GK MB200 humming issue when grounded

[SSCBA]

Hi,
I’m trying to diagnose a low-frequency hum on a GK MB200 (around 12-15 years old) and I’m curious if anyone has seen similar behavior.

Symptoms:

• Hum is audible from the speaker only (15” bass cabinet - from woofer).
• Hum does not change with gain, EQ, contour, or master settings.
• Hum is present with absolutely nothing connected to the amp.
• Hum disappears completely when the amp is not referenced to earth ground.
• Hum appears immediately when the chassis becomes earth-referenced.

Examples:

• Amp plugged into an ungrounded outlet: no hum.
• Same amp plugged into a grounded outlet: hum.
• Amp in an ungrounded outlet, but connected to a grounded device (MacBook + MiniFuse): hum.
• Amp in an ungrounded outlet, but I touch the chassis while standing barefoot on the floor: hum (lower) appears immediately.

Additional tests:

• Shorting the input jack does not change the hum level.
• Connecting a cable to the AUX input increases the hum.
• Touching the tip of the AUX cable increases the hum even more.
• Headphone output is clean; I cannot hear the hum in headphones.
• The hum is only present through the speaker output.
• FFT analysis shows a dominant 50 Hz component with harmonics.

Measurements:

• Earth pin of the IEC inlet has continuity to the chassis.
• Chassis to earth measures essentially 0 V (about 0.03 VAC).
• Neutral-to-earth at the outlet measures about 8 VAC.
• Ground rod was recently installed and tested.

I contacted GK support and they replied that “there should be no leakage to the chassis when ungrounded” and suggested having the amp checked.

The amp works perfectly otherwise and I don’t know whether this is:

1. A normal characteristic that I never noticed because I always used it ungrounded,
2. An issue in the power supply / EMI filter section,
3. A grounding/mass reference issue inside the amp,
4. Something else entirely.

Has anyone seen an MB200 (or another Class D bass amp) behave this way?

Thank you very much.

 

[agedhorse]

Most likely cause is a contaminated ground external to the amp.

Have you tried the amp in another location (different building/service) on a grounded receptacle?

This is not necessarily a class D issue.

 

[SSCBA]

Most likely cause is a contaminated ground external to the amp.

Have you tried the amp in another location (different building/service) on a grounded receptacle?

This is not necessarily a class D issue.

As my wiring is old and my house is not grounded, I installed a ground rod specifically for this outlet, and the only thing plugged in to that outlet was the amp.

Haven't tried in another place yet.

In my country AC is 220v 50hz and ground connection is not neutral bonded (TT) - if it matters.

 

[fdeck]

It seems like you tried this, but one of my two MB200 amps is fairly old, and had some hum that went away when I shorted the aux input jack. I used a length of some kind of heavy brass wire that was sitting around. I got a deal on the amp from a TB'er who disclosed the problem and the fix to me. I never investigated it more deeply than that, and the amp has served me for years.

 

[Lo-E]

It seems that when your amp is connected to ground it forgets the words….

 

[agedhorse]

As my wiring is old and my house is not grounded, I installed a ground rod specifically for this outlet, and the only thing plugged in to that outlet was the amp.

Haven't tried in another place yet.

In my country AC is 220v 50hz and ground connection is not neutral bonded (TT) - if it matters.

I think you created the problem as you are injecting noise (ground is only a descriptive term) into the ground of the amp itself.

In your country, neutral and ground are most certainly bonded (standard MEN system) unless you have a TT installation AND a residual current interruptor device (which is also called a high resistance ground/bond), otherwise the PE (protective earth) system wouldn't work and no fault would clear. I think you have a misunderstanding of how this works and this would be a good time to consult with a licensed professional (which I think your national code requires anyway, maybe for this sort of reason). You can't just add a ground rod and wire to the receptacle because you will then have two different ground potentials and this causes circulating currents to flow between the two ground points (that will never be completely equi-potential).

Here's a one-line diagram of how your MEN system is implimented. Normally for a house with a single panel, only the top left (main switchboard) panel connections would apply and you can clearly see the bonding conductor between the neutral and earth (ground) buses. Outbuildings are a little different in that this is where the "multiple" part comes into play:

When I design amps for certification to Australia C-Tick safety regulations, there is a ground fault current test that we have to pass because a ground fault MUST be able to return to the neutral bus on the panelboard if a live (hot) conductor contacts any portion of the amp's PE system.

In a TT system, there's still a bond between neutral and ground but it's located (generally) at the distribution transformer on the pole and it may not be a solid bond but have a known resistance inserted. Because of this, a separate earth and neutral wire are carried into the main distribution panel AND an RCD must be present.

So, adding the floating ground to the system created the issue by creating circulating currents that induce a small 50Hz voltage onto the amplifier's ground which then creates a floating noisy reference. IF you have a legal RCD installed in your service, your best bet is to leave the added ground disconnected.

Hope this helps.

 

[SSCBA]

It seems like you tried this, but one of my two MB200 amps is fairly old, and had some hum that went away when I shorted the aux input jack. I used a length of some kind of heavy brass wire that was sitting around. I got a deal on the amp from a TB'er who disclosed the problem and the fix to me. I never investigated it more deeply than that, and the amp has served me for years.

