Connecting EP2500 - humming problem

[mvc]

I recently bought a used JL12W7 car subwoofer with JL audio enclosure (HO-112R-W7) from craigslist to replace my current JBL subwoofer used for my home theater system. Based on many posts (especially from WmAx), I bought a Behringer EP2500 to drive this subwoofer. To connect my Denon 3805 sub pre-out to the amp’s balanced input, I used Neutrik NA2MPMF MXLR-FRCA connector. However, when I plug the FRCA end of the Neutrik into the RCA cable coming out of Denon's sub pre-out, it creates humming. Please note that the XLR end of the Neutrik is not even plugged into the amp yet. The humming comes through rest of the 5 speakers that are connected to the Denon. As soon as I unplug the Neturik from the subwoofer cable, the humming is gone. I am looking for suggestion to connect the RCA cable to the XLR of the Behringer without any humming issue.
Thanks.

 

[jvgillow]

I think your problem is that pin 3 on the Neutrik adapter is connected to ground. All the recommendations I've seen on avsforum say pin 2 should be connected to signal and pin 1 should be connected to ground. Never connect pin 3 to ground directly.

If you can "open up" that Neutrik connector you can snip the connection to pin 3.

 

[highfigh]

I recently bought a used JL12W7 car subwoofer with JL audio enclosure (HO-112R-W7) from craigslist to replace my current JBL subwoofer used for my home theater system. Based on many posts (especially from WmAx), I bought a Behringer EP2500 to drive this subwoofer. To connect my Denon 3805 sub pre-out to the amp’s balanced input, I used Neutrik NA2MPMF MXLR-FRCA connector. However, when I plug the FRCA end of the Neutrik into the RCA cable coming out of Denon's sub pre-out, it creates humming. Please note that the XLR end of the Neutrik is not even plugged into the amp yet. The humming comes through rest of the 5 speakers that are connected to the Denon. As soon as I unplug the Neturik from the subwoofer cable, the humming is gone. I am looking for suggestion to connect the RCA cable to the XLR of the Behringer without any humming issue.
Thanks.

RCA isn't balanced. The closest it can come is "quasi-balanced", by using a twisted pair to carry the audio and a shield that's grounded on one end. XLR uses pin 1 as ground and RCA uses the shield, which corresponds to that. This is why pin 3 should be lifted, as jvgillow mentioned.

 

[WmAx]

That enclosure is tuned for car audio use. It may not be appropriate for home use - and result in insufficient deep LF and boosted upper bass response.

As for the humming problem, try the solutions posted here, and if it's still a problem, you may need to get a ground loop isolator such as the ART DTI box. You would plug RCAs into the DTI on the input, and the DTI has balanced XLR outputs to feed to the amp. This assumes your pre-outs from the receiver have sufficient voltage to drive the amp, of course.

-Chris

 

[TLS Guy]

RCA isn't balanced. The closest it can come is "quasi-balanced", by using a twisted pair to carry the audio and a shield that's grounded on one end. XLR uses pin 1 as ground and RCA uses the shield, which corresponds to that. This is why pin 3 should be lifted, as jvgillow mentioned.

Unfortunately none of the above is correct. You have to construct what is called a floating line.

You can either connect three and one to ground at the RCA or the XLR. Two connects to the hot pin of the RCA.

Here is a good illustration.

 

[Haoleb]

frankly I dont think any of the above is correct when corresponding to your particular problem. You say you are getting this hum with only the adapter plugged in and nothing else. Either your rca outputs chassis and signal ground are different and causing you problems (unlikely I would suspect) or perhaps one of the adapters is wired wrong and is shorting your preamp outputs to ground or something. Otherwise i do not see how plugging in just the adapter will make your reciever hum through all channels as electrically, it would be just the same as if it wasnt plugged in at all.

I know the Neutrik adapter like yours that I bought came unassembled. If you put it together yourself go back and check your wires because of their ridiculous design you must screw the assembly together once its all wired which in turn twists all the wires up and if you dont use small gauge and long enough length you'll have problems. If you didnt have to assemble it. You probably should make sure its actually assembled.

 

[highfigh]

Unfortunately none of the above is correct. You have to construct what is called a floating line.

You can either connect three and one to ground at the RCA or the XLR. Two connects to the hot pin of the RCA.

Here is a good illustration.

How is "The closest it can come is "quasi-balanced", by using a twisted pair to carry the audio and a shield that's grounded on one end." different from your comment that "You can either connect three and one to ground at the RCA or the XLR. Two connects to the hot pin of the RCA."? It's still not balanced. Pin 1 is XLR ground, Pin 2 is Signal, non-inverted and Pin 3 is Signal, inverted. Just connecting a balanced audio feed to an RCA jack is NOT the best way to do this- using an impedance matching device that addresses the three to two conductors, is.

