Category: Headphone Amplifiers

In this post I’m about to show my new BUF634 based headphone amp which also has a crossfeed filter. This gives a better listening experience. The BUF634 IC from TI is an exceptional piece of technology and offers excellent performance. It is very suitable for headphone applications. I have used it in another of my projects with very satisfactory results. For my application the amplifier will have the task of providing some hefty voltage swings because I use 600 Ohm headphones, namely the Sennheiser HD 540 Reference. These are simply gorgeous and I do love them. Check out the HD540 at Head-Fi.

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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.

The PSU:

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 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!!!

Here is a truly classical implementation of the diamond buffer. A headphone amplifier.
There isn’t much to be said about this topology. It’s pretty popular and is used in variety of applications. One application is buffering an opamp’s output so that it can drive low impedance high capacitance loads like a pair of headphones. Here is a schematic showing the diamond buffer as a current booster for an opamp:

The gain is set by the resistors R5 and R2. If you are using NE5534 the C6 capacitor is a must when gain below 3 is set. For more info on this read the 5534 opamp datasheet.

The buffer is biased in pure class A at about 300mA 30mA. That will give a plenty of headroom for plenty of voltage swing. To calculate the quiescent current use the following equation:

This amp will output a substantial amount of power so it must be used with care. Always switch it on with a volume pot set to minimum. It’s very easy to damage you hearing so be warned!

The PCB artwork is available for download HERE. You need two PCB’s for a stereo application.

Yet another TAP6120 based headphone amplifier you’d say. Well maybe you are right. This one started as a group project involving myself and few other guys forum mates. So here it is – the UBIQUITOUS TPA6120 amp.
The TPA6120 chip is a rather strange beast. Looking at the TI’s portfolio of headphone amplifiers you would not find any other amp similar to this. Especially if you look at the slew rate. So why is that? The answer is rather simple. The TPA6120 chip is actually a re-branded high speed line driver that had failed to meet the requirements. This is a common practice among the manufacturers. Many so called “audio opamps” are actually the same thing – lower grade opamps that had failed to meet the criteria.

However failing to comply with the high speed line driver requirements does not make the TPA6120 chip a useless junk. Believe it or not it’s an excellent performer. Many people believe that it can outperform much more expensive headphone amplifiers. I don’t want to put any thoughts on this mater though.

Some facts about the TPA6120 chip – as it’s a high speed IC it requires a careful PCB layout. Any parasitic capacitance may cause the amp to go unstable; – the chip is rather hard to run alone. The simple truth is that you may destroy your headphones if you don’t put any attention to this. The chip will have a severe DC offset at the output depending on the volume pot setting. This can go up to several volts. You wont find this in the datasheet so BE CAREFUL!

This being said I’ve decided to go safe. This design uses an input opamp based stage. It’s not an ordinary two stage project though. This design is based on the so called COMPOSITE opamp design. This approach uses the best of both CFB and VFB design worlds. More info on this matter can be found here:
http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sboa002&fileType=pdf&track=no

I went for the non inverting design.

So my input stage is based on a precision opamp. Few candidates here:
OPA2132 – http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sbos054a
LT1057 – http://cds.linear.com/docs/Datasheet/10578fc.pdf
LT1215 – http://cds.linear.com/docs/Datasheet/12156fb.pdf

I’ve decided to use opamps in DIP package for an easy experimenting. However you could use just about anything you find suitable and use a DIP->SOIC adapters. Just remember that you need a PRECISION opamp in this stage. This would help keeping the DC offset low.

Few words about the project realization. This one is completed on a two-sided PCB using smd components and TH metalization. A special attention was paid on the bottom ground plane to minimize the parasitic capacitance. More info on this matter can be found in the TPA6120’s datasheet:

www.ti.com/lit/ds/symlink/tpa6120a2.pdf

“A ground plane should be used on the board to provide a low inductive ground connection. Having a ground plane underneath traces adds capacitance, so care must be taken when laying out the ground plane on the underside of the board (assuming a 2-layer board). The ground plane is necessary on the bottom for therma reasons. However, certain areas of the ground plane should be left unfilled. The area underneath the device where the PowerPAD is soldered down should remain, but there should be no ground plane underneath any of the input and output pins. This places capacitance directly on those pins and leads to oscillation problems. The underside ground plane should remain unfilled until it crosses the device side of the input resistors and the output series resistor.

The power supply regulation is complete on the board itself near the TPA chip. This requires a +/-18 to +/-21VDC prefiltered.

The complete schematic for this project can be downloaded HERE.

Here is how the PCB looks like:

Since it was the first batch of PCB’s I had some problems with the silkscreen printing. However it’s not a big issue at the moment. In order to keep the return paths as short as possible I was forced to use jumpers. This could be easily solved with a 4-layer PCB. These come at a price though.

WARNING: This project was brought to the DIY community for free. It is intended for personal DIY needs only. Commercial usage of this project is not allowed!