Noisy Purifi amplifiers

[mocenigo]

Still waiting an answer to my so called "nice question"...

And when I was about to reply something like "nobody doubts that there is noise, the matter is audibility, and I will repeat it for the last time - you also have no proof that the reported hiss problems are from the amp or induced by external causes" ... the user was deleted? I might have added also "if you think you can design a Class D amp better than the Mueta folks, better than Putzeys, or how to solve a problem of your invention, then please show us your pudding" but it is too late now. Oh, well, I can live with that, but it is a bit sad.

 

[thenamelessone7]

I have the audiophonics purifi based stereo amp.

My signal chain:
SMSL-SU9 -> audiophonics purifi stereo amp via XLR -> KEF R3s.

All components are hooked to a surge protector with a built-in EMI filter. (not sure if it plays any role)

I cannot hear a thing even at 5cm from the tweeter. I cannot hear anything with the DAC On or Off.

I am fairly youngish (35) and can still hear frequencies up to 17kHz. So I don't think we can be blaming my hearing loss for not hearing anything.

 

[Armand]

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?

Disconnecting the source leaves the input open and it is just the amps internal ground reference that keeps the signal to ground. Depending on the implementation of the input buffer and the ground reference this will give various results and most certantly increased noise.
The only way to really test the amps noise is to short the inputs.
Remember that the inputs are "shorted" when connected to a low impedance source.

Properly designed Hypex and Purifi amps are dead quiet. I have tested both on 107dB SPL/W horn speakers.

 

[vco1]

I just did a small test with, at least to me, a remarkable result.

The noise completely disappears when I switch the amp to the rca input which is shorted (by means of an Oehlbach cover connector closed). When I remove the shorted rca plug, still using the rca input, the noise reappears, although less than with the switch set to the xlr input.

What could this mean for the cause of the (quite loud) noise? And what would be next steps to take?

I don't have a shorted xlr plug, so testing with that is a bit hard at the moment.

 

[Armand]

To short the XLR you can simply use any short length of bare copper wire and insert it into pin2 and pin 3. Totally safe.
I am 99% sure it will be silent on XLR also.

The conclusion is that the noise comes from the source. Buy a better source

 

[mSpot]

The noise completely disappears when I switch the amp to the rca input which is shorted (by means of an Oehlbach cover connector closed). When I remove the shorted rca plug, still using the rca input, the noise reappears

In the second case, do you mean that you listen to an open RCA input with nothing connected? It wouldn't be as quiet as a shorted plug because it could pick up RF/EMI noise from the surrounding environment.

 

[vco1]

Buy a better source

I never thought the Topping DX7Pro was that bad...

 

[vco1]

In the second case, do you mean that you listen to an open RCA input with nothing connected? It wouldn't be as quiet as a shorted plug because it could pick up RF/EMI noise from the surrounding environment.

Exactly.

 

[TimoJ]

And what would be next steps to take?

Solder resistors to amp input/inside connector so that input impedance is lower. Between XLR - and + (or between RCA hot to ground if you use RCA input). Test with 10Kohm.

 

[vco1]

Solder resistors to amp input/inside connector so that input impedance is lower. Between XLR - and + (or between RCA hot to ground if you use RCA input). Test with 10Kohm.

I have to thank you a lot. Your reply pointed me in the right direction. And the noise is completely gone!

Since you were writing about resistors, and lowering the input impedance, I went back to the Audiophonics website. Their Purifi amps have a custom input card with 3 gain settings. The default setting - according to their website - should be +20dB gain for a source signal of approx. 4.2V RMS. And that's the voltage of the balanced output of my Topping DX7Pro.

Audiophonics being a French company (sorry to all French people) I was concerned this may not be the actual default setting. And after opening the amp that feeling turned out to be right. It was set to "Configuration 2" for approx. 2.25V RMS. So I removed the gain jumper to get to the 4.2V RMS configuration. And gone was all the noise. Like magic.

Finally, I must say I'm a bit disappointed Audiophonics didn't point me in this direction. I raised a service request and told them about my setup. Not a single word about checking, let alone changing, the configuration. And since changing that configuration is not an obvious thing to do (changing jumpers inside the amplifier), it's not something a new user would immediately check.

Anyways, the amps are now dead quiet. And I'm very happy with my new setup.

