This headphone amp was due for many years. It was many years ago when I start fiddling with the idea.
After many experiments and measurements, I concluded that a SET 300B headphone amp is best fit to my sound taste and design preference.
With few things left over or repurposed from previous projects, I’ve managed to finish a C3m – 300B headphone amplifier. It is a one enclosure amplifier using all Lundahl transformers and choke and the long lasting EML300B. If you don’t recognize the C3m in the pictures is because I cut out the aluminum screening can.
The power supply is using a Lundahl power transformer and choke and a hybrid bridge. The rectifier bridge is using SiC 1200V diodes for the negative and two 6CJ3 TV dampers for the positive. The vacuum tube rectifiers are ramping the B+ in a very slow manner ( about 20 seconds). The filtering is C-L-C with 50uF first cap and 220uF second. I am using a Neurochrome Maida 21st Century regulator which provides 385V with extremely low ripple, and protection at 300mA. This accomplished two of many requirements : a super quiet amplifier and the 300B/output transformers are protected. The filaments of the 300Bs are DC powered from a separate supply using its own transformer and it is regulated. Also the C3m filaments are DC powered from a separate supply using its own transformer and it is regulated. I am very careful about the design of the power supply which is a key to a good sounding SET amplifier.
The main amplifier is using the LL2765AM/70mA wired as 12:1 step down. The 300B is biased at Ua=310-320 V and Ia=65-70 mA, dissipating about 22W on the plate. This gives a lot of headroom for this amp and also a long life for the power triodes. The driver/gain stage C3m is loaded on a cascode CCS and it is capacitor coupled to the 300B using very good sounding Miflex caps. Both tubes are self biased and there no NFB applied.
The enclosure is a excellent built and sturdy by Landfall Systems. I used blood wood hardwood for the front and sides. The top plate was redone, CNC at Front Panel Express for a more professional/pleasant look.
The amplifier frequency response is 25Hz-25kHz at -1dB, the overall gain is 14-15dB and THD with 1Vrms input is 0.02%, delivering 1W on 32 ohms or 100mW on 300 ohms. The FFT, at a 2Vrms output shows only the 2nd harmonic, all other harmonics are hidden bellow the noise floor. This gives this amplifier an extreme SET DHT sound, very clear, pleasant and clean.
The Zout is 8 ohms. The headphone amplifier can also be used as a preamplifier delivering very clean signal to a lower gain input sensitivity amplifier. Given the very low output impedance, it can drive heavy capacitive loads ( long cables/lines) and amplifier low input impedances.