March Audio P502 Stereo Amplifier Measurements

[Thomas savage]

Had a few conversations with Amir and I think I have got to the bottom of the 60Hz issue. Its an interesting one !

I have performed a few more tests this morning and I can replicate the issue Amir sees. It does appear to be related to the measurement system screen connection.

So, whilst the amp input is balanced, the output is ground referenced. So the modules speaker out negative is connected to the whole system screen, xlr pin 1 and the chassis. Being powered by an SMPS I have no doubt there are mains leakage currents through caps into the ground, chassis and therefore the speaker negative.

In Amirs test set-up my understanding is that the AP input balanced screen was not connected. If I do the same I get a very similar problem.

Excuse my lash up test system.

measurement screen disconnected - note its 50Hz mains here in Oz as opposed to 60Hz in the US.
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screen connected to speaker negative (system ground reference) - problem gone!
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Note that there is no mains safety earth connection, the Hypex modules are class II double insulated and amp construction follows relevant guidelines. I went this way to minimise the potential for ground loops from single ended source connections. Note that a mains safety earth connection may reduce the issue but is not a correct and potentially problematic solution. See below.

If I release the measurement screen but connect my scope ground clip (which as you know will be connected to mains safety earth) to the amp ground (in this case the screened cable chassis connection) the problem also disappears. Interesting to see some 8kHz usb packet noise spring up This will be due to the screen connection going a torturous route through the mains wiring, back to the PC and probably through the USB screen to the measurement system.

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So on the face of it this seems a similar to the problem you can get connecting single ended audio sources into balanced inputs. Its really a measurement anomaly and not an amp fault or design issue. It's really the age old "ground loop" problem. It won't manifest in the speaker.

Hypex talk about this where the screen (in this instance) should be taken to the far source end and terminated with the balanced low so the ground noise currents dont flow in the signal low.

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BTW it should be noted that unless you know the amp design you need to be cautious connecting screen to an output. I gather that some class D designs are floating and can go bang! So sensible starting point for Amirs testing.

Good job , Mike Smith has had account suspended.

No doubt the sad twat will sign up and play pretend again at some point.

 

[pma]

Good job , Mike Smith has had account suspended.

No doubt the sad twat will sign up and play pretend again at some point.

May I ask what was the reason? Was it a multiple identity or just the fact he had different than common opinion on MA approach? Do you protect specific producers, for whom the forum is a kind of advertising, or is it just my wrong feeling about the situation?

 

[PierreV]

Well... I am not a covert competitor and have no agenda other than being interested in Hypex stuff given how happy I am with my NC400... but this non-review has left me a bit confused on a few issues.

The good thing first: thanks to Amir for transparently publishing "not so good but still good" measurement results in a transparent way. In terms of objectivity, it is great to sometimes see excellent results from brands that had bad ones before and not so good results from "friendly" brands. This reinforces, imho, the overall objectivity of Amir's approach here.

The not so good things now: as I understand it, once the results were published, @March Audio quickly found the cause and replicated the issue on another sample. This leads to two linked questions 1) why wasn't it found before? 2) are all of those specific Hypex modules affected by the same issue? Most of them? Some of them only? Is that a fundamental issue in the design or something linked to bad/under spec components?

Then, on the issue measured itself

Its really a measurement anomaly and not an amp fault or design issue. It's really the age old "ground loop" problem. It won't manifest in the speaker.

Fair enough, I don't have the technical ability to dispute that statement anyway, but what does that mean for the significance of measurements? (Of this device and others that were previously measured) How should I now know that some stuff that was shown in the past isn't simply a measurement issue? How should I know if what was measured will reach the speaker or not?

Color me seriously and sincerely confused here...

 

[restorer-john]

Do you protect specific producers, for whom the forum is a kind of advertising, or is it just my wrong feeling about the situation?

Pavel, there's no protection for forum members who have transitioned to marketing or selling a product/s. If anything, there is more rigor in trying to keep things totally above board. Scrutiny is greater if you ask me. March has taken plenty of hits on the chin and done so mostly with a decent attitude.

Just wait until I bring a product to market, I will get assassinated.

The issue here is simply grounding. Balanced, floating, double insulated and connections to earthed common devices. It's been a problem since Adam was a boy. You work with it.

I do think the interchannel differences need to be investigated, and I'm sure March will do that and report back. It could be a runt sample or something else.

 

[Mnyb]

May I ask what was the reason? Was it a multiple identity or just the fact he had different than common opinion on MA approach? Do you protect specific producers, for whom the forum is a kind of advertising, or is it just my wrong feeling about the situation?

I think he is recocuring troll , multiple identies sign up with a new handle now and then .
Reading the post in this tread after just three forum post he adressed restorer-john diectly ? who is familar with all the forum regulars after a couple of posts ? You can lurk for a while as did I . But you usually present yourself and do not jump rigth in to the fray with something really negative .

I think this forum has at least two of these or are they the same ? The guy that wants to bash March Audio and that other one that atacks amir and the forum in general

 

[Thomas savage]

May I ask what was the reason? Was it a multiple identity or just the fact he had different than common opinion on MA approach? Do you protect specific producers, for whom the forum is a kind of advertising, or is it just my wrong feeling about the situation?

It's a disingenuous account.

Stuff gets measured, if issues are found the decent audio companies investigate and post their findings . Others chose to throw mud and slag everything off.

Readers of the forum can make up their mind on how they feel march audio has done.

And yes your feeling is quite wrong , how you come to those conclusions ( accusations) is something for you to consider and reflect on.

 

[BillH]

May I ask what was the reason? Was it a multiple identity or just the fact he had different than common opinion on MA approach? Do you protect specific producers, for whom the forum is a kind of advertising, or is it just my wrong feeling about the situation?

