Circuits of hearing aids using transistors. Homemade hearing aid with aru
Recently, someone close to me needed a hearing aid. In many countries, such hearing aids are provided free of charge to people with hearing loss, but free cheese...
Here is one of the free devices, these can be bought in China for 2-3 dollars.
There are good devices, but their price is clearly more than 2-3 dollars.
The factory device was not satisfactory because it had low sensitivity and unstable gain, and it was also battery powered, which meant extra costs for purchasing batteries.
Having opened the Chinese device, it became clear why it has low sensitivity. As you can see, the circuit is primitive, built on a pair of transistors, although the sound quality is not bad.
As a result, I made several similar devices.One of these devices is quite bulky, but has very good sensitivity and auto gain control.The final stage is built on the MC34119 chip, which can deliver up to 250 milliwatts of power to the load, so literally any headphones can be connected to such a device.This device was equipped with a fairly capacious lithium-ion battery; one charge is enough for several days of operation. Unfortunately, I didn’t save any diagrams or photographs.
The second device is no less good; it is built entirely on transistors.
A pair of pre-amplification stages, then an automatic volume adjustment stage and a final stage in the collector circuit of which a high-impedance earphone is connected.
How does automatic volume control work? At the initial stage, the voltage from capacitor C5, which is charged by resistor R8, is supplied to the base of the first stage transistor in order to provide maximum gain.
As the input signal increases, for example during a loud conversation near the device, the signal at the output of the second preliminary stage also increases, and as soon as the voltage reaches the unlocking value of the silicon transistors, which is 0.6 - 0.7 Volts, the transistor is triggered through its open transition and resistor R6, capacitance C5 will be discharged, this lowers the voltage at the base of the first transistor and reduces the sensitivity of the device, the gain gradually increases after a few seconds, as capacitor C5 charges.
This function should be present in any normal hearing aid; it provides stable sound in the earphone, regardless of the volume of the conversation.
The disadvantage of this device is that it is critical to headphones; here it is needed with high impedance. Headphones from smartphone headsets have an impedance from 20 to 35 ohms, but I had to work hard to find a headphone with a coil impedance of 50 ohms, but with such a headphone the sound is simply perfect, although the device can work with lower-impedance headphones.
There are many advantages: high sensitivity, loud noise suppression function, low-voltage power supply and low current consumption.
Such a device can be powered by just one battery, and if you manage to find a small-sized nickel metal hydride battery, then your device may well compete with its Chinese counterpart in terms of ergonomics.
I used a small-sized lithium-ion battery from a Bluetooth headset. Another feature of the circuit is the wide range of input voltages. In the range from 0.8 to 4 volts the device works perfectly.
The printed circuit board of this option was laid out for DIP components, the board turned out to be quite compact.
Later I decided to try with SMD mounting
One of my friends shared his problem with me - he began to hear poorly and began to notice that when watching TV, speech was unintelligible, he had to increase the volume, which created inconvenience for others. Previously, his service took place at the airfield, he was engaged in the maintenance of jet aircraft, and in his youth he did not pay attention to hearing protection. As a result, hearing loss is 40%, especially in such cases, the perception of high sound frequencies of the speech spectrum from 1000 Hz and above is lost. Industrial hearing aids are very expensive, and I decided to help him - I assembled a hearing aid with my own hands from simple and affordable parts. A diagram of the assembled device is shown below.A hearing aid is a simple sound amplification device consisting of a microphone, an input amplifier, a final amplifier and a telephone. The input amplifier is assembled on two transistors T1 and T2 according to a circuit with direct connections between the stages and is covered by a common negative DC feedback in order to stabilize the gain and improve the amplitude-frequency characteristics. Setting the modes of transistors T1 and T2 is carried out using resistors R3 and R6. It is important to use the low-noise transistor P28 in the first stage of the amplifier. In addition, the operating mode of this transistor (Ik = 0.4 mA, Uke = 1.2 V) also ensures minimal noise. The amplifier provides uniform signal amplification in the frequency band of the conversational spectrum 300...7000 Hz. From the collector of transistor T2, the signal goes to potentiometer R7, which acts as a gain regulator. Instead of the P28 transistor, you can use: MP39B, GT310B, GT322A, silicon KT104B, KT203B, KT326B, but low-noise transistors of the KT342, KT3102 and KT3107 series give especially good results. The final stage is assembled on transistor T3 according to an amplifier circuit with a floating operating point, which makes it possible to sharply reduce the current consumed by the stage in silent mode.
