Internal ground loop in Apollon NCMP6350 (Hypex NC502MP)

[audioBliss]

So a while back I bought an Apollon NCMP6350 (3 x hypex nc502mp modules inside) amp and I waited for a couple of months or so to get it. I was really excited when it finally arrived but right out of the gate I noticed that there was a fairly loud ground loop noise coming from the speakers even when no XLR inputs were connected. When connected to my AVR the noise is less but it's still there. I've had quite a long email conversation with Apollon but in the end they stopped replying and marked my order as completed. I basically have just not had enough energy to send it back or try and fix it since then but it is sort of bothering me. Apollon is of the opinion that the amp is AES-48 grounded and since my AVR is unbalanced and I'm using RCA to XLR cables that might be the issue. But the ground loop is even louder when no input cables are attached!

Is there any chance this issue will go away with a balanced AVR or should I try and send it back as a warranty issue? I mean the issue seems to be an internal ground loop. I've opened the amp very carefully and had a look and everything is very neat and tidy and looks like a very nicely put together amp with no signs of transport damage or anything. The amp was very well packaged when it arrived. The only thing that struck me as odd was that the input cables and output cables are zip tied together. However since I hear the issue without any inputs that should not affect anything. Is there something I could do myself here? Any connections I should check? The mains ground looks very solid. The XLR input connectors are grounded directly to the chassi I'm told but it's very hard to if they are. In either case I have the issue without any XLRs attached.

Any help is appreciated.

 

[Apollon Audio]

So a while back I bought an Apollon NCMP6350 (3 x hypex nc502mp modules inside) amp and I waited for a couple of months or so to get it. I was really excited when it finally arrived but right out of the gate I noticed that there was a fairly loud ground loop noise coming from the speakers even when no XLR inputs were connected. When connected to my AVR the noise is less but it's still there. I've had quite a long email conversation with Apollon but in the end they stopped replying and marked my order as completed. I basically have just not had enough energy to send it back or try and fix it since then but it is sort of bothering me. Apollon is of the opinion that the amp is AES-48 grounded and since my AVR is unbalanced and I'm using RCA to XLR cables that might be the issue. But the ground loop is even louder when no input cables are attached!

Is there any chance this issue will go away with a balanced AVR or should I try and send it back as a warranty issue? I mean the issue seems to be an internal ground loop. I've opened the amp very carefully and had a look and everything is very neat and tidy and looks like a very nicely put together amp with no signs of transport damage or anything. The amp was very well packaged when it arrived. The only thing that struck me as odd was that the input cables and output cables are zip tied together. However since I hear the issue without any inputs that should not affect anything. Is there something I could do myself here? Any connections I should check? The mains ground looks very solid. The XLR input connectors are grounded directly to the chassi I'm told but it's very hard to if they are. In either case I have the issue without any XLRs attached.

Any help is appreciated.

Hello,

I'm very sorry that we overlooked your last email but it somehow eded up in the spam folder. I just found it.

I really thought that this issue was fixed since I haven't seen your last email.

I would suggest that we organise a pick up of your amplifier so that we can check it in our factory. If the issue lies in the amplifier we will fix it in the same day and we will send the amplifier back to you. We sold hundreds of our NCMP multichannel amplifiers and haven't heard of such issue before. My guess is that the 16 pin amphenol audio signal input connector became loose a bit from the amplifier module due to the transport and this could cause such issue. I'm just guessing. It can also be a faulty NC502MP amplifier module.

Please let me know when is the best time for you so that we can organise the pick up with UPS.

Thank you.

Best regards

Tibor

 

[audioBliss]

Hello,

I'm very sorry that we overlooked your last email but it somehow eded up in the spam folder. I just found it.

I really thought that this issue was fixed since I haven't seen your last email.

I would suggest that we organise a pick up of your amplifier so that we can check it in our factory. If the issue lies in the amplifier we will fix it in the same day and we will send the amplifier back to you. We sold hundreds of our NCMP multichannel amplifiers and haven't heard of such issue before. My guess is that the 16 pin amphenol audio signal input connector became loose a bit from the amplifier module due to the transport and this could cause such issue. I'm just guessing. It can also be a faulty NC502MP amplifier module.

