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1-Chip Microphone Audio Compressor This simple project can be used for a number of audio effects, including compression, automatic level control, sustain and limiting. It can be used with a guitar, microphone or any other low-level signal source. By JOHN CLARKE Many audio enthusiasts would argue that a signal shouldn’t be altered in any way from its original source, whether it is from a guitar, a microphone or any other source. However, in many cases it is necessary to change the signal so as to provide the very best intelligibility or simply to produce a sound effect to add life to a musical score. A microphone in a PA setup, for example, can be called upon to respond 56  Silicon Chip to a huge variation in sound levels. At one extreme, you have people who speak very softly at some distance from the microphone while at the other you have people who speak very loudly and get quite close to the microphone. This means that some type of automatic level control is necessary to maintain a relatively constant audio output level, regardless of the volume from the person speaking. Generally, this automatic control takes the form of signal compression, whereby the lows are made louder and the highs are made softer. Set correctly, signal compression can greatly in­crease the intelligibility of the amplified signal. In many cases, it may even be necessary to prevent severe signal overload (and the high distortion that results). As well as signal compression, this unit can be used for other special effects. Guitarists, in particular, are always keen to add effects to their music – the more controls and adjustments the better, it seems. To this end, we have designed a versatile compression unit which has controls to allow for adjustment of the major parame­ters. This includes the amount of compression, ranging from 1:1 where there is no effect on the signal up to a 15:1 compression. The threshold and limiting signal MAIN FEATURES • Low noise • Low distortion • Adjustable compression ratio • Adjustable limiting level for large signal clamping • Adjustable minimum level for compression • • • • Adjustable gain Adjustable output level Signal bypass switching Facility for electret microphone supply level positions are also ad­justable and there is an overall gain control facility. So the four controls, from left to right, are: (1) Gain; (2) Threshold; (3) Compression Ratio; and (4) Limit. The compression setting produces a range of effects on the signal. Low compression settings, ranging from say 2:1 to 5:1, will restrict the dynamic range of the signal but there will still be some variation in volume. This effect is usually called “compression” or “dynamic range control”. Higher compression ratios will produce a sound level that’s reasonably constant, regardless of the input level. This effect is called “automatic level control” or “sustain”. The Limit control effectively produces a constant output level even if the input level is increasing. It is useful for preventing excessive noise levels from being amplified, as can occur if a microphone or a guitar is dropped. The Threshold control operates at the other signal level extreme and prevents compression from occurring below a preset input level. This reduces noise and hum on the output when little or no signal is present. Finally, the Gain control allows a wide range of signal levels to be tailored to the compressor circuit. It can provide extra gain, ranging from 0dB (x1) up to 20dB (x100). Fig.1 shows the response of the compressor for different compression ratios. Below the noise gate threshold, the signal is “downward expanded”, which means that the signal is attenu- Fig.1: this graph shows the response of the compressor for different compression ratios. The limiting threshold is adjustable and sets the point where compression ceases and limit­ing occurs Fig.2: block diagram of the SSM2166 preamplifier/compressor IC. The buffer stage accepts the input signal and in turn applies a sample signal to the level detector. The level detector then produces a DC voltage output and this controls the internal voltage controlled amplifier (VCA). ated below its normal level. The noise gate threshold is adjustable and above this is the compression region. Note that you can adjust the compression between the ex­ tremes shown (from 1:1 to 15:1). The limiting threshold is also adjustable and sets the point where compression ceases and limit­ing occurs. Any gain added to the compressor simply shifts the graph upwards by the gain value. Block diagram Fig.2 shows the block diagram for the Microphone Compres­sor. It’s based on a single SSM2166 preamplifier/compressor IC which includes a buffer, a level detector, a control circuit and a voltage controlled amplifier (VCA). The buffer stage accepts the input signal and in turn applies a sample signal to the level detector. The level detector then produces a DC voltage output that follows the buffer output signal. The output from the level detector charges an “average” ca­pacitor which is connected to pin 8 and this in turn sets the voltage applied to the control circuit. Note that the value of the MARCH 1999  57 Fig.3: the complete circuit for the microphone compressor. R1 is only necessary if an electret microphone is to be used, while C1 should be 22µF for voice signals and 2.2µF for music signals (eg, from a guitar). “average” capacitor sets the attack and decay times for the compression response. Finally, the control circuit