[LAU] Hum pickup in DI boxes

Rick Green rtg at aapsc.com
Tue Jun 8 14:43:55 CEST 2021

Headphone outputs are a strange beast.  Unbalanced, for sure (a single signal 
lead for each channel, with a common ground return), but much lower impedance 
than you would expect for an unbalanced line out.
   I've struggled as well, trying to find a consistently reliable DI for this 
application.  There are several on the market which specifically address this 
issue. Search for 'laptop interface' or 'A/V adapter' to find some of the ones 
I've tried.
   I've also resorted on occasion to abandoning the headphone jack altogether, 
and using an external USB soundcard/interface instead, which provides proper 
balanced outputs.

Rick Green

On Tue, 8 Jun 2021, Brandon Hale wrote:

> Hello,
> Have you tried plugging in the laptop without the DI in between? I think 
> laptop outputs are balanced, low-impedance, so you shouldn't need the DI in 
> between. Even if this is not true, it's at least worth a shot.
> Brandon Hale
> On 6/8/21 7:43 AM, Bill Purvis wrote:
>> Not strictly a Linux problem, but I hope for advice.
>> We're using a Behringer X32-Rack mixer for our church PA system. One of the 
>> inputs that we need is
>> for people to plug in laptops - usually via the headphone socket into a 
>> Stagg dual DI box. We seem
>> to get a lot of hum pickup on that, which I think is down to the proximity 
>> to the laptops. We have
>> another identical DI box at the other end of the room which takes keyboard 
>> and bass and I've had no
>> problems with that. Is there an alternative method we can try? We do raise 
>> the ground lift switch on
>> the DI box, which reduces the hum somewhat but still get enough hum to be 
>> noticeable when nothing
>> else is playing. I wondered if the matching transformers in the DI box are 
>> acting as pickups for the
>> RF noise generated by the laptops. The mixer on EQ display with RTA shows 
>> noise across the audible
>> spectrum, but most of the audible sound is mains hum.
>> Bill
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Rick Green

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