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Acoustic amplifier for those who hear differently

yego666 8715 68

TL;DR

  • A custom acoustic amplifier for people with hearing problems uses headphones, a LiIon battery, and a small PCB in a 3D-printed enclosure.
  • The circuit was redesigned in LTSpice from a borrowed base schematic to reduce noise and resting current while keeping the gain flat from about 80Hz to 10kHz.
  • The final version draws less than 3mA at rest, and the measured idle current was about 3.2mA.
  • It picks up sounds from about 5 metres and does not distort them in the audible range.
  • Replacing the BC109C input transistor with a 2SC9014 lowered the noise well below the expected value, but the LiIon battery charging method is not recommended.
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  • #61 21145025
    CMS
    VIP Meritorious for electroda.pl  
    acctr wrote:
    that the insect would fall into the bat's mouth.


    And here is an interesting fact unrelated to the topic, but related to bats. They more often catch insects with their "hand" than with their muzzle.
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  • #63 21145061
    CMS
    VIP Meritorious for electroda.pl  
    Well, because he has fingers on his elbow :) .
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  • #64 21145293
    bratHanki
    Level 39  
    acctr wrote:
    bratHanki wrote:
    If you are standing next to someone talking can you only hear them when you are in front or when you are standing next to or even behind them?.
    .
    And how are you supposed to hear? only straight ahead? why not sideways? this is anatomy and drawing conclusions about the nature of sound based on it is a bit infantile.

    Well, that's right, I hear sideways because there's something to hear on the sides too, because that's the nature of sound wave propagation in the low ranges. The example with netcopters is not quite right because they locate their prey by ultrasound and these are eminently directional. The lower the frequency, the more difficult it is to locate the sound source, which is why music systems have a common bass channel that can be placed anywhere and why satellites reproducing midrange and treble should be positioned at specific locations in relation to the listener because, thanks to the physiology of our hearing, we are able to accurately locate sound sources at higher frequencies.
    Adult males have an average fundamental frequency of around 120 Hz, while adult females have an average of around 210 Hz, a range that is not particularly directional, meaning that two microphones placed next to each other, no matter at what angle, will pick up almost the same signal.
    Two years ago I had my hearing tested. The audiologist was surprised that, at 64 years of age, I could still hear 12-13 kHz. So she checked me on the bone and, to her surprise, I repeated the result.
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  • #65 21151213
    PiotrPitucha
    Level 34  
    Hello
    I would approach this from a different angle, this circuit is like from the time of Janusz Wojciechowski, that is from the 1960s.
    First of all, I would do it on an operational amplifier, preferably quadruple, necessarily R2R, because all designs like TL082 with power supply from a single Lion cell can be put between the fairy tales.
    For example, something from the MCP6042 or 6044 series, resting current 600nA (nanoamperage), supplying 1.2V to 6V, bandwidth up to 14kHz is completely sufficient for such instruments.
    With the relatively high efficiency of today's headphones, you can plug directly into the amplifier output without additional terminal transistors.
    With four amplifiers, you may be tempted to use frequency correction and/or tone mapping.
    Regards
  • #66 21151475
    Janusz_kk
    Level 39  
    PiotrPitucha wrote:
    the bandwidth up to 14kHz is quite sufficient for such instruments.
    .
    Well when not, at a gain of 10 such an amplifier will have ~1.5khz bandwidth:( this is far too low.
    Better would be MCP6L04T, it has admittedly higher current because 85uA but also 1Mhz bandwidth, and it is definitely cheaper :) because it doesn't cost 12 zł only 3:) and the same has power supply 2.7 to 6V.
    https://www.tme.eu/pl/details/mcp6l04t-e_sl/wzmacniacze-operacyjne-smd/microchip-technology/
    But in general you are right, compression and filters are needed and this is hard to do without an opamp.
  • #67 21151481
    yego666
    Level 33  
    @ PiotrPitucha .
    Your proposal is thoroughly modern and undoubtedly achieves excellent performance, but it has several drawbacks.

    1. lack of a diagram
    2. the need to purchase new parts to make the device
    3. no demonstration of a working and tested device.

