03 Apr 200 W Hybrid Power amplifier with Nutube Valve

200 W Hybrid Power amplifier with Nutube Valve (Purifi Audio)

As a follow up to our blog post on February 11, 2020, we have started testing the Nutube Valve buffer connecting it to one of the two power modules it was designed for: the Purifi Audio 1ET400.

First of all, we have to take into account that, with this hybrid amplifier design, we want to achieve the maximum power, dynamics, damping factor and efficiency typical of class D amplifiers, and mix it with the musical warmth that the valves provide. To achieve these results in an amplifier that only works with tubes we would need a large number of valves, transformers of considerable size and high energy consumption. This would result in an amplifier of large dimensions, very heavy and with a lot of heat generation (let’s not forget that we are talking about obtaining powers of more than 100 W_RMS).

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

The first tests carried out have been with the 1ET400 module from Purifi Audio. This module was the last to come onto the market and has taken advantage of all the accumulated experience of Bruno Putzeys in the design of 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 V for maximum output power.

They are ideal to use with our buffer. 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

After many adjustments and measurements, we have managed to obtain quite interesting results:

• We have achieved, for a +/-1 V_RMS differential input signal, an output voltage of 90 V_PP before clipping. The total harmonic distortion + noise (THD+N) is 9.26%. The distortion of the second harmonic (2H) is 0.45%. With this numbers, we have the equivalent of a 370 W_RMS tube amplifier @ 4 Ω, with a total gain of almost 38 dB.
• As it can be seen in the graph, for a 100 W_RMS amplification the distortion is just 1%.

Fig.1 Power vs THD+N

In the next graphs you can see some measurements of the THD+N at 1 kHz and its harmonics depending of the output voltage:

Fig 2. FFT 1 kHz (5 W @ 4 Ω)

Fig 3. FFT 1 kHz (13.38 V_RMS @ 4 Ω)

This buffer is available as an option for the EPM-450 Mono and EPM-700 Mono power amplifiers and as an individual kit for those users who want to use this kit in their modules.