back to
www.audiodesignguide.com

To get more information contact me at: webmaster@audiodesignguide.com


Headphone Amplifier 2021 started in the 2021

INTRODUCTION

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.

More specifications depend by the bias current used see table.

Single Ended pure class A Amplifier without feedback
Vout = 11Vrms with low thd
Zout = 2ohm to have a damping factor also for 32ohm headphones.
Ft(-0dB) = 10Hz-800Khz with 5687 or E182CC
Ft(-0dB) = 10Hz-400KHz with 5814/E82CC/12AU7
KHz

The phase of the signal is inverted on output so you should correct on your source if you think this is necessary, in my test there is no difference.d  10Hz-65KHz at -3dB

 

 

SCHEMATIC

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)
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.

     

 

 

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.


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