In our church sound system we have a microphone cable running to the pulpit. Being less formal these days nobody has preached from the pulpit in years, but when I wanted to feed the digital piano into the sound system, it was convenient to tap into that cable. So I needed a simple passive DI box with pass-thru on the balanced output (mic) side. Nothing simpler. Using a 10k:600Ω audio transformer I could do it for pocket money. (Schematic attached.)

But after disabling background noise suppression on Zoom and audio processing on Windows in order to improve the piano sound quality, an annoying background buzz became evident.

Originally I had the XLR ground connected to the piano input ground. That'll be the problem, I thought. But disconnecting it made no difference. I tried connecting the transformer frame to XLR ground but that made no difference either.

With the piano unplugged at the mains and from the DI box, the buzz is absent, or near enough.

With the piano switched off, pugging it into power OR plugging in the piano jack to the DI box (but not both) the buzz is practically unchanged.

Plugging in both (still with the the piano off) increases the buzz markedly.

Now switching on the piano, the buzz is icreased by about the same again, to the level that was complained of.

If, however, I plug the piano into the pulpit mic socket via a jack - XLR converter (tip -> +, ring -> - and gnd), at the same time switching that input on the sound system from Mic to Line, the buzz is absent (or at least, insignificant compared to the higher line level signal).

That is the obvious solution, but I'm still puzzled. I've checked there's no DC path across the transformer. The transformer isn't shielded, but the closed magnetic circuit should make it reasonably immune to hum pick-up. The nearest mains cable is a copper-sheathed fire-resistant mains cable maybe a foot away, and the hearing loop is a similar distance.

Can anyone offer a coherent explanation?

 

If your drawing is an accurate representation ..........
The Noise is 100% Inside the Piano,
or,
you have not set all the Levels correctly.

The Piano is very likely to have a small amount of Noise.

Is the Piano-Output Adjustable ?, Is the DI-Box turned all-the-way up ?,

They both need to be at 100% to start with,
and then, if necessary, Padded-down,
first at the "Mixer-Input-Pad",
( which could be a Switch or a Pot ),
( the Mixer-Input must be set to "Line", NOT "Microphone-Level" ),
If the Mixer Line-Input is being over-driven, or distorting,
and you can't Pad-It-down any further,
then you can reduce the Gain with the Pot on the DI-Box
just enough to stop over-driving the Mixer-Input.

If the Piano has a Line-Out Level-Pot, keep it at 100% at all times.

This will maximize the "Signal-to-Noise-Ratio".

Don't just assume that the DI-Box is wired as per your drawing, check it.
See if it has a "Ground-Lift" Switch, if it does, it must be set to "Lift".

As per your Drawing,
you may not want a High-Impedance to Low-Impedance Transformer.
You want to blast that Mic-Cable with the hottest Signal you can get,
and then Pad-it-down at the Mixer.

There is a remote chance that the Piano has a Balanced-Line-Output which
simply needs a "Stereo"-Phone-Plug to be functional, ( T-R-S, as opposed to just T+S ),
if this is the case, you just need the "Tip" and "Ring" connections,
never connect the Ground, or "Sleeve" on the Piano, to the Ground on the Mic-Cable, ever.
.
.
.

 

The unshielded transformer is not immune to EM pickup, especially as it has a high impedance winding. Have you tried rotating or re- locating the transformer?

 

I had a similar problem. Probably you have some lights with dimmers someplace. That buzz will travel on the green wire ground and bother every audio device connected. The fix is one of those ground isolator plugs. AND this will instantly set off all of those folks who go into a panic mode about disconnecting the green wire ground. Just make sure that the electronic piano player is not standing in water or washing their hands while playing. Probably they should also wear dry shoes. The only way around the green wire problem was a separate conduit run, isolated from all other electrical everything until it got to the main panel, with an insulated ground wire tied directly to the main ground in the panel. And a separate circuit for the audio stuff, all run on that one 20 amp breaker, on the other side phase from where all of the dimmer lights were fed. Running thinwall 80 feet in a masonry building is not much fun. But that alone was the alternative to just disconnecting the green wire ground. I could have simply added a GFCI to that circuit and then there would be no more threoreticaly potential shock hazard.

 

I disagree...
Whatever you do...don't isolate the sound system power from ground (like cutting off the ground prong on the AC plug).
It creates a an easy way for a singer or speaker to get shocked or electrocuted...

 

The "Sound-System" is never the problem,
( what ever "The Sound-System" is that you are referring to ),
99.9% of any noise problems originate from poorly designed Musical-Instrument-Electronics.
Shock-Hazard problems have the same exact same source.
The worst offenders are Tube-Amps that the owner is trying to keep "all-original".
.
.
.