 

[Chester]

Sorry to resurrect a thread but can someone help clarify something for me. Some are saying the Purifi should be absolutely silent, others are reporting it’s silent until really close to the speaker.

While appreciating that individual setups and implementations will impact things, what is the expected behaviour of a well implemented Purifi amp? Absolutely dead silent (even with ear close to the tweeter) or almost silent?

Thanks

 

[Chrispy]

Sorry to resurrect a thread but can someone help clarify something for me. Some are saying the Purifi should be absolutely silent, others are reporting it’s silent until really close to the speaker.

While appreciating that individual setups and implementations will impact things, what is the expected behaviour of a well implemented Purifi amp? Absolutely dead silent (even with ear close to the tweeter) or almost silent?

Thanks

Would that be entirely amp-dependent? Are there differences in pre/signal with the others?

 

[Armand]

Sorry to resurrect a thread but can someone help clarify something for me. Some are saying the Purifi should be absolutely silent, others are reporting it’s silent until really close to the speaker.

While appreciating that individual setups and implementations will impact things, what is the expected behaviour of a well implemented Purifi amp? Absolutely dead silent (even with ear close to the tweeter) or almost silent?

Thanks

The noise from the modules themselves are about 20uV. That is dead silent. Even with the ear just next to the tweeter.

 

[restorer-john]

The noise from the modules themselves are about 20uV. That is dead silent. Even with the ear just next to the tweeter.

Purifi specify 11.5uV A-WTD.

No power amplifier is 'dead silent', especially when in close proximity to tweeters.

 

[mocenigo]

Sorry to resurrect a thread but can someone help clarify something for me. Some are saying the Purifi should be absolutely silent, others are reporting it’s silent until really close to the speaker.

While appreciating that individual setups and implementations will impact things, what is the expected behaviour of a well implemented Purifi amp? Absolutely dead silent (even with ear close to the tweeter) or almost silent?

Thanks

There is residual noise for any amplifier, and there is noise from upstream devices that gets of course amplified.

Let us restrict to the amp’s noise only.
This level is very low (12uV for the module, I estimate about 16-17uV in my build with the Neurochrome buffer) and that means that the result will be VERY faint at best…

… and it will depend on the speaker.

With a 108db/2.83V/m compression driver I hear a faint hiss when I am VERY close only (as in: head inside the horn), with a 102Db Beyma TPL-150H I barely hear some traces of that hiss (I wrote previously “no hiss at all” - well I should have written that it is so close to the threshold of audibility that I am never completely sure what I am hearing) , with my ear nearly touching a 93Db SEAS Excel tweeter I hear nothing, same of course with my 84Db Elac little speakers.

Not even in the first case was noise audible at all if I was 30cm away (in the dead of the night).

 

[SIY]

Sorry to resurrect a thread but can someone help clarify something for me. Some are saying the Purifi should be absolutely silent, others are reporting it’s silent until really close to the speaker.

While appreciating that individual setups and implementations will impact things, what is the expected behaviour of a well implemented Purifi amp? Absolutely dead silent (even with ear close to the tweeter) or almost silent?

Thanks

My speakers are medium efficiency (87 db/2.83V/1m), and I cannot hear any sound whatever from them with the Purifi hooked up, even with my ear close up to the tweeter.

 

[daniboun]

Reminds me of a case we had with a Purifi amp...we had a slight hiss and when we connected the XLR ground (good practice) to the case everything was perfect!

 

[antcollinet]

Reminds me of a case we had with a Purifi amp...we had a slight hiss and when we connected the XLR ground (good practice) to the case everything was perfect!

Whose wrong/recommended practice is that? When you look around there seems to be significant difference of opinion.

I also suspect that which works better will depend significantly on the specifics of equipment grounding design.

 

[SIY]

Whose wrong/recommended practice is that? When you look around there seems to be significant difference of opinion.

I also suspect that which works better will depend significantly on the specifics of equipment grounding design.

I'd go with Bill Whitlock.

 

[daniboun]

Whose wrong/recommended practice is that? When you look around there seems to be significant difference of opinion.

I also suspect that which works better will depend significantly on the specifics of equipment grounding design.

Indeed, we will find a lot of debates concerning this point... personally I solved my problem like this