A yes/no possible ground loop (?) question

[Trilobita]

Hello,

I have just bought a ART Tube MP/C Project Series preamp, and usually I test all functions as soon as I get new electronics. In this case everything seems to be ok but I have a question about possible ground loops (?).

My bass goes into the ART preamp input and the amp unbalanced (TS) output goes to my USB-only Steinberg interface (there is no alternative way to power it), which has hybrid balanced/unbalanced jacks. When I set everything with unbalanced (TS) cables, the connection is very silent. However, just out of curiosity I decided to test the connection using the balanced (XLR) output as well. In this case, as soon as I connect it to my interface I get hum which sounds like ground loop like ground loop hum samples I've found on YouTube.

As far as I understood correcting ground loops can be a nasty business and I am not actually trying to fix it. My question is way simpler. Given the following symptoms:

- If I keep the XLR cable running from the preamp output to my interface input but disconnect the unbalanced (TS) cable coming from my bass from the ART preamp instrument input, the hum disappears;
- If instead of plugging an actual unbalanced (TS) cable into the preamp instrument input I plug a loose TS plug with no length of cable attached to it, the hum never appears in the first place;
- Even with the hum caused by using the XLR cable, when I connect the bass to the instrument input the audio signal is passed normally (together with the hum);
- When my bass is connected and the preamp is humming, touching the bass strings increases the volume of the hum and makes it sound slightly brighter as well;
- Fussing with other power sources connected to the energy strip (PC monitor, a headphone amp etc.) to which the ART preamp is connected noticeably changes the quality and/or intensity of the hum.

Can I be pretty much assured that the ART preamp XLR output is working ok as far as the preamp is concerned and that, whatever the origin of the hum, it is not related to a problem with the ART unit itself?

In short, as long as I can rule out any actual problems with the unit, I don't care about what's happening and I am not going to bother fixing it right now. I just want to make sure that the unit is working correctly while I can still return it if there are any problems with it.

Thanks!

 

[byacey]

I don't know the signal levels and gain of the various inputs and outputs of your gear, but the fact that the hum disappears when you unplug the bass makes me think it is not a ground loop issue. Your bass would have to be connected to a second ground source for a ground loop to occur.

Also, if it was a bonafide ground loop, it should show up when using unbalanced high impedance connections too. Likely, it's more of an excessive gain issue amplifying the residual noise- ie. +4db line level out on the ART preamp, going into a mic level input on your Steinberg interface.

 

[agedhorse]

Is there a way to lift the pin 1 ground connection on the XLR output?

 

[Trilobita]

Well I just kept testing and noticed that if all the connections are made as above using the ART's instrument (TS) in for the bass and the XLR out into my Steinberg interface XLR in, the hum occurs even when the ART unit is completely turned off and even unplugged from power. If I touch the metal chassis of the unit, the hum reappears. Also, even when I unplug the bass so as to make the hum go away, touching the metal chassis of the unit makes the hum come back.

I also unscrewed the tips of my only XLR cable and found that pin 1 (the one on the upper left if you looking directly to the prongs of the male connector on a fully assembled cable) connects to the hot (red) wire whereas pin 2 (just beside it) connects to the ground (shield) wire on both ends of the cable. I am posting a picture of both ends - in this picture, you are seeing the wiring slightly from above (meaning the two parallel pins that are next to each other are closer to your eyes, whereas the lone cold pin is closer to the surface of the table).

I guess these pins 1 and 2 should be wired the other way around, right? Or should it simply not matter because both ends are wired the same kind of wrong? The manual of the ART preamp specifically says that it expects pin 1 as ground and pin 2 as hot. I couldn't find this sort of information about my Steinberg interface. (I'll be happy to lift pin 1 and report back during the re-soldering process once someone tells me what's the best way to go from here.)

 

[Redbrangus]

It does appear from the photo that the wiring to pins 1 and 2 is reversed -- and yes, that could make a big difference. The shield of that cable would be carrying the 'hot' signal.

You didn't happen to purchase that cable at Monoprice, did you?

