BUF634 Headphone Amplifier, Driver. PCB Project.

Here is a project describing a very high quality headphone amplifier. This one is built around the TI’s buffer IC – BUF634. The project features a complete design including PSU section, input section, volume control and output connector.
The whole design is completely DC coupled form input to output. The BUF634 IC was used to boost the output current of the op-amp forming the input stage of the amplifier.

In order to keep the DC offset at the output low enough it is recommended to use precision op-amps like OPA132. FET input is preferable. This gives you high input impedance and low gain error. Of course for really low cost application one can you the traditional NE5534. However you may need to set a small compensation capacitor across pin 5 and pin 8 of 5534. A value of about 10pF would be enough. The other way is to have the stage gain set at higher value. Resistors R9/R8 and R14/R13 set the gain of the stage. With the default values the gain is set at about 15dB or G=5.7

The schematic for this project looks like this:

BUF634 AMPLIFIER SCHEMATIC
BUF634 AMPLIFIER SCHEMATIC

Few words about the schematic. The input potentiometer is ALPS RK27 series. Dual pot logarithmic. Value 20K.

The input signal is passed through a low-pass RC filter formed by R5/C15 and R10/C22. Omitting this filter will cause a small overshoot when testing with square wave. Bare in mined that BUF634 is a wide bandwidth IC (so are the input op-amps). Some form of high frequency suppression is good to have as we are only dealing with audio frequencies here.

The output connector is Neutrik NMJ6 PCB mount series.

The PSU:

BUF_634_HEADPHONE_AMPLIFIER_POWER_SUPPLY_SCHEMATIC
BUF634 HEADPHONE AMPLIFIER POWER SUPPLY

The power supply is a bit nontraditional. It uses two bridge rectifiers to form the two power supply rails and the GND. This means you will need a transformer with TWO SEPARATE secondary winding. 30VA transformer providing about 2x17VAC is sufficient to power this amplifier. The decoupling capacitors around the optional. Those are suppose to reduce the noise coming from the diodes. I have never actually been able to hear any audible difference between “bypassed” and “non bypassed” bridge rectifiers so I leave that to your own personal judgement. Mounted or not one thig is for sure – they do no harm. The rest of the PSU is pretty straightforward. It is regulated power supply using the traditional LM317/LM337 regulators.

From the power supply each rail is then delivered to the op-amps trough individual LC filtering.

The PCB:

buf634_amplifier_headphone_pcb
BUF634 HEADPHONE AMPLIFIER PCB

The PCB is two sided with large GND copper pour for improved noise and parasitic interference shielding. The two input op-amps are in DIP package to allow for further experimentation. Those are in many of the cases, a subject to personal taste. However I’m giving a small list of op-amps that in my opinion are suitable for this project:
LT1022, LT1468, LT1028, OPA602, OPA604, OPA227

Complete parts list:
Partlist exported from C:/Program Files (x86)/EAGLE-5.11.0/projects/BUF634/BUF634_HP_SIMPLE.sch at 25.7.2012 г. 14:21:49 ч.

