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Very weird grounding issue


kusee pee
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OK, so just bought a new ashdown rootmaster evo ii 500.  First gig with it tonight and the DI is causing a big buzz.  Indeed, having exhausted various possible sources, it still buzzes when the amp power socket is out and nothing is plugged into it except the DI cable!  Luckily, I can DI out my pedal for tonight but can the basschat experts tell me any solution other than I've got unlucky and need to box it up to send it back?

 

Hopefully the gig will take my mind off it for a couple of hours!

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I'm not sure about Ashdown, but my Mesa amp has a poorly designed DI output. Even though it's not a cheap amp by any stretch of the imagination, they really cut some corners on the DI output. It does not provide any isolation, which is one of the main reasons to have a DI box. In other words, you're probably better of with a separate DI box than with the one built into your amplifier. The sound tech can usually provide you with one, or get one yourself to be fully self-sufficient.

 

For the people who enjoy some more technical details:

For a DI you can use an audio-transformer, which is big, expensive, but provides you with galvanic isolation. In combination with a ground-lift switch you can get audio across without any buzz.

On the other hand, you can have a balanced output without a transformer. These are usually called transformer-less or electronically-balanced. This is considerably cheaper than using a transformer, but does not provide any isolation, so it may lead to all kind of buzz and problems.

On stage you do want galvanic isolation. So either make sure you amp has a transformer based DI output, or get a separate proper transformer based DI box.

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Which amps actually have a transformer-isolated DI these days though?

 

I think it was an add-on option on my old Trace combo if I remember correctly, but it wasn’t fitted.

 

I agree that true galvanic isolation plus a ground lift is the only guaranteed way of breaking a ground loop. A proper DI box should always be a fallback in case the built-in DI hums and can’t be cured with a ground lift.

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Cheers guys, a DI box is a consideration but the buzz was so loud (and present even when the amp power supply was disconnected which was weird) that it's not going to do the trick for me long term.  So I'm moving on, probably back to Markbass where I've been for the last ten years.  Strangely, not a single member of the audience commented on the fact I was DI'ing out of my pedal.  Ignorant lot 🤣

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On 12/09/2021 at 19:29, nilebodgers said:

Which amps actually have a transformer-isolated DI these days though?

 

……

Aguilar DB751 - pic of a Jensen Di box transformer on the pcb: https://www.talkbass.com/threads/aguilar-db751-di-output-too-low-question-for-owners.1404614/

 

 Nice. Very expensive component though. (Doesn’t help if the sound crew are too dim to either switch the pad off or turn the gain up on the desk though as per that thread contents.)

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