2. amps-preamps
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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
audio2design
1,351 posts
 
Oh george, you must realize by now that everyone knows you are the looney bird here. Quack quack.
georgehifi's avatar
georgehifi
10,312 posts
 

You really are delusional, over and out sunshine. 

dougsat
374 posts
 
“You really are delusional, over and out sunshine”

Seems rather condescending and passive aggressive in tone.
Why the axe to grind?
tweak1
3,645 posts
 
ricevs

I am thinking more about how much better my EVS1200 + Oppo 105 with PS mods and Audio Alchemy DDP-1 + PS 5 could sound off grid.  Some days the sound is magical, other less so, probably fluctuations in the grid and/or varying humidity. 2 weeks ago low temp and humidity, this past week mid to upper 80s and high humidity resulting in sticky skin 


The amp is plugged into 20amp dedicated line, the other two into a CorePower 1800. What do you suggest?
atmasphere's avatar
atmasphere
11,772 posts
 
No, it is not, especially with the Hypex amps. The input is meant to be driven by a very low impedance source.

In order to make the comparitor work properly (no offsets, no oscillation) with all the comparitors we tried we did wind up with an input impedance of about 2K. Through a lot of our prototype process, we simply drove this with our MP-1 (or MP-3) preamp, which has no problem at all driving a load like this. Of course, it was obvious that the circuit needed more gain and an easier load to work with most preamps. But it worked fine since we were not including the input buffer in the feedback loop anyway. FWIW our MP-1 can drive 32 ohm headphones- its got a miniature tube power amp (with direct-coupled output) as its line stage.


So it is possible that George was telling the truth, but that would mean that he had a pretty gutzy preamp on hand. Most preamps I can think of wouldn't drive a load like that, so I do think his comments should be held in abeyance until he can explain how he did it.


Since you really don't need much gain from the input buffer, almost any opamp that can drive 600 ohms (and most can these days) can do the job since your gain only need be a value of 2 or 3. So it would not surprise me in the slightest that the kind of opamp used has an inaudible effect. A proper buffer will have more than just one opamp of course, especially if you are executing a balanced input. But they would not need much gain so the feedback on the opamp would be very high! IME its when you are asking more than about 20dB of gain that you start hearing differences between opamps.
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