Headphone Amplifier 2025

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After the big result obtain with the Headphone Amplifier 2021 I have develop a new project able to drive also low impedance headphone.
I have long experiences with Beyerdynamic DT911, DT880, Sennheiser HD600 and Stax SR303 but here I want to test a product based on a different technology.
The target headphones are the HiFiMan Planar Magnetic products with a true hi-end performances and an acceptable price.

Many of the HiFiMan headphones have a 32ohm impedance but this does not mean that you can drive them with the normal headphone output of a laptop or a smartphone because these need a high output level.

HiFiMan Sundara

Frequency Response : 6Hz-75kHz
Impedance : 32Ω
Sensitivity : 92dB

HiFiMan HE-400S - highest efficiency planar phones

Frequency Response : 20Hz – 35KHz
Sensitivity : 98dB
Impedance : 22 Ohms

The Sundara have a low efficiency so need a dedicated amplifier near to 1W.

The HE-400S can be driven with smartphone or most any portable audio device but the sound performances are much more limited.

120 dBSPL are acceptable for short periods so we can calculate per power necessary.

Power (mW) = 10^[(loudness - sensitivity) / 10]

Voltage(V) = √ (Power(W) * Impedance)

   Current(class A Ibias) = Voltage (Vrms) * 1.41 / Impedance(ohm)

so for Sundara

   Power(mW) = 10^[(120 - 92) / 10] = 630mW

   Voltage(V) = √ (0.63 * 32) = 4.5Vrms

   Current(class A Ibias) = 4.5 * 1.41 / 32 = 198mA

and for HE-400S

   Power(mW) = 10^[(120 - 98) / 10] = 158mW

   Voltage(V) = √ (0.158 * 22) = 1.87Vrms

   Current(class A Ibias) = 1.87 * 1.41 / 22 = 119mA

This project is an hybrid amplifier composed by a tube voltage amplifier followed by a single ended mosfet current amplifier with an exclusive configuration.

This amplifier integrate the unique feature of the Power Follower with discoupling of the signal current from power supply.

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In my design the first feature making a big difference in the sonic results is the negative power supply and the signal ground tied to the drain of source follower.
With this design we achieve:

The discoupling of the signal current from power supply because the current generator has a huge impedance (an open circuit for the signal, in theory)
The constant current flow from the power supply also during the music peaks that prevent any stress of the power supply.
The simmetric distribution of the power dissipated under static and dynamic conditions both on the current generator and the source follower, so I have used the same type of devices.

I have published several articles about this my current amplifier for loudspeakers and this design was the winner of the award in the Circuit ideas in May 2000 on Electronics World which is the most important electronic magazine in the world.

To read these articles search on the web page: www.audiodesignguide.com/doc/index1.html

This project collects all the experiences made in these years then it born with all these characteristics:

output voltage until 10Vrms to drive also 600ohm headphones

output current until 0.2A to drive also 22ohm headphones

no global feedback design

passive components without any compromise on any section

only few parts on the signal path

very short signal path

very low output impedance (near to 0.1ohm) for high damping factor

ultra fast slew-rate

large frequency band, near to 1MHz

high separation from power supply for all the sections

power supply separated for each channel

zero noise

ultra low distortion in the current amplifier

no fuse on the signal path

no relay on the output signal

only a single vacuum tube for the voltage amplifier

only one interstage capacitor on the signal path

high input impedance 110Kohm

high voltage gain able to be driven directly by any source

voltage stage and output stage working in pure class A operation

compact chassie

R1,R2,R11,R12,R23,R24,R33,R34   220K 1/4W 1%
R3,R4,R13,R14                    2K2 1/4W 1%
R5,R6,R8,R9,R15,R16,R17,R18      39K    2W 5%
R9,R19                           100 1/4W 1%
R10,R20                           47 1/4W 1%
R29,R25                          6k8    2W 5%
R25,R38                          3.9 - 2.7 1/4W  1%    bias resistors
R26,R31                          3K3  1/4W 1%
R27,R32                          4k7 1/4W 1%
R21,R22,R36,R37                  470 1/4W 1%
R30,R40                          220 1/4W 1%
R41,R42,R43,R44                  6k8 1/4W 1%
R28,R39                          2k2 multiturn trimmer