    Maybe if I had one of the circuits you mentioned, I would be tempted to try to make such a device, but I find it more appealing to make things based on elements I have at hand (usually recycled ones), which greatly reduces the time of production and probably costs as well.
    Instead of buying something new, I can make a circuit giving a "second life" to working components doomed to extinction.

    Maybe one of my colleagues, who is not satisfied with the presented construction, will make and present an amplifier based on your rather imprecise description.

    I suppose that in some time some tube enthusiast will propose a tube-based design. And good, because maybe it will inspire someone :) .
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  • #68 21151528
    PiotrPitucha
    Level 34  
    Hello
    An amplifier like an amplifier, you can try it on any operational amplifier, amplifiers on operational amplifiers work as simple as voltage dividers :) only instead of dividing they amplify the signal.
    I could cite dozens of solutions, but they all boil down to one solution, like this one for example:
    Link .
    The only difference is in the circuit I was proposing because you can feed it with a lower voltage.
    Compressors unless they are made on dedicated circuits also have a similar design, the rectified voltage from the output controls a component that limits the voltage coming from the microphone, for example:
    Link ]
    By connecting such blocks together, we can build circuits with a gain tailored to the specific microphone and headphones you have.
    The circuit can be simulated in any analysis software, although with operational amplifiers, the amplification can be seen on the schematic as if on the palm of your hand.
    So that you don't have to bother, there are ready-made circuits that take care of compression , noise gates, etc. For example:
    Link
    Greetings

    Added after 17 [minutes]:

    Hello
    Janusz_kk with the bandwidth you are right, I gave the amplifier as an example of an R2R amplifier with a low supply voltage, I myself used the MCP607 for similar purposes, which in addition has excellent DC performance.
  • #69 21151790
    Janusz_kk
    Level 39  
    PiotrPitucha wrote:
    To take the headache out of it, there are ready-made circuits that take care of compression, noise gates, etc. , for example:
    Link
    .
    And that's it, one small cube and it takes care of the whole thing, only the headphone amp and the filter are missing, something really needs to be done here.
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Topic summary

✨ The discussion revolves around the construction of a custom acoustic amplifier designed for individuals with hearing difficulties. The author shares their experience in building the device, which utilizes a modified circuit based on the MAX9814 amplifier. Key improvements were made to address issues such as high resting current and noise, achieved through simulations in LTSpice. Participants suggest enhancements, including dual-channel configurations with two microphones and headphones for better sound localization, and the use of operational amplifiers like TL071 and TL072 for improved performance. Various components and configurations are discussed, including the use of transistors, diodes, and integrated circuits to optimize sound quality and reduce distortion. The conversation also touches on practical challenges, such as isolating the microphone from unwanted noise and the emotional impact of improved hearing on the user.
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FAQ

TL;DR: With 3.2 mA quiescent current and pickup from about 5 metres, this DIY hearing amplifier shows that "the goal has therefore been achieved" for people who need a simple portable speech booster. It suits makers building a low-power analog aid from spare parts, not a full medical hearing aid. [#21129428]

Why it matters: This thread turns a one-off hobby build into practical guidance on noise, battery life, microphone mounting, and safer upgrade paths for real users.

Approach Supply target Idle current / power note Main thread takeaway
Discrete transistor build Single Li-Ion cell 3.2 mA measured Proven, quiet enough, built and tested
TDA2822M 3.7 V discussed About 5 mA idle Simpler, but worse for battery life
TL07x / TL072 idea 9 V or at least 4.5–6 V Not optimized for 1-cell Li-Ion Better suited to higher supply
OP193 / low-voltage op-amp idea Single-cell friendly Very low idle current claimed Promising, but not built in-thread

Key insight: The most useful improvement was not extra complexity. It was matching the circuit to one user: lower noise, low standby drain, and mechanics that the user could actually wear and switch on easily.

