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Earth hum through monitors - isolation transformer needed?

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Earth hum through monitors - isolation transformer needed?

Postby paulfromcamden » Wed Apr 01, 2020 12:25 pm

I'm using a pair of JBL LSR305 monitors with a Focusrite 2i2 connected to my Macbook and getting a quite noticeable earth hum through them.

Disconnecting the power cable from the Macbook fixes it so I think I have a ground loop.

What's the simplest/best/cheapest way of solving this?

I wondered about an Art DTI isolator - but I'm not sure if it's the right thing. And if I can not spend £50 that would be even better! :)

Thanks
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Re: Earth hum through monitors - isolation transformer needed?

Postby Mike Stranks » Wed Apr 01, 2020 12:38 pm

Yup! Almost certainly iso-transformers will fix it. I had similar problems with monitors in the past.

I'm a great fan of the DTI - as is Mr Robjohns. It does it's job and does it well. It's a versatile little gizmo too.

If budget is tight then I also have one of these: https://www.gear4music.com/Recording-and-Computers/SubZero-Hum-Destroyer/27BH that I bought to dig me out of a hole on a live gig. It did the job, but if you want quality then it costs...

And for the avoidance of doubt for anyone else reading this, an iso-transformer is not the same as a DI box. Different beast for a different problem...
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Re: Earth hum through monitors - isolation transformer needed?

Postby Hugh Robjohns » Wed Apr 01, 2020 12:42 pm

Yep, that laptop disconnection test implies it's a ground-loop alright.

There are a couple of things you can do at zero cost that might help:

1. Make sure the laptop PSU, both active speakers, and the interface (if appropriate) are all plugged into adjacent sockets in a mains plugboard. Keeping the mains safety earth connections of these devices as close to each other as possible minimises the size of the ground-loop, and thus minimises the level of the hum. Sometimes this is enough to fix the problem on its own, but it's always good practice even if it doesn't.

2. Make sure you're using proper balanced cables between the interface and speakers.

3. Assuming you're using balanced cables already, and if you're up for a bit of DIY, then open up the XLR/TRS connectors (if possible) at the speaker ends of the cables from the interface and either desolder or snip the cable screen connections to pin 1 /sleeve. You will probably need some insulating tape to ensure the cable screen can't come into contact with anything post surgery! Also, label the cables in some way as having the screen cut at the destination end, especually if they are XLR-XLR cables, since they will no long be capable of use with phantom-powered mics.

The alternative, if you don't want to hack your cables, is to invest in something like the ART DTI box -- there are other transformer line-isolation boxes around, but I like the DTI for its comprehensive set of connectors which make it extraordinarily versatile and convenient, and its proven audio quality. The ART CleanBox 2 (not the Cleanbox Pro) uses the same transformers and is £5 cheaper, but it only has TRS sockets in and out, and that lack of flexibility seems a lot to lose for the sake of a fiver!

If you're going down this route you connect the interface outputs into the box, and the box outputs into the speakers, using balanced cables throughout. Make sure the box is kept well away from mains cables and anything with a big mains transformer inside!
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Re: Earth hum through monitors - isolation transformer needed?

Postby ef37a » Wed Apr 01, 2020 1:12 pm

A few months ago I bought, for an entirely different reason, a Duracell DRAC 9006 power supply. 19V and 90W and comes with a figure 8 mains input. It is 'universal' and comes with several adapter plugs. No idea what a mac wants but Amazon are very good at returns if it does not suit.
Cost me 40 quid from a shop but I have seen them online for less. Slight saving on the transformer and it fixes the problem 'at source'. You are also spared the virtually undetectable but always present quality loss from a transformer.

General NB? More systems are now going 'earth free' so such a power supply could result in MORE hum rather than less!

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Re: Earth hum through monitors - isolation transformer needed?

Postby Zukan » Wed Apr 01, 2020 1:13 pm

If you're still struggling after all the great advice given here let us know and we will send round our resident hum finder. Martin loves these challenges and was only confiding in me yesterday 'Ed, keep this a bit quiet mate but I'm really, really missing locating hums for forum members.'
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Re: Earth hum through monitors - isolation transformer needed?

Postby Martin Walker » Wed Apr 01, 2020 2:52 pm

And if you believe that you'll believe anything :D


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Re: Earth hum through monitors - isolation transformer needed?

Postby Zukan » Wed Apr 01, 2020 3:23 pm

lol
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Re: Earth hum through monitors - isolation transformer needed?

Postby paulfromcamden » Wed Apr 01, 2020 6:51 pm

Thanks for the suggestions. Moving the plugs nearer to each other on the strip doesn't seem to make much difference (but is a handy tip to know!). It's an Olsen PDU so hopefully decent quality.

I'll have a tinker with disconnecting pin 1 and see if that helps before handing over any cash :thumbup:
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