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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
speakermaster
738 posts
 
I consider a good class ab amplifier any well designed high power consumption power amplifier that takes at least 1000 watts from the wall there are many to choose from but i can tell tell you that the best are of the bipolar variety.
madavid0 OP
214 posts
 
After well over 200 hours of burn-in of the Weiss opamps I did my first focused listening test. The low-quality / distorted / unnatural midrange has come back. I could have sworn that issue was largely resolved by the Sparkos, but maybe I was misremembering it or I wasn't paying attention to midrange quality? Anyway it's pretty bad. Sounds sort of metallic, "buzz-y", which is why I want to call it a distorted sound. I switched to a $300 NAP200 clone just to make sure it wasn't something else in my chain -- nope, good quality midrange.

I HAVE to ask those of you who own this amp if you really can't hear this low-quality midrange? It's pretty obvious in vocals and horns.
t_ramey
632 posts
 
@jaytor 

How would you describe the sound difference between the Purifi amp and the Neurochrome class a/b you've built?
atmasphere's avatar
atmasphere
11,834 posts
 
Sounds sort of metallic, "buzz-y", which is why I want to call it a distorted sound.
That can be caused by an offset problem at the input of the comparitor- if its something like that I would regard it as a malfunction.
jaytor's avatar
jaytor
765 posts
 
That can be caused by an offset problem at the input of the comparitor- if its something like that I would regard it as a malfunction.
Seems unlikely that this would be the case in both channels (assuming that's the case). Since this is a stereo amp, perhaps an issue with the power supply?

I am not noticing anything particularly egregious in the midrange in my amps, but I want to spend some quality listing time comparing to my other amps now that I've got a couple hundred hours on the Purifis.

It's been a few weeks since I listened to my Neurochrome Mod-286 amps, and a few months since I used my Parasound JC-5. I am hoping to get some time this weekend to do some critical listening and comparisons, and will report back after that. 

I ordered a pair of VTV's tube input buffers with Weiss op amps to compare with the Neurochrome buffers. These should arrive later this week, but will require some rewiring since the connectors are not compatible. I may have to order some connectors if I can't find any that fit in my stash, so it may be a couple weeks before I can try these out.
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