2. amps-preamps
  3. 166076-3592

hum when hooking up sub to preamp

I have a Jolida Fusion Preamp and it's kind of noisy. It does have a musical full bodied sound which I like. I am hooked up to a Parasound A21 which has gain knobs. I have the gain knobs dialed back about a 3rd and this eliminates the hum to where you can only hear it from several inches away. When I hook up a sub from the XLR on the Jolida now there is hum coming from the main speakers again. I guess my question would be is there some kind of on way transmission xlr or rca cable where the noise would not travel back into the preamp? Would this even help. Any other suggestions to get rid of the hum here? 

Thanks
mofojo
oldhvymec
5,101 posts
 
Have you swapped XLRs? 

Have you turned down the gain on the Parasound, all the way. I read in one thread, with the gain turned UP it was causing an issue. When turning it all the way down, it cured the problem.. I think it was a pop issue or might have been noise...

Do you have the gain set real HIGH on the sub plate?

Is there a switch to flip and it's in the wrong position for RCA, XLR?

Do you have a single ended RCA to try? Makes me wonder if the XLRs are truly balanced and have hot/cold noise canceling.

Regards


pragmasi
212 posts
 
It sounds like you're describing a ground loop but it shouldn't be possible if the xlr is implemented correctly in both the pre and the sub... however it might be worth seeing if the hum is still there with everything plugged into the same outlet (if they're not already). It is entirely possible that a ground loop between the sub and pre would be audible in the main speakers.



russ69's avatar
russ69
3,543 posts
 
They have ground breaking rca cables. For a sub application it might work OK. At any rate you will learn where the ground loop is.


EDIT: Will your sub accept speaker level inputs? 
mofojo OP
2,307 posts
 
I think I may have tried regular RCA but can't remember for sure so I will try that again. All the outputs from the Jolida Pre are on all the time, no switch. 
Hum does not change in volume from the gain setting on the sub.
If I turn the gain all the way down on the Parasound there is nothing when the sub hooked up coming out of the main speakers. 
I have 2 dedicated power lines in the system and the sub plate is on the other line. I'll try and put them on the same line and see what that does. 
mofojo OP
2,307 posts
 
Sub will not accept speaker level inputs. It's actually a Paradigm Servo 15 I blew up years ago. Using a Crown XLS 1500 to power the woofer directly. Also have a JL F113 I need to get fixed. I think an input cap is bad. 
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