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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
djones51
4,947 posts
 
I tried a VTV purifi amp months ago but sent it back. It actually sounded fine but the workmanship was a bit lacking and I didn't feel like cleaning the thing up.
georgehifi's avatar
georgehifi
10,312 posts
 

Does not mean it’s junk just that there is a system mismatch.
230ohm into 2.2kohm is fine, 1:10 ratio, and if he has enough gain there is no problem.
"unless" there is a coupling capacitor on the preamp's output that's too small, and rolling off the bass, because the output impedance goes even higher than 2.3kohm at low frequency. This will cause the thin sound.

Cheers George
djones51
4,947 posts
 
But it's not 2.2kohm. That's the OEM amp module without a buffer board.
madavid0 OP
214 posts
 
I bought it without the custom input board. It uses the standard Purifi board's input and buffer, which apparently isn't bad. I didn't bother spending a bunch on custom input boards and buffers because I figured this will at least give me a fundamental grasp of the sound. Custom buffers shouldn't make or break the sound, should they?
mapman's avatar
mapman
22,277 posts
 
My Audio Research sp-16 has 240 ohm output impedance into Bel Canto ref1000m amps. These have 100kohm input impedance with the BC input stage versus 10k ohm for the stock Icepower module used solo on the ref1000 model. That is to provide good performance with most any preamp including higher output impedance tube preamps. So yes special input stages are done for a reason and do provide significant better performance in many cases.
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