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Yet another headphone amplifier ty.ytka

tytka 3195 14
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  • Headphone amplifier in a silver case with control buttons. .

    I hope I don't bore you, with my next headphone amplifier. But somehow it just so happened that I've had the verve for such projects lately.

    This headphone amplifier was created to be used with a phone or laptop. Hence my decision to power it from the built-in batteries (for easier and more versatile use), and to only equip it with a USB digital input.

    The schematic of my amplifier itself is overwhelmingly taken from the datasheet of the LME49600 chip. This chip is a very interesting buffer, ideally suited to the construction of such a device. It has been enhanced here with a USB DAC realised on the PCM2705 chip, which is easy to apply and works with various operating systems without problems. In addition, the amplifier has a battery charging circuit, as well as a cell balancing circuit. The latter two, are in the form of off-the-shelf modules. I made this decision mainly because these modules were already lying in a drawer, waiting to be used.

    Electrical schematic of a headphone amplifier with USB. .

    The circuits used, in terms of supply voltage, can work when powered by two cells connected in series. I, however, chose to power the device from a battery made up of four cells. And why did I choose these cells? Initially, I was thinking of using smaller sized cells, but as 18650 are probably the most popular, their price is more affordable than the smaller ones. Besides, on these, the amplifier can work for a really long time.

    Before I go any further IMPORTANT NOTE for possible role models! .
    The ground of the charging module, has a different potential than the ground of the amplifier circuit. They cannot be combined!
    For this reason, an insulator has been glued into the charging connector hole located on the rear panel, in order to prevent accidental short-circuiting of the grounds when connecting the charger plug.

    Close-up of a panel with a charging port and the label IN 5V 2A. .

    Back to my design. I have designed the following board:

    PCB design for a battery-powered headphone amplifier. PCB design for a headphone amplifier. .

    Which I commissioned from a company (I think) in a known part of the world.

    As for the casing. I think you will already recognise it in part. The main part of it is taken from one of my previous projects, which underwent a metamorphosis some time ago and received a new enclosure. So, these I have used here to make useful.
    In completing the project on the mechanical side, a few more small components were needed. These include:

    Metal components and potentiometers on a white background. .

    In order not to bore you with descriptions, hereafter just a few photos taken during the project.

    Prototype headphone amplifier board with built-in batteries. .
    Printed circuit board of a headphone amplifier with electronic components. .
    Electronic headphone amplifier board with components. .
    Headphone amplifier on a yellow circuit board with slots for four cells. .
    Printed circuit board with a headphone amplifier circuit and four 18650 batteries. .
    Headphone amplifier board with built-in modules. .
    Rear view of the headphone amplifier case with visible USB socket and battery connectors. .
    Metal casing of a headphone amplifier with volume knob and headphone jack on the front panel. .
    Headphone amplifier in a metal case on a wooden background. .
    Side view of a silver headphone amplifier on a wooden surface.
    Headphone amplifier enclosure with metallic finish. .
    Metal enclosure of a headphone amplifier with visible USB ports and labels. .
    Silver headphone amplifier with volume knob and headphone output. .
    Headphone amplifier in a metal casing with four rubber feet. .
    Silver headphone amplifier on a wooden surface. .

    Below I still include the results of the tests carried out. Looks to the fact that, as in the design of my amplifier in the mains-powered version, the converter block from the PCM2705 affects the measured parameters in a negative way. i will still review this carefully.

    RightMark Audio Analyzer test report for headphone amplifier .
    Graph showing distortion (%) versus frequency (Hz) for a headphone amplifier. .

    Greetings and feel free to comment.

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    About Author
    tytka
    Level 22  
    Offline 
    tytka wrote 684 posts with rating 1745, helped 8 times. Live in city Pabianice. Been with us since 2006 year.
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  • #2 21303473
    -rafal-
    Level 15  
    I have read the whole description and looked at the schematic and I still don't know what the circuits on the output are :) .
    Why USB type A? I don't think this is their typical use (being an "input") I guess a "printer" type B would fit better here. Or generally putting it on a single USB-C connector.
  • #3 21303535
    tytka
    Level 22  
    Sorry, the symbol for the output circuits will not appear on the schematic, but they are listed in the description, it is LME49600.