D1,D2,D3,D4                      zener 18V 1W

C1,C10                           220uF 6.3V Nichicon UFG or any other audio grade
C2,C12                           Vishay MKP 50uF 500V Vishay MKP 1848H or any other audio grade
C3,C8                            0.68uF min 400V min MKP audio grade
C6,C11                           1000uF 35-50V Nichicon KG audio grade
C5,C9                            220uF 16V
C4,C7                            47uF   50V

T1,T2                            BD139
Q1,Q2,Q3,Q4                      IRF822
U$2                              E180F tube or compatible

On the PCB all the resistors on the audio signal are duplicated to put 2 of them in parallel out of phase in order to minimize the parasitic inductance.

This is the PCB develop for this project, all the power supply are separated to build a complete dual mono configuration.

pcb files to produce this modue lver1q_2025-07-12.zip

Remember to connect the 6.3VDC to anodic ground with a little jump (see the pcb near the FIL input).

TUBES

All these tubes are compatible with this projects, the Siemens NOS are my first choice.

E180F - CV3898 old Mullard

E180F - 6688 Siemens Gold Pins New Old Stock

6Z9P - 6ZH9P - E180F

The operating point of the tube was chosen for minimum distortion.

Distortion with an output level of 5.5Vrms, a very good result for a vaccum tube without feedback

AUDIO CAPACITORS

This project is made with few components but these must be carefully chosen to achieve the best sound result.
As interstage capacitors I used Icel PHC 0.68uF 400V, alternatively I recommend Audyn Cap Reference or Zantzen Audio Superior Z-cap.
In the output use only Nichicon KG 1000uF 35V or 50V.

HEATSINKS

This project need a good heatsink able to dissipate for each channel this power.

Pd = Ibias * V = 0.2A * 24V = 4.8W Rbias = 2.7ohm

My choice is this model got on Aliexpress.

Heatsink 160mm * 62mm * 32mm

PROTECTION CIRCUIT and DELAY

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

There are 3 power supply,one for vacuum tubes filaments (6.3VDC), one for the anodic (270-280VDC) and one for the mosfet (24VDC).

After the good result obtain in the Inpol / Mofo project and last Power Follower I have decided to use a switching power supply for the mosfet used in the current amplifier section.

In my last Power Follower I have got the same sound of the normal version with a linear power supply and this result is only possible only in this type of configuration with the decoupling of the signal current from the power supply.

For both channels I have one MEAN WELL LRS-50-24.

In this power supply there are integrated protections for short circuit, overload, over voltage and over temperature.

ANODIC 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.

NEW POWER SUPPLY

R1,R2        100     2W   5%
R3,R11      4700   1/4W   1%
R12         470K   1/4W   1%
R13          10K     2W   5%
R14         150K     3W   5%
R45          68K   1/4W   1%
R46          220   1/4W   1%
R48           1K   1/4W   1%
R47          100   multi-trn trimmer

C1,C6               100u 400V
C2                   33u 400V
C7                 4700u   16V
C8,C10mc11          220u   16V
CY5,CY6,CY7,CY8    0.01u 500V Vishay MKP 1848H

U$1,U$3           IRF840
D1,D2,D3,D4       1N5407
D5,D6             zener 10V 1W
B1                diode bridge 100V 2A 2KBP01
U$19,U$16         1N4007
T3                2N2904
U$6,U$3           IRF840
U$15              LM317

KK1,KK4 Heatsink type SK104 or EA-T220-38E compatible L=38mm Thermal resistance = 9.5K/W

KK2 Heatsink type SK104 or EA-T220-38E compatible L=38mm Thermal resistance = 7.5K/W

The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser 571-63862-1-CT, cod. RS 718-7987)

file to produce this pcb

PHOTOS AND DETAILS

In my case there are only little bypass MKP on anodic power supply because on the power supply there are large capacitors after the mosfet.

SUNDARA