Here is a project describing a very high quality headphone amplifier 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|
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.
|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.
|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 1.0uH/92mA/6.90Ω L_0805 L0805 INDUCTOR
L2 1.0uH/92mA/6.90Ω L_0805 L0805 INDUCTOR
L3 1.0uH/92mA/6.90Ω L_0805 L0805 INDUCTOR
L4 1.0uH/92mA/6.90Ω L_0805 L0805 INDUCTOR
L5 1.0uH/92mA/6.90Ω L_0805 L0805 INDUCTOR
L6 1.0uH/92mA/6.90Ω L_0805 L0805 INDUCTOR
L7 1.0uH/92mA/6.90Ω L_0805 L0805 INDUCTOR
L8 1.0uH/92mA/6.90Ω 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
THANK YOU FOR READING!!!
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.
Steve@PCB Design Consultancy
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.
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 🙂
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
This headphone amplifier can compete with Lehmann Linear Cube ?
Hi @ Becker –
>>This headphone amplifier can compete with Lehmann Linear Cube ?<<
On paper – yes – even exceed it with the right gain and choice of servo op-amp. My simulator shows good results with extremely low distortion.