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Where does the hum come from?



 
 
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  #31  
Old February 11th 21, 09:30 PM posted to rec.audio.pro
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Posts: 105
Default Where does the hum come from?

John Williamson wrote:
===================
>
> He claims that with an open circuit, he gets the normal white noise we
> all expect, but when he connects the speaker input to the output on the
> converter, he gets an audible hum. There is a pair of domestic standard
> (And quality) RCA phono connections on the converter, so the only thing
> I can suggest is that the shields on the two phono leads are making an
> earth loop connecting the two speaker inputs and the two phono sockets,
> which share an earth on the adaptor circuit board. The speakers will be
> sharing the safety earth of the whole system with each other. Going from
> a single ended output to a balanced input with no isolating transformer
> won't be helping.


** That is the likely scenario.

I did not expect balanced input, JBL self powered monitors with SMPSs to be other than class 2.

https://jblpro.com/en-US/site_elements/lsr305-back

**** poor design if this is true - cos it wont matter how you connect them.


...... Phil


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  #32  
Old February 11th 21, 10:15 PM posted to rec.audio.pro
geoff
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Posts: 1,771
Default Where does the hum come from?

On 12/02/2021 2:02 am, Scott Dorsey wrote:
> geoff > wrote:
>>
>> Look at the picture in the OP link. RCA unbal outputs.

>
> I have no graphics here. So RCA on one end, what is on the other end?


No graphics ? This *is* the 21st Century ! Or are you working on one of
these ? ;- )

https://www.facebook.com/photo/?fbid...57424488371333

(for the insecure the link is to a picture on Facebook of a computer).

>
> If the RCA connector is run to pins 2 and 3 of an XLR on the other end
> with pin 1 lifted (as is a common and reasonable configuration), then
> the case of the powered speaker won't be connected to the case of the
> DAC until power is applied. So in that case the thing would hum in the
> configuration the original poster describes.
>
> But without knowing what the cable is, you don't know for sure.
> --scott
>

Well can be sure it isn't a twin untwisted heavy-gauge speaker cable !

The LSR305 has bal (and equally unbal) jack and XLR inputs. At least
mine have, and no hum even when connected via several intermediate
devices to a PC, a mixture of bal and unbal.

Would help to know the 'quality' and make-up/configuration of the
RCA-to-whatever cable.

geoff
  #33  
Old February 11th 21, 10:21 PM posted to rec.audio.pro
geoff
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Posts: 1,771
Default Where does the hum come from?

On 12/02/2021 5:05 am, Unsteadyken wrote:
> In article >,
>
> John Williamson says...
>
>> Going from
>> a single ended output to a balanced input with no isolating transformer
>> won't be helping.
>>

> I suspect the OPs "speaker wire" is probably an RCA cable with a mono 2
> pole TRS adapter on the end.
> What connection that is making within the 3 pole TRS input is anybodies
> guess.
>
>
>



Presumably pin 1 screen, pin 2 hot, and pin 3 bonded to pin 1.

Maybe the XLR shell also bonded to pin 1.

geoff
  #34  
Old February 11th 21, 10:26 PM posted to rec.audio.pro
geoff
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Posts: 1,771
Default Where does the hum come from?

On 12/02/2021 10:15 am, geoff wrote:
> On 12/02/2021 2:02 am, Scott Dorsey wrote:
>> geoff¬* > wrote:
>>>
>>> Look at the picture in the OP link. RCA unbal outputs.

>>
>> I have no graphics here.¬* So RCA on one end, what is on the other end?

>
> No graphics ?¬* This *is* the 21st Century ! Or are you working on one of
> these ?¬* ;- )
>
> https://www.facebook.com/photo/?fbid...57424488371333
>
>
> (for the insecure the link is to a picture on Facebook of a computer).
>
>>
>> If the RCA connector is run to pins 2 and 3 of an XLR on the other end
>> with pin 1 lifted (as is a common and reasonable configuration), then
>> the case of the powered speaker won't be connected to the case of the
>> DAC until power is applied.¬* So in that case the thing would hum in the
>> configuration the original poster describes.
>>
>> But without knowing what the cable is, you don't know for sure.
>> --scott
>>

> Well can be sure it isn't a twin untwisted heavy-gauge speaker cable !
>
> The LSR305 has bal (and equally unbal) jack and XLR inputs. At least
> mine have, and no hum even when connected via several intermediate
> devices to a PC, a mixture of bal and unbal.
>
> Would help to know the 'quality' and make-up/configuration of the
> RCA-to-whatever cable.
>
> geoff


'Slight' correction, mine are LSR705P.

geoff
 




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