This hearing aid amplifier circuit is characterized by an effective shift of the cascade operating point and, accordingly, small nonlinear distortions. When a signal is applied to the input from resistor R7 through capacitor C6, the signal is sent to the base of transistor T3. The signal amplified by the transistor from the collector T3 through capacitor C8 is sent to a doubler rectifier on diodes D1 and D2. The rectified voltage accumulates on capacitor C7 and is applied to the base of transistor T3, shifting its operating point towards opening.
Resistor R8 sets the initial cascade current. The hearing aid is powered by a voltage of 9 volts from the Krona element. LED D3 serves to indicate power on. Any miniature dynamic or condenser microphone can be used as a microphone. If you are using a condenser microphone, you must supply power to it through a 3–5 kOhm resistor. You can use TM-3, TM-4 as a telephone. A suitable plastic case was selected for the hearing aid, which houses the printed circuit board and power supply. When setting up, you must first set the currents of all transistors. resistors R4 and R6 current T1 and T2, then resistor R8 with the microphone turned off, set the quiescent current of Transistor T3 to 2-2.5 mA. A signal with a frequency of 1000 Hz and an amplitude corresponding to the maximum signal amplitude at the collector of transistor T3 is supplied to the base of transistor T3 from the generator. Use resistor R9 to achieve undistorted signal amplification. In this case, the collector current of the transistor should have a value of 15-17 mA. Select the capacitance of capacitor C3 according to the best sound, the absence of harsh sounds. Author: Shimko Sergey.
Today we will make a hearing aid. The video will consist of two parts. In this one I will describe the electrical part, and in this one I will describe the manufacture of the case and installation of electronics into it.
A little background
My grandfather is entering his ninth decade. Over time, he became hard of hearing. For a couple of years he used a miniature BTE hearing aid from Siemens. Until I successfully lost it. It would seem that they bought a new one and forgot about the problem, but I decided to get confused and make a homemade one. There are several reasons for this decision. Firstly, the price issue. As the dollar has risen, hearing aids have increased in price significantly. Secondly, the mentioned specimen had an extremely short operating time on button batteries. 
They had to be changed once or twice a week. Thirdly, wearing a headdress caused extraneous noise that made it difficult to hear the conversation. Fourthly, the monoblock design and close proximity of the speaker and microphone caused constant squeaks at maximum volume, but nothing was audible at an average sound level. Therefore, I decided to kill all the birds with one stone and make a device using AAA batteries, consisting of several blocks. 
The earphone and microphone will be located according to the principle of a wired headset. And the case, which will contain batteries and a printed circuit board, will be carried in a trouser pocket or on a belt. It will have to perform the function of the screen and protect the internal elements from damage due to accidental falls or steps on it.
Hearing aid diagram
I looked up the circuit design on the radioskot website http://radioskot.ru/publ/unch/karmannyj_slukhovoj_apparat/6-1-0-627 The circuit is quite confusing. I tried to simplify it.
The main amplifier is a Motorola MC34119 microcircuit. Let's take a look at the datasheet. The mikruha consists of 45 transistors and can operate on 2V, for example from 2 NiMH batteries, which, when completely discharged, will have a voltage of 1V each, i.e. the total amount we need is 2V. At the same time, the microcircuit consumes extremely little. 2.7 mA stated. And it can deliver up to 250 mW of power per 32-ohm headphone. Quite good indicators.
The simplest inclusion option has minimal weight. But I, like the author of the mentioned circuit, realized during experiments on a breadboard that it is best to use the option with suppression of high-frequency sounds.
I empirically selected an electret microphone from an old Philips phone based on sound quality; it turned out to be noticeably better than other microphones.
My version
I managed . (KiCAD project file) A few words about the preamplifier. Because I didn’t have the preamplifier parts indicated on the diagram, so I decided to experiment with what I had. And in stock I had the banal KT315B. During the experiments, it turned out that the very first option with just one transistor turned out to be the most successful, and all subsequent ones had poor sound quality and poor amplification. But at the same time, if a common power supply was applied to the microphone and the preamplifier, the preamplifier began to self-excite. All my attempts to solve this problem only led to worse sound. In the end, after weighing all the pros and cons, I decided that the elegance of the technical solution had less priority over the sound, and used a second battery pack to power the preamplifier. Yes, my weight and dimensions have increased, but I am making the first experimental sample and this is forgivable. This is the compromise.
