It could be any other lead connected to your PC carrying ground. Like the VGA cable or maybe even a Ethernet lead that uses shielding.

You get ground loops through monitor and power cables (in my case KVM is the major source) so it more or less is unavoidable when there are multiple devices. If you really want to get rid of them without harming sound (those isolators typically use a transformer and it will limit low freq response and also massively increase THD at the lower freqs, and usually they limit high freq response too) you need a mixer or switcher with differential inputs (even when sound output devices don't have them), so that the additional ground connections through other paths will become meaningless and only reference is the ground at the sound card output connector (which is then connected to the negative input at the mixer end of the cable). Custom cabling is probably necessary then though...

Hi. Ground loops were quite common in hi-fi applications.
Galvanic insulated connections often fixed the issues. Opto-couplers.. Toslink.. AF transformers..
Rotating one of two of the AC plugs also helps, sometimes. 😉

Thank you all for your input . The DPS12 does not have differential inputs, but does have balanced inputs using a TRS connector for each audio channel .

The Rolls HE18 units I have on order have the option of converting unbalanced signals to balanced . Would that help with the ground loop while having less of an impact on audio quality than simply using them as isolators on unbalanced signals in and out ?

There is a schematic in the manual for the HE18, if that helps answer my question .

The isolator will do its thing regardless of what sort of inputs you'll use on it. To avoid having to use the isolators you need a mixer or a switcher with differential/balanced inputs. Audio signal goes to positive input of the receving device, ground goes to ground and negative input, this creates a pseudo differential connection which still allows ground loop issues to be cured.

Tiido wrote on 2020-08-11, 08:36:

The isolator will do its thing regardless of what sort of inputs you'll use on it. To avoid having to use the isolators you need a mixer or a switcher with differential/balanced inputs. Audio signal goes to positive input of the receving device, ground goes to ground and negative input, this creates a pseudo differential connection which still allows ground loop issues to be cured.

Thanks for the additional info . Unfortunately, what you describe seems to be the way I am currently connected . I used RCA to TR phone connectors, which means that Ring and Sleeve are already connected together and to the source's ground .

Tiido wrote on 2020-08-11, 08:36:

The isolator will do its thing regardless of what sort of inputs you'll use on it. To avoid having to use the isolators you need a mixer or a switcher with differential/balanced inputs.

You can also use a audio patchbay with balanced connectors, AFAIK. You can buy those new (with 6.3 mm plugs) starting at 50 bucks...

yawetaG wrote on 2020-08-12, 10:16:

Tiido wrote on 2020-08-11, 08:36:

The isolator will do its thing regardless of what sort of inputs you'll use on it. To avoid having to use the isolators you need a mixer or a switcher with differential/balanced inputs.

You can also use a audio patchbay with balanced connectors, AFAIK. You can buy those new (with 6.3 mm plugs) starting at 50 bucks...

The Akai DPS12 that I am using already has balanced inputs . Would it be worth converting the sound cards'/modules unbalanced outputs to balanced and feeding the DPS12 with that ? If so, what would you suggest I use (other than the Rolls HE18, which can do the conversion, but also have isolation transformers)?

darry wrote on 2020-08-12, 13:25:

yawetaG wrote on 2020-08-12, 10:16:

Tiido wrote on 2020-08-11, 08:36:

The isolator will do its thing regardless of what sort of inputs you'll use on it. To avoid having to use the isolators you need a mixer or a switcher with differential/balanced inputs.

You can also use a audio patchbay with balanced connectors, AFAIK. You can buy those new (with 6.3 mm plugs) starting at 50 bucks...

The Akai DPS12 that I am using already has balanced inputs . Would it be worth converting the sound cards'/modules unbalanced outputs to balanced and feeding the DPS12 with that ? If so, what would you suggest I use (other than the Rolls HE18, which can do the conversion, but also have isolation transformers)?

I don't know. The whole balanced vs. unbalanced thing tends to give me a headache. The only thing I know it that one of my effect units had a grounding issue causing a hum, and the solution was either a custom cable or plonking a balanced patchbay in between (which effectively changed the connection in such a way that it emulates the custom cable).

yawetaG wrote on 2020-08-12, 17:21:

darry wrote on 2020-08-12, 13:25:

yawetaG wrote on 2020-08-12, 10:16:

You can also use a audio patchbay with balanced connectors, AFAIK. You can buy those new (with 6.3 mm plugs) starting at 50 bucks...

The Akai DPS12 that I am using already has balanced inputs . Would it be worth converting the sound cards'/modules unbalanced outputs to balanced and feeding the DPS12 with that ? If so, what would you suggest I use (other than the Rolls HE18, which can do the conversion, but also have isolation transformers)?

I don't know. The whole balanced vs. unbalanced thing tends to give me a headache. The only thing I know it that one of my effect units had a grounding issue causing a hum, and the solution was either a custom cable or plonking a balanced patchbay in between (which effectively changed the connection in such a way that it emulates the custom cable).

I will visit my local music store and see if I can get something to convert unbalanced to balanced . Probably should have done that before plonking down dough on the best rated "affordable" isolation transformers I could find online .

I have learned that unbalanced to balanced converters are called DI boxes (direct injection boxes) and exist in passive (unpowered) and in active (battery or phantom powered) types . Both types can provide so-called "ground lift" to break a ground loop between devices .

Passive DI boxes use isolation transformers for ground lift whereas active DI boxes typically do not use transformers for ground lift (there are apparently exceptions).

My Akai DPS12 does not provide phantom power, so to get transformerless ground lift, I would need to use battery powered (yuck) active DI boxes or get myself some sort of phantom power injector (maybe wall wart powered active DI boxes exist?) . Alternatively, I could replace my Akai DPS12 with a mixer that does provide phantom power. I would also need to make sure the DI box can handle line level signals . Additionally, the phantom PSUs (injectors) I have seen so far are meant for passing through microphone signals only, not line level .