 

[highfigh]

I recently bought a used JL12W7 car subwoofer with JL audio enclosure (HO-112R-W7) from craigslist to replace my current JBL subwoofer used for my home theater system. Based on many posts (especially from WmAx), I bought a Behringer EP2500 to drive this subwoofer. To connect my Denon 3805 sub pre-out to the amp’s balanced input, I used Neutrik NA2MPMF MXLR-FRCA connector. However, when I plug the FRCA end of the Neutrik into the RCA cable coming out of Denon's sub pre-out, it creates humming. Please note that the XLR end of the Neutrik is not even plugged into the amp yet. The humming comes through rest of the 5 speakers that are connected to the Denon. As soon as I unplug the Neturik from the subwoofer cable, the humming is gone. I am looking for suggestion to connect the RCA cable to the XLR of the Behringer without any humming issue.
Thanks.

If the Behringer has onlybalanced inputs, look on the back for a ground lift switch. Assuming the adapter is assembled correctly, this switch may eliminate the hum. Check the adapter first, though.

 

[TLS Guy]

How is "The closest it can come is "quasi-balanced", by using a twisted pair to carry the audio and a shield that's grounded on one end." different from your comment that "You can either connect three and one to ground at the RCA or the XLR. Two connects to the hot pin of the RCA."? It's still not balanced. Pin 1 is XLR ground, Pin 2 is Signal, non-inverted and Pin 3 is Signal, inverted. Just connecting a balanced audio feed to an RCA jack is NOT the best way to do this- using an impedance matching device that addresses the three to two conductors, is.

You can't balance the connection, unless both ends are balanced. The only way to balance it is with a transformer. Otherwise you have to float. I have done this frequently over the years and I know how you have to do this.

I don't know how his connector is connected internally, but if the OP uses a meter he can find out.

If it is connected as per the diagram in my link, it should not hum when he plugs it into the amp.

If it is connected correctly but still hums, he will have to balance with a transformer at his LFE out. That should not be necessary.

 

[TLS Guy]

I think your problem is that pin 3 on the Neutrik adapter is connected to ground. All the recommendations I've seen on avsforum say pin 2 should be connected to signal and pin 1 should be connected to ground. Never connect pin 3 to ground directly.

If you can "open up" that Neutrik connector you can snip the connection to pin 3.

I have mentioned that this is completely wrong advice. If you do it you will only get half the wave form!

The signal is between pins two and three with no potential between ground in a balanced system. Pin two has the positive part of the deflection and pin three the negative portion. When floating if you don't ground pin three, it is just the same as feeding the signal though a diode.

The above advice needs correcting fast.

 

[mvc]

Thanks everyone for all the help. I will get the ART DTI box and will report the result back. Also thanks for pointing out that the car optimized enclosure may not be appropriate for home audio. May be after few months, I will try to make a sealed enclosure following the specifications given in JL Audio's website.

 

[OttoMatic]

I have mentioned that this is completely wrong advice. If you do it you will only get half the wave form!

The signal is between pins two and three with no potential between ground in a balanced system. Pin two has the positive part of the deflection and pin three the negative portion. When floating if you don't ground pin three, it is just the same as feeding the signal though a diode.

The above advice needs correcting fast.

Yeah, I think you do have to ground pin 3. It has to be connected to the shield of the cable that attaches to the case of the RCA plug. The differential is between pins 2 and 3 in a balanced system, so there needs to be a reference. If pin 3 is floating, there is no reference, and it will be screwed up.

Now, I'm still not sure how all this affects your humming... If the RCA cable you connect to the sub output causes hum with nothing else attached at the other end (the XLR end), then I would guess that there's a problem in the way things are connected inside the connectors (and it's probably not about grounding pin 2 or 3 or whatever). I would suspect that you have shorted the center pin of the RCA connector to ground somewhere...

Good luck!

 

[jvgillow]

In a properly balanced system with fully differential output, pins 2 and 3 have inverted signals. Each one does not carry half-waveform like you implied. In a quasi-balanced system one carries the signal and the other is connected to ground through a matched impedance, there is still no half-wave effect.

Either one of the signals compared to ground with give you normal unbalanced output. In this case he only has one signal line coming in on the unbalanced signal coming in on the RCA anyway. I'm not sure why you think the XLR converter will somehow act as a diode if you leave pin 3 floating.

http://www.tvtechnology.com/article/12238

A last resort for interconnecting balanced and unbalanced equipment is the use of adapter cables. The problem with adapters is that these provide neither level nor impedance transformation.

Pre-wired adapters or cables can be purchased. Check the wiring, especially the shield. The shield can be connected in a few ways--the sleeve of the unbalanced connector to pin 1 of an XLR (or the sleeve of a TRS connector); the sleeve to pin 3 of the XLR (or ring of the TRS); the sleeve to both pins 1 and 3 of an XLR (or ring and sleeve of a TRS connector). On some equipment, one type may work better than another.

The shield does not have to be connected to both pin 1 and 3.

 

[TLS Guy]

In a properly balanced system with fully differential output, pins 2 and 3 have inverted signals. Each one does not carry half-waveform like you implied. In a quasi-balanced system one carries the signal and the other is connected to ground through a matched impedance, there is still no half-wave effect.