The circuit also contains another transistor - BC547, which switches the amplifier chip to a low-power mode when the supply voltage drops below 2.0V. This prevents the main batteries from being completely discharged. This doesn’t work with preamplifier batteries, and although this issue could have been solved, I decided that it was not so critical. Because Measurements of current consumption showed that the preamplifier consumes 10 times less current, namely 0.6 mA and 6.3 mA, respectively. Taking this into account, we can make the assumption that the preamplifier batteries will be charged once every ten charges of the main batteries, which is quite acceptable. Having a main battery capacity of 1000 mAh, we have about 160 hours of continuous operation. We can make the assumption that this charge will be enough for 2-3 weeks of work for 8-10 hours a day. Which is quite a good indicator. The circuit also contains a volume control that regulates the voltage on the electret microphone.
Signet
Everything according to the plan. Let's move on to the seal. Because I initially assembled the parts on a breadboard, I decided to rearrange them on the board without any problems, so our board is THT - pin mounting - much more convenient for experimental things. It is quite possible that there will be an SMD option later. The board was laid out in the KiCAD program, then exported to SVG and printed from a vector editor. I used single-sided fiberglass. The drawing was translated using the LLT method. 
Those. I printed on photo paper using a laser printer, and warmed it up using a laminator. Initially I tried to separate the paper using isopropyl alcohol - I always used this option with thermal transfer paper, but then I failed. The second time I used soaking in water, I cleaned off the remaining film with a toothbrush and toothpaste. It turned out very well. Etched in ferric chloride. Covered with Rose alloy in boiling water. I drilled with a machine using a microscope. There were no special problems with the installation, but as always, I messed up with the mirror image, so the mikruhi’s legs had to be turned inside out.
In the next episode
A simple hearing aid with automatic gain control in a 3D printed housing
Since the case is not miniature, you can use a more capacious (compared to miniature models) Li-Ion battery, which allows the device to operate autonomously for a longer time. Another advantage compared to miniature Chinese models is the independence from the type of headphone. Since the earphone here is the external part, you can choose the earphone individually without changing the hearing aid itself, since the performance of the entire device greatly depends on the characteristics of the earphone itself. It is advisable to select headphones depending on the user’s hearing characteristics. Of course, if you purchase an expensive hearing aid in a specialized store, a qualified worker will make audiograms and adjust the device to the patient’s hearing characteristics, but if you purchase cheap models in Chinese online stores, this is impossible.
The body of the device has a volume control, a headphone jack, an electret microphone capsule and a jack for connecting a charger. The hearing aid is charged using a five-volt cell phone charger. The case is printed on a 3D printer. You can download the 3D models from the link at the end of the article along with the printed circuit board and other files for this project. To charge the Li-Ion battery, a miniature cheap controller board, ordered from Aliexpress, is used.
!!! Do not try to charge a Li-Ion battery directly from a voltage source without a controller board! This is dangerous for the battery and may cause it to catch fire!
Schematic diagram of a homemade hearing aid with automatic gain control. Click on the diagram to enlarge it
An electret microphone (not shown in the diagram) is connected to the mic and GND_mic contacts. The positive terminal of the capsule must be connected to the mic contact, and its second, negative terminal must be connected to the GND_mic contact. Typically, the capsule has this terminal connected to its body. Used electret condenser capsule type WM-61A from China from Aliexpress:
The supply voltage is supplied to the capsule through resistor R1. Next, through capacitor C2, the signal is supplied to the first amplification stage, made on transistors Q1 and Q2. The automatic gain control unit is assembled on transistor Q3 and field-effect transistor Q4. The field-effect transistor controls the gain of the first stage by shunting resistor R13 in the collector circuit of transistor Q2 with alternating current, reducing the gain of the stage as the input signal level increases. Thus, a relatively constant signal level is maintained at the output of the cascade when the input level changes over a wide range. Here, diode D3 rectifies the alternating voltage of the audio signal, converting it into a pulsating voltage, which is amplified by transistor Q3 and then smoothed by electrolytic capacitor C7.
From the output of the pre-amp stage, the signal is fed to the volume potentiometer. The potentiometer is not shown in the main diagram. How to connect it to the board contacts is shown in the small diagram below. A 10 kOhm potentiometer is used.
