Hu is a headphone amplifier. Headphone amplifier circuit based on low-noise operational amplifiers Circuits of the best headphone amplifiers

If you are the lucky owner tube amplifier , then most likely, if you want to listen to your favorite songs alone, through headphones, you are faced with the inconvenience caused by the lack of output to headphones.

And owners of expensive or not very expensive smartphones and tablets also have a hard time - these devices are most often not able to pump high-quality high-impedance headphones. Therefore, your favorite compositions sound completely different from how they sound on professional equipment.

Of course, if you true music lover and music is more valuable to you than money, then nothing will stop you from buying a preamplifier for $6,000, a headphone amplifier for $5,000, and the headphones themselves for $2,000. And plunge into nirvana... However, if the money situation is not so rosy, or you like to do everything yourself, then it turns out that you can build a high-quality headphone amplifier for only... $30.

Why do you need it???

Do you need a precision amplifier? It depends on your musical preferences and habits. If you are used to listening to music “on the run”, that is, with portable devices while walking, jogging, in the gym and other similar places, then the project described below is not for you. Just try to choose headphones that match your device with the most suitable characteristics and sound.

You should do the same if you like musical styles where there is strong signal distortion, such as rock, heavy metal and the like.

However, if you prefer to listen to music in a quiet, comfortable environment at home or in the office, and your tastes gravitate toward live and natural music such as classical, jazz, or clean vocals, then you will appreciate the sound quality and accuracy of the mix. precision amplifier plus high quality headphones.

Options

Let's say you decide that you need a headphone amplifier. What's the next step? On the Internet you can find a lot of projects using the ubiquitous LM386. The microcircuit has become popular due to its high reliability, low cost, ability to work with single-supply power and a small number of external elements. Such amplifiers usually work well with inexpensive headphones, but all these advantages pale when compared to the noise and distortion levels of the LM386 and a well-designed discrete or ASIC amplifier.

If you have about $30 and are not afraid of working with surface mount elements (SMD elements), then the project presented here is exactly what you need.

Ideas and scheme

When designing this scheme, the following points were taken into account:

• The amplifier must be driven by the relatively high impedance output of a tube preamp or electric guitar amplifier. In other words, the input impedance must be easily tunable for sources with different output impedances.
• small number of components. Therefore, microcircuits were chosen instead of transistors.
• low gain and power. Needs to be rocked sensitive dynamic headphones, not the speaker system.
• The amplifier must be able to handle high impedance headphones. The author uses Sennheiser HD 600 (resistance 300 Ohms).
• get the lowest possible noise and distortion.

Schematic diagram precision headphone amplifier shown in the figure:

Click to enlarge

When developing this design, microcircuits from such manufacturers as National Semiconductor, Texas Instruments and others were considered. Weight useful information was found on Headwize resources and DiyAudio forums.

As a result, the choice fell on a precision headphone driver from Texas Instruments TPA6120A2 and operational amplifiers AD8610 from Analog Devices for the input buffer.

The circuit turned out to be relatively simple, with bipolar power supply. If you are sure that there is no DC component at the output of your signal source, then the coupling capacitors (C24 and C30) can be excluded from the path using jumpers H1 and H2.

The power supply provides ±12V output voltage at a load of up to 1A. Its diagram is shown in the figure:

Click to enlarge

Often in audiophile designs, the cost of the power supply is several times higher than the cost of the amplification part itself. Here it turned out a little better - the cost of the elements for the power supply is approximately $50 and the most expensive elements here are the transformer and electrolytic capacitors. You can save a little if you replace the toroidal transformer with a regular W-shaped one, abandon the LEDs and fuses at the output of the unit.

We tested a version with separate stabilizers for each TPA6120A2 channel (the microcircuit has separate power pins for each channel). It was not possible to hear or measure the difference, which made it possible to significantly simplify the power supply.