Quick Facts

  • Measured quiescent current was about 3.2 mA, and the author reported useful sound pickup from about 5 m without audible-band distortion. [#21129428]
  • The finished unit used an about 800 mAh Li-Ion cell under the PCB, giving roughly about a week of runtime if left on continuously. [#21130913]
  • Charging was done from an old Nokia charger at about 100 mA through a 20 Ω resistor for 10 hours, which worked in practice but was explicitly described as not recommended for Li-Ion cells. [#21129428]
  • Headphones of 16 Ω, 32 Ω, and 64 Ω were all reported as workable, with the author using 32 Ω earphones in series and cable-mounted volume control. [#21129428]
  • Replacing 1N4148 bias diodes with transistor-connected Q5/Q6 removed visible output-stage switching artifacts identified later as crossover distortion. [#21130748]

How was this homemade hearing amplifier circuit improved over the original Talking Electronics design to reduce noise and cut quiescent current below 3 mA?

It was improved by simulating the borrowed Talking Electronics circuit in LTSpice and then reworking it for lower idle drain and less noise. The author states the original had very high resting current and much noise, while the revised version achieved a flat gain response from about 80 Hz to the limit of audibility, around 10 kHz, with under 3 mA intended idle current and about 3.2 mA measured. That makes the redesign a practical optimization, not just a copy. [#21129428]

Why did replacing the BC109C input transistor with a 2SC9014 noticeably reduce noise in this hearing amplifier?

Replacing the BC109C with a 2SC9014 reduced input-stage hiss in this specific build. The author tested both parts and reported that BC109C made the noise too high, while the 2SC9014 dropped noise well below the expected level, so it stayed in the final version. The thread does not provide lab noise figures, but it clearly records a before-and-after hardware change on the microphone input stage. [#21129428]

What is crossover distortion in a class-AB transistor output stage, and why did using transistors Q5 and Q6 instead of 1N4148 diodes help eliminate it?

"Crossover distortion" is output-stage distortion that appears when complementary transistors hand over conduction near zero crossing, creating a notch or gap if bias is too low. Using Q5 and Q6 as diode-connected transistors raised the effective bias above what 1N4148 diodes provided, so the output pair no longer showed the visible "saddle" on the oscilloscope. A later reply explicitly named the fault as crossover distortion and tied it to insufficient resting current with plain diodes. [#21130756]

How do you properly charge a single-cell Li-Ion battery in a DIY audio device, and what are the risks of using a Nokia phone charger with only a series resistor?

You should use a dedicated Li-Ion charging circuit, not only a phone charger and resistor. The build used an old Nokia charger, about 100 mA charge current, and a 20 Ω series resistor for roughly 10 hours, but the author clearly said this is not the recommended method. The practical risks in the thread are simple: the method is crude, it bypasses a proper charger, and the author advised adding an external charging module instead. [#21129428]

What's the best way to make a hearing amplifier dual-channel with two microphones and two headphones so the user can better locate the direction of sound?

Make it fully dual-channel, with two microphones and two separate ear outputs positioned to preserve left-right cues. One reply says bilateral users need two microphones and two headphones so they can locate sound, and suggests two independent amplifiers mounted in the earcups, with shared power from a pendant battery. That layout is the clearest thread-backed path to better directional hearing than a single-microphone mono pendant. [#21129479]

How can I reduce knocks, rubbing noise, and enclosure resonance picked up by an electret microphone inside a 3D-printed case?

Move the microphone mechanically off the enclosure and give it a damped acoustic path to the outside. The most actionable thread method is: 1. suspend the microphone on rubber or soft supports, 2. avoid placing it loose inside the resonant box, 3. connect it to the outside through a short acoustic tunnel or channel. Several replies say the case acts like a resonance chamber, while old phone designs used rubber mounting plus a few-millimetre channel to isolate internal noise. [#21133970]

What is an audiogram, and how can it help tailor a DIY hearing aid or speech amplifier to a person's hearing loss?

"An audiogram" is a hearing test chart that maps hearing threshold versus frequency for each ear, showing where amplification or correction is needed. In the thread, it is presented as a practical design input because it gives separate left and right ear curves and can include both air and bone conduction results. That lets a builder decide whether speech needs treble lift, bass restraint, or different treatment per ear instead of flat broadband gain. [#21129799]

How does a dynamics compressor or limiter work in a hearing amplifier, and why is it useful for suppressing overly loud sounds while boosting quiet speech?

