of your system?
  3. Did you ever confirm that the mains is properly grounded? That will mess things up in a maddening way if there isn't a proper ground on the mains circuit you are using...
  4. What power conditioner are you using?
 
sweetbeats said:
Are the 8 channels to/from the 48 handled on a snake or snakes?
Click to expand...

Yes.

Think of a car. The ground strap connects to the engine and frame, the +terminal goes to all sorts of places that eventually also connect to the frame. What happens if you disconnect the ground strap from the battery? Nothing works. This would be the same scenario if you were to set your DMM to continuity and probe the shield and pin on a jack on the 48 and find no continuity. Or a light bulb...how do you find out a bulb is dead? Check for continuity between the terminals of the bulb and if there is none it is dead...there is no way for energy to pass through the device and thus power it. Think of the input or output jack on the 48 is the "power source" for that side of that circuit and like the car the shield is connected to the chassis and the pin goes all sorts of places that ultimately lead to the chassis. I'm putting "power source" in quotes because of course there is power from the mains that energizes the active components of that same circuit, but my analogy is focusing on the passive circuit entered at the jack. Now in a car with the power source being a battery, the battery would die if the path to ground was connected all the time so the car is designed to have the path to ground broken in all places when the vehicle is not in use...but open a door (dome light turns on) and voila! There is continuity between the positive and negative terminals of the battery. The circuit doesn't work unless there is a way for the electrons to move from the positive potential to ground. In an unbalanced system the shield is providing RF shielding as well as the path to ground for its circuit. The car is setup so that the battery doesn't die when the vehicle is not in use...all paths to ground are broken (unless you are my daughters and you leave your map lights on...then the battery in the van dies). The circuit you are probing at the jack is not powered by a battery. No reason to have a mechanism to break the path to ground at any point so it is good that there is continuity because if there was no continuity it would be like the broken bulb.
Click to expand...

I'll mull this over later tonight. Thanks! I'm supposed to be working right now... ;)
 
sweetbeats said:
I'm wondering if there is a way to isolate the 48 and see if the source of the hum is the 48 or if it is a system problem (I apologize if I am missing something addressed above).
Click to expand...

I will try my best to do this.

You still have your M-30 and can you connect that up to your 48 and connect the 48 and the M-30 directly to each other and directly to the mains to totally bypass the rest of your system?
Click to expand...

My M-30 is still in pieces. Maybe I should get that back together.

I also have a M-216 that a friend gave to me for free. I should try with that.

Did you ever confirm that the mains is properly grounded? That will mess things up in a maddening way if there isn't a proper ground on the mains circuit you are using...
Click to expand...

I'm not really sure if the outlet is properly grounded. My outlet tester says that everything is OK, but the outlet only has two wires connected to it (from the conduit). Do you recommend a sure-fire way of testing the ground?

What power conditioner are you using?
Click to expand...

Tripp Lite LC1800. Here's info from Tripp Lite's website
 
I also have a M-216 that a friend gave to me for free. I should try with that.
Click to expand...

You suck. Yeah, try that.

I'm not really sure if the outlet is properly grounded. My outlet tester says that everything is OK, but the outlet only has two wires connected to it (from the conduit). Do you recommend a sure-fire way of testing the ground?
Click to expand...

If your outlet only has two wires going to it, it is not grounded and I highly suggest you deal with that first before doing anything else. To do it right you have to have a grounding stake (available at a hardware store) driven into the ground and then that connects with a solid core copper wire the same guage as your household wiring to the ground lug of the receptacle (or ideally to the neutral buss back at the breaker box). The wire that comes into the outlet box...does it only have two wires or is there a third that is disconnected? Your outlet tester will say everything is okay because likely that is looking for differentials between the hot and neutral legs of the receptacle, not whether or not the ground is present or healthy. Try taking a voltmeter and putting one probe in the ground socket and testing the other two sockets...I'm betting you get nothing and it should 120VAC.

Tripp Lite LC1800. Here's info from Tripp Lite's website
Click to expand...

Too bad it doesn't have a ground lug on it...that would be handy. Anyway, am I correct that the green "LINE OK" light is lit and the red "LINE FAULT" light is off? My Monster PowerPro 2500 has a lamp to indicate helathy power as well as proper ground presence.
 
sweetbeats said:
You suck. Yeah, try that.
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Ha ha ha. It needs a LOT of love. He should have given me money to take it from him :)

If your outlet only has two wires going to it, it is not grounded and I highly suggest you deal with that first before doing anything else. To do it right you have to have a grounding stake (available at a hardware store) driven into the ground and then that connects with a solid core copper wire the same guage as your household wiring to the ground lug of the receptacle (or ideally to the neutral buss back at the breaker box).
Click to expand...

Gosh, this could be a real task. I think I'll have to put the ground stake into the soil outside of my bedroom window, and then run a wire through the window and into the receptacle.

The wire that comes into the outlet box...does it only have two wires or is there a third that is disconnected?
Click to expand...