Since all the microcircuits used in the amplifier have a low sensitivity to noise and interference in the power supply circuits, as well as a high level of suppression of common-mode interference, the use of standard integrated stabilizers in the power supply turned out to be sufficient to obtain high performance.

TPA6120A2

The Texas Instruments TPA6120A2 is a high-quality, high-fidelity headphone amplifier. It uses an amplifier architecture with differential input, single-ended output, and current feedback. It is thanks largely to the latter that low distortion and noise, a wide frequency band, and high performance are obtained.

The microcircuit contains two independent channels with separate power pins. Each channel has characteristics:

• output power 80 mW into a 600 Ohm load with ± 12 V power supply at distortion + noise level 0,00014%
• dynamic range over 120 dB
• signal/noise level 120 dB
• Supply voltage range: ±5V to ±15V
• output voltage slew rate 1300V/µs
• protection against short circuit and overheating

For comparison, the distortion + noise level of the “folk” LM386 microcircuit is 0.2%. Although, of course, high parameters do not guarantee high-quality sound. To obtain maximum results, you must take into account the manufacturer’s recommendations on the selection of external elements and topology printed circuit board. All this can be found in technical documentation for this microcircuit.

AD8610

The AD8610 chip from Analog Devices is an operational amplifier with field-effect transistors at the input, which gives low offset and drift voltages, low noise levels, and low input currents. In terms of noise level and slew rate of output voltage, these operational amplifiers are in perfect harmony with the TPA6120A2.

However, don’t be lazy and try replacing them with other op-amps. According to the pinout arrangement, the AD8610 is compatible with other audiophile microcircuits. Moreover, many music lovers claim that they hear a difference in the sound of the op-amp!

Passive Components

Not all resistors are the same! And if your budget allows, use metal film resistors in this design, which are somewhat more expensive, but have lower noise and higher stability. If you want to save money, metal film resistors should be installed at least in the input circuits (for the AD8610), where the sensitivity to noise is the highest.

It is better to install film capacitors on the signal path C23, C24, C29, C30. The manufacturer recommends ceramic capacitors for power supply circuits of microcircuits.

The main requirement for signal connectors is reliable contact. In his design, the author used a regular “jack” to connect headphones and gold-plated RCA connectors with Teflon insulation to connect the signal cable.

On schematic diagram shows a version of the amplifier for operation from a tube preamplifier, in which the volume is adjusted. If the design is intended to be made more flexible and universal, then, of course, it is advisable to provide its own volume control at the input. For achievement maximum quality and in order not to degrade the characteristics of the amplifier, a high-quality potentiometer should be used here.

The budget version can be products from Alpha or RadioShack costing about $3. For $40 you can purchase an audiophile-grade product from ALPS. The best solution will use a strip attenuator from DACT or GoldPoint. Their cost is approximately $170. By the way, on eBay you can find similar Chinese-made attenuators for only $30. The potentiometer rating can be in the range of 25-50 kOhm. The use of a step attenuator, in addition to the convenience of volume control, additionally guarantees identical adjustment in both stereo channels, which is especially important in a headphone amplifier.

Design

All structural elements (except for the power transformer) are placed on one printed circuit board. If you decide to use an external power supply or assemble it in a different way, about 70% of the PCB will remain free.

The layout of the elements is shown in the figure:

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The figure shows a drawing of the printed circuit board from the parts side:

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The figure shows a drawing of the bottom side of the printed circuit board:

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Printed circuit board drawings in the popular SLayout format can be picked up

The main installation feature: on the case on the bottom side of the TPA6120A2 there is a contact pad of approximately 3x4mm. She must be soldered to the area on the printed circuit board under the chip, which serves as a heat sink.

Photo of the finished structure:

When you turn it on for the first time, you should remove the two fuses at the output of the power supply and make sure it is working. If the output voltages are normal, replace the fuses. The amplifier itself does not need adjustment.

The board can be placed in a case of suitable dimensions, preferably metal, to shield it from external interference.