has facilities to allow ad­justment of the three affects parameters – ie, the compression ratio, the rotation point (or limit) and the noise gate threshold. Its output in turn controls the voltage controlled amplifier (VCA), which ad- justs its gain accordingly. In addition, the VCA is fitted with a separate gain control facility, so that its overall gain can be adjusted. Circuit details Refer now to Fig.3 for the full circuit details of the Microphone Compressor. Apart from the SSM2166 preamplifier/com­pressor, it consists of four Specifications Gain control Anticlockwise 0dB; mid-setting 10dB; clock­wise 20dB Threshold control at 0dB gain Anticlockwise at noise floor; mid-setting 0.2mV; clockwise 30mV Ratio control Anticlockwise 1:1; mid-setting 7:1; clockwise 15:1 Limit control at 0dB gain 1:1 ratio Clockwise 600mV; mid-setting 10mV Total Harmonic Distortion at minimum gain before limiting 0.16% at 1kHz and 200mV input; 1.2% at 10kHz and 200mV input; 0.32% at 1kHz and 200mV input; 3% at 10kHz and 500mV input Frequency response -3dB at 30Hz and -1dB at 30kHz Signal-to-noise ratio with respect to 300mV, input threshold anticlockwise 1:1 ratio and 600mV limit: 75dB with 20Hz to 20kHz filter, 78dB A-weighted. 15:1 ratio: 60dB with 20Hz to 20kHz filter, 64dB A-weighted 58  Silicon Chip pots, a couple of 6.35mm jack sock­ets and a handful of minor parts. The input signal is fed in via a jack socket and applied to the pin 7 input (Buffer In) of IC1 via a 0.1µF capacitor. Resis­tor R1 (2.2kΩ) is included to provide for an electret microphone input (an electret microphone requires a bias current in order to function). The buffer amplifier has a gain of -1, as set by two 10kΩ feedback resistors. One of these resistors is connected between pins 5 & 6 (ie, between the buffer amplifier output and its inverting input), while the other is connected between ground and the inverting input via a series 1µF capacitor. This 1µF capaci­tor provides low-frequency rolloff below 16Hz. Different values are used for the “average” capacitor at the output for the level detector (pin 8), depending on whether the circuit is to be used for speech signals or music signals. If the circuit is used predominantly for speech signals, a value of 22µF is used. Conversely, if the circuit is used mainly for music signals, a value of 2.2µF is best. If the circuit is to be used for both music and speech on a regular basis, you can add a switch to select between two dif­ferent capacitors. Potentiometer VR1 sets the VCA gain, while VR3 between pin 10 and ground sets the compression ratio. Similarly, VR2 sets the noise gate threshold, while VR4 sets the limit. The output from the VCA appears at pin 13 and is fed to the output socket via VR5, a 1µF capacitor and switch S1. S1 is a bypass switch – it simply switches the compressor circuit in or out of circuit. In the OUT position, the signal at the input is fed straight through to the output socket, bypassing IC1. VR5 is a level control. This trimpot is adjusted during the setting up procedure so that the output from the compressor matches the sensitivity of the amplifier that’s being used. Power for the circuit is derived from a 12V DC supply (eg, a plugpack or a battery). Diode D1 provides reverse polarity pro­tection, while the 470µF capacitor provides filtering of the supply line. Regulator REG1 then provides a 5V rail for IC1, while LED1 is the power indicator. Construction Building it is easy since all the parts are mounted on a PC board coded 01303991 and measuring 104 x 57mm. Note that IC1 is available in two versions – either as a normal 14-pin DIP IC or in a surface-mount package. In the latter case, a second small PC board (coded S0-14) is required to mount the IC. This board is then mounted on the main board in the normal IC position (see photo). This technique allows the main board to accommodate both versions of the IC. Start the construction by checking the PC board against the published pattern. Repair any broken tracks or shorts that may be evident. If you have the surface-mount version of IC1, this can now be mounted on the small S0-14 board using a fine-tipped soldering bit. You will need keen eyesight and preferably a magnifying lamp for this job. To mount the IC, position it so that Fig.4: install the parts on the PC board and complete the wiring as shown here. The bypass switch (S1) is optional and can be left out if not required. If you do leave it out, be sure to link the IN and COM terminals on the PC board. Take care when installing the potentiometers, as their values differ. Resistor Colour Codes        No. 2 2 2 1 2 1 Value 100kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 680Ω 4-Band Code (1%) brown black yellow brown brown black orange brown yellow violet red brown red red red brown brown black red brown blue grey brown brown 5-Band Code (1%) brown black black orange brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown blue grey black black brown MARCH 1999  59 This photo shows how the bodies of the potentiometers are connected together and earthed using a single length of tinned copper wire. This is done to prevent hum injection into the signal whenever a pot is touched. its pins contact the pads on the top of the board and lightly solder each pin in turn. Once this has been done, insert short lengths of tinned copper wire into the holes down the outside edges of the board and solder these in position. The assembly can now be installed on the main PC board, just like a regular 14-pin IC. Alternatively, if