Noisy Purifi amplifiers

[vco1]

To get rid of the humming transformers in my Cambridge amp I bought 2 Purifi monoblocks. Happy with the way they look and sound. Until I came close to my speakers... Both amps cause (some) noise from my speakers. If I had to describe it, I'd say a slight hiss, with a touch of static. Audible from about 30cm to 40cm (approx. 1ft). It's (obviously and fortunately) not loud enough to be noticeable when music is playing.

Tried to move the amps away from the speakers. Plugged one of them in a different wall outlet. Tried different speaker cable. Unplugged the source (xlr). No change. As soon as the amp is switched on, the noise is there. Same noise is on both speakers. So both amps show this behavior.

Is this considered 'normal'? And if not, what could be causing this noise? And even more important, how to get rid of it?

 

[Matias]

To get rid of the humming transformers in my Cambridge amp I bought 2 Purifi monoblocks. Happy with the way they look and sound. Until I came close to my speakers... Both amps cause (some) noise from my speakers. If I had to describe it, I'd say a slight hiss, with a touch of static. Audible from about 30cm to 40cm (approx. 1ft). It's (obviously and fortunately) not loud enough to be noticeable when music is playing.

Tried to move the amps away from the speakers. Plugged one of them in a different wall outlet. Tried different speaker cable. Unplugged the source (xlr). No change. As soon as the amp is switched on, the noise is there. Same noise is on both speakers. So both amps show this behavior.

Is this considered 'normal'? And if not, what could be causing this noise? And even more important, how to get rid of it?

If disconnected from the source and they still hiss then it is self noise from the amp itself. Which Purifi monoblocks did you buy? Can you open them and post an inside picture?

 

[vco1]

They are Audiophonics MPA-M400ET.

 

[daniboun]

I solve my Hiss issue with some Hypex amps adding ferrite in speaker signal path. Works fine for me
You can also ground the XLR to the case, helps a lot

 

[Killingbeans]

Is this considered 'normal'? And if not, what could be causing this noise? And even more important, how to get rid of it?

Any amp will have some residual noise. How audible it is depends on the sensitivity of your speakers.

But the Purifi modules have very low residual noise, so even though it's normal, it might not be what you are experiencing.

 

[Matias]

They are Audiophonics MPA-M400ET.

So this is how they look inside. Don't know how to try to improve them.

AUDIOPHONICS MPA-M400ET Mono Class D Amplifier Purifi 1x400W 4 Ohm - Audiophonics

Class D mono power amplifier based on a PURIFI 1ET400A module and a HYPEX SMPS600A400 power supply.

www.audiophonics.fr

 

[DVDdoug]

Audible from about 30cm to 40cm (approx. 1ft). It's (obviously and fortunately) not loud enough to be noticeable when music is playing.

And obviously not audible from your normal listening position...

And even more important, how to get rid of it?

Your going-down a "rabbit hole" trying to reduce noise that you can't (normally) hear. But of course, less efficient speakers would help.

Hiss usually comes from the amplification circuits and there's not much you can do without a re-design. And like Killingbeans says, there is always some noise from any analog electronics.

Hum or whine are usually "external" and sometimes they can be reduced.

​

 

[Chrise36]

To get rid of the humming transformers in my Cambridge amp I bought 2 Purifi monoblocks. Happy with the way they look and sound. Until I came close to my speakers... Both amps cause (some) noise from my speakers. If I had to describe it, I'd say a slight hiss, with a touch of static. Audible from about 30cm to 40cm (approx. 1ft). It's (obviously and fortunately) not loud enough to be noticeable when music is playing.

Tried to move the amps away from the speakers. Plugged one of them in a different wall outlet. Tried different speaker cable. Unplugged the source (xlr). No change. As soon as the amp is switched on, the noise is there. Same noise is on both speakers. So both amps show this behavior.

Is this considered 'normal'? And if not, what could be causing this noise? And even more important, how to get rid of it?

Are there any other devices powered from the same outlet except the amps? Do you use a pre amp or head amp? How sensitive are your speakers?

 

[Matias]

Does the hiss gets more or less depending on the gain position you choose? If so, this is caused by the Audiophonics Interface Card. If not, then it is inherent of the amp module.

 

[sarumbear]

To get rid of the humming transformers in my Cambridge amp I bought 2 Purifi monoblocks. Happy with the way they look and sound. Until I came close to my speakers... Both amps cause (some) noise from my speakers. If I had to describe it, I'd say a slight hiss, with a touch of static. Audible from about 30cm to 40cm (approx. 1ft). It's (obviously and fortunately) not loud enough to be noticeable when music is playing.

Tried to move the amps away from the speakers. Plugged one of them in a different wall outlet. Tried different speaker cable. Unplugged the source (xlr). No change. As soon as the amp is switched on, the noise is there. Same noise is on both speakers. So both amps show this behavior.