Conclusion

Subjectively, the amplifier sounds on par with professional studio equipment. When compared to the LM386, this design showed a smoother, cleaner and more detailed sound.

The scheme turned out to be quite flexible and easily customizable to suit various needs. For example, the author himself assembled two copies of the amplifier. One according to the above diagram for operation in conjunction with a tube preamplifier. The second copy was designed to work with a smartphone and a guitar amplifier, so it was supplemented at the input with a high-frequency noise filter and a volume control. In addition, to increase the gain (the smartphone produced an insufficient signal level), the values ​​of resistors R6 and R14 were changed to 2 kOhm.

By changing the values ​​of these resistors, you can change the gain within a wide range.

A variant of the amplifier printed circuit board from our “Martian friends”, designed for installing elements in “standard” packages (there are no DIP packages used in the design of microcircuits):

Animated demonstration of the board from all angles

High-tech casing made from electrical tape. Initially, I made the board under a heat-shrinkable tube - but literally a millimeter was not enough, it didn’t fit. Well, nevertheless, I like it.

Price issue

Total - this is exactly 11.06 rubles for me, and about 61.06 rubles if you buy everything :-)

results

You can solve this problem either by getting rid of the audio connection (USB DAC) or the power supply (battery or other power supply). I was completely satisfied with using a power supply with a USB output due to the fact that they are available everywhere and are standard.

The end result is beyond any expectations. No complaints about quality, absolutely 0 noise, comfortable volume level - from 22 to 40%, and a reserve for “pulling out” quiet recordings. The sound is richer (the main thing to remember is that the bass here starts from 0Hz) and all that, and in general - audio devices made by yourself always sound especially good :-)

It differs from ready-made Chinese devices (like the FiiO E3) by more low price(sic!), assembly with spare components, absence of capacitors in the audio path, greater power when working with high-impedance headphones (300 Ohms) due to a higher supply voltage, and the sound quality in theory promises to be higher (in practice I would probably didn't hear the difference).

PS. As I mentioned above, an amplifier is needed not to spoil your hearing with ultra-high volume (not to mention torn headphones), but to drive “heavy” headphones with low sensitivity, if the output sound card too dead. Well, extract quiet recordings/movies without software...

PS2. The difference between pluses and “added to favorites” is 4 times, a record :-)

An amplifier without gain, sounds strange doesn't it? But nevertheless, we will talk about him.

I'll start with a little background: having bought new headphones, I began to notice that there was not enough volume, but not so much that it would be much. Purely in recordings at which the volume level is low, you want to add a little more, and that’s the limit. And subjectively, it seemed that the sound card began to sound somewhat “compressed” with them, low frequencies left much to be desired. I think many people have encountered this.

Looking ahead, I can say that I myself have come to the conclusion that a headphone amplifier is needed. And not even so much as a sound amplifier, it is more needed to unlock the potential of the headphones. During the assembly of this amplifier, I noticed that the headphones began to play better than just from the sound card input. Perhaps this is due to the fact that there is some kind of “consistency” between the relatively low impedance of the headphones and the output of the sound card.

The headphone amplifier itself will be built on just one op-amp (operational amplifier), in this case the OPA2134 from burr brown. Since the voltage of the signal source does not need to be particularly amplified, the op-amp is switched on by a voltage follower. The repeater gain is equal to unity, or, to put it another way, there is no gain at all. Then why do we need such a scheme? Here it is quite appropriate to remember that there is a transistor circuit - an emitter follower, the main purpose of which is to match cascades with different input resistances. Such cascades (repeaters) are also called buffer cascades. Now the name “Amplifier without amplification” becomes clear.

In a word, when choosing a circuit, I decided to make a buffer, and yes, I know, turning on an op-amp at unity gain is not very good, but the OPA2134 copes with this, just like the NE5532 and TL072. And the output current is sufficient to “drive” the headphones. You don't need much there, do you? Well, 20mA, maximum 40mA, I don’t take into account especially sensitive headphones.

