1124p...More Hum after adding XLR-to-RCA cable? - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 9 Old 12-12-11, 10:42 PM Thread Starter
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1124p...More Hum after adding XLR-to-RCA cable?

I recently just finished a DIY XLR to RCA cable for my 1124p in order to try and get rid of the cheater plug. Without the cheater plug there is a slight hum at my normal subwoofer gain of 10 o'clock. With the cheater plug its dead silent and I was hoping for something similar with the new cable. However, once I got it hooked up there was actually a lot more hum present. I double checked the cable with my meter and resistances are all at zero between the connection ends and the soldering points seem to look fine (although I am somewhat of a newbie solderer I've tutorialed and practiced pretty thoroughly).

I've included a picture of my cable in case someone notices something is up with how its constructed.

I connected XLR pin 2 to the RCA center pin, XLR 1 to RCA sleeve, XLR 3 to wire shield and no connection on RCA

I also moved all the components to the same outlet which should be grounded correctly since I just installed it not too long ago. I still have to inspect the main breaker grounds but I think they are connected to the outside grounding rod and the cold water pipe entrance.

Anyone have any ideas whats going on or a solution?
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post #2 of 9 Old 12-13-11, 10:29 AM
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Re: 1124p...More Hum after adding XLR-to-RCA cable?

I have modified cables to convert an "RCA output into a Balanced Input" as from and AVR to the 1124p. If that is what you are doing then this may help.

Solder a jumper wire between pin 3 and pin 1 on the XLR connector (the white wire pin to the shield pin). This will short out the negative side of the balanced input. Likely it is picking up hum because the negative side is floating with that long white wire "antenna" attached and thus picking up hum from stray fields?

I wired mine this way originally to avoid that possibility and have had no issues with hum. [Actually, I also cut the white wire off pin 3 just to be sure, but I donít think it should really matter if it is connected or not once the jumper is in place.] There are other sources of hum however so there is no guarantee that this will solve all hum issues. My electrical skills are limited, but I think this is likely to help you get back to the same hum level that the adaptor setup provided. Itís easy to try, as a soldering iron is your friend, right?

Let me know if it works for you.
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post #3 of 9 Old 12-13-11, 10:37 AM
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Re: 1124p...More Hum after adding XLR-to-RCA cable?


Hey John (xfiddle),

With unbalanced (RCA) connections, the signal (-) rides on the sleeve. You sent that signal (the white lead) from the RCA sleeve to the XLR’s pin 1, which is the shield (ground) connection for a balanced input. With the XLR, pin 3 is the signal (-), and you don’t have pin 3 connected to anything on the RCA end. On top of that, you have lots of loose strands from the shield floating around, which might be grounding out on the connector's barrel when you put it on (hopefully you're using the protective paper or plastic sleeve to prevent that?).

Here’s what to do: On the RCA end, tie the shield and white together, and solder it to the sleeve. On the XLR end, connect the shield to pin 1, and the white to pin 3. If that doesn’t stop the hum, try floating the shield connection on the XLR end (i.e., disconnect it from pin 1).

By the way, I have a tutorial on DIY cable making that you can find by clicking the Tech Articles link in my signature.

Regards,
Wayne



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post #4 of 9 Old 12-13-11, 11:07 AM Thread Starter
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Re: 1124p...More Hum after adding XLR-to-RCA cable?

Thanks for the assistance guys. I have a much clearer picture now of what went wrong and how to fix it.

BTW, thanks Wayne for that great cabling tutorial! I had only watched a few basic soldering tutorials beforehand and tinned a few wires prior to tackling this cable (guess not as thoroughly as I imagined ). For my first soldering project I think it was OK but after browsing the link you sent me I think I can definitely improve the cleanliness even more.

Time to cut the wire, desolder, and try again.
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post #5 of 9 Old 12-13-11, 05:36 PM Thread Starter
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Re: 1124p...More Hum after adding XLR-to-RCA cable?

Well here is my second attempt at soldering an actual cable.




Plugged it in and the sub is just as quiet as the BFD with cheater plug. Thanks! I'd also be gracious for comments regarding any flaws you see with my second soldering attempt here.

Last edited by xfiddle; 12-13-11 at 06:48 PM.
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post #6 of 9 Old 12-13-11, 07:15 PM
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Re: 1124p...More Hum after adding XLR-to-RCA cable?

Looks Good ( much better ) !

One nagging question ;

Are you sure this cable still passes audio ?

Your pic seems to show a metal tab running between pin 2 & pin 3 ( if so , that'll surely silence things,,,everything ).

Hopefully it's just the bad angle of the pic and I'm seeing things that aren't really there .

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post #7 of 9 Old 12-13-11, 10:42 PM
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Re: 1124p...More Hum after adding XLR-to-RCA cable?


Quote:
xfiddle wrote: View Post
Well here is my second attempt at soldering an actual cable.
I'd also be gracious for comments regarding any flaws you see with my second soldering attempt here.
Wow John, thatís 1000% better. Looks as good as anything I would have done! Congrats!


Quote:
EarlK wrote: View Post
Are you sure this cable still passes audio ?

Your pic seems to show a metal tab running between pin 2 & pin 3 ( if so , that'll surely silence things,,,everything ).
Earl, Iím sure itís just the chassis ground tab on the other side of the connector (180ļ from Pin 3).

Regards,
Wayne



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post #8 of 9 Old 12-14-11, 11:56 AM
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Re: 1124p...More Hum after adding XLR-to-RCA cable?

Quote:
wayne wrote:
Earl, Iím sure itís just the chassis ground tab on the other side of the connector (180ļ from Pin 3).
Ahhh, I'm sure your right there Wayne .

Just consider my post a caution to others ( against inadvertently tying pins 2 & 1 together ) & a corollary to your mention of tying pins 1 & 3 together .

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post #9 of 9 Old 01-09-12, 04:21 PM
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Hi Guys,
I thought I would throw this in here for your consumption considering the application. Here is a link to Rane's RaneNote #110 Sound System Interconnection.

http://www.rane.com/note110.html

It is a very good paper to keep handy. Cheers!

"Any company that cannot test and prove it's designs on proper test equipment will never get my hard earned Aureus." ~ Maximus Decimus Meridius
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