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Purifi Class D: Junk?

So, from the previous thread about high-end class D the Purifi module was brought up. I decided to get a cheap example from VTV, a simple stereo unit with a single Purifi module and matching Hypex SMPS. Standard input buffer. I got it in yesterday. First impression wasn't what I was expecting: weak, congested dynamics is what stood out to me. I expected greater expression through my ProAc D30Rs. The other problems such as poor soundstage, thin / boring character, etc, I marked up to needing burn-in before evaluating. So it's been 24 hours, I would still expect to get at least the high control / damping of high end class D and dynamic power, but it's just not present.

Could it be an impedance mismatch? Other manufacturers selling the Purifi with their custom input buffers are reporting 47k Ohms. VTV doesn't say in the manual or on the site. I checked the Purifi data sheet which reports...2.2k Ohms on SE???? That can't be right?? That's absurdly low! Am I reading the right spec? My preamp has an output impedance of 230 Ohms. Can someone confirm that the stock Purifi has this ultra-low input impedance?
madavid0
georgehifi's avatar
georgehifi
10,312 posts
 
Data sheet: Each output drives a 150Ω load to ±7.5V with ±15V supplies
The amplifiers are stable with any capacitive load making them useful in buffer or cable driving applications.

No they are the output buffers in the rear of the MSB discrete R2R dac, which can swing 7.5v and do it with ease into 2kohm, with no temp rise or voltage drop or heat sink needed into the raw NC500 module

And the proof is how it sounds, far better than having "any additional buffers" at the input of the NC500 module/s.

And everything is direct coupled with no DC offset from dac outputs right through to speakers. ("small very fast settling" switch on low level bump from the dac) so amps go on last how it should be anyway.

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
Datasheet: The output drives a 100Ω load

Sorry, just looked at my notes, the MSB’s output buffer could be the LT1819
https://www.analog.com/media/en/technical-documentation/data-sheets/18189fb.pdf

Cheers George
audio2design
1,351 posts
 
atmasphere,

The issue is performance, and without a proper low impedance buffer, at the input, not 6 feet of cable, with capacitance and inductance, the performance suffers in my testing (with actual test equipment).


I don't doubt he did it, just pointing out that it is not an optimum implementation.
atmasphere's avatar
atmasphere
11,834 posts
 
No they are the output buffers in the rear of the MSB discrete R2R dac, which can swing 7.5v and do it with ease into 2kohm, with no temp rise or voltage drop or heat sink needed into the raw NC500 module
Did you test to see if they actually do that in this application? There are a lot of variables here, not the least of which is how surprising 1 watt of power actually is and how distortion can affect your perception of sound pressure, to say the least.
The issue is performance, and without a proper low impedance buffer, at the input, not 6 feet of cable, with capacitance and inductance, the performance suffers in my testing (with actual test equipment).


I don't doubt he did it, just pointing out that it is not an optimum implementation.
As you might know our MP-1 is a tube preamp but can drive this through 30 feet of balanced cable with no worries- but that is our module, not a Hypex or the like to which I have no exposure. I would think that due to the low impedances involved the interconnect issues wouldn't be an issue. But there is a lot to unpack here; IMO the whole thing can be taken with a grain of salt without more information.

georgehifi's avatar
georgehifi
10,312 posts
 
Did you test to see if they actually do that in this application?
Yes, that why I said it.

I would think that due to the low impedances involved the interconnect issues wouldn’t be an issue.
And to have the MSB buffer at the Hypex input or 1mt away with quality interconnect, is no difference at all and you know that.
But what is worse, is having one buffer in series with another buffer. When if one "can" if loud enough do the job perfectly and yield even better sonic results.

You have probably already seen this, but again for the masses, from the Hypex NC500 data sheet.
Audio input
The INH/INC inputs form a differential pair. Note that the input impedance is fairly low meaning that, minimalist discrete circuits or valve input stages won’t work. All op amps commonly used in audio can handle them though

Input Conditioning / Buffering
Unlike in many other Hypex products, no input buffer is present as manufacturers of audiophile equipment tend to bypass it anyway and design their own. The NC500 is practically devoid of any sonic signature so this external buffer is a good way of tuning in a “house sound”.
"good way of tuning in a “house sound”" Take that as I would as colouration distortions, countering what they say earlier in the same sentence as "The NC500 is practically devoid of any sonic signature ”
And this is what you hear by not! having an extra buffer in series that’s not needed.
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