Is this considered 'normal'? And if not, what could be causing this noise? And even more important, how to get rid of it?

Sound levels in a quite studio is around 20dBSPL. Loud club (disco) level is around 100dBSPL at 1m. It translates to 110dBSPL at 10cm.

Assuming that is the maximum volume your system is set to you should start hearing noise from an amplifier that has a noise figure of 90dB or less. Purifi spec is 116dB SINAD, 26dB better.

You shouldn’t be hearing any noise. Most likely the implementation you have is not optimum.

 

[Deleted member 27948]

That hissing is essential for class-D amplifiers. That's how they work.

Measurements procedures and weighting have been developed for class A/B amplifiers with 1/f noise, no one cared at the time about the possibility of hissing amps. So the objective numbers for class-D may be astonishing ... but slightly disappointing in reality. Only slightly - because [Purifi] amp designers are not fools and know about the issue very well.

 

[sarumbear]

That hissing is essential for class-D amplifiers. That's how they work.

Measurements procedures and weighting have been developed for class A/B amplifiers with 1/f noise, no one cared at the time about the possibility of hissing amps. So the objective numbers for class-D may be astonishing ... but slightly disappointing in reality. Only slightly - because [Purifi] amp designers are not fools and know about the issue very well.

Please explain why the hiss is essential?

Also, do you think that Purifi is lying with that 116dB SINAD?

 

[Deleted member 27948]

I am not aware of shortcuts to knowledge. I googled professional presentations on the issue, and 1st choice was http://publish.illinois.edu/grainger-ceme/files/2014/06/ClassDB05a.pdf. You could have done the same.

No, I have no grounds to doubt Purifi competence nor to conclude that Purifi is lying. However, the world of electronics is not simple. Nothing is fully described by 1 number. You have to account for spectra and statistical distribution. A 116dB SINAD number is meaningless if taken out of the context, if not accommodated by the full, complete description of measurement procedures, and btw, interquartile width of intra-batch deviations. Nor room noise is simple. Nor human ear with its amazing adaptability and variability is simple. It's a way easy to jump to conclusions based on oversimplified presentations.

 

[Buddelpudding]

It's funny to find this thread now, because I got the same model monoblocks from Audiophonics very recently and noticed the same problem yesterday.

I could even hear the irregular crackling noise from my listening position. But later I concluded that this strong interference was caused by the power strip mounted to my desk and sitting just above the amps (some power cables almost touching their tops). I know this setup is theoretically not ideal, but it's only temporary anyway. However, if I move the desk up (standing desk) to increase the distance, most of the noise disappears,leaving only a faint background noise playing from my Kef R5s, but this would only be audible from very close (few cm).

So in real world usage this should not be much of a problem as long as sources of EM are kept at some distance. Still, this behaviour is kind of annoying given the price tag and assumed state of the art technology of this amp. Or what do you guys think?

 

[Killingbeans]

I googled professional presentations on the issue, and 1st choice was http://publish.illinois.edu/grainger-ceme/files/2014/06/ClassDB05a.pdf.

That's a "fully digital Class-D amplifier", DAC and amp in one circuit? Not really representing a normal Class-D amp?

 

[Vini darko]

It's funny to find this thread now, because I got the same model monoblocks from Audiophonics very recently and noticed the same problem yesterday.

I could even hear the irregular crackling noise from my listening position. But later I concluded that this strong interference was caused by the power strip mounted to my desk and sitting just above the amps (some power cables almost touching their tops). I know this setup is theoretically not ideal, but it's only temporary anyway. However, if I move the desk up (standing desk) to increase the distance, most of the noise disappears,leaving only a faint background noise playing from my Kef R5s, but this would only be audible from very close (few cm).

So in real world usage this should not be much of a problem as long as sources of EM are kept at some distance. Still, this behaviour is kind of annoying given the price tag and assumed state of the art technology of this amp. Or what do you guys think?

Having induction from close mains wires isn't particularly suprising. Metal cases are not much of an impediment in my experience.

 

[Killingbeans]

Or what do you guys think?

That Audiophonics didn't do a great job on the implementation.

EDIT: Scratch that. What Vini says above makes more sense than my hasty conclusion.

 

[Deleted member 27948]

to Killingbeans: Please feel free to google references you like more. There are lots of them, explaining all the details from any angle.

to Buddelpudding: for what I know, the Purifi amp performance is stellar and the price to performance ratio is unbeatable in that quality range.

 

[Killingbeans]

to Killingbeans: Please feel free to google references you like more. There are lots of them, explaining all the details from any angle.

I'm not sure what angle I should be looking for? Residual noise is Residual noise? Any "crappy" amp will hiss, regardless of Class-designation?

Are we talking about lousy Class-D implementations that let the switching noise bleed into the audible spectrum somehow?

 

[Deleted member 27948]

May I suggest starting with the theory of noise shaping? There is nothing lousy about it