Some of the approx costs in CAN$ are as follows (new equipment):

300$ for a mixer with enough inputs and phantom power
3 x (50 to 100)$ for active DI boxes
6 x (13 to 20)$ for required TRS or XLR cabling.

Total 528 to 720 CAN$ plus taxes and eventual shipping costs for everything .

TLDR : NOPE!

I really hope the Rolls HE18 sound transparent enough, because I do not want to go down the "balanced" rabbit hole .

Phantom power only gets used for microphone inputs, AFAIK. Did you specify you were looking to convert unbalanced line inputs to balanced ones? Otherwise they might just have assumed you were talking about a microphone or guitar signal...

Music shops tend to be rather clueless these days (try Gearslutz instead).

And yeah, it gets expensive...I just cabled a whole 16U 19" rack - total cost for just the cabling (audio + MIDI for 2 synths and 4 effect units): about 300 bucks.

O no, phantom power is pretty common on all XLR inputs on a mixer. Unless you call XLR inputs mic inputs, that’s incorrect. DI boxes are designed to take phantom power because this is so ubiquitous and they are also used to connect keyboards, electric drumkits and laptops that are used more and more on live setups. Most active DI boxes have an attenuation switch to take line levels or sometimes even speaker levels.

Just to be clear, I went info gathering using Google, Wikipedia and various audio sites before i went to my local store . The very nice salesperson person pretty much confirmed and complemented what I had learned beforehand .

I did no go there to drain the poor dude's knowledge and buy nothing (bought a Roland switching PSU actually rated at more than 650mA for my MT-32, the Roland recommended one is current limited at 500mA ). If I ever do make the jump to a new mixer etc (if my DPS12 dies irrepairably), I will almost certainly be buying at least the mixer itself locally even if a bit more expensive than online because I really like the store .

I know realize how lucky I am to have gotten a non working 50 CAN$ DPS12 that only needed an affordable (at the time, one of these https://www.artmix.com/SATA_SCSI_AZMN_II_1.html ) SCSI to SATA adapter and a spare hard drive to come back to life . Decently featured mixers and even used multi-track recorders are not cheap .

EDIT : I stumbled upon one of my old posts from 3 years ago where I mentioned seeing sub-200 CAN$ multi-track recorders . I really should have considered buying one back then . Hindsight is 20/20 .

yawetaG wrote on 2020-08-13, 06:03:

Phantom power only gets used for microphone inputs, AFAIK. Did you specify you were looking to convert unbalanced line inputs to balanced ones? Otherwise they might just have assumed you were talking about a microphone or guitar signal...

Music shops tend to be rather clueless these days (try Gearslutz instead).

And yeah, it gets expensive...I just cabled a whole 16U 19" rack - total cost for just the cabling (audio + MIDI for 2 synths and 4 effect units): about 300 bucks.

Yes, I did specify I needed line level signals converted. I checked specs on packaging of DI boxes to be sure . I also made sure to look only at 2 channel ones (didn't want to consider getting 6 single channel ones).

digistorm wrote on 2020-08-13, 06:20:

O no, phantom power is pretty common on all XLR inputs on a mixer. Unless you call XLR inputs mic inputs, that’s incorrect. DI boxes are designed to take phantom power because this is so ubiquitous and they are also used to connect keyboards, electric drumkits and laptops that are used more and more on live setups. Most active DI boxes have an attenuation switch to take line levels or sometimes even speaker levels.

I'm pretty sure that connecting a keyboard with XLR outputs to a XLR input with phantom power enabled will cause damage to the keyboard.

yawetaG wrote on 2020-08-13, 21:25:

digistorm wrote on 2020-08-13, 06:20:

O no, phantom power is pretty common on all XLR inputs on a mixer. Unless you call XLR inputs mic inputs, that’s incorrect. DI boxes are designed to take phantom power because this is so ubiquitous and they are also used to connect keyboards, electric drumkits and laptops that are used more and more on live setups. Most active DI boxes have an attenuation switch to take line levels or sometimes even speaker levels.

I'm pretty sure that connecting a keyboard with XLR outputs to a XLR input with phantom power enabled will cause damage to the keyboard.

I would not be surprised .

These seem to be some good rules of thumb :
https://www.sounddevices.com/the-danger-of-ap … apply%20phantom.

yawetaG wrote on 2020-08-13, 21:25:

digistorm wrote on 2020-08-13, 06:20:

O no, phantom power is pretty common on all XLR inputs on a mixer. Unless you call XLR inputs mic inputs, that’s incorrect. DI boxes are designed to take phantom power because this is so ubiquitous and they are also used to connect keyboards, electric drumkits and laptops that are used more and more on live setups. Most active DI boxes have an attenuation switch to take line levels or sometimes even speaker levels.

I'm pretty sure that connecting a keyboard with XLR outputs to a XLR input with phantom power enabled will cause damage to the keyboard.

Let me clarify that: I talked about connecting those instruments through a DI box (my "they" referred to DI boxes, not XLR inputs). Although I have seen a lot of non-phantom powered things do just fine when they got phantom power because if the output is engineered right they don't use it because they should only use the differential voltage between the two signal lines. Many dynamic microphones don't break a sweat when they get phantom power, in the same way I saw a Roland drumkit survive and also a MOTU audio interface. Still, better don't do it, but it is usually cheap crap or damaged leads that can kill the equipment when they receive phantom power.

I received my 3 Rolls HE18 units today . I do not think that I will be double blind testing them . I will give them a subjective listening test with some Flacs from my collection, though . If I don't hear anything obviously annoying, I will leave it at that and be happy the hum is gone .