 

[Wasnex]

Well I just kept testing and noticed that if all the connections are made as above using the ART's instrument (TS) in for the bass and the XLR out into my Steinberg interface XLR in, the hum occurs even when the ART unit is completely turned off and even unplugged from power. If I touch the metal chassis of the unit, the hum reappears. Also, even when I unplug the bass so as to make the hum go away, touching the metal chassis of the unit makes the hum come back.

I also unscrewed the tips of my only XLR cable and found that pin 1 (the one on the upper left if you looking directly to the prongs of the male connector on a fully assembled cable) connects to the hot (red) wire whereas pin 2 (just beside it) connects to the ground (shield) wire on both ends of the cable. I am posting a picture of both ends - in this picture, you are seeing the wiring slightly from above (meaning the two parallel pins that are next to each other are closer to your eyes, whereas the lone cold pin is closer to the surface of the table).

I guess these pins 1 and 2 should be wired the other way around, right? Or should it simply not matter because both ends are wired the same kind of wrong? The manual of the ART preamp specifically says that it expects pin 1 as ground and pin 2 as hot. I couldn't find this sort of information about my Steinberg interface. (I'll be happy to lift pin 1 and report back during the re-soldering process once someone tells me what's the best way to go from here.)

View attachment 4247194

The cable appears to be wired correctly to me. That is no guarantee the cable is not the problem. Try another cable.

Your cable uses white instead of green: Image edited

Sometimes you get hum when going from XLR to XLR. Not every piece of gear uses a real balanced circuit, rather one of the circuits may be impedance balanced. My guess is the XLR out on your preamp is impedance balanced, and pin 3 is connected to ground through a resistor.

This circuit shows an impedance balance output and a differential balanced input.

Ref: Balanced Line Technology (douglas-self.com)

You could try a 1:1 isolation transformer like an Ebtech HE-2 XLR

There have been times when even an isolation transformer did not solve the problem for me though. Under those circumstances I do the best I can with gain staging.

For example, I have seen situations were a Radial JDI hummed and a Countryman Type-85 was dead silent. Since the JDI is a passive transformer and the Type-85 is active, I was very surprised.

As far as testing the output of your preamp, try the XLR out with another piece of gear. Check to make sure you AC outlet is properly wired. Check the preamp and Steinberg interface at another location. The preamp and recording interface may work fine in other locations, or they may simply not get along (personality conflict ).

 

[Wasnex]

It does appear from the photo that the wiring to pins 1 and 2 is reversed -- and yes, that could make a big difference. The shield of that cable would be carrying the 'hot' signal.

You didn't happen to purchase that cable at Monoprice, did you?

I am not seeing that... the best thing to do is plug the ends together, then you can see what is going on more easily.

 

[Redbrangus]

[Edit] This post is no longer valid -- the incorrect illustration referenced was corrected.

Was...I'm holding a Switchcraft D3F (female 'XLR') in my hand: viewed from the rear (solder) side, pin 1 is on the left, pin 2 is on the right.

Did that illustration come from Monoprice?

Edited for clarity

 

[Wasnex]

Was...I'm holding a Switchcraft D3F in my hand: viewed from the rear (solder) side, pin 1 is on the left, pin 2 is on the right.

Did that illustration come from Monoprice?

It depends upon if the connector is male or female. They are mirror image. The image was generated from a Google search. Here is a another image, and it's consistent with the other one.

Ultimately the colors of the wires don't matter...pin to pin connections are what is important. 1 to 1, 2 to 2, 3 to 3.

 

[Redbrangus]

This one is correct, but isn't consistent with the first one. Or maybe my eyes are just getting old.

[Edit] This post is no longer valid -- the incorrect illustration referenced was corrected.

 

[Trilobita]

I am not seeing that... the best thing to do is plug the ends together, then you can see what is going on more easily.

My plugs have actual numbers printed on the outside of the plastic. If the cable is fully assembled and ready to use, by looking straight-on at the prongs or holes you can read the following pinout on each plug:

MALE
(1---2)
(--3--)

FEMALE

(2---1)
(--3--)

Where (the way it came wired) 1 is hot, 2 is ground and 3 is cold.