It monitors signal level and reduces gain when sound gets too strong, so quiet speech stays audible without letting loud peaks become uncomfortable. One reply says a proper hearing device should include a limiter or compressor that amplifies quiet sounds, passes loud sounds, and suppresses very loud ones. Later, another poster describes a simple analog approach: rectify the output signal and use it to control an input attenuator. That makes compression a practical safety and intelligibility feature, not just an audio effect. [#21144450]

TDA2822M vs a discrete transistor amplifier vs an op-amp design like TL072 or NE5532 — which approach is better for a low-power hearing amplifier?

For the exact goal in this thread, the tested discrete transistor design is the best proven low-power option. A reply says TDA2822M draws about 5 mA quiescent current at 3.7 V, which is worse than the author’s sub-3.2 mA result. TL072-style ideas are simpler to commission, but later posts note TL07x is unsuitable for about 3 V single-cell operation. NE5532 was only suggested as a concept, not demonstrated. So the transistor build wins on verified battery life, while op-amps remain upgrade ideas. [#21130894]

Which low-voltage op-amps discussed in the thread, such as OP193, TL062, LMV358, MCP6044, MCP607, or MCP6L04T, are most suitable for a single-cell Li-Ion hearing amplifier?

The most suitable discussed parts are the truly low-voltage options, especially OP193, MCP607, and MCP6L04T. The thread rejects TL07x for single-cell Li-Ion because quoted minimum supply was 4.5 V or 6 V, and it criticizes MCP6044 because at gain 10 its usable bandwidth would drop to about 1.5 kHz. LMV358 was suggested for 2.7–5 V use, but another reply says it is not a low-noise audio part. MCP6L04T was favored for 2.7–6 V and 1 MHz bandwidth. [#21151475]

How do you choose the right headphones impedance for this kind of portable hearing amplifier, and what changes when using 16, 32, or 64 ohm headphones?

Choose the lowest impedance that gives enough volume without stressing the output stage or battery. The author directly reports that 16 Ω, 32 Ω, and 64 Ω headphones all work, and uses 32 Ω earphones connected in series with volume control on the cable. The thread gives no measured output power, so the practical takeaway is compatibility, not optimization. Lower impedance usually demands more current, while 64 Ω is easier on the amplifier but may need more voltage for the same loudness. [#21129428]

What is the occlusion effect in hearing devices, and why can a user start speaking more quietly after hearing their own voice more clearly?

The user can start speaking more quietly because better self-hearing changes their internal loudness reference. In the thread, regular use let the author’s mother hear both the environment and herself better, so she began lowering her voice and others had trouble hearing her. A later reply linked this to the occlusion effect discussion and suggested relocating the microphone rather than immediately redesigning the amplifier. The behavioral change appeared after about a week of regular hospital use. [#21144213]

How would I design a simple anti-local or self-voice reduction circuit using two microphones and a differential amplifier for a wearable hearing amplifier?

Use one microphone aimed at ambient sound and a second microphone aimed at the wearer’s own voice, then subtract them with a differential stage. One reply proposes placing the extra microphone on top of the box to catch what the user says and subtracting it from the front microphone so self-voice is reduced. The main failure case is also in the thread: if the microphones sit too close and hear nearly the same thing, the subtraction will be weak or unstable. [#21144260]

What microphone mounting method works best in small enclosures: rubber suspension, felt, foam isolation, an acoustic tunnel, or placing the microphone outside the case?

Rubber suspension plus an acoustic path to the outside is the strongest thread-backed choice. Foam and sponge were tried first and gave poor or unnoticeable improvement, while several replies say the microphone should not sit inside the resonant enclosure. The most concrete proven pattern comes from older devices: hang the microphone on rubber, connect it with thin wires, and use a short acoustic tunnel through the case. Felt was suggested, but not confirmed as the winning fix here. [#21133695]

How do I design and 3D-print a compact enclosure for a DIY hearing amplifier so the battery fits under the PCB and the microphone stays mechanically isolated?

Design the case around the finished PCB, leave a battery pocket under the board, and keep the microphone off the rigid shell. The author printed the housing as separate bottom and top parts, then confirmed the Li-Ion cell sits under the PCB. For the microphone, the thread’s best guidance is mechanical isolation: suspend it in rubber or mount it in a damped tunnel rather than hard-fixing it to the 3D-printed wall. That avoids turning the case into a contact-noise amplifier. [#21129987]
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