High pitched noise from Hypex NC502MP based amps. Please help!

[chucky7]

I am helping a friend out.

His 2 Hypex NC502MP based amps, a VTV 4x500W amp and a Monolith M8250x amp, once powered on for around 3 minutes, will start making his surround speakers (JTR Noesis 110HTs) produce high pitched noise, even when they are not connected to the HT processor. His other amps, a 1 ch Hypex NC500MP based amp, and 2 other class A/D amps, do not. The aforementioned Hypex NC502MP based amps do not produce high pitched noise on his other speakers (JTR Noesis 215RMs, JBLpro AWC82s, and Klipsch speakers RP-600Ms).

The following sound clips are recorded at the MLP:
Once a Hypex NC502MP based amp powers on (w/o input):

audioclip-1676651039000-22649.mp4

drive.google.com

After 3 minutes:

audioclip-1676651072000-20343.mp4

drive.google.com

He lives in Asia and the receptacles in the HT are not grounded (very common in Asia). He already tried the cheater plug. That didn't work.

Any suggestions?

 

[restorer-john]

There’s possibly an issue with the surround speakers cable run. It’s likely to be long and perhaps adjacent to some form of coupling/interference.

 

[chucky7]

That was one of my suspicion as well. However, he hooked up the surround speakers to the front speaker wires (powered by the 2 Hypex NC502MP based amps) and still got the same high pitched noise.

 

[restorer-john]

So the surround speakers are the issue it would seem...

It would be interesting to investigate exactly what type of load they are presenting to the Hypex amps.

Are they a conventional 2 way/ 3 way small box speaker/wall speaker? Is this a new thing (the noise) or has it always been an issue with that combination? It's somewhat possible they may be presenting a load which is causing some parasitic oscillation or instability. You could try placing some low value caps (100nF-220nF) over the amp/speaker terminals as an experiment to see if that makes the noise diminish.

 

[chucky7]

The surround speakers, JTR Noesis 110HT, are 2 way, 8 ohm, rated at 95db/2.83V/free air and have a FR of 110Hz~20KHz in free space so they are pretty easy to drive. I had them as surrounds before and drove my whole HT with just a mid level AVR without any issues.

My friend just received the 110HTs along with the main speakers and immediately heard the high pitch noise once he hooked them up. Only the combination of 110HTs + NC502MP based amps would result in the noise. His Audiophonics NC500MP Mono amp and other amps won't produce the noise.

Another friend had experienced the same noise with the same combination (110HT + Buckeye NC502MP amp). He switched over to VTV NC252MP amp and no more noise.

 

[restorer-john]

The surround speakers, JTR Noesis 110HT, are 2 way, 8 ohm, rated at 95db/2.83V/free air and have a FR of 110Hz~20KHz in free space so they are pretty easy to drive. I had them as surrounds before and drove my whole HT with just a mid level AVR without any issues.

My friend just received the 110HTs along with the main speakers and immediately heard the high pitch noise once he hooked them up. Only the combination of 110HTs + NC502MP based amps would result in the noise. His Audiophonics NC500MP Mono amp and other amps won't produce the noise.

Another friend had experienced the same noise with the same combination (110HT + Buckeye NC502MP amp). He switched over to VTV NC252MP amp and no more noise.

They are clearly very efficient, about 8-10dB more than a typical speaker. That may be the reason- the residual noise simply is too great. But it's something I think would have come up before. I think the extreme efficiency of the compression tweeter is highlighting the poor noise floor of the amplifier.

Good luck tracking it down.

 

[KKQ]

From a Hypex module amp manufacturer:

Buckeye Amps: New US based Hypex multichannel amplifier builder, line-up announcement!

IMO it would be a waste of a Purifi amp to power a sub.

www.audiosciencereview.com

Read this and his next responses. Perhaps?

 

[chucky7]

From a Hypex module amp manufacturer:

Buckeye Amps: New US based Hypex multichannel amplifier builder, line-up announcement!

IMO it would be a waste of a Purifi amp to power a sub.

www.audiosciencereview.com

Read this and his next responses. Perhaps?

Unfortunately, as I posted before, the high pitch noise can be heard coming from the 110HT speakers, once a Hypex NC502MP based amp is powered on for 3 minutes without input connected. Besides, my friend is using a XLR-XLR cable from his HT processor to the power amp.

 

[pma]

What happens if you short pins of the input connector? (All of them)

 

[DVDdoug]

I'd guess it's power supply noise. Does it have a switching power supply?

Switching power supplies tend to have a high-pitch whine (when they have noise) whereas linear power supplies tend to have lower-frequency hum or buzz (when they have noise).

 

[chchrlam]

I had this same high pitched noise on one of my Hypex FA503 DSP plate amplifiers.
Big spikes after 5 minutes at 10.5, 13 and 19.5 KHz. Your friend has one spike at 10 KHz from my inspection of the files you posted.

The FA503 amp has two NC502MP and one NC100HF on board.

The FA503 is connected to a partially activated Visaton Vox 253 MTI floor standing speaker which I am testing and converting at the moment to fully activate.
On the analogue crossover the LPF for the bass is removed as is the HPF for the twin mids and the tweeter. The LFP and HPR between mid and tweeter remains.

At the crossover the minus wires from the amplifiers are connected together(I measured 0 Ohms between the grounds Black and Grey).
Being lazy and impatient I used the existing 4 pole post connectors on my speakers to feed bass, mid and high and just connected all the minus wires togehter at one post.

The noise instantly dissapears when I connect a cheap tweeter directly across the terminals before the crossover.
The noise also instantly dissapears when I seperate the two minus wires.

My best guess is a combination of a complex analogue crossover and my adventureous grounding produces this problem.

Currently I have completley removed the analog crossover and I love it. No wierd noises.

 

[pma]

Amp stability issues, the reasons already mentioned.

 

[chchrlam]

Amp stability issues, the reasons already mentioned.

Yes, confirmed in two threads in avsforum and also here. I just thought it might be worth mentioning that a brand new Hypex FA503 made in Week 2 2023 has the same issue and that it might be related to grounding. My two pennies might save someone a headache or two.

 

[Sokel]

Spectrum of the OP's noise:

not very nice.

 

[Sokel]

Maybe a coincidence but it looks very similar with @pma's recording:

NC252MP (class D) vs. A250W4R (classAB) burst measurements into 4ohm//2.2uF load

Thank you @pma for this very interesting measurements. As I see that you are using a Picoscope, there is an interesting measurement that could be added. i hope that the Picoscope is able to do a FFT. With the same test conditions as before, it it possible to inject a 6khz signal and make the...

www.audiosciencereview.com