subwoofer hum - RCA in

jsmithe

Well-Known Member
Joined
Nov 5, 2012
Messages
640
Points
63
Location
Bangalore
I notice a hum when I connect the RCA inputs to my sub. No hum when I use the balanced inputs. What is weird is that the hum is present even when the other end of the RCA cable is not connected to anything. Wondering what could be causing this. Any ideas?
 

DB1989

Well-Known Member
Joined
Apr 6, 2020
Messages
1,646
Points
113
Location
New Delhi
I notice a hum when I connect the RCA inputs to my sub. No hum when I use the balanced inputs. What is weird is that the hum is present even when the other end of the RCA cable is not connected to anything. Wondering what could be causing this. Any ideas?
What about without any rca connected? Or a different rca cable? Does it still produce a hum in the aforementioned two situations?
 

jsmithe

Well-Known Member
Joined
Nov 5, 2012
Messages
640
Points
63
Location
Bangalore
What about without any rca connected? Or a different rca cable? Does it still produce a hum in the aforementioned two situations?

Thanks for the quick reply.

Without any RCA connected to the sub inputs there isn't any hum at all. Just tried again with a new RCA cable to the sub input but without anything on the other end and there still is a hum.

Finally tried with the new RCA cable to the sub input with the source also connected and the hum is gone. Wonder what's going on.

I've been using the balanced inputs to the sub and did not experience the hum until now when I needed to use the RCA inputs.
 

DB1989

Well-Known Member
Joined
Apr 6, 2020
Messages
1,646
Points
113
Location
New Delhi
Thanks for the quick reply.

Without any RCA connected to the sub inputs there isn't any hum at all. Just tried again with a new RCA cable to the sub input but without anything on the other end and there still is a hum.

Finally tried with the new RCA cable to the sub input with the source also connected and the hum is gone. Wonder what's going on.

I've been using the balanced inputs to the sub and did not experience the hum until now when I needed to use the RCA inputs.
“Source also connected” - Could you clarify what you mean’t by this?

Also, i think it could be an issue with grounding.
 

jsmithe

Well-Known Member
Joined
Nov 5, 2012
Messages
640
Points
63
Location
Bangalore
“Source also connected” - Could you clarify what you mean’t by this?

The source is a DAC via RCA out.

My normal chain is PC Audio interface ==> active sub ==> active speakers with TRS from the interface going to balanced on the sub and then balanced back to the active speakers. Working fine for ages. No hum, nothing.

Replaced the Audio interface with a DAC that only has RCA outs and the hum started.

Also, i think it could be an issue with grounding.

Not sure how to rule that out, but the DAC and the sub are on the same wall socket set.

I first noticed a hum when the DAC and the sub were on different wall socket sets. Migrated the DAC to the same power socket set as the sub to eliminate that possibility and then noticed that I was getting a hum with the sub on and the RCA plugged in but without being connected to the DAC. Took a chance, connected the other end of the RCA to the DAC and hum gone even without powering on the DAC. Powered on DAC and no hum, so good for now, but the mystery remains as to the reason for the hum.
 
Last edited:

DB1989

Well-Known Member
Joined
Apr 6, 2020
Messages
1,646
Points
113
Location
New Delhi
The source is a DAC via RCA out.

My normal chain is PC Audio interface ==> active sub ==> active speakers with TRS from the interface going to balanced on the sub and then balanced back to the active speakers. Working fine for ages. No hum, nothing.

Replaced the Audio interface with a DAC that only has RCA outs and the hum started.



Not sure how to rule that out, but the DAC and the sub are on the same wall socket set.

I first noticed a hum when the DAC and the sub were on different wall socket sets. Migrated the DAC to the same power socket set as the sub to eliminate that possibility and then noticed that I was getting a hum with the sub on and the RCA plugged in but without being connected to the DAC. Took a chance, connected the other end of the RCA to the DAC and hum gone even without powering on the DAC. Powered on DAC and no hum, so good for now, but the mystery remains as to the reason for the hum.
I know what you’re talking about. I face the same issue with my REL T9i when i sm powering the front LR channels through an amplifier connected to my preouts of the AVR whereas my high level connection of the sub runs from the leads of the AVR. connecting the leads from the avr to the speakers resolves the hum, even when i don’t have them powered on. Theres a constant hum if the leads are hanging free and i have to switch off my sub manually cuz that hum doesn’t allow it to go into standby.
 

