17 Nov Purifi Audio. 300 W Hybrid Power amplifier with Korg Nutube Valve V2

Posted at 13:28h in Power Amplifiers by atm-audio 0 Comments

As a follow-up to our blog post on February 11, 2020, we continue to test the Nutube valve buffer, now in version 2, connecting it to the Purifi Audio 1ET400.

As we said before, with this hybrid amplifier design we want to achieve the maximum power, dynamics, damping factor and efficiency of class D amplifiers, and mix it with the musical warmth that the tubes provide. We are talking about obtaining powers of more than 100 WRMS.

Well, with our buffer we can put all this in a stylish light, cold chassis, no bigger than a shoe box.

The Purifi 1ET400 module was the latest to hit the market and has drawn on all of Bruno Putzeys’ accumulated experience in designing the Hypex NC500 module. Its most important characteristics are:

Input impedance of 4.4 kΩ.
Voltage gain Gv of 12.8 dB
Input sensitivity in differential mode of +/- 4.8 Vpp for maximum output power.

They are ideal to use with our buffer V2. To do this, we have connected the module to the Hypex model SMPS1200/400 switched mode power supply. This power supply provides us with a voltage of +/-63 VDC and auxiliary voltage of +/-22 VDC for the internal modulator of the 1ET400, which will also is valid to supply the double Korg Nutube triode of the buffer.

Purifi Audio 1ET400 module with Buffer Nutube V2

After the necessary adjustments, we have obtained these results:

For a differential input signal of +/- 500mV RMS, we have an output voltage of 98 VPP before clipping. The total harmonic distortion + noise (THD + N) at this point is 1.53%. The distortion of the second harmonic (2H) is 0.037%. With these numbers, we have the equivalent of a 300 WRMS tube amplifier at 4 Ω, with a total gain of 34 dB.
As can be seen from the graph, for 100 WRMS amplification the distortion is only 0.35%. Up to 50W the distortion is less than 0.2%

Fig.1 Power vs THD+N