Anyone who’s spent much time searching the web for interesting circuits is likely to have found at least one TL431 based audio amplifier, the circuit being based on the principle that any comparator can be used in linear mode if it’s rolled off with enough negative feedback. Although the TL431 is often referred to as a programmable or adjustable zener, it is in fact a comparator with it’s own 2.5 V reference all neatly wrapped up in a TO92 package.
The problem with the TL431 amplifiers to be found on the web is that they simply roll it back with large nfb and leave it at that, which results in ver y low gain, to make mat ter s worse some such circuits make a bit of a hash of biasing the control input.
The problem with the TL431 amplifiers to be found on the web is that they simply roll it back with large nfb and leave it at that, which results in ver y low gain, to make mat ter s worse some such circuits make a bit of a hash of biasing the control input.
Electret Mic Booster Circuit Diagram
The circuit presented here takes care of the low gain by adding an AC shunt to the feed-back path and using an electret mic for the input the 2.5 V set on the control input at stable operating condition suits an electret mic per fectly. The first prototype had a 35 ohms loudspeaker as a load (RL), this gave good results although the TL431 ran a bit warm with a Vccof 12 V. An old 130 ohm telephone earpiece is likely to present a less stressful load. AC shunt C2 (100 µF) has to be a quality component in terms of its ESR specification don’t just use a scruffy capacitor lying about as you may experience RF sensitivity. It was necessary to add a series resistor (R3; about 100 ohms) or in extreme cases an inductor (L1; 100 – 220 µH).
Components C1 & R1 are entirely optional to selectively feed some unshunted feedback to reduce noise; 1.5 k? & 5.6 nF are as good a place as any to start off with. Initial set-up depends on the current drawn by the electret mic and the value for RL any-where between 200 and 2,000 ohms is good. R2 allows the TL431 cathode to swing despite the AC shunt, 1.2 k? was found to be satisfactory, P1 can be a 47 k? trimpot and is used to set the voltage drop on RL. In the case of moving coil speakers a compromise between volt-age swing and prebiasing the cone should be sought, with a resistive load adjust for 0.5 Vcc, once the operating point is determined P1 can be measured and replaced by an equivalent fixed resistor.
The circuit has a couple of handy features, firstly it wor k s ver y well on the end of a twisted-pair the output can be tapped off at the wiper if RLis a pot at the power supply end, secondly by salvaging the JFET from an old electret mic (some common types of JFET will work but not quite as well), just about any piezo electric element can be used as the transducer. Brass disc sounders give a good output (handy as vibration sensors if glued to a structure); even the quartz discs from clock crystals give some output, a phono crystal cartridge gives a high output and the piezo-ceramic pellet from a flintless cigarette lighter gives a huge output... the range of possible applications is awesome!
A surprising application is the ability to test the microphonic sensitivity of ordinary capacitors! Disc ceramic types don’t need to be tapped very hard to produce an output but rolled metalised foil types produce some out-put too. Link
The circuit has a couple of handy features, firstly it wor k s ver y well on the end of a twisted-pair the output can be tapped off at the wiper if RLis a pot at the power supply end, secondly by salvaging the JFET from an old electret mic (some common types of JFET will work but not quite as well), just about any piezo electric element can be used as the transducer. Brass disc sounders give a good output (handy as vibration sensors if glued to a structure); even the quartz discs from clock crystals give some output, a phono crystal cartridge gives a high output and the piezo-ceramic pellet from a flintless cigarette lighter gives a huge output... the range of possible applications is awesome!
A surprising application is the ability to test the microphonic sensitivity of ordinary capacitors! Disc ceramic types don’t need to be tapped very hard to produce an output but rolled metalised foil types produce some out-put too. Link
The quad opamp is the same rail-to-rail TS924IN, made by ST. The turnover frequency of the filter (3rd order 1 dB Chebyshev) has been increased slightly to improve the intelligibility of speech and is now about 5.5 kHz. The filter now amplifies the signal by a factor of 10. In practice it is possible that due to various tolerances and the fact that the opamp is not perfect, the filter characteristic shows some deviation from that required. In our prototype it was necessary to change R15 into 2k7 to straighten the response curve. The DC current variation at the output of the OTA and the resulting offset variation at the output of current/voltage converter IC2d is such that the gain of IC2d has to be substantially smaller than in the ‘old’ design.
The minimum gain is about 20 dB (10 Ω). The OTA is then slightly overdriven and the distortion becomes several percent! With a fixed gain selected (P1 shorted) the gain is about 42 dB (125 ×). The middle curve was measured with P1 in its central position. The curve drawn for a fixed gain (the straight line) doesn’t finish at the edge of the graph because the end of the line corresponds to the maximum possible output level, which is 25 dBr (≈1.76 V or 5 / 2√2). 
For a larger safety margin R9 is calculated with the full supply voltage and a current of 2 mA: (5 – 0.7) V / 2 mA = 2k2 (rounded upwards). Of course the regulation will then be different (a little less gain). This circuit and the transmitter module can therefore be fed from the same 5 V supply. Because the transmitter requires a DC offset at its input, a resistor is connected to +5 V via a jumper, which biases the output to half the supply voltage. With the jumper open R17 functions as a load resistor when the output is not connected, because C9 still has to charge up even without a load. If you’re designing a PCB for this compressor then it makes sense to include the transmitter module as well. The current consumption then increases by about 10mA.