Kannan

Well-Known Member
Joined
Jan 13, 2008
Messages
2,197
Points
113
Location
Chennai
@jsmithe is the power plug 3-pin or 2-pin. Id 3 pin, insulate the earth pin and try.
In all probability, the hum should go.
Since most plugs are molded, you can tightly fit a cello or insulation tape over thr earth pin on the plug side. If it solves the problem, open the wall board and disconnect the earth pin for this socket alone.

Regards
 

jsmithe

Well-Known Member
Joined
Nov 5, 2012
Messages
640
Points
63
Location
Bangalore
@jsmithe is the power plug 3-pin or 2-pin. Id 3 pin, insulate the earth pin and try.
In all probability, the hum should go.
Since most plugs are molded, you can tightly fit a cello or insulation tape over thr earth pin on the plug side. If it solves the problem, open the wall board and disconnect the earth pin for this socket alone.

Regards

The sub is powered by a 3 pin plug, via a Schuko adapter though, so double-checking whether the Schuko earth connector is properly contacted is something to look at.

insulate the earth pin and try.
Reading again, more carefully I see that you are suggesting that I block out the earth on the 3 pin plug and try. Will give that a shot too.
 

jsmithe

Well-Known Member
Joined
Nov 5, 2012
Messages
640
Points
63
Location
Bangalore
I know what you’re talking about. I face the same issue with my REL T9i when i sm powering the front LR channels through an amplifier connected to my preouts of the AVR whereas my high level connection of the sub runs from the leads of the AVR. connecting the leads from the avr to the speakers resolves the hum, even when i don’t have them powered on. Theres a constant hum if the leads are hanging free and i have to switch off my sub manually cuz that hum doesn’t allow it to go into standby.

Ahh, same ting here. I wonder if some signal is looping around somewhere and connecting equipment to the RCA earths it somehow/somewhere. These things need some careful debugging. Between debugging and listening, guess what I take over the short term :)
 

DB1989

Well-Known Member
Joined
Apr 6, 2020
Messages
1,646
Points
113
Location
New Delhi
Guys,
Don't block out the earth pin.
Subs have plate amps and grounding is a safe thing to have.
Just in case ...

Cheers,
Raghu
When the avr is on, it provides the grounding through the rca cable connected from the lfe out of the avr to the lfe in of the Sub. Thats what Rel also suggests if one gets a hum which is why I identified it as a grounding problem. However, this does not work when the avr is switched on which is why i switch off the sub when the avr is not in use.
 

raghupb

Well-Known Member
Joined
Sep 11, 2015
Messages
3,317
Points
113
Location
Bengaluru
When the avr is on, it provides the grounding through the rca cable connected from the lfe out of the avr to the lfe in of the Sub. Thats what Rel also suggests if one gets a hum which is why I identified it as a grounding problem. However, this does not work when the avr is switched on which is why i switch off the sub when the avr is not in use.
I agree that these methods can be used to figure out the ground loop.
Then work on ways to eliminate them; same wall socket, different wall socket, etc.
But from a safety point of view, the earthing connector should be engaged.
These plate amps can potentially carry lethal currents.
In case of a snafu, one wants a short path through the 3-pin earth as compared to ones body.

Cheers,
Raghu
 

jsmithe

Well-Known Member
Joined
Nov 5, 2012
Messages
640
Points
63
Location
Bangalore
Thanks gents.

The earth prongs on the Schuko adapter weren't contacting the earth strips on the Schuko plug. Damn, I've been without earth on those actives all these days :eek: Lucky I guess. I ought to have checked as soon as I bought those MX Schuko adapters.

A word of caution, if using Schuko adapters, please check to ensure the earth prongs on the adapter make contact with the strips on the plug. If they don't, just bend the prongs on the adapter inward a bit so that they make contact. A multimeter in continuity testing mode comes in handy.
 
Purchase the Audiolab 6000A Integrated Amplifier at a special offer price.
Top