50 hz noise in audio system

[Neu1986]

Hi, I'm new of the forum. I have a poor knowledge of electronics so I'll try to explain my problem as best as I can...
I play soft synths on a laptop controlled by MIDI keyboard. The laptop is connected to an external USB sound card with two 1/4'' balanced jack outputs. The problem is, when I connect the jacks to a mixer sending signal to the PA, I can hear a very high 50 Hz hum (I live in Italy).

I tried the following:
- replacing the PSU with a stock one, less noisy, the noise lowered but it's still on
- improve the SNR inside the laptop (basically turning the digital volume to the maximum possible without distortion), it worked but as soon as I need a litte more volume the noise can be heard again
- putting a jitter filter between the laptop and the USB cable, the sound is far better, all the other noises have disappeared (circuitry, mouse ecc) but the 50 Hz noise is still on place
- using a DI-box: the SNR is better, the sound is better, but the noise is still relatively loud
- running the laptop on battery only: sometimes the noise goes away, sometimes not, depending on where I am.

The problem is not always present: when I am in locations (venues, studios etc.) that I know are properly grounded, there's no noise, but in other places the noise is very high.

I don't know what else can I do: I was thinking of powering the sound card via a battery-powered USB hub which removes totally the power supplied by the laptop, maybe changing the USB cable with a better one, but honestly I am tired of experimenting and so I would like to have your opinion on this matter

thanks in advance!!

 

[graz_lag]

... I was thinking of powering the sound card via a battery-powered USB hub which removes totally the power supplied by the laptop, maybe changing the USB cable with a better one, but honestly I am tired of experimenting and so I would like to have your opinion on this matter ...

It looks the hum / noise is picked up via a ground loop : https://en.wikipedia.org/wiki/Ground_loop_(electricity)

If you intend to power the sound card by means of a 5V battery, which could be a good idea anyway, you will need an "Y" USB cable, where one plug transfers the "data only" from the laptop to card, the other gives the power to the card. Pins #1 and #4 should be disabled (covered with a tape strip ...) in the plug that transfer the data so power does not go from the laptop to the card.

 

[Patrick1958]

You could try a usb/spdif bridge between laptop and soundcard. A good constructed bridge would have galvanic isolation, eleminating hum that could have origin in the laptop.

 

[Neu1986]

Thanks. My sound card is powered by a USB A/B cable, is there anything similar for this type of cable?

 

[graz_lag]

Thanks. My sound card is powered by a USB A/B cable, is there anything similar for this type of cable?

https://www.ebay.fr/itm/C2G-81578-2...Cable-Black-/113173909548?hash=item1a59b0d02c

I bought mine last year for 10 EUR or so from this seller : https://www.ebay.fr/usr/coolercastleltd?_trksid=p2057872.m2749.l2754

The RED A is for power only from the battery.
The BLACK A is from the laptop. In this, you tape with a little strip the contact #1 and #4, so the power is interrupted, and it becomes "data only".

 

[amirm]

Get a sound card with balanced output. Otherwise, get one of these transformer based converters: http://www.jensen-transformers.com/pro-audio/

 

[AnalogSteph]

My hunch is that you may be dealing with mixers without any balanced inputs (traditional DJ mixers tend to be all-unbalanced, and on less expensive mixers only part of the inputs may be balanced, so watch out). In this case I would run balanced cabling into the DI box / line isolator (which one is it, the classic Behringer HD400?) followed by (unbalanced) instrument cable to the mixer. Balanced = shielded twisted pair with XLR or TRS plugs, unbalanced / instrument = coax with TS plugs. Twisted pair makes subpar unbalanced cable, much like coax isn't much good for balanced. Long runs in an electrically noisy environment (as often found in stage applications) may well exhibit issues.

So if you can, always look for a balanced input first. Maybe have cabling for both XLR and TRS handy.

As an additional measure, I would advise on grounding the laptop to protective earth - look up whether your laptop manufacturer has a compatible bigger power supply (>75 W, those pretty much have to have a 3-prong power cord), otherwise one can always rig up something. You don't want to have the leakage currents from the power supply's mains filter traveling down your audio ground or taxing your line isolator's common-mode rejection. Those would be some of the cases where battery operation helps. If it doesn't, my bets are on stray radiation caught by unsuited cabling.
(Now let's just hope you don't come across any boneheaded installations where the PA has no PE connection at all but the outlet you are using does.)