 

The "Sound-System" is never the problem,
( what ever "The Sound-System" is that you are referring to ),
99.9% of any noise problems originate from poorly designed Musical-Instrument-Electronics.
Shock-Hazard problems have the same exact same source.
The worst offenders are Tube-Amps that the owner is trying to keep "all-original".
.
.
.

I disagree. I can't tell you how many time I've gotten shocked on stage only to find out "oh that. I cut that off..."

 

But that has nothing to do with "The Sound-System".
Generally, no part of the Sound-System is "plugged-in" on Stage.
The only thing you've ever been shocked by, was attached to a Tube-Amp in some way.
Every piece of Equipment in the House-System has an Isolation-Transformer, without exception.
Power-Grounds are a totally different animal when
compared to Low-Voltage, Line-Level, and Mic-Level Grounds.
.
.
.

 

I disagree...
Whatever you do...don't isolate the sound system power from ground (like cutting off the ground prong on the AC plug).
It creates a an easy way for a singer or speaker to get shocked or electrocuted...

In order for a shock to happen there must be a path for the current to flow. In addition, there must be a path to the microphone outside for the person to touch. And note that this is not a garage band playing on a cement floor, where there actually would be a possibility of being grounded.

In addition, it is a simple matter to provide a ground fault interrupter device in the power feed to the whole sound system . And that device does not need any ground connection to function, no matter what superstition s you have.

Andonce again I suggest trying it with all of the electonic light dimmers switched off. That noise will get into any and every signal level circuit around. And it does travel around on that green wire ground as well.

 

In order for a shock to happen there must be a path for the current to flow. In addition, there must be a path to the microphone outside for the person to touch. And note that this is not a garage band playing on a cement floor, where there actually would be a possibility of being grounded.

In addition, it is a simple matter to provide a ground fault interrupter device in the power feed to the whole sound system . And that device does not need any ground connection to function, no matter what superstition s you have.

Andonce again I suggest trying it with all of the electonic light dimmers switched off. That noise will get into any and every signal level circuit around. And it does travel around on that green wire ground as well.

Nice theory....but "that ain't what happens". Shock happens...
Plenty of conductive surfaces like sweaty hands, sometimes sweaty feet, soilled beverages, sweaty bodies, beer...

 

Only time I’ve been shock was using an old Fender tube amp with a mic and a guitar. In any case, nothing serious just annoying. As I recall that old amp had a ground lift.
unbalanced cables are a horrible source of noise.

 

All my sound systems use unbalanced shielded audio cables. Good quality cables worked very well but cables from The Dollar Store and from Radio Shack (ebay today) had a very simple shield made from a few strands of thin wire and produced hum and interference.

 

Perhaps that added cable shown from that boxin post #1 should be shielded and then the shield can be the ground connection for the transformer frame.

 

Ive usually had this issues at extremely high gain using the highest quality cables.

 

It seems that some folks posting have no understanding at all as to how a ground fault interrupter works.

 

All the audio wires must be in a shielded audio cable so they are not antennas that pickup hum and other interference.
Sometimes the shield is connected only at the receiving (amplifier) end.

 

All the audio wires must be in a shielded audio cable so they are not antennas that pickup hum and other interference.
Sometimes the shield is connected only at the receiving (amplifier) end.

That single ended shield connection would not work well with phantom power unless a separate ground wire was also present.

 

(I'm sorry, I haven't been here for a bit - I thought I was subscribed to this thread but only got notified of the first reply. Thank you to all of you for your contributions.)

Update - the link to the piano only seems to be one of a whole shoal of red herrings. Here's a schematic of the whole system:

There are two ways in which I can detect the problem: running Audacity on the laptop (audio recording and editing software), or by putting my hearing aid on Telecoil, and they can give different results. Audacity gives easily quantifiable results but my hearing aid much less so, not least because it's quite sensitive to the angle with respect to the hearing loop.

We have 4 physically separated blocks of equipment:
- The sound system: a 19" rack containing the mixer, equaliser, power amp, loop amp, aux panel and 100V amp for the creche room.
- The laptop on which we run Zoom (or Audacity, for tests). This has a combined headset socket and we use a (passive) CTIA splitter to split out the headphone and mic function. Replacing this with a USB headset adapter with separate headphone and mic sockets has been known to improve things, but it's not consistent. There's also a USB webcam connected to the laptop via a long USB cable with active repeaters, but disconnecting this or reconnecting it is one of the few things that's never made the slightest difference!
- The piano and DI box.
- The projector is connected to the laptop VGA output using CAT5 cable and using VGA/RG45 converters, powered from the projector end. Unfortunately he projector and its converter are very hard to access, being suspended from a beam which is not horizontal and hence not safe to lean a ladder against.

It seems changing almost anything can affect the problem. Most consistently, disconnecting the VGA connector at the laptop or switching off power to the projector and converters eliminates the problem. Then another day it doesn't! I fitted a clip-on ferrite filter the the VGA cable at the laptop, with zero effect.