Part  Value         Device         Package        Description                                                         
B1    DBL201G       DBL201G        DB      Single Phase 1.0 AMP Glass Passivated Bridge Rectifier              
B2    DBL201G       DBL201G        DB      Single Phase 1.0 AMP Glass Passivated Bridge Rectifier              
C1    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C2    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C3    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C4    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C5    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C6    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C7    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C8    10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C9    1000u         CP_E-050X125   CE-050X125     POLARIZED CAP                                                       
C10   1000u         CP_E-050X125   CE-050X125     POLARIZED CAP                                                       
C11   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C12   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C13   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C14   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C15   150p          C_1206         C1206          NON-POLARIZED CAP                                                   
C16   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C17   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C18   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C19   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C20   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C21   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C22   150p          C_1206         C1206          NON-POLARIZED CAP                                                   
C23   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C24   10u           CP_SV-B        CSV-B          POLARIZED CAP                                                       
C25   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C26   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C27   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
C28   10n           C_0805         C0805          NON-POLARIZED CAP                                                   
D1    LL4148        DIODE-MINIMELF MINIMELF       DIODE                                                               
D2    LL4148        DIODE-MINIMELF MINIMELF       DIODE                                                               
IC1   LM317         LM317TS        317TS          VOLTAGE REGULATOR                                                   
IC2   LM337         LM337TS        337TS          VOLTAGE REGULATOR                                                   
IC3   OPA132        OPA134P        DIL08          Operational Amplifiers                                              
IC4   BUF634U       BUF634U        SO08           250mA High-Speed Buffer                                             
IC5   OPA132        OPA134P        DIL08          Operational Amplifiers                                              
IC6   BUF634U       BUF634U        SO08           250mA High-Speed Buffer                                             
L1    BEAD          L_0805         L0805          INDUCTOR                                                            
L2    BEAD          L_0805         L0805          INDUCTOR                                                            
L3    BEAD          L_0805         L0805          INDUCTOR                                                            
L4    BEAD          L_0805         L0805          INDUCTOR                                                            
L5    BEAD          L_0805         L0805          INDUCTOR                                                            
L6    BEAD          L_0805         L0805          INDUCTOR                                                            
L7    BEAD          L_0805         L0805          INDUCTOR                                                            
L8    BEAD          L_0805         L0805          INDUCTOR                                                            
POT1  RK27-DUAL-20K RK27-DUAL      RK27-DUAL-UNIT                                                                     
R1    240R          R_0805         R0805          RESISTOR                                                            
R2    2.7k          R_0805         R0805          RESISTOR                                                            
R3    2.7k          R_0805         R0805          RESISTOR                                                            
R4    240R          R_0805         R0805          RESISTOR                                                            
R5    1k            R_1206         R1206          RESISTOR                                                            
R6    22k           R_1206         R1206          RESISTOR                                                            
R7    10R           R_X0207/10C    0207/10C       RESISTOR                                                            
R8    1k            R_0805         R0805          RESISTOR                                                            
R9    4.7K          R_0805         R0805          RESISTOR                                                            
R10   1k            R_1206         R1206          RESISTOR                                                            
R11   22k           R_1206         R1206          RESISTOR                                                            
R12   10R           R_X0207/10C    0207/10C       RESISTOR                                                            
R13   1k            R_0805         R0805          RESISTOR                                                            
R14   4.7K          R_0805         R0805          RESISTOR                                                            
R17   10R           R_1206W        R1206W         RESISTOR                                                            
R18   10R           R_1206W        R1206W         RESISTOR                                                            
   
Complete manufacturing files are available for download HERE

THIS PROJECT IS AVAILABLE FOR DIY AND PERSONAL USE ONLY. COMMERCIAL USE IS NOT ALLOWED WITHOUT AUTHOR’S EXCLUSIVE PERMISSION!

THANK YOU FOR READING!!!

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5 Comments

  1. Posted August 7, 2012 at 2:51 PM | Permalink

    The circuit diagram seems to be a difficult to read. It seems like its an image of the sheet on which you had the diagram.

  2. Posted August 7, 2012 at 4:22 PM | Permalink

    Hi,
    yes looking at the images trough you browser is a bit difficult. If you save those to you PC you can open them. If you are still having troubles reading them please drop me an e-mail and I will send you a PDF file.

    Regards.

  3. Mincior
    Posted August 19, 2012 at 6:40 PM | Permalink

    I don’t get it why people insist on the diagram’s visibility when the option of downloading all the necessary information at the end of the article is crystal clear :)

  4. Posted August 21, 2012 at 12:15 PM | Permalink

    Hi, This is a good post, indeed a great job. You must have done good research for the work, i appreciate your efforts. Looking for more updates from your side. Thanks

  5. Becker
    Posted March 20, 2014 at 2:44 PM | Permalink

    This headphone amplifier can compete with Lehmann Linear Cube ?

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