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
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.
atmasphere's avatar
atmasphere
11,834 posts
 
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 agree. I don't know how the Purifi module works, but if its anything like ours there is a low voltage side of the module where the comparitor and triangle wave processor reside. If the power supply has a regulator for all that, it could be that something with it is amiss- for example, the regulator is oscillating. At any rate it seems a good idea to get it checked out.
jaytor's avatar
jaytor
765 posts
 
If @madavid0  is using the VTV buffer boards with Weiss opamps, these have Sparkos discrete regulators for the low voltage power, and these are independent for each channel. 

It's possible, though, that the SMPS supply is configured for regulated Vaux output which could certainly cause problems. 
markw1951
51 posts
 
I’m listening to a Chinese made bought on eBay Icepower amp for $150. Sounds perfectly acceptable with a tube preamp, which is a Bellari PA555. Total investment of $425.  The amp sounded not very good until it had run a day or so.  
djones51
4,947 posts
 
I don’t think there’s anything wrong with the amp unless he messed it up popping opamps in and out. Try a blind listening test and get back with the results.
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