Connection diagram of the volume control potentiometer to the board contacts
From the volume potentiometer slider, the signal is supplied to the final amplifier assembled on the MC34119 () chip. The chip is an audio power amplifier capable of operating from a very low supply voltage, starting from 2 volts and is ideal for operation in our hearing aid powered by a Li-Ion battery. In fact, the chip contains two output power amplification stages that operate in antiphase to implement bridge mode. The load is connected between the two outputs of the final amplifiers of the chip, and not between the output and ground, as in most other integrated amplifiers. That is, we connect the headphones to pins 5 and 8 of the microcircuit. None of the headphone pins should be connected to ground; you should pay attention to this if you are using a headphone jack in which one of the pins is connected to a metal case. such a socket must be isolated from the common wire of the device.
Powering the device.
As a power source, you can use any small Li-Ion battery with a voltage of 3.7 V, suitable in size. I made a case for a “little finger” Li-Ion battery, I was too lazy to go buy one, and in the end I used a small battery from a children’s toy - a helicopter.
To charge the battery, I used this board, ordered from Aliexpress. 10 pieces of such boards cost around 150 rubles, I ordered a lot of 10 pieces, the boards are useful in amateur radio and cheap.
Since the controller board has a standard micro USB connector, you can use a regular charger from any cell phone to charge the hearing aid.
Diagram of connecting batteries to the hearing aid board
Printed circuit board created in the DipTrace program. You will find the PCB drawings in the archive with the project files.
VR Kaplun, Severodonetsk
One of the readers in his letter drew attention to the fact that there is a certain problem for people with reduced hearing associated with the purchase of a hearing aid due to its rather high cost. After analyzing a number of hearing aid circuits given in the literature, the basic requirements for an amplifier for a hearing aid were determined:
1) amplification of the signal from the microphone to a level of at least 0.5-1 V at a load with a resistance of 80-100 Ohms;
2) presence of AGC (desirable);
3) minimum volume, weight, cost;
4) operation from a galvanic cell with a voltage of no more than 3 V (preferably) with minimal current consumption.
A distinctive feature of the circuit shown in Fig. 1 is the construction of the amplifier output stage. Essentially, this is a push-pull bridge circuit that allows you to get maximum output power with minimum current consumption.
Resistors used in the circuit: R1 – 4.7k; R2 – 270k; R3 – 10k; R4 – 620k; R5 – 3.3k; R6 – 22k; R7 – 68k; R8 – 6.8k; R9, R10 -330; R11, R16 – 30; R12, R13 – 22k; R14 – 5.1k; R15 – 110; R17 – 110; capacitors C1, C3 – C7 – 20.0 μ 6.3 V; C2 – 0.47 μ 16 V; dio
dy VD1-VD3 – KD522; transistors VT1 – KT3102E; VT2 -KT3107Zh; VT3, VT5, VT6, VT8 – KT361; VT4, VT7, VT9 – KT315.
Transistors VT1, VT2, VT4 are used as a pre-amplifier, covered by an AGC (transistor VT3). The stage on transistor VT5 is a phase inverter that provides antiphase signals necessary for the operation of the bridge output stage on transistors VT6-VT9. Negative feedback through resistor R14 provides the necessary stability of the DC pre-amplifier mode. Local feedback in the output stage through resistors R12, R13 stabilizes the DC mode and improves the quality of amplification of weak signals.
The amplifier is set up by selecting resistor R14 in order to obtain simultaneous and symmetrical limitation of the output signal at the collector and emitter of transistor VT5.
The “transparent” version of the printed circuit board is shown in Fig. 2. Resistors can be used with a power of 0.125-0.25 W. Electrolytic capacitors are small-sized imported ones for a voltage of 6.3 V. A small-sized imported electronic microphone is used as a microphone; BA1 earphone – from an industrial hearing aid.
This amplifier was tested in the Radoamator laboratory and showed good performance. The circuit operates stably at a supply voltage of 2.4 V (two D-0.25 batteries). Chinese-made headphones connected in series with a resistance of 32-42 Ohms were used as a load. In this case, the current consumption was 16-19 mA. The author’s circuit additionally includes capacitor C8 and resistors R11, R16. The resistance of resistor R14 can be changed within wide limits. There is a slight noise in the headphones, which could not be completely removed. The noise decreases somewhat when replacing the type of transistor VT1 with KT315. According to a person with hearing loss, “the device works well, but if the noise were reduced it would be even better.”