07 Sep 500 W Hybrid Power amplifier with Nutube Valve for Hypex NC500

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 Hypex NC500.

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 second tests carried out have been with the NC500 module from Hypex. This module has taken advantage of all the accumulated experience of Bruno Putzeys in the design of Hypex UCD modules. Its most important characteristics are:

• Input impedance of 1.8 kΩ.
• Voltage gain Gv of 12.4 dB
• Input sensitivity in differential mode of +/-5.4 V for maximum output power.

They are ideal to use with our new buffer V2. To do this, we have connected the module to the Hypex model SMPS1200/700 switched mode power supply. This power supply provides us with a voltage of +/-85 VDC and auxiliary voltage of +/-21 VDC for the internal modulator of the NC500, which will also is valid to supply the double Korg Nutube triode of the buffer.

Hypex NC500 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 141 V_PP before clipping. The total harmonic distortion + noise (THD+N) is 2.34%. The distortion of the second harmonic (2H) is 0.27%. With this numbers, we have the equivalent of a 625 W_RMS tube amplifier @ 4 Ω, with a total gain of almost 36,19 dB.
• As it can be seen in the graph, for a 2-100 W_RMS amplification the distortion is just below 0.17%. In the range between 100-500W the distortion is less than 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 (100 W @ 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.