Below is a diagram of the buffer itself:

The diagram shows the op-amp itself and, literally, not a large handful of parts for its wiring. At the input there is a variable resistor of 50 Ω, then behind it there is a film capacitor C4 and C5 of 1.5 μF each, you can put more, say 2.2 μF, which is even better. Resistors R7, R8 and capacitors C2, C1 are needed as a kind of filter that serves as protection against the penetration of radio frequencies, and noise from the computer, maybe not significantly, but so be it. Resistors R5, R6 protect the op-amp input, it is better to install them, I installed them at 100 Ohms, but you can go up to 1 KOhm. Resistors R4, R3 protect the op-amp output, you can set them from 10-30 Ohms, you can do more, but why? Resistors R1, R2, or rather their resistance does not affect the “unity” of the gain, I personally set it to 30 KOhm, but you can set it to 47KOhm and it will work. Although the op-amp is not the fastest 20 V/µs, nevertheless, the power supply on the board was fully supplied. 100 nF and 1000 μF each, from the minus and plus legs of the power supply to ground, and also as close as possible to the op-amp, between plus and minus there is another capacitor, 100 nF. That's all the details, just a little bit.

When assembled, this buffer scarf looks like this:

It is also necessary to solder the wires from ground to the resistor body so that there is no hum when touching it, I did it like this:

All that remains is to make the power supply; I didn’t want to tinker too much with the power supply, so I made it according to the standard scheme on the LM7812 / LM7912. The only thing I selected was stabilizers so that there would be more or less similar tension in the shoulders. Actually the scheme:

Nothing special, I just added a couple of details, in the form of a varistor and an interference suppression capacitor, parallel to the transformer input. In the rectifier, I used SF26 diodes, HER107 could also be used. Yes, I understand, it seems, but why? You can install something simpler, but the price for them is not very high, and you don’t need a lot of them. And it seems to affect the sound, so I installed it. I also used 2 PTC 250 mA fuses, they were clean. I decided to put it on as a safety net, but I don’t have to put it on.

The power board looks like this, it turned out to be quite miniature:

As a result, this headphone buffer looks like this:

Now the actual result of the work done, there is basically no background, you can’t hear it, which I was very happy about after assembly :) The sound became better, I really felt it, not that it was somehow beautifully colored, purely subjectively it had added lows and mids frequencies, and the compression has passed. The volume is now enough for me, paradoxically from an amplifier without amplification. In a word, I’m happy with the work of the buffer; with such a simple design, you just need to try to assemble it :) Moreover, it works right away, and you don’t need to make any settings. I advise those who want to hear their headphones in a new way, but do not want to assemble something complicated yet. Personally, now I’m thinking about how to make a body for it.

List of radioelements

As they say, everything ingenious is simple. This amplifier consists of a minimum of parts, allowing the signal to pass through a minimum of elements, and thereby protecting it from the distortion that these elements can introduce.

The amplifier has a power of 500 mW. The calculated level of distortion when using a chip like OPA2134 is 0.001%. Load resistance 32-300 Ohm.

A volume control is assembled on R1 and R2, or rather it is one double resistor. At the input there is a sandwich of 4.7 and 0.47 µF capacitors, which allows you to achieve maximum linearity. Inverting amplifiers with a gain equal to 4 are assembled on IC1.1 and IC1.2. Next are transistor repeaters. OOS is formed by R6 and R5. R11 and R12 limit the current flowing from the op-amp to the bases of the repeaters; this makes life easier for the op-amp, and there is a little less distortion. R7, R8, R9, R10 limit the current of the repeater transistors and protect them from through currents. The circuit is powered by bipolar voltage and has built-in filtering circuits on stabilizer chips 7812 and 7912. At the output there are capacitors that prevent direct voltage from reaching the output.