    And why USB-A?
    Due to lack of space, I couldn't manage to use a dedicated B-type connector for such purposes (the batteries take up almost the entire width of the case).
    The Type-C connector, on the other hand, I did not want to use for the simple reason of not confusing the input socket with the charging socket.
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  • #4 21303723
    remiorn
    Level 18  
    I wonder why such poor performance. Over 20 years ago I made a similar circuit, only on a PCM2900 chip. And the parameters were much better, inter-channel transfer -90dB, noise -96dB, which is practically as much as can be achieved with a 16-bit converter. Maybe the later chips are cheaper and worse.
  • #5 21305237
    djfarad02
    Level 19  
    It is not known how it was measured. I presume some other card of unknown grade.
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  • #6 21305432
    tytka
    Level 22  
    remiorn wrote:
    More than 20 years ago I did a similar circuit, only on a PCM2900 chip. And the performance was much better
    .
    djfarad02 wrote:
    Not sure how it was measured.


    Just out of curiosity I measured the DAC with the PCM2902. This is what the Behringer UCA222 I own is made on.
    I did this, the USB input from the UCA222 plugged into the computer, and its analogue output connected to the line input on my computer.
    Here the result of this measurement:

    Screenshot of a RightMark Audio Analyzer report for PCM2902 DAC. .

    And here the result of my measurement card, i.e. my reference point:

    RightMark Audio Analyzer test report for Xonar D2X device. .

    I don't know why, but with me the DACs of these PCMs, come out poorly in the tests.

    Today, I slightly improved the power supply condition of the PCM2705 chip in my amplifier, which resulted in some improvement in the performance obtained in tests.

    RightMark Audio Analyzer test report for HA-025 audio device. .

    And here are the test results of my headphone amplifier minus the PCM, i.e. for the analogue part only.

    Screenshot of RightMark Audio Analyzer test report with audio test results. .
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  • #7 21305441
    djfarad02
    Level 19  
    I used to apply Wolfson's WM8776 cubes. Theoretically also with excellent performance. Equally, I encountered the problem of quite high noise levels. The chips were from aliexpress - maybe rejects.
  • #8 21305465
    tytka
    Level 22  
    djfarad02 wrote:
    maybe it's discards.


    Yours I cannot rule out. Maybe, in fact, this is the cause here.
  • #9 21305474
    andrzejlisek
    Level 31  
    Very nice, well made and certainly satisfying, so the purpose of the construction was fully achieved.

    However, at first glance it seems that, since this is an amplifier for headphones, its dimensions are, in my humble opinion, far too large. At home or there in the office it's not a problem, worse outdoors and on transport. I understand that the laptop and phone played too quietly, hence the need to build an amplifier.

    Like a dozen years ago I had the same problem, i.e. the need for an amplifier for headphones, and in choosing the components and how to build it I was guided by the following criteria:
    1. It has to be cheap and simple to build.
    2. the amplifier itself with dimensions similar to a matchbox.
    3. battery powered separately on wires.

    I settled on using two 6LR61 or 6F22 batteries for the power supply, which did the job.