I can't recall. I think there were only two wires when I looked in there. The outlet's ground might be floating freely.

Too bad it doesn't have a ground lug on it...that would be handy. Anyway, am I correct that the green "LINE OK" light is lit and the red "LINE FAULT" light is off? My Monster PowerPro 2500 has a lamp to indicate helathy power as well as proper ground presence.
Click to expand...

This is correct. It says "LINE OK". It said "LINE FAULT" when I first brought it home, which lead me to fix the outlet. Whoever installed it had reversed hot + cold, so I set it straight.
 
Aww...I like mixers that need a lot of love. That M-216 now has some lo.fi.love.

Man, Jeff...SERIOUSLY...if the hot and neutral legs were swapped on the receptacle and there's no ground to the receptacle, you really NEED to get some proper grounding. I would be awfully surprised if that didn't address your hum issue. You could even test out the theory by taking some of that solid core copper wire, shoving it in the ground, running it through your window and connecting it to the ground-lug on the outlet. Try it. If there is a ground wire not connected in the receptacle then test it with the voltmeter. If it is there and is connected to ground (i.e. if you get voltage on the HOT side to ground...{I goofed before...you won't get 120VAC from ground to neutral, just ground to HOT}), don't be surprised if the hum issue isn't taken care of when you hoook that ground wire up inside the receptacle because a lot of times in an older dwelling the ground wire might just be connected to the nuetral buss in the breaker box and there is no ground stake. If you know where the breaker box is at your place you can look outside where the meter is and see of you can find the ground stake. If you don't see it I would be suspicious. This is why I'm thinking it would be valuable to test the theory and just shove some wire in the ground and run that to the ground lug on the receptacle and if that takes care of your problem and there is a ground wire in the receptacle but that doesn't resolve the hum issue then I'd say the houehold does not have a proper ground and yer gonna have to go cowboy on it.
 
Cory,

THANK YOU for your suggestions. There's a lot of 'sketchiness' in our house... botched repairs, some problems left to fester... but I plan on doing a full investigation on this tonight when I get home. I'm sure that this outlet/receptacle isn't completely without a proper ground.

Anyway. I'll write more tonight.

sweetbeats said:
Aww...I like mixers that need a lot of love. That M-216 now has some lo.fi.love.

Man, Jeff...SERIOUSLY...if the hot and neutral legs were swapped on the receptacle and there's no ground to the receptacle, you really NEED to get some proper grounding. I would be awfully surprised if that didn't address your hum issue. You could even test out the theory by taking some of that solid core copper wire, shoving it in the ground, running it through your window and connecting it to the ground-lug on the outlet. Try it. If there is a ground wire not connected in the receptacle then test it with the voltmeter. If it is there and is connected to ground (i.e. if you get voltage on the HOT side to ground...{I goofed before...you won't get 120VAC from ground to neutral, just ground to HOT}), don't be surprised if the hum issue isn't taken care of when you hoook that ground wire up inside the receptacle because a lot of times in an older dwelling the ground wire might just be connected to the nuetral buss in the breaker box and there is no ground stake. If you know where the breaker box is at your place you can look outside where the meter is and see of you can find the ground stake. If you don't see it I would be suspicious. This is why I'm thinking it would be valuable to test the theory and just shove some wire in the ground and run that to the ground lug on the receptacle and if that takes care of your problem and there is a ground wire in the receptacle but that doesn't resolve the hum issue then I'd say the houehold does not have a proper ground and yer gonna have to go cowboy on it.
Click to expand...
 
Update:

There is NO ground! Only two wires come into the receptacle. And there isn't any soil near the window that I could drive a grounding rod into. Out of luck.

Well, my room mate and I have been talking about getting a new place, and I think this seals it.

What a relief, though, to know that this is likely to be the cause of the humming issues. I plugged the 48 into my M-216 and there was no hum. I think it's the accumulated ground current buildup in the system that is causing things to hum, since there's no way for it to be discharged anywhere.

OK. Time to look at Craigslist for a new place... !
 
I think it's the accumulated ground current buildup in the system that is causing things to hum, since there's no way for it to be discharged anywhere.
Click to expand...

That's not how the grounding works...it would only be a discharge path if the neutral leg failed, so there is no "buildup".

The hum comes from a differential in the ground potential in two devices not connected to a common ground (i.e. two devices with two-pole mains plugs). You can test this by checking for voltage between the chassis of two devices...devices not connected to a common ground can have different ground potentials because a system's ground is a ground reference for that system and it may be slightly different between systems. You can alleviate that by connecting all systems to a common ground which is what we're shooting for.

The common ground is the first half of the battle, and then comes the cabling. That's a whole rat's nest because audio systems often have the chassis ground, the audio ground and sometimes a third system ground...depends on the device and the power rails...sometimes a power rail needs to have an isolated ground within the system. And there has been disagreement in the audio industry about standards for grounding.

But first, always, get a good solid common ground for your mains feed.

I also like to get my lighting and non-audio-related equipment on a separate circuit if possible.
 
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