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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
 
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.
oldhvymec
5,101 posts
 

From Nord Acoustics;

All Purifi 1ET400A are modular, PSU, AMP, and buffer. Making them easily upgradable in future developments of our Input Buffer. They are also easy and open to maintain.

​

The REV D Buffer board uses modern versions of an old Op-Amp used primarily in Studio mastering and mixing equipment. Their development was focused solely on Audio performance and has been bought up to SOTA design and performance. Rev D will also accept the previous 8 pin Sparkos and Sonic Imagery Op Amps. Details of Op Amps Click


Regards
ricevs
1,543 posts
 
The VTV input buffer board is pretty much the same as Nord's.  However, VTV offers more op amp options and the amps cost less and have a 30 day money back in the US...............and you can buy their version of the Purifi evaluator amp for less than $1000 delivered.
oldhvymec
5,101 posts
 
The VTV input buffer board is pretty much the same as Nord’s. However, VTV offers more op amp options and the amps cost less

<<<<<<<<<<<<<<<<<<<<<,>>>>>>>>>>>>>>>>>>>>>>>>>

Actually I did a visual side by side. The Nord is a much better made board, by no small margin either, NO copper foil resistors, and SLOPPY, solder joints. Second the Nord Rev. D has the greater options. You can not use the Sparco 2590s, or other pro op amp options.

Either buffer can use the smaller op amps, SI, or Sparco, a few of the Class A op amps..

The Wiess op amp is HUGE and has a proprietary mount, that only it can use. The cost of that buffer and opamp is the most expensive you can buy. The funny part the guy that sells the VTV boards admits the Nord with 2590s sound better and cost LESS. 1/3 less. 274.xx (Euro) 323.xx usd

He took back the VTV board, I got from him. I thought the option for Wiess was for TWO.
It is for one board 442.xx usd. Over 100.00 usd difference AND the guy that makes the VTV likes the Sparco better, ASK HIM... I think both use Sparco Voltage regulators. Nord does..

Regards
oldhvymec
5,101 posts
 
The one thing I did dig out of the thread is, I need to pick up a pair of Nord built Purifi 1ET400A, with Nord buffers and Sparco 2590. He'll give me a deal.

I own 9 of his NC500. I bought 5 from him. 3 pairs of MBs and one X3 NC500 with trigger and 3 x PS (no sharing). ALL use 1200/700 PS. All but 2 have rev. d boards (rev c). Let's say we have a rapport. 
My 3X was the first to roll off the line with rev d boards and multi Mono rail within a single enclosures, withOUT shared (or compromised) PS 1200/700. Serious piece of gear..

Now to modify the fuse circuit. It needs to be inline and remove the onboard 15 and add a 20 amp iec copper/silver non fuse female.

These are great sounding units with small planars MUCH better than any of the earlier 400/500/700. I really didn't like that sound as ALL.

The buffer made all the difference in the world for me.. BUT then I like tone control, Mcintosh and VMPS speakers. I'm one of "those guys".

To top it all off, kenjit, kicked me off his forum, I'm heart broken.. (took a while to work). Ok I all better now.. I'll quit yipin', I mean typing. :-)

Regards
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