BECUBE AMPLIFIER FOR HEADPHONE
We have been working hard during the last weeks on the prototypes of headphone amplifier to get ready for listening tests at the last week Munich Hi End show. The development took almost one year as we went through different versions, measurements and listening tests with a number of headphones and beta-testers: sennheiser HD800 and HD800s, focal Utopia, fostex TH900, denon AH-D7200 and 2000, AKG K1000 (yes, the old and maybe unsurpassed K1000) and 501, grado 325, beyerdynamic DT990.
The underlying design resembles a small solid state power amplifier with two direct coupled complementary stages and minimal feedback, never-the-less our H/P amplifiers comes also with a convenient preamp output on RCA connectors on the back panel. We use low noise jFET's from linear systems at the input and TO220 MOSFET's at the output for increased current drive and reduced output impedance. The devices are accurately matched and controlled against thermal runaway, an additional super fast and high sensibility protection circuit disconnect the headphone in case the power stage drifts too much from the factory bias point. This solution was taken after observing a very stable and reliable output offset and preferred over servo bias.
Gain is +3dB and +9dB selectable with a toggle switch on the back. But that switch is not only changing the gain of the amplifier, it is also adjusting the current flowing through the power mosfets and making them run hotter at high gain position: this gives more headroom for hostile headphones even thou the general performance of the amplifier do not change.
Power supply is based on a low flux toroidal transformer with faraday shield, SiC Schottky diodes, a tuned passive filter and a discrete low noise regulator.
In terms of performance, the prototypes showed a very low residual noise and distortion that stays below 80dB even at 1W on a 32 ohm loads, it is mainly second order. Bandwidth is very wide from DC up to the MHz region.
In the picture (below), an output signal of 1mVrms at 633Hz is left as reference...there is practically no trace of noise at 100Hz after the power supply or irradiated noise from transformer at 50Hz. This amplifier is dead quite and can offer the highest resolution even with the most sensitive headphones.
How does it sound: we are super happy after the Munich Hi End... the beta testers and visitors loved it with rock, jazz and pop as it returns the rhythm and pace of drums and midbass with ease and power, they reported a dynamic and neutral timbre, male and female voices sound natural and true-to-life, the very quite and silent background allows to enjoy the pianissimo's to full extent even with high sensitivity headphones but it still has a power reserve for demanding headphones.
The BeCube Headphone amplifier will be soon available on the market.
Author: Gianluca Sperti
Product Development & Co-founder