120Hz hum from Logitech Z313 satellite speakers

[DonR]

My small satellite speakers on our Z313 2.1 system (used for a TV) are emitting a 120Hz hum whether connected or not to the TV. The subwoofer emits no such hum. Here is a look at the main circuit board.

Satellite speakers are fed with the blue/yellow/red wires going to the ferrite core. The black wire at the top goes to the remote volume control and muting switch. Power comes in on the left from a linear supply. Main filter cap checks out fine for capacity and ESR. Sub wires at coming out at the bottom. Most of the caps appear to be associated with the crossover network for the sub. I have a second unit that exhibits the same symptom to a lesser degree.

 

[AnalogSteph]

Hmm, just one power amplifier IC for a 2.1? Or is the one for the satellites on the back?

My initial hunch would be that the sub is driven in BTL but the satellites are SE, so PSRR would be markedly different. I might look at ripple on the main power rail with a scope if you have one.

Failing that, a PCB layout issue is a possibility as well.

The variability between units does suggest that capacitors might have a hand in this, just not necessarily the big guy. Is there a second supply for opamps?

 

[DonR]

Hmm, just one power amplifier IC for a 2.1? Or is the one for the satellites on the back?

My initial hunch would be that the sub is driven in BTL but the satellites are SE, so PSRR would be markedly different. I might look at ripple on the main power rail with a scope if you have one.

Failing that, a PCB layout issue is a possibility as well.

The variability between units does suggest that capacitors might have a hand in this, just not necessarily the big guy. Is there a second supply for opamps?

It's a 4-channel chip amp, an STA240. Yes, sats are SE and sub is BTL. I am also beginning to think layout issue. Only one power rail, scopes out with very low ripple of a few mv. Xformer and rect diodes are on another PCB and then filtering is done on main PCB. I will check out other caps and see if I can reroute power wires. Backside of PCB has lots of surface mount passives and some SMD op amps.

 

[DonR]

Grounding the negative supply to earth has eliminated most of the hum (but not all). Voltage difference of 11VAC from negative to earth. Definitely something deficient in the design but I paid C$15 for it 12 years ago so par for the course.

 

[JaapD]

Does this linear power supply have a 2 pin or a 3 pin connection to the wall? If it has a 3 pin earth connection then you could try (first as experiment) to isolate / disconnect the earth connection, then see if that solves the hum. The idea behind this is that I suspect a parallel signal return path, flowing through the mains connection back to the TV.

Cheers,
JaapD.

 

[DonR]

Does this linear power supply have a 2 pin or a 3 pin connection to the wall? If it has a 3 pin earth connection then you could try (first as experiment) to isolate / disconnect the earth connection, then see if that solves the hum. The idea behind this is that I suspect a parallel signal return path, flowing through the mains connection back to the TV.

Cheers,
JaapD.

Both the speaker system and the TV have no earth return. The voltage differential appears whether the speaker is connected to the TV or not. I already have an ground loop isolator on the TV output.

 

[peplum]

Hello,
I have a Z213 2.1 that was the previous generation that worked very well with a desktop PC. But very sensitive (normal ?) sound answer when line jack is being plugged or unplugged, or just touched by one finger (ground action?).
It produces no hum noise as soon as plugged to the desktop PC or a laptop HDMI plugged PC to an extenal screen.
Curiously it has a hum (from the subwoofer) when I unplugg the HDMI connection.
At this moment, at low volume I hear a boring hum noise.
It seems the HDMI ground connecter cancel the hum. But the problem is sometimes I cannot connect the external screen.

Now for test I've just tested a brand new Z313. When jack is unplugged : almost no hum contrary to the Z213.
But when plugged on a desktop PC that is shutdown, or just but not playing audio : hum from the satellites ! Possibly more from left one than from the right one.

I'm really embarrassed with these low volume hum issues. I tried an ground noise canceller jack adapter but excepting to high pitch the noise : not working.
An idea ?

 

[DonR]

Is it the 313 you have an issue with, the 213 or both?

 

[peplum]

Actually both.
213 when used with a simple laptop. Desktop PC use is ok. Laptop use is OK when using an HDMI connector plugged
313 when used with a desktop PC (not tested with a laptop)

Both were also tried using this adaptor : https://www.amazon.fr/dp/B06XQYN77L?psc=1&ref=ppx_yo2ov_dt_b_product_details with no improvement
Not sure if I should give a try with this one : https://www.amazon.fr/gp/product/B00BBE59WS/ref=ox_sc_act_title_1?smid=A3BFA3WHDHQKAP&psc=1

 

[Philbo King]

If you have a DMM, measure for any AC voltage present on the DC supply. If AC is present, the power supply filtering is inadequate. If no AC is present you likely have inadequate grounding on the speaker line inputs.

 

[peplum]

I suppose I will break the warranty if I open the sub for this test. Because the 213 work fine and not the 313 on the same PC, I suppose the Z313 PS woud be bad filtered (?).
What is strange is the hum noise end when I let the input line plugged on the PC but I disconnect the power supplies except the speakers system one.
Sound board is typical integrated motherboard sound module.

 

[DonR]

On the 313, ground the middle top screw (it connects to the shield ground internally) of the rear panel with another solid ground source like an amp or PC chassis. I used some crocodile clip leads, it eliminated most of the hum. I had about 11V AC of difference between this screw and solid ground. The PSU is crappy and I think it is coming from the transformer. Note that not all Z313 PSUs are the same, some are linear and some are switch-mode.

 

[peplum]

You're right. I've connected the black painted screw just above the power cord of the Z313 sub (not sure the middle top worked) to a ground pin of the VGA mass connector and the hum "magically" disappeared.
So dirt design !
Too bad designed this device does not include a grounded 3 connectors power cable to cancel this noise !
By the way the hum is more when volume is set very low. Not very loud but enough to be eared when silent PC and silent room used.

Even this workaround does work I don't feel very confortable to provide this device to a child/other person with a non std connection.

So I would préferably use a Z213 this is also more than sufficient in term of power for my simple use than the Z313 and the Z213 also feature a sub filter setting knob on the sub the Z313 does not have. The last boost toom much the bass also.

Z213 : do you know how I could simply cancel the hum that is naturally cancelled using the HDMI connector plugged in without any electrical risk ?
This hum happens of laptop when HDMI is not connected and when laptop is discharging. When laptop is on charge : no hum
Regards

 

[DonR]

You're right. I've connected the black painted screw just above the power cord of the Z313 sub (not sure the middle top worked) to a ground pin of the VGA mass connector and the hum "magically" disappeared.
So dirt design !
Too bad designed this device does not include a grounded 3 connectors power cable to cancel this noise !
By the way the hum is more when volume is set very low. Not very loud but enough to be eared when silent PC and silent room used.

Even this workaround does work I don't feel very confortable to provide this device to a child/other person with a non std connection.

So I would préferably use a Z213 this is also more than sufficient in term of power for my simple use than the Z313 and the Z213 also feature a sub filter setting knob on the sub the Z313 does not have. The last boost toom much the bass also.

Z213 : do you know how I could simply cancel the hum that is naturally cancelled using the HDMI connector plugged in without any electrical risk ?
Regards

You must connect the ground (either via audio or screw) to a solidly grounded device. I hacked in a 3-prong jack and connected the ground pin to the internal shield (which is grounded). You could also hard-wire in a 3 pin cord or use an eyelet from the screw to a cable ground connection to ground the screw. I think the screws are the only way to safely ground with minimal invasion.