To get more information contact me at: firstname.lastname@example.org
started in the 2021
After the good result obtain in the last hybrid Amplifiers for loudspeakers using high trasconductance vacuum tube, often pentode in triode connection, and mosfet I have think about a new Headphone amplifier with the same configuration.
depend by the bias current used see table.
It is very simple class A single ended amplifier with only few components on the signal path.
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)|
The output stage is like the
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.
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
The socket pins are not the same for
The output stage use the IRF820 chosen for the low input capacity near to 300pF and high Drain-source breakdown voltage 500V.
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.
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.
Attection9VAC used for the filaments are not enought for this module so use separated secondaries or for both 11 or 12VAC.
Modify the Eagle file for your heatsink model.
MEASUREMENT WITH 5687
MEASUREMENT WITH E182CC
MEASUREMENT WITH 12AU7/5814/E82CC
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||Alpine 12 Passive||Alpine||12||24|
|1||10K ALPS R27 / 10K DACT||15||15|