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.
Category: Headphone Amplifiers
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:
, , , 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:
I went for the non inverting design.
So my input stage is based on a precision opamp. Few candidates here:
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:
“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!