Finally, if you are certain you've got a balanced input and proper cabling but there still is hum, the input may be afflicted by the Pin 1 Problem. In this case a line isolator with balanced cabling on both sides should get the issue sorted (but beware: some connect pin 1 through, which defeats the purpose, so make sure yours doesn't). Alternatively, you could make a "ground lifted" balanced cable that leaves shield unconnected on one end.

It's hard to come up with a one-size-fits all kind of solution if you are dealing with multiple different issues, and it sounds like you are. You always have to address the problem in question. Fortunately the number of permutations is limited.

Just to be sure, which interface are you using? Just to make sure its outputs are actually balanced. (It wouldn't be a super big deal if it isn't, but you'd have to make your own balanced adapter cable. Unbal signal --> hot, unbal ground --> cold, unbal ground --> shield. Otherwise, run instrument cable into line isolator and balanced cable out of that.)

 

[Neu1986]

My hunch is that you may be dealing with mixers without any balanced inputs (traditional DJ mixers tend to be all-unbalanced, and on less expensive mixers only part of the inputs may be balanced, so watch out). In this case I would run balanced cabling into the DI box / line isolator (which one is it, the classic Behringer HD400?) followed by (unbalanced) instrument cable to the mixer. Balanced = shielded twisted pair with XLR or TRS plugs, unbalanced / instrument = coax with TS plugs. Twisted pair makes subpar unbalanced cable, much like coax isn't much good for balanced. Long runs in an electrically noisy environment (as often found in stage applications) may well exhibit issues.

So if you can, always look for a balanced input first. Maybe have cabling for both XLR and TRS handy.

As an additional measure, I would advise on grounding the laptop to protective earth - look up whether your laptop manufacturer has a compatible bigger power supply (>75 W, those pretty much have to have a 3-prong power cord), otherwise one can always rig up something. You don't want to have the leakage currents from the power supply's mains filter traveling down your audio ground or taxing your line isolator's common-mode rejection. Those would be some of the cases where battery operation helps. If it doesn't, my bets are on stray radiation caught by unsuited cabling.
(Now let's just hope you don't come across any boneheaded installations where the PA has no PE connection at all but the outlet you are using does.)

Finally, if you are certain you've got a balanced input and proper cabling but there still is hum, the input may be afflicted by the Pin 1 Problem. In this case a line isolator with balanced cabling on both sides should get the issue sorted (but beware: some connect pin 1 through, which defeats the purpose, so make sure yours doesn't). Alternatively, you could make a "ground lifted" balanced cable that leaves shield unconnected on one end.

It's hard to come up with a one-size-fits all kind of solution if you are dealing with multiple different issues, and it sounds like you are. You always have to address the problem in question. Fortunately the number of permutations is limited.

Just to be sure, which interface are you using? Just to make sure its outputs are actually balanced. (It wouldn't be a super big deal if it isn't, but you'd have to make your own balanced adapter cable. Unbal signal --> hot, unbal ground --> cold, unbal ground --> shield. Otherwise, run instrument cable into line isolator and balanced cable out of that.)

As far as I know I have already acted according to your suggestions... my sound card is an Alesis iO2 express which has two 1/4" balanced TRS jack outputs, so the fact is, when the signal goes into the DI-box (it's a Radial Trim Two with XLR output) the ground SHOULD have an independent path and the noise SHOULD go away when I use the DI's ground lift.. unfortunately, the ground lift has no effect. T_T
The power supply is three-pronged and original from the laptop manufacturer.
It seems that there is some noisy component inside the motherboard, while I have also been told that the noise can come from the phantom power of the sound card, which has a "resonator" (I don't know the name in English) which resonates at 50 hz. Could it be the case?
I don't know anything about the Pin 1 problem, can you tell me more about that? thanks a lot

 

[Speedskater]

With a balanced XLR interconnect system, you can try using cables with the shields connected at the send end only. Don't cut into expensive cables, just make two short male/female cables.

 

[DonH56]

^^^ The DI box's ground lift should do that. Sounds like a ground loop but the path may not be through the XLR's. I would normally suggest using a Hum-X or trying cheater plugs to track down which device is the culprit but have no idea how that works in Italy.