You can use LM358 as IC1 as the most affordable option, but for high-quality sound I advise you to use a more expensive analogue.

The printed circuit board includes all elements except the connectors. Its dimensions are only 50x50mm. This size was chosen with the goal of ordering boards from the Chinese in the future, fitting into the cheapest lot measuring 5x5cm. In general, this project was originally planned to be used as a commercial development, but I still decided to make it publicly available.

The first board was made using the plotter application method:

The shaft is small, so fastening is carried out using a standard variable resistor nut. The assembled device looks like this:

List of radioelements

Due to the purchase of a new sound card without a headphone output, I had a need for a decent quality headphone amplifier capable of driving my favorite TDS-4. The amplifier had to be compact, easy to assemble and set up, with low level noise and distortion. Eventually, assembled amplifier met all the above requirements.

The characteristics of the amplifier were measured using the RMAA 6 program. A single channel layout was tested (the program worked in MONO mode), measurement results:
Frequency response unevenness (in the range 40 Hz - 15 kHz), dB: +0.05, -0.74
Noise level, dB (A): -90.9
Dynamic range, dB (A): 90.9
Harmonic distortion, %: 0.0014
Intermodulation distortion + noise, %: 0.010
Intermodulation at 10 kHz, %: 0.0084

The amplifier is built according to the op-amp + output transistor buffer circuit. Op amp provides high open loop gain feedback, necessary for suppression nonlinear distortion using deep OOS. The output buffer performs current amplification by matching the low resistance of the headphone coil to the low-power output of the op amp. The circuit uses dual high-speed op-amp K574UD2. The signal from the source through the isolation capacitor C3 and resistor R1 is supplied to the non-inverting input of the op-amp. Resistor R4 sets the operating point of the amplifier according to DC. Elements C1, C2, R2, R3 provide frequency correction of the op-amp. The output buffer is made according to a “parallel” circuit. This circuit was chosen because it lacks the transient distortion associated with conventional push-pull circuits. When using transistors with similar parameters, the voltage drops at the base-emitter junctions of the pre-final and final cascades are mutually compensated. Buffer transistors, being installed on a common heat sink, mutually thermally stabilize each other. The op-amp and buffer cascade are covered by a total 100% environmental protection in constant and alternating current, the gain of the circuit is 1.

It is advisable to use film capacitor C3. C1, C2, C6, C7 – ceramic. All resistors are MLT-0.125 type (or imported analogues). Transistors VT1 KT315G, VT2 KT361G, VT3 KT815G, VT4 KT814G. It would be preferable to use transistors KT815G and KT814G as VT1 and VT2, for reasons of identical parameters and the ability to easily organize thermal contact of all four buffer transistors. The op-amp can be replaced with any other high-speed one with a corresponding change in the set of corrective elements and the layout of the printed circuit board. The amplifier is powered from a bipolar unstabilized power supply. The power supply uses a 220/20 transformer with a center tap secondary winding. Any diode bridge for voltage 50V and current up to 1A. It is possible to use diodes of the 1N4001-1N4007 series. The capacity of capacitors C4, C5 is at least 1000 µF (I used 4700 µF)
A properly assembled amplifier does not require adjustment. It is necessary to check the current consumption (about 30 mA for a two-channel amplifier) ​​and the constant voltage at the output.

The parts of the amplifier and power supply are placed on a common board measuring 35x78mm. The transistors of each channel are attached through insulating gaskets to a common U-shaped heat sink. The area of ​​the heat sink is unimportant, the main thing is that it ensures thermal contact of the transistors.

The printed circuit board is single-layer with jumpers, laid out in Sprint Layout 5. In the author’s version, non-foil PCB was used, the parts were installed in holes, the pins were connected with copper wire.

Literature:
Amplifier block of an amateur radio complex. A. Ageev, Radio No. 8 1982
The Sapphire Desktop Headphone Amplifier - http://phonoclone.com/diy-sapp.html

List of radioelements