Purifi Class D: Junk?

As far as I know the only company not using a dedicated buffer for the input of the Purifi module is Mivera....they made a streamer last year that used just one gain stage between the DAC chip (1 volt out per phase AKM DAC chip) and the Purifi module. Normally the DAC has a gain stage and so does the Purifi.....by removing one gain stage and having it all in one box you get some serious transparency. Most Purifi amp users will want a built in gain/buffer stage. You will not know if your high gain preamp/DAC can drive the 2.2K input impedance very well, unless you listen. The only way to listen is to buy the VTV amp with the Purifi input board that has the jumper on it so you can use the built in OPA1612 op amp or run it without. However, if you like it with the 1612 buffer (read Sarajen’s review in 6 moons......he liked the buffer over his preamps direct).....then you have no choice for better op amps. The VTV amp with the Purifi buffer cannot be changed to the VTV version with the discrete buffer board.....they won’t fit....its a smaller chassis.

Here is an interesting factoid. The IceEdge amp I made last year had 47K input impedance when I was first modding it. I raised the input impedance to 150K and it sounded noticeably better. I am using 150k on my mods to the VTV Purifi amps. My source was a discrete output stage with a 50 ohm output impedance......The cable in between was low capacitance and two feet long.

So, if you have 500 ohms output impedance and you are driving 2.2K.....then maybe you should try lowering that 500 ohm resistor to say 10 ohms......I could sound much better. You do not need a 500 ohm resistor in series with anything unless it needs it for stability (which might indicate a bad design).  Most high end solid state preamps have an output impedance around 100 ohm........The Gryphon Pandora is 7 ohm.

No not really, the output buffers have .7ohm output impedance, but use a 50ohm series resistor for protection

@georgehifi  Are you saying you built a preamp or buffer circuit using these ICs?

Data sheet: Each output drives a 150Ω load to ±7.5V with ±15V supplies
The amplifiers are stable with any capacitive load making them useful in buffer or cable driving applications.

No they are the output buffers in the rear of the MSB discrete R2R dac, which can swing 7.5v and do it with ease into 2kohm, with no temp rise or voltage drop or heat sink needed into the raw NC500 module

And the proof is how it sounds, far better than having "any additional buffers" at the input of the NC500 module/s.

And everything is direct coupled with no DC offset from dac outputs right through to speakers. ("small very fast settling" switch on low level bump from the dac) so amps go on last how it should be anyway.

Cheers George

Datasheet: The output drives a 100Ω load

Sorry, just looked at my notes, the MSB’s output buffer could be the LT1819
https://www.analog.com/media/en/technical-documentation/data-sheets/18189fb.pdf

Cheers George

atmasphere,

The issue is performance, and without a proper low impedance buffer, at the input, not 6 feet of cable, with capacitance and inductance, the performance suffers in my testing (with actual test equipment).


I don't doubt he did it, just pointing out that it is not an optimum implementation.