Please let me know when is the best time for you so that we can organise the pick up with UPS.

Thank you.

Best regards

Tibor

I've sent you an email.

 

[JohnYang1997]

When testing, inputs needs to be shorted instead of being left open/floating.

 

[Burning Sounds]

I have an Apollon NC8350 - disconnect a XLR lead and it will hum in that channel - that's normal, not a fault. I wonder how your RCA to XLR leads are wired? They could be the culprit here. Anyway, nice to see @Apollon Audio quick response.

 

[audioBliss]

I have an Apollon NC8350 - disconnect a XLR lead and it will hum in that channel - that's normal, not a fault. I wonder how your RCA to XLR leads are wired? They could be the culprit here. Anyway, nice to see @Apollon Audio quick response.

Thank you for your input.

Ok so I have had a pretty lengthy conversation over email with Apollon about this and I have shared my result both with text and video a while back. Not once was it mentioned that it would be normal for a channel to have a ground loop type sound with no XLR connected. I have specifically asked this question and the answer is that it was not normal. You have to specify what you mean with hum because with my nc400 monoblocks there will be a faint white noise when no XLR is connected - this I find normal(I have this connected the same way with XLR to RCA cable). With the Apollon though there is a distinct ground loop sound.

Now with all XLRs connected to the Apollon every other channel will have the buzzing sound i.e. channels 1, 3 and 5. The other three channels will be normal. So basically one channel per Hypex module will have the issue. I am not an expert on aes-48 grounding so I have no idea how this is suppose to work, all I know is that the amps in my AVR are more quiet than this. Sounds very strange that I would need an AES-48 compliant AVR connected to the amp for the amp not to output a ground loop?

Granted it could be that I need a balanced AVR for this to work then but no manufacturer is able to tell me if they are AES-48 compliant so how do I know how to even find a compatible AVR?? I mean people just don't know this kind of stuff in support.

I have no idea how my XLR->RCA cables are wired internally btw.

 

[audioBliss]

When testing, inputs needs to be shorted instead of being left open/floating.

I have asked Apollon how I should test and this has not been mentioned. Also it's above my knowledge level of electronics to attempt to start shorting things out. I have asked if it's normal to have an internal ground loop with AES-48 grounded amps with no XLRs connected but was told it was not normal. I'm actually in the market for a balanced Processor like the HTP-1 but I have no idea if it's AES-48 compliant and they can't tell me if it is or not. So I would not know if that would solve the issue. As you understand it's a lot of money with amps and processors like this so I want to take one thing at a time here and make sure what I already have is actually working properly.

 

[Burning Sounds]

Thank you for your input.

Ok so I have had a pretty lengthy conversation over email with Apollon about this and I have shared my result both with text and video a while back. Not once was it mentioned that it would be normal for a channel to have a ground loop type sound with no XLR connected. I have specifically asked this question and the answer is that it was not normal. You have to specify what you mean with hum because with my nc400 monoblocks there will be a faint white noise when no XLR is connected - this I find normal(I have this connected the same way with XLR to RCA cable). With the Apollon though there is a distinct ground loop sound.

Now with all XLRs connected to the Apollon every other channel will have the buzzing sound i.e. channels 1, 3 and 5. The other three channels will be normal. So basically one channel per Hypex module will have the issue. I am not an expert on aes-48 grounding so I have no idea how this is suppose to work, all I know is that the amps in my AVR are more quiet than this. Sounds very strange that I would need an AES-48 compliant AVR connected to the amp for the amp not to output a ground loop?

Granted it could be that I need a balanced AVR for this to work then but no manufacturer is able to tell me if they are AES-48 compliant so how do I know how to even find a compatible AVR?? I mean people just don't know this kind of stuff in support.

I have no idea how my XLR->RCA cables are wired internally btw.

The alternate channels issue is very strange - and given that Apollon have agreed to look at the amp that is obviously your best route.

