Acoustic amplifier for those who hear differently

yego666 23 Jun 2024 16:43 68 8715 Cool? (+33)

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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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Similar topics have already been discussed on the Forum, but my device is the most mine, so I decided to share a report of its construction with those who either have hearing problems or would like to make a similar device for someone with such a problem.

I'm not an acoustician and I don't do acoustics at work, so professionals versed in the art may find the presented device technically poor, incompatible with the art and Be at all, but it has an undeniable utility value confirmed by the person concerned, with whom I can now communicate freely.

I know you can buy a fairly decent amplifier on the MAX9814, however, where is the fun and learning here?
So I borrowed the base schematic from the website : Link .
I made this device unchanged, but it had some significant drawbacks such as a very high resting current ( battery life ) and a lot of noise.
So I decided to simulate this circuit under LTSpice, which pointed me in the direction of improvements.
This resulted in a circuit with relatively low noise, a flat gain characteristic from about 80Hz to the limits of audibility ( probably around 10kHz ) and a negligible resting current of less than 3mA.
The final shape of the circuit is shown in the diagram below:

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and here is a revised version of the final layout form:

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What the layout is like everyone can see. A board was also created, of course :

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and then it was enhanced with the relevant electronic components:

It's not a master craftsmanship, but it works and follows the schematic.
After attaching the LiIon battery, headphones and switch, I performed tests and basic measurements.
Actually the resting current is about 3.2mA and the noise is very negligible. The circuit picks up sounds well from about 5 metres and, most importantly, does not distort them in the audible range of sound waves. The goal has therefore been achieved.
Now only the casing remained to be made. Since I acquired a 3D printer, this is no longer a problem and instead of adapting the device to the housing I have, I make a housing for the circuit I made. Very convenient and quick, and I don't have to search for the right box.

The box consists of the bottom and of course the top :

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And after printing it looks like this:

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Now the previously made layout goes into a new box :

and that's pretty much it.
One more thing, I originally used a BC109C transistor in the input stage, but the noise was too high, so I replaced it with a 2SC9014. The noise dropped well below the expected value, so that's how it stayed.

I use an old power supply from a Nokia phone as a charger. The battery is charged with about 100mA through a 20 ohm resistor. Ten hours is completely sufficient to fully charge the battery. This is not the recommended way to charge LiIon batteries, but if someone wants to, they can connect an external circuit purchased from MyFriends instead of a simple phone charger..

I use 32 ohm headphones connected in series on the output, but both 16 and 64 will work well too. Volume control is via a potentiometer on the headphone cable.

I don't expect applause, but if this device is useful to someone, I think I'll consider the goal achieved.

I encourage esteemed colleagues to enrich the topic with their valuable comments and ideas.

About Author

yego666 wrote 2175 posts with rating 564 , helped 239 times. Been with us since 2004 year.

Comments

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md 23 Jun 2024 17:24

I suggest doing a little test to show that the mere fact that the 'interested person' hears better or hears at all is already something, but something will be missing. Just hide from the 'interested person'... [Read more]

gulson 23 Jun 2024 18:34

Thanks, I for one am glad to see hearing aid designs still appearing. It reminds me of the beautiful days of electrode from the early days of the service. Respect to you and for presenting the construction. Write... [Read more]

Mateusz_konstruktor 23 Jun 2024 20:57

"Helpful post" clicked. The type of circuit breaker used by the Fellow has the drawback of a rather flimsy slider design. I suggest replacing the switch currently used with one from the photo below. This... [Read more]

TechEkspert 23 Jun 2024 21:06

Very interesting analogue design, hearing aids used to be made like this at Omig, on germanium transistors: http://bee.mif.pg.gda.pl/Oktoda/index.php/AS-571 https://a.allegroimg.com/s1024/0c44b3/18d71e9a4772b0b8b948573a334b In... [Read more]

yego666 23 Jun 2024 23:30

You are right in the general case, and undoubtedly anyone wishing to make such a device on their own should take your advice into account for the benefit of the user of such a device. However, in this... [Read more]

TechEkspert 23 Jun 2024 23:37

I checked out of curiosity and there is a free 'hearing test' app anyone can easily assess the state of their hearing organ. [Read more]

kris8888 24 Jun 2024 15:13

Why transistors Q5 and Q6 and not ordinary diodes in this location? Does this have any significant effect or influence on anything (on e.g. noise)? [Read more]

yego666 24 Jun 2024 16:54

In the first version I used 1N4148 diodes, which is what was recommended, but I noticed on the oscilloscope a waveform skip ( actually it was a saddle) when switching the output transistors. After replacing... [Read more]

kris8888 24 Jun 2024 17:09

. Yes, this is crossover distortion. It's just that when you used diodes there was too little resting current for the terminal transistors. Apparently transistors Q5 and Q6 as diodes have a minimally... [Read more]

yego666 24 Jun 2024 18:42

Thanks for the expert explanation and definition. "Cross-cutting" - nice word :) . I did not pair transistors Q5 and Q6 but Q3 with Q5 and Q4 with Q6 to match the voltage drops across the connectors... [Read more]

kris8888 24 Jun 2024 19:13

This is also unnecessary because all that matters is the sum of the voltage drops on Q5 and Q6. And in what proportions and whether they correspond exactly to the E-B voltage drops of transistors Q3 and... [Read more]

LEDówki 24 Jun 2024 19:16

Have you not thought about the TDA2822M? It will replace 5 transistors. [Read more]

kris8888 24 Jun 2024 19:30

. It draws about 5mA of resting current at 3.7V supply. More than the author managed to wring out in this title project. And this, as I understand it, was also one of the design goals, i.e. to reduce... [Read more]

yego666 24 Jun 2024 19:48

I considered various configurations and circuits, both integrated and less integrated. In the end, the spirit of the experimenter and a foolish belief in my abilities in a field in which I had no experience... [Read more]

LEDówki 24 Jun 2024 19:57

The TDA2822 does not do resting current problems. Classical class AB power stages have current regulation and a transistor to maintain the correct voltage between the bases of the power transistors. I... [Read more]

acctr 24 Jun 2024 20:36

If not a 2822/7050 then perhaps a ne5532 opamp plus a complementary AB power amplifier would suffice, all covered by the feedback. Minimal components, low noise and low resting current. [Read more]

yego666 24 Jun 2024 21:39

I have noticed that there have been proposals for designs based on a variety of integrated circuits, but the proposals are so far very general. My problem - and I think a lot of people have a similar... [Read more]

acctr 25 Jun 2024 17:27

The idea is more or less as shown in the diagram https://obrazki.elektroda.pl/6165262800_1719329224_thumb.jpg . [Read more]

yego666 25 Jun 2024 18:15

The amplifier schematic looks logical and is uncomplicated, however, have you tested it? Furthermore, it seems to me by the output transistors that these headphones can have even a few watts of power,... [Read more]

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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