2. amps-preamps
  3. 162284-76532

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
 
Have you heard them in combination? I have, and I found them not a good match together.

Are you saying it sound like **** like ricevs, and it’s a bad design bad sounding buffer?

Or the impedance match is no good as a buffer? then any buffer will have the same problem.

Do you think Putzey made a bad one on purpose?

Because it is a "simple buffer", made by someone who could make one in his sleep, using good sounding opamps, which is hard to get wrong.
He could teach just about anyone how to make one in 15 mins, even adjusting the gain of it
Here is one using the opa1632 opamp https://ibb.co/njDHP1H Pin 6 is a misprint, should be 5. Pins 3 and 6 are then for + and - from the powersupply.

No, like I said there’s something else the OP is hearing, and all that’s left is the Purifi itself.

Cheers George
jaytor's avatar
jaytor
765 posts
 
The power supply has at least as much influence on the sound as does the input buffer, so "all that is left" is not just the Purifi module.

Further, the Hypex input buffer was never intended as a finished product. It is about as simple a circuit as can be used to provide impedance matching and gain. While the op amps used (LM4562) are decent, they are designed to be operated at a higher voltage than is used on the Hypex buffer to get optimal performance. The 7812/7912 voltage regulators used on this board are decades old and far from state-of-the-art when it comes to noise and response speed, so there is a lot of room for improvement. And there is no frequency limiting and EMI/RFI filtering circuits, so there is lots of opportunity for environmental electrical noise to cause problems. 

And, of course, there are improvements that can be had from wiring, connectors, layout, etc, but these will likely be more subtle compared to the power supply and input buffer design.

Bottom line, this amp as currently built is about the worst case scenario for the Purifi module. You can only go up from here. 
georgehifi's avatar
georgehifi
10,312 posts
 

Wow you need to look at the LM4562 a bit better than what your waffling on about.
Texas Instruments designed it to work it’s optimum between 5-17v, and from what i can find it works with +-12v with the Purifi
+-12v gives .00004thd +-17v gives .00003thd sweet      all in it.https://www.ti.com/lit/ds/symlink/lm4562.pdf?ts=1609274706414&ref_url=https%253A%252F%252Fwww.ti...
jaytor's avatar
jaytor
765 posts
 
That was the least of the issues I pointed out. But oh well..
atmasphere's avatar
atmasphere
11,834 posts
 
Using King Crimson's track to make a final evaluation ? Ridiculous.
Formantera Lady, from King Crimson's Islands LP; very nicely recorded. I play it at audio shows quite often as it can really show off a system!
More to discover