Hypex have an application note primarily dealing with the XLR Pin1 issue, but it also covers AES-48 and RCA to XLR wiring. Apologies if you are already aware of it.

https://www.hypex.nl/img/upload/doc/an_wp/AN_Legacy_pin_1_problems.pdf

 

[audioBliss]

The alternate channels issue is very strange - and given that Apollon have agreed to look at the amp that is obviously your best route.

Hypex have an application note primarily dealing with the XLR Pin1 issue, but it also covers AES-48 and RCA to XLR wiring. Apologies if you are already aware of it.

https://www.hypex.nl/img/upload/doc/an_wp/AN_Legacy_pin_1_problems.pdf

Ive read the document before but to me this is only applicable if the ground loop is introduced when connecting the amp to other equipment. In my case it is present with no XLRs or trigger wire connected. If anyone could explain to me the expected behaviour in this scenario I’d be grateful.
Apollon has sent UPS to pick up the amp this week. Apollon has been helpful in trying to sort all this out. I hope they find the issue.

 

[Silver1omo]

If you have nothing connected to the input leads (RCA or XLR) they act like antennas and pick up noise. As stated by others, you want to short the input.
If no noise is present with inputs shorted then, in supper generalized terms, the amp is good.
If you have noise with a shorted input, then you have a noisy amp.

 

[Silver1omo]

You can actually google RCA shorting cap or XLR shorting strap/pin and see some examples.

 

[audioBliss]

I’m surprised that this was not mentioned when I was in contact with Apollon. Now the amp is packed up and being shipped out soon.
Doing a quick google search I can see that this is definitely a thing. However I still have the issue when XLRs connected but that could be a cable issue then? Either way feel good that Apollon will have a look at it.

Many sites that sell these shorting straps recommend to consult the manufacturer before use.

 

[Silver1omo]

I mean, the manufacturer is stepping in and doing the right thing. Let them check it and follow up.

 

[audioBliss]

For anyone that's as picky as me and having this issue I can say that Apollon has tested the amp now and what they found was quite interesting imo! I don't think Apollon would mind me sharing since then maybe they don't get these kind of questions in the future from other customers

So today we tested your amplifier. We have managed to replicate your setup and came to these conclusions.

All the channels have this high frequency hum when there is no input connected. Usually we don't test these modules this way, without any connections connected. We tried other brand new MP series modules and all Hypex MP series modules have this. We tried the same thing with NC400, NC500 and NC1200 modules and there is no hum on high frequencies when there are no input connections. Only MP series modules have this when there are no input connections present. It seems like the MP series is more sensitive when there are no connections present.

When a balanced connection is connected this hum goes away and all the channels are dead silent.

Like I've mentioned before our amps are grounded after the AES48 standard. This means that with rca connections you need to have the right cable in order to avoid ground loops and hum. If you connect the (-) from rca and shield from the cable to pin1 this could cause problems. My guess is that your rca-xlr cables have the - and shield of the cable connected directly to pin1 which is not the right way to do this. This is even mentioned by Bruno Putzeys writing in the Hypex Application note. https://www.hypex.nl/img/upload/doc/an_wp/AN_Legacy_pin_1_problems.pdf

The right rca-xlr adapter cable should look like this:

As you can see the - and shield from the cable are connected with a 100nF and 100Ohm resistor in parallel to pin1. This way there are no problems and the amp performs dead silent.

If your cables have the - and shield connected to the pin1 you can also try cutting away the wire that goes to pin1 on the xlr side. Maybe this will solve the problem without any capacitors and resistors in parallel needed.

To summarize the test of your amplifier, your amplifier performs without any problems and all the channels perform and measure great.

So basically the answer is that it is "normal" to have a high pitched hum with no XLRs connected and the same hum but lower level when "normal" XLR-RCA cables are used unless you build your own cables, the the noise should go away. Now it just so happens that I will have access to a balanced AVP now so when I get the amp back I will see how all this works out with balanced but I trust that it will work out fine But if you don't have a balanced pre-amp be ready to build cables. I was surprised by this issue as I found that my nc400 did not have this issue with an unbalanced pre-amp, I'm glad Apollon could also replicate and confirm this as well.