Signal ground is common throughout (including to the projector - the RJ45 adapters don't provide galvanic isolation) unless I insert an isolation transformer in the laptop mic input (the headphone output already goes through one) - which, if anything, makes the problem worse!

Thank you @MisterBill2 - I hadn't thought of the lights, but none of the lights are on dimmers. However, we replaced the CFL lamps with LED last Autumn so I'll certainly try switching them off next time I'm down there. Neither sort tend to have exemplary PF.

Thank you also @LowQCab for your extended contributions. Having, for nigh on 70 years, been messing with batteries and bulbs, not long after with electronics and only a little after that with computers, long experience tells me that statements like "The sound system is never the problem" are a good way of disproving your own existence. When you've eliminated or discounted everything, only two possibilities remain: either the problem doesn't exist (and we know it does) or you don't exist!

Thank you @Audioguru again - all audio cable runs outside the the sound system are balanced except to and from the laptop. The first thing I did was to replace the ancient low quality (though balanced) cable run to the pulpit and piano by a good quality cable. The two wired mics use good quality balanced cable and are completely silent. (The lapel and hand held are UHF radio mics with the receivers on top of the sound cabinet.)

As I said, signal ground is isolated from power ground (though I could usefully double-check) and the sound system is plugged into a wall socket via a mains filter. But there's sure to be capacitive coupling at multiple points, in paticular, the sound system, the laptop, the piano and the projector. It makes no difference to run the laptop on its internal bettery.

I will continue to beat my brains out over this problem, will welcome any further suggestions and will provide an update on any material and sustained progress.

 

There are two ways in which I can detect the problem: running Audacity on the laptop (audio recording and editing software), or by putting my hearing aid on Telecoil, and they can give different results. Audacity gives easily quantifiable results but my hearing aid much less so, not least because it's quite sensitive to the angle with respect to the hearing loop.

- The laptop on which we run Zoom (or Audacity, for tests). This has a combined headset socket and we use a (passive) CTIA splitter to split out the headphone and mic function. Replacing this with a USB headset adapter with separate headphone and mic sockets has been known to improve things, but it's not consistent. There's also a USB webcam connected to the laptop via a long USB cable with active repeaters, but disconnecting this or reconnecting it is one of the few things that's never made the slightest difference!
- The projector is connected to the laptop VGA output using CAT5 cable and using VGA/RG45 converters, powered from the projector end. Unfortunately he projector and its converter are very hard to access, being suspended from a beam which is not horizontal and hence not safe to lean a ladder against.

It seems changing almost anything can affect the problem. Most consistently, disconnecting the VGA connector at the laptop or switching off power to the projector and converters eliminates the problem. Then another day it doesn't! I fitted a clip-on ferrite filter the the VGA cable at the laptop, with zero effect.

Signal ground is common throughout (including to the projector - the RJ45 adapters don't provide galvanic isolation) unless I insert an isolation transformer in the laptop mic input (the headphone output already goes through one) - which, if anything, makes the problem worse!

The Laptop sounds suspicious...

Have you tried removing the laptop power supply and running the laptop using only its internal battery?
Have you tried removing the laptop from the system?
For either of the above, is there still noise (hum..or?) with audio?

 

(I wrote and previewed my previous reply yesterday but evidently didn't click Post.)

Today's update:

The lights make no difference whether on or off. Good thought though.

The mixer, equaliser, power amp and loop amp are all fed through 3 pin IEC connectors and there is continuity between the earth pin of each, its chassis and its XLR, phono or jack ground. (But I didn't disconnect all the XLR cables so it's possible not all units connect signal and power ground.)

There is no connection between power ground and signal ground on the laptop itself.

I can't get to the projector or the (powered) RJ45-VGA converter adjacent to it to test for continuity between their signal and power grounds.

But (and this is where it gets interesting), pulling the VGA cable out of the laptop I find there is 0.25V between the cable shield and the laptop frame! There must therefore be a current flowing from the projector VGA input, or more likely the (powered) RJ45 converter, through the RJ45 cable run and the (remotely powered) RJ45 converter, through the laptop via the VGA output and the mic iput, and hence to the sound system. I guess this current is not clean and the noise is getting impressed on the mic input to the laptop. An isolation transformer in the mic input makes no difference, if anything making it worse - presumably there's sufficient capacitive coupling between the windings for the noise to get through anyway.

As an experiment, perhaps I should try connecting a thick wire between the VGA connector frame on the RJ45 converter at the laptop end and a solid ground on the sound system. That should divert the bulk of this noise current away from the mic input on the laptop.

There are actually 2 mains fuseboxes in the building, notionally one for the church and the other for the (attached) church hall but I'm not sure the segregation has been strictly adhered to over the last 50+ years - it's just possible the projector is on a different circuit from the sound system and laptop, and if not, they are on different spurs. That woluld be pretty hard to sort out without rewiring the whole building!