I also previously witnessesed a similar incident with a "new" member jumping into a discussion to attack March. It was a DIY discussion about ncore or other class d amps and the input buffer supplied for OEM development. March was trying to point out that the development (buffer) board was wide open (unfiltered) to enable testing but that it wasn't appropriate for general use. The troll tried to claimed March was hiding things, making unfounded claims and selling snake-oil products that used the exact same unfiltered input buffer. I was confused by the whole thing until all the telltale signs were made evident.

 

[restorer-john]

And yes your feeling is quite wrong , how you come to those conclusions ( accusations) is something for you to consider and reflect on.

Thomas, I do think Pavel was asking, more than accusing. There is a language issue, to consider.

If I'm wrong, just whip me...again.

 

[pma]

I think I will be continuing to contribute (if) only with measurements or comments on measurements, or maybe rather with nothing. I believe I have my own right to comment on measurements, based on my long time experience in the field. And I will not restrict my measurements to 20kHz, though it is not appreciated by some. At least, I do not use comments like "FUD", "rubbish", "trolling" etc. to someone's address in case I do not agree or do not like some findings. However, to my surprise, such comments seem to be well accepted and are not supposed as violating the rules.

 

[Willem]

Relax. Just like John I noted a somewhat strident tone of voice, and equally blamed it on the linguistic barrier. Just view it as the excuse it was surely meant to be.

 

[Purité Audio]

Pavel , do you sell your designs commercially?
Keith

 

[pma]

Pavel , do you sell your designs commercially?
Keith

I stopped it in 2012. Before, I sold 500+ PCBs and preamp boards mostly to DIY community, worldwide.

 

[Purité Audio]

Thanks Pavel.
Keith

 

[Thomas savage]

Thomas, I do think Pavel was asking, more than accusing. There is a language issue, to consider.

If I'm wrong, just whip me...again.

Well as questions go they are rather undignified.

 

[March Audio]

Well... I am not a covert competitor and have no agenda other than being interested in Hypex stuff given how happy I am with my NC400... but this non-review has left me a bit confused on a few issues.

The good thing first: thanks to Amir for transparently publishing "not so good but still good" measurement results in a transparent way. In terms of objectivity, it is great to sometimes see excellent results from brands that had bad ones before and not so good results from "friendly" brands. This reinforces, imho, the overall objectivity of Amir's approach here.

The not so good things now: as I understand it, once the results were published, @March Audio quickly found the cause and replicated the issue on another sample. This leads to two linked questions 1) why wasn't it found before? 2) are all of those specific Hypex modules affected by the same issue? Most of them? Some of them only? Is that a fundamental issue in the design or something linked to bad/under spec components?

Then, on the issue measured itself


Fair enough, I don't have the technical ability to dispute that statement anyway, but what does that mean for the significance of measurements? (Of this device and others that were previously measured) How should I now know that some stuff that was shown in the past isn't simply a measurement issue? How should I know if what was measured will reach the speaker or not?

Color me seriously and sincerely confused here...

The issue is not actually an issue . It's a measurement anomaly that would not manifest with a speaker. I can try and cover this in more layman terms to explain what's happening in further posts if that helps.

Well, it has to be noted that a most amp measurements do appear to have a surprising amount of 60 Hz and harmonic pick up. However Amir is absolutely correct to be cautious in connecting amp outputs to screen.

The measurement results will depend on how the amp is configured. For example if I had chosen an earthed configuration as opposed to class II this would almost certainly would not be seen.

 

[Thomas savage]

The issue is not actually an issue. It's a measurement problem that would not manifest with a speaker.

Ooo I smell a cover up ..

 

[March Audio]

May I ask what was the reason? Was it a multiple identity or just the fact he had different than common opinion on MA approach? Do you protect specific producers, for whom the forum is a kind of advertising, or is it just my wrong feeling about the situation?

Pavel, if you take a look over time I have been trolled on multiple occasions, one positively identified as a competitor with the same suspicion here. It follows the same sequence, behaviour and tactics every time. New account, going straight to slinging mud. Same sort of characteristic language etc. Its obvious from the off so I call it straight away.

 

[March Audio]

I think I will be continuing to contribute (if) only with measurements or comments on measurements, or maybe rather with nothing. I believe I have my own right to comment on measurements, based on my long time experience in the field. And I will not restrict my measurements to 20kHz, though it is not appreciated by some. .

Pavel, of course you have the right to comment but you have to see that your comments above were pretty inflammatory. You were basically saying I was faking measurements. That's pretty offensive.

The technical explanation, to an experienced EE such as yourself, shouldn't be in any way contentious. In fact it's very simple once you have established what's going on.

Your technical input is appreciated but you have to temper what you are doing with relevance to what happens in the real world and not base it on some theoretical and highly unlikely possibility. That's been my criticism of your previous contributions. Without that it loses any relevance. If you try hard enough you can break anything. So being stroppy about my concerns of some of your extreme and unrealistic tests and trying to undermine my findings here without genuine technical consideration won't get you anywhere.

 

[Deleted member 9286]

@March Audio I don’t understand well enough to contribute much to this thread but is your explanation something that Amir could adjust for and then re-test?

That way we would have an accurate view of measurements and that would silence the sceptics/mud slingers for good.

 

[March Audio]

@March Audio I don’t understand well enough to contribute much to this thread but is your explanation something that Amir could adjust for and then re-test?

That way we would have an accurate view of measurements and that would silence the sceptics/mud slingers for good.

Yes I appreciate this is impossible for people who haven't got the technical background to draw conclusions.

In my conversations with Amir I suggested bypassing the input filter and connecting directly to the Audio Precision as I have shown in my previous posts. He said he didn't have a bare wired xlr lead to test with.

You best ask Amir. I don't know the schedule for returning the amp to the owner or Amir's other work load.