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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
georgehifi's avatar
georgehifi
10,312 posts
 
My preamp has an output impedance of 230 Ohms.


A preamp with 230ohm output impedance will drive anything, even a low gain one, if you get high enough volume.
And even drive a buffer with 2.2kohm input impedance "if correct", I can’t believe it is though. 47kohms has been the industry standard since time began, and tube amps were 100kohm

Cheers George
mapman's avatar
mapman
22,277 posts
 
Well if that’s the spec that would certainly help explain the result.
Does not mean it’s junk just that there is a system mismatch. Inexpensive Class D amps usually just provide a stock circuit in a box
I have an $80 Fosi Class D integrated amp (with Bluetooth) in a small setup driving a pair of refurbed Boston A40s. It uses a stock TI Class D chip. The sound is very good there in a suitable application.
djones51
4,947 posts
 
The 2.2kohm is single ended input to ground, what the amp sees. That’s simply the amp module without an input board connected. The evaluation board that manufacturers use on the amps has a 10.2kohm differential input impedance. In other words you need to know what board VTV used to know what impedance the preamp sees.
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
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