Thanks for the suggestion, I was missing that test.
I've just tried that and the hum decreased considerably.

 

[SSCBA]

I think you created the problem as you are injecting noise (ground is only a descriptive term) into the ground of the amp itself.

In your country, neutral and ground are most certainly bonded (standard MEN system) unless you have a TT installation AND a residual current interruptor device (which is also called a high resistance ground/bond), otherwise the PE (protective earth) system wouldn't work and no fault would clear. I think you have a misunderstanding of how this works and this would be a good time to consult with a licensed professional (which I think your national code requires anyway, maybe for this sort of reason). You can't just add a ground rod and wire to the receptacle because you will then have two different ground potentials and this causes circulating currents to flow between the two ground points (that will never be completely equi-potential).

Here's a one-line diagram of how your MEN system is implimented. Normally for a house with a single panel, only the top left (main switchboard) panel connections would apply and you can clearly see the bonding conductor between the neutral and earth (ground) buses. Outbuildings are a little different in that this is where the "multiple" part comes into play:
View attachment 7518471

When I design amps for certification to Australia C-Tick safety regulations, there is a ground fault current test that we have to pass because a ground fault MUST be able to return to the neutral bus on the panelboard if a live (hot) conductor contacts any portion of the amp's PE system.

In a TT system, there's still a bond between neutral and ground but it's located (generally) at the distribution transformer on the pole and it may not be a solid bond but have a known resistance inserted. Because of this, a separate earth and neutral wire are carried into the main distribution panel AND an RCD must be present.

View attachment 7518478

So, adding the floating ground to the system created the issue by creating circulating currents that induce a small 50Hz voltage onto the amplifier's ground which then creates a floating noisy reference. IF you have a legal RCD installed in your service, your best bet is to leave the added ground disconnected.

Hope this helps.

Thank you very much for your detailed response! Very insightful.

I live in Argentina, and our national code mandates a TT system instead of the MEN (TN-C-S). Bonding Neutral and Earth inside a house is not allowed here and and we have to run our own completely isolated ground rod for the house PE.

That's why, as my house is ungrounded because the wiring is old, I grounded one outlet where I connect my laptop, studio monitors, etc. and that's the outlet where I tried the amp, without anything else plugged in. Only the amp.

In that outlet I measured the following:
Fase to neutral: 221v
Fase to ground: 226v
Neutral to ground: 8v
So there's some potential difference - don't know if that can contribute to the problem.

What I discovered today thanks to fdeck's comment (who had the same problem with the same amp of similar period) is that shorting the AUX IN input reduces the hum almost completely, but I know that's not a permanent solution.

 

[agedhorse]

Thank you very much for your detailed response! Very insightful.

I live in Argentina, and our national code mandates a TT system instead of the MEN (TN-C-S). Bonding Neutral and Earth inside a house is not allowed here and and we have to run our own completely isolated ground rod for the house PE.

That's why, as my house is ungrounded because the wiring is old, I grounded one outlet where I connect my laptop, studio monitors, etc. and that's the outlet where I tried the amp, without anything else plugged in. Only the amp.

In that outlet I measured the following:
Fase to neutral: 221v
Fase to ground: 226v
Neutral to ground: 8v
So there's some potential difference - don't know if that can contribute to the problem.

What I discovered today thanks to fdeck's comment (who had the same problem with the same amp of similar period) is that shorting the AUX IN input reduces the hum almost completely, but I know that's not a permanent solution.

Sorry, your profile says Australia.

In Argentina, TT is the most common along with an RCD trip device. The neutral is still bonded to ground but at the service transformer rather than at the service panelboard.

The cause of your voltage between neutral and ground comes from two sources. The first is that when current is drawn from the supply the voltage drop on the neutral (which is grounded) causes a different voltage (or potential) at that point of the ground/earth compared with the unreferenced electrode ground at the house. The second is that any potential difference, for whatever reason, will cause current to flow and a voltage gradient between the two grounds.

Since this is what you have, and presumably you have RCD protection, operating ungrounded is your better (and still safe) option.

 

[eniac]

If Agedhorse ever decides to quit this place, we're all in some pretty deep doodoo.

 

[SSCBA]

Well, after further testing, bridging de aux im helps (reduces hum by 70% I would say) but doesn't completely solve the problem. Also tried it on a different location with newer grounded wiring and nothing changed.

I recall reading there was a defective batch of mb200 from 2010/2011 and one was yours @fdeck ?
If you still have yours, is it completely quiet by bridging the aux in and do you have any other information regarding this problem?

Thank you all

 

[agedhorse]

I wouldn't read too much into the claims of there being a defective batch, generally that's just unfounded rumors.

 

[fdeck]

Well, after further testing, bridging de aux im helps (reduces hum by 70% I would say) but doesn't completely solve the problem. Also tried it on a different location with newer grounded wiring and nothing changed.

I recall reading there was a defective batch of mb200 from 2010/2011 and one was yours @fdeck ?
If you still have yours, is it completely quiet by bridging the aux in and do you have any other information regarding this problem?

Thank you all

Completely quiet.

 

[agedhorse]

Completely quiet.

You are also operating on a solidly grounded power system which is almost always going to be electrically quieter.