I've tested for continuity with a meter and got what was expected: 1 connects to 1, 2 to 2 and 3 to 3. I am going to de-solder, test it with a lifted ground and then reverse pins 1 and 2 to see what happens. Expect me to report back soon.

 

[agedhorse]

It’s hard to tell from the photo, but by chance does one of the wires connect to the shell terminal of the female connector?

That picture is consistent of inexpensive off-shore XLR cables, commonly riddled with issues.

 

[Redbrangus]

My plugs have actual numbers printed on the outside of the plastic. If the cable is fully assembled and ready to use, by looking straight-on at the prongs or holes you can read the following pinout on each plug:

MALE
(1---2)
(--3--)

FEMALE

(2---1)
(--3--)

Where (the way it came wired) 1 is hot, 2 is ground and 3 is cold.

I've tested for continuity with a meter and got what was expected: 1 connects to 1, 2 to 2 and 3 to 3. I am going to de-solder, test it with a lifted ground and then reverse pins 1 and 2 to see what happens. Expect me to report back soon.

The incorrectly wired cable (which you do have) will test properly on a continuity tester since it is consistent end-to-end. Do you have a different XLR cable that you can substitute?

 

[agedhorse]

Pin 1 is ALWAYS the shield.

 

[Redbrangus]

Pin 1 is ALWAYS the shield.

This is what I'm saying. That cable is wired back-asswards.

 

[Trilobita]

Ultimately the colors of the wires don't matter...pin to pin connections are what is important. 1 to 1, 2 to 2, 3 to 3.

Initially I also thought that, but don't forget that the red (hot) wire is supposed to be insulated behind the metallic foil, and that the shield (ground) wire is supposed to be outside it. If you reverse hot and ground on both ends of the cable you still get a pinout where 1 goes to 1, 2 to 2 etc., but the insulation is reversed. I am going to test it right now and report back!

 

[Wasnex]

Initially I also thought that, but don't forget that the red (hot) wire is supposed to be insulated behind the metallic foil, and that the shield (ground) wire is supposed to be outside it. If you reverse hot and ground on both ends of the cable you still get a pinout where 1 goes to 1, 2 to 2 etc., but the insulation is reversed. I am going to test it right now and report back!

See post #12. There should be no continuity between any of the wires are any of the pins. The shield goes around the two signal wires. If one of the signal wires is connected to the shield, you are not working with a balanced cable.

The only ground should be the shield. The shield is always connected to pin 1 (ground).

The red and white wires in this image both carry signal in a balanced circuit. These wires go to pins 2 and 3.


This image illustrates where balanced circuits get their noise cancelling properties from.

A differential signal is use. This means one conductor goes positive and the other goes negative. When the signal gets to the input it needs to be decoded. Note that one input is inverting (-) and one input is noninverting (+). Because the inputs have opposite polarity, the two out of phase signal add back together. into one composite signal.

The two signal wires are floating with respect, to ground. The shield, which is not shown here, sucks some of the noise into ground so it does not reach the signal wires. The noise that does reach the signal wires is equal and of the same polarity on each wire. But remember the inputs are out of phase (the green signal is flipped 180 degrees). Since the noise is equal and in polarity on the wires, it cancels in the input circuitry.

 

[Trilobita]

Lifting pin 1 (which the way this was soldered connected to the red wire) left everything the same: I can still hear the audio signal which goes through the shield wire (pin 2), but the hum is horrible. I am now going to re-solder this properly.

 

[Redbrangus]

Again, if you use that incorrectly wired cable, you will be putting the 'hot' signal onto the shield of the cable -- and the subsequently induced hum problem is likely exacerbated by the fact that one or both of the devices being interconnected is/are likely to not be truly balanced. I'm saying again -- that cable is incorrectly wired, Google-search generated images to the contrary notwithstanding.

 

[Wasnex]

Lifting pin one (which the way this was soldered connected to the red wire) left everything the same: I can still hear the bass signal which goes through the shield wire (pin 2), but the hum is horrible. I am now going to re-solder this properly.

Check pin to pin for continuity. No shorts from 1 to 2 or 3; 2 to 3 or 1; 3 to 1 or 2.