Either one of the signals compared to ground with give you normal unbalanced output. In this case he only has one signal line coming in on the unbalanced signal coming in on the RCA anyway. I'm not sure why you think the XLR converter will somehow act as a diode if you leave pin 3 floating.

http://www.tvtechnology.com/article/12238


The shield does not have to be connected to both pin 1 and 3.

Balanced circuits were developed in the age before opamp chips. There was a transformer at each end. Pins two and three were connected across the secondary of the outputting device and the primary of the inputting device. So if you did not ground pin 3 you had no sound at all. If you grounded pin two, then phase was reversed.

Transformers are still around, and quite a few balanced inputs and outputs use them. Now the usual practice is to balance using dual op amp chips, for input and output. Pin 2 goes to one half and three to the other. So if you leave a line not connected, you get no signal from one half of the op amp chip, with quote a few circuits, and actually the best ones.

Balanced circuits were developed in the broadcast and engineering industries, many, many years ago. The reason was noise prevention, especially RF penetration. The long cables, will pick up identical interference. So the interference arriving at the primary connected to pins 2 and 3 cancel in the primary of the transformer.

If the circuit in a modern piece of equipment is a true replica of the transformer balanced circuit, there will be problem if pin three is not grounded. I would always connect pin three to ground, when going from balanced to unbalanced, or better yet, balance the unbalanced output with a transformer. Radio Shack sell very high quality transformer adapters for doing just that.

 

[Votrax]

As mentioned in several posts here's an example of balanced and unbalanced connections.

 

[TLS Guy]

As mentioned in several posts here's an example of balanced and unbalanced connections.
View attachment 6638

That is the correct way to do it, just like the link I posted.

 

[highfigh]

Balanced circuits were developed in the age before opamp chips. There was a transformer at each end. Pins two and three were connected across the secondary of the outputting device and the primary of the inputting device. So if you did not ground pin 3 you had no sound at all. If you grounded pin two, then phase was reversed.

Transformers are still around, and quite a few balanced inputs and outputs use them. Now the usual practice is to balance using dual op amp chips, for input and output. Pin 2 goes to one half and three to the other. So if you leave a line not connected, you get no signal from one half of the op amp chip, with quote a few circuits, and actually the best ones.

Balanced circuits were developed in the broadcast and engineering industries, many, many years ago. The reason was noise prevention, especially RF penetration. The long cables, will pick up identical interference. So the interference arriving at the primary connected to pins 2 and 3 cancel in the primary of the transformer.

If the circuit in a modern piece of equipment is a true replica of the transformer balanced circuit, there will be problem if pin three is not grounded. I would always connect pin three to ground, when going from balanced to unbalanced, or better yet, balance the unbalanced output with a transformer. Radio Shack sell very high quality transformer adapters for doing just that.

I would edit my post about lifting Pin 3 if it was possible, but it is true that Pin 3 needs to be connected to the RCA shield.

Even without op-amps, they were still able to use one half of a tube as a phase inverter.

 

[TLS Guy]

I would edit my post about lifting Pin 3 if it was possible, but it is true that Pin 3 needs to be connected to the RCA shield.

Even without op-amps, they were still able to use one half of a tube as a phase inverter.

Any circuits from that era that I was involved with used transformers. For one thing you had to balance the impedance, as tubes are very high input impedance, and balanced lines are 600 ohm. No transformer will output anything unless both ends of both coils are connected to something. I don't think there is any confusion in this post now.

 

[WmAx]

Thanks everyone for all the help. I will get the ART DTI box and will report the result back. Also thanks for pointing out that the car optimized enclosure may not be appropriate for home audio. May be after few months, I will try to make a sealed enclosure following the specifications given in JL Audio's website.

The DTI is a transformer balancing device. It is passive, and does not add any gain. Just make sure your pre-amp outs have enough gain to get the subwoofer amplifier to driver the woofers properly without having to turn the gain knobs on the amps up so high that it causes noise. Do note the DTI has a slight bass attenuation. It has -2dB at 30hz, and about -3dB at 20Hz, based on the sample that I measured. This is a typical problem it appears in these devices. The active devices, like the Art Cleanbox, have even more attenuation. The passive box has only slight attenuation and if anything, that slight attenuation slope will make the bass sound a little bit 'tighter' subjectively.

BTW, you really don't need 2 of the EP2500s for this application unless you simply want to have maximum SPL potential. The Ep2500 outputs a clean 650 x 2 into 4 Ohms. This is sufficient. BTW, for a touch more, you could buy a Yamaha P7000S instead of 2x EP2500s. The P7000S outputs almost 1,000 watts x 2, both channels driven, 20hz-20000Hz. This is plenty of power for the Kappa Perfect units. It's fan will not activate in home use. You will need to swap the Ep2500 fans to low noise fans.

-Chris

 

[TLS Guy]

This transformer from Radio Shack also works very well.

You will of course need an RCA to 1/4 inch phone jack adapter also.

As stated before, floating the line as per the diagrams should be satisfactory.

If you use a transformer, put it as close to your receiver as possible. Use good quality balanced microphone cable to go from XLR to XLR.

One of the nice things about balanced cable runs, is that they can be extremely long without any signal degradation, hundreds of feet in fact.