Three years ago I build the first version of Nocturne – a transformer coupled tube headphones amplifier – equipped with 6C45p triode and then last year a second version with a more performing ( and more gain ) D3a triode strapped, both intended to drive with excellent performance a AKG K550 back closed headphones ( 32 ohms, 114 dB sensitivity ).
A few months ago I got to try a planar magnetic open back type Hifiman HE400i ( 35 ohm, 93 dB ) and I was truly impressed with his sound, but due their low sensitivity, I thought they needed one more muscular amplifier. There is a double dissimilar triode family born in 1950-60 years for television use with excellent features also in the audio area like 6FD7, 6DR7 and so on. These tubes include in the same bulb a mid-high mu triode like 12AT7/12AU7 family and a low mu, low plate impedance triode ( 800 ohms like a 2A3 ! ) a perfect match for a two stage transformer coupled headphones class A tube amplifier.
The power supply section need a main transformer with 3 secondaries: 320 V at 100 mA CT for HT, 6,3 V at 3 A for EY80 filaments, 8 V at 2 A for DC 6FD7 filaments power supply. The main HV power supply is implemented around a double LC filter followed by a power HV mosfet ( STP6NK90Z zener protected ) regulated stage per channel to ensure the minimal residual noise. Due to very low plate impedance of output triode ( 800 ohms ) its sufficient a 3kohms primary impedance for SE output transformer, 50mA max current. Secondary side must be 32 ohms or less for best matching Hifiman HE400i load. No global feedback was used ( only local in the first stage ).
Hint of components: first capacitor in PSU section must be a paper in oil or MKP type, on the first stage cathode ( pin 8 ) use a carbon composition resistor while on the anode ( pin 6 ) a non inductive 3/5 W Resista or similar. For the coupling capacitor choose the best possible quality for your wallet (for those who can afford it I suggest Intertechnik Audyn MKP True Copper ). The 1 KΩ resistor on cathode of power triode may be a Caddok 25 W TO220 case on adeguate heathsink bypassed by a Elna Silmic II ( 50V min ) capacitor. For the last capacitors in the HV power supply ( 220 uF-450 V ) I use an Nichicon KX audio series. Remember: this capacitor is in series with the audio signal as well as the one on the cathode.
As mentioned above there are other excellent tube candidates ie 6DR7. Look at the specs they are very similar.
headphone listening is naturally a very intimate and personal experience but I think Nocturne is really detailed, warm and very good balanced sound amplifier. Voices are very fine lifelike and excellent dynamics attacks. Every single detail from tonal instrument color to a random cough from a performer is brought to the foreground. It’s amazing!!!