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 has been upgraded with a hybrid bridge rectifier ( 2 x EY80 plus 2 x UF4700 ), a double LC filter and finally 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 ). Please note the ground path separation technique. At last all ground converge in a single point SG and then at chassis ground through a separator ( bridge rectifier plus a RC cell: 10 ohms carbon composition resistor and 10 nF ceramic capacitor ).
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 Audyn MKP True Copper ). The 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. 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!!!