    I just searched for my topics where I discussed what and how to make such an amplifier:
    https://www.elektroda.pl/rtvforum/topic602671.html#3114667
    https://www.elektroda.pl/rtvforum/topic1180464.html#5918700
    I never tested or measured my amplifier, the main thing was that it played and gave a satisfactory volume in the headphones. I had a volume control on every device that played sound, so no potentiometer in the amplifier was needed.
  • #10 21305527
    acctr
    Level 38  
    andrzejlisek wrote:
    At home or there in the office this is no problem, worse outdoors and on transport.
    .
    What is the point of using an amplifier in such surroundings?
    The noise from outside nullifies the quality obtained from it. And those who want to cut out the noise use headphones with active noise reduction, usually with bt.
    Helpful post? Buy me a coffee.
  • #11 21305577
    andrzejlisek
    Level 31  
    acctr wrote:
    What is the point of using an amplifier in such surroundings?
    .
    The only, and primary, point is to "shout out" the noise of the vehicle or ship during the journey. Even a 'muted' train inside is in fact loud. The headphones, were the cheapest ones, pressed into the ears. The cheapest because, in my experience, with heavy use, such headphones often break from stretching the wires and there was no point in overpaying. They are pressed into the ears so that they are small and easy to put in the pocket when not needed. Active noise cancellation was unlikely to exist in budget equipment. It was a case of use it, don't bother. and if it breaks, throw it away and buy new ones.

    The Gameboy I write about in those topics had the disadvantage that the sound produced even at maximum volume was too quiet and the only point and use of an amplifier was to increase the range of possible volume.
  • #12 21306482
    remiorn
    Level 18  
    >>21305432 .
    My experience with these circuits was that good PCB design and manufacture was critical.
    A circuit with improperly routed ground was able to exhibit noise levels some 20-30 dB higher.
    And of course good power supply filtering. I was doing the circuit as part of my thesis, so I had plenty of time to test various options. It turned out that moving the 100nF capacitor literally a few millimetres away from the circuit worsened the signal-to-noise ratio by a few dB.
    Nevertheless, the most puzzling thing to me is the poor channel separation. Although utilitarian, in my opinion, this is of relatively minor importance. With vinyl records, for example, we get about 20 dB and this is sufficient for stereo to be heard.
  • #13 21306492
    acctr
    Level 38  
    andrzejlisek wrote:
    The only and also basic sense is to "shout out" the noise of the vehicle or ship while travelling.
    .
    This is a very bad method because only hearing suffers.
    When I used to travel on Polish rolling stock I used earplugs and over that earphones.
    Helpful post? Buy me a coffee.
  • #14 21311948
    _J23
    Level 1  
    The amplifier looks very good, and the DAC measurements suspiciously poor. Isn't packing batteries into such a simple design an excess of form over substance...? If it were the headphone amplifier itself then an independent power supply would be most advisable, here we have a DAC that takes data and is powered by USB. This 5V, 500mA can be used to power the DAC + headphone amp - simpler, better, mini.
    As for the measurements, perhaps the ground loop is responsible for the drop in form ;) of the DAC and it doesn't matter how the analogue part is powered, because the DAC itself is powered from usb. Try connecting the DAC via galvanic separation - your tests should improve. It seems that the pcm2702 has better performance and sounds more analogue.
  • #15 21312709
    tytka
    Level 22  
    The PCM2702 chip, has not been in the TI range for some time now. So it is also more difficult to obtain it.

    Yes my amplifier, especially with those rather large batteries, may seem like an overdesigned design. But from my assumptions, it wasn't meant to be a mini, mobile type of design in the sense of walking around. It's meant to provide me with good drive for my headphones at home or when out and about, to be fairly easily portable and independent in terms of power. And that's what I was able to achieve.

    As for the poor performance of the DAC's measurements, I've tracked a few things, have some suspicions and will be correcting/checking this. On the results of my actions, I will keep you informed.
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Topic summary

The discussion revolves around a custom-built headphone amplifier designed for use with phones and laptops, powered by built-in batteries and featuring a USB digital input. The amplifier utilizes the LME49600 chip as a buffer and incorporates a PCM2705 USB DAC. Users express concerns about the performance of the DAC, comparing it to older models like the PCM2900, and discuss issues related to noise levels and channel separation. The design choices, including the use of a USB-A connector due to space constraints and the overall size of the amplifier, are debated. Some participants suggest improvements in PCB design and power supply filtering to enhance performance, while others question the necessity of such an amplifier in noisy environments.
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