The impedance of the headphones is very variable from one model to another, those born for listening on mobile devices or PCs are often 32ohm while the hi-end ones that are used with a dedicated amplifier are in the 100-600ohm range. 
So it is not easy design a SE class A amplifier for all this range and it's a waste to use high bias current if we will use only 250-600ohm headphones.
Follows the table to calculate the bias current necessary on each load for the most common impedance and output power.
Normally the max power of the headphones is 100mW so here the values has been calculated for the double (200-203mW).
The output devices used are mosfet with a low input capacitance (about 360pF) and a max power dissipation of 40-50W so these have no problems to be used for any of the following configurations.
To increase the bias is necessary change a single resistance (R2) and increase the heatsink size.
I have used R2=10ohm so the bias is near to 56-60mA and the amplifier will be able to drive any headphone in the range 100-600ohm.
The heatsink is a passive CPU Cooler for Intel given for max 45W but with fans.

Ibias(A) = Vbe of Q1 / R2 = 0.6 / R2

Follows a table to calculate the min bias current to get 200mW on any load from 32 to 600ohm.

Vout (Vrms)	Load (ohm)	Io (Arms)	Pout (mW)	Ibias (mA)	Vpower	Pd mosfet (W)	Pd each ch (W)	heatsink (°C/W)	Tamb (°C)	T heatsink (°C)	Ft(-3dB)	Cout min (uF)
2.55	32	0.080	203	112	270	15	30	1.5	30	75.5	10	498
5.5	150	0.037	202	52	270	7	14	1.5	30	50.9	10	106
7.1	250	0.028	202	40	270	5	11	1.5	30	46.2	10	64
11	600	0.018	202	26	270	3	7	1.5	30	40.5	10	27

The output stage is like the Power Follower but here without the interstage capacitor and share the same power supply of the tube.
On the signal path there is only the tube U1, the mosfet M2 and the output capacitor.
The mosfet M1 and the transistor Q1 are used to create a current generator to lock the bias current.
There are many benefit to use a current generator instead of a simple power resistor.

VACUUM TUBE

For this design is necessary a tube with a medium amplification factor >= 17x and low internal resistance <= 7000ohm.
If your load are only low impedance headphones near to 32ohm you can use other tubes like the direct heating types with low amplification factor .

The socket pins are not the same for all these tubes.

     

• JAN 6189W near to 12AU7
• JAN 5814A near to 12AU7
• E182CC
• JAN 5687WB same socket pins of the E182CC

 

 

MOSFET

The output stage use the IRF820 chosen for the low input capacity near to 300pF and high Drain-source breakdown voltage 500V.

 

HEATSINK

The following heatsink is good for 600 to 100ohm headphones so with max 50mA bias.

Only 12 euro on Amazon shop.

It is necessary to work on these heat sinks to make the three 3mm threaded holes.

	Tamb = 25 °C Ibias = 50mA Vsupply = 278V Pd = 14W Theatsink = 48 °C without any cover

  

CHASSIE

 

PROTECTION CIRCUIT

This design need a relay to keep the output short to ground during the switch-on phase for 1min.

I have decided to use a AIYIMA 2.0 Digital Power Amplifier Speaker Protection Board Delay Relay Speaker Protection available on Alixpress online shop. 

This module use 2 optoisolator PC817 for each input and are necessary only some little changes to increase the start-up time, to increase the accepted input voltage and to obtain a faster reset of timer.

Attection 9VAC used for the filaments are not enought for this module so use separated secondaries or for both 11 or 12VAC.

    

 

 

POWER SUPPLY

A result to always be achieved in headphone amplifiers is the total absence of noise.
Here the result has been obtain with a simple virtual battery power supply used in many of my projects.

• Eagle schematic file
• Eagle board file

Modify the Eagle file for your heatsink model.

 

 

PHOTOS

  

   

    

 

 

 

MEASUREMENT WITH 5687

 

MEASUREMENT WITH E182CC

 

MEASUREMENT WITH 12AU7/5814/E82CC

Ra=41Kohm
Rk=1k2ohm

 

 

COST

For the my version able to driver headhones with impedance in the range 100-600ohm.
To driver 32ohm probably is necessary change chassie and heatsinks or add a fan.
 

Qty	model	brand	unit price (euro)	total price (euro)	shop
1	Slim Line 02/230 2U 10mm BLACK	Hi-Fi 2000	90	90
1	Fully vented aluminium cover for Slim Line 230 series	HI-FI 2000	32	32
1	Power transformer	to define	50	50
1	Jensen 100+100uF electr		35	35
2	Jensen 220uF		35	70
2	Alpine 12 Passive	Alpine	12	24
1	5687/E182/12AU7/...		20	20
1	Potection module	Alixpress	11	11
1	Connectors		12	12
1	10K ALPS R27 / 10K DACT		15	15
1	varios components		41	41
	total			400