 

[boXem]

So basically the answer is that it is "normal" to have a high pitched hum with no XLRs connected and the same hum but lower level when "normal" XLR-RCA cables are used unless you build your own cables, the the noise should go away. Now it just so happens that I will have access to a balanced AVP now so when I get the amp back I will see how all this works out with balanced but I trust that it will work out fine But if you don't have a balanced pre-amp be ready to build cables. I was surprised by this issue as I found that my nc400 did not have this issue with an unbalanced pre-amp, I'm glad Apollon could also replicate and confirm this as well.

Properly designed RCA to XLR cable like these work perfect with the NCxxxMP. No absolute need to build your own cables. The RC network in Bruno's paper is nice but can usually be replaced by a short, as he explains.

 

[audioBliss]

Properly designed RCA to XLR cable like these work perfect with the NCxxxMP. No absolute need to build your own cables. The RC network in Bruno's paper is nice but can usually be replaced by a short, as he explains.

Good information which will be useful for people trying to run the NCxxxMP with unbalanced pre. Imo it is very hard to know how cables are built and I have two different brand cables at home (one of which were pretty expensive) and none of those worked. One of the cables is hand built by Nord Acoustics and the other is a Hosa cable(both work great with non MP Hypex amps). So if the ones in your link are confirmed working that is great!

Because I hope you have tested exactly those cables with NC502MP?

 

[boXem]

Not with nc502mp, but with nc122mp and nc252mp.
Not sure if you already mentioned this: when no input cable is wired to your amp, can you hear hum/hiss?

 

[audioBliss]

That's literally what the thread is about. Yes, when no input cables at all are connected there is a loud ground loop noise. Mind you this is not the normal white noise but a high pitched ground loop noise. Like a ground loop noise but more annoying. Apollon said is was not normal and no one on basically the whole internet has heard about this. It is the reason I sent the amplifier back to Apollon. But now after testing they found that this is indeed normal for the MP series but not for the rest of the ncore series of amps(just like I have found myself).

Now, this noise disappears for channel 2, 4 and 6 when I connect my XLR-RCA input cables but for channels 1, 3 and 5 I can still hear the same noise but more quiet. Apollon assures me after their testing that there will be no issues when using a balanced connection. So I will test this when I get the amp back but from their testing they found no issues.

 

[Panelhead]

Look up the Rane Corporation notes on cable construction. The one for your application is diagram 4. Single ended source, balanced amplifier. Diagram 6 also should work.
Building your own cables is the way to go here. More expensive than Amazon, Blue Jeans, or eBay. Cheaper than Audio companies.
All you need is a good soldering iron, cutters, solder, pocket knife, and a multi meter to check cable after building, And of course audio cable. This allows you to check if connecting the shield to the RCA is quieter than floating.
Lots of great products available. Most are Pro audio products; cheap, reliable, and effective.

 

[boXem]

That's literally what the thread is about. Yes, when no input cables at all are connected there is a loud ground loop noise. Mind you this is not the normal white noise but a high pitched ground loop noise. Like a ground loop noise but more annoying. Apollon said is was not normal and no one on basically the whole internet has heard about this. It is the reason I sent the amplifier back to Apollon. But now after testing they found that this is indeed normal for the MP series but not for the rest of the ncore series of amps(just like I have found myself).

Now, this noise disappears for channel 2, 4 and 6 when I connect my XLR-RCA input cables but for channels 1, 3 and 5 I can still hear the same noise but more quiet. Apollon assures me after their testing that there will be no issues when using a balanced connection. So I will test this when I get the amp back but from their testing they found no issues.

Sorry for not carefully reading your first post and asking stupid questions.
In fact, there was at least someone on the internet knowing about this issue, me .
The "problem" affects all NCxxxMP. The issue is that a few SMPS components are radiating 50 Hz and harmonics really close to the inputs. As you can see below, input 1 is more subject to the issue than input 2 since it's closer from these components. As soon as the impedance between the two input wires becomes high (no input cable plugged in example), the two wires become nice antennas, catch the 50 Hz and harmonics, that you can hear in your speakers.

There is a secret sauce to avoid this, and before people start masturbating with quality of cables, I use ribbon cable and reach hum levels far below audibility with no input cable plugged.