you have the DIP version of the IC, solder it in instead. In either case, make sure that the IC is oriented correctly, with pin 1 adjacent to the 100µF capacitor at the back of the board. Next, install the diode (D1), the resistors and the link in the locations shown. You should also install a link between the “IN” and “COM” pads (near the output socket) if you don’t intend installing a bypass switch. Note that D1 must be oriented with the polarity shown. The banded end is the cathode (K). R1 is only installed if an electret microphone is to be used. Table 1 shows the resistor colour codes but it is a good idea to also measure them using a digital multimeter. Install the PC stakes now, followed by the capacitors. Apart from the 0.1µF unit adjacent to the input socket, the capacitors are all electrolytic types so make sure they are correctly oriented. Use a 22µF capacitor for C1 if you intend using the compressor with Fig.5: the full-size etching pattern for the PC board. The section labelled “S0-14” is required only if you have the surface-mount version of the SSM2166 IC. 60  Silicon Chip a microphone. Alternatively, make C1 2.2µF if you intend using the unit with a guitar or other music sources. The regulator can be mounted next, then trimpot VR5 and the four potentiometers (VR1-VR4). Take care when mounting the pots to ensure that you use the correct type and value in each posi­tion. In particular, note that VR1 is a logarithmic pot, while VR2-VR4 are linear pots. It’s quite easy to tell them apart – log pots are marked with an “A”, while linear pots are marked with a “B’. Use the 50kΩ log pot for VR1, the 1MΩ linear pot for VR2, and the 50kΩ linear pots for VR3 and VR4. The LED and the two 6.35mm jack sockets can go in next. Watch the orientation of the LED – its anode lead (which is the longer of the two) goes towards the nearby wire link. Finally, the PC board assembly can be completed by connect­ing the bodies of the pots together using a length of tinned copper wire. One end of this wire is then connected to the GND PC stake adjacent to the input socket. This measure is necessary to prevent hum from being injected into the signal whenever a pot is touched. Fig.4 shows how the bypass switch (S1) is connected, using shielded cable. This will usually be required for guitar use and with line level inputs but not when the compressor is used with a microphone. In the latter case, simply short the IN and COM terminals by installing a wire link, as described previously. Testing The circuit can be powered up using a battery or power supply which can deliver 9-12V at about 50mA. Check that the voltage between pins 1 and 14 is 5V and that LED1 illuminates when power is applied. Next, feed a signal into the input (either from a guitar, a line level source or a microphone) and connect the output to an audio amplifier. This done, set VR1, VR2 & VR3 fully anticlockwise and VR4 fully clockwise. Trimpot VR5 should also initially be set to its full clockwise position. Now check that the signal can be heard. At this stage, the sound will not appear any different from normal because the compression is 1:1. Assuming that a signal can be heard, you can now adjust VR3 for the desired compression effect. Parts List 1 PC board, code 01303991, 104 x 57mm 1 PC board (S0-14) for surface-mount 14-pin IC (S version only) 2 6.35mm PC mount mono or stereo jack sockets 1 16mm 50kΩ log pot (VR1) 1 16mm 1MΩ lin pot (VR2) 2 16mm 50kΩ lin pots (VR3,VR4) 1 10kΩ horizontal trimpot (VR5) 1 SPDT toggle switch (S1) 1 500mm length of 0.8mm tinned copper wire 1 500mm length of single shielded cable 7 PC stakes Semiconductors 1 SSM2166P or SSM2166S preamplifier with variable compres­sion (IC1); available from Insight Electronics, phone (02) 9585 5511 1 78L05 low power regulator (REG1) 1 5mm red LED (LED1) 1 1N4004 1A diode (D1) Capacitors 1 470µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic (C1) – see text 2 10µF 16VW PC electrolytic 1 2.2µF 16VW PC electrolytic (C1) – see text 2 1µF 16VW PC electrolytic 1 0.1µF MKT polyester Resistors (0.25W, 1%) 2 100kΩ 1 2.2kΩ (R1) 2 10kΩ 2 1kΩ 2 4.7kΩ 1 680Ω Protect Your Valuable Issues Silicon Chip Binders REAL VALUE AT $ 12 +$5 ea.95 P&P Or buy 5a get th nd postag em e free  Each binder holds up to 14 issues  Heavy board covers with 2-tone green vinyl covering  SILICON CHIP logo printed in gold-coloured lettering on spine & cover Just fill in & mail the handy order form below; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Yes! Please send me ________ SILICON CHIP binder(s) at $A12.95 each plus $5.00 p&p. Australia only – not available elsewhere. Enclosed is my cheque/money order for $­__________ or please debit my  Bankcard    Visa Card    MasterCard Card No. Signature­­­­­­­­­­­­_________________________ Card expiry date______/______ Name _____________________________________________________ Street _____________________________________________________ Suburb/town __________________________ Postcode______________ SILICON CHIP PUBLICATIONS PO Box 139, Collaroy Beach, NSW 2097, Australia. Phone (02) 9979 5644 Fax: (02) 9979 6503. ✂ VR2 (the Threshold pot) is adjusted to reduce the noise level with no signal. Don’t set it too high though, otherwise it will adversely affect the compression process at low signal levels. VR4, the Limit control, is adjusted anticlockwise to allow compression up to a selected level before limiting occurs. Finally, VR1 (Gain) is adjusted to give the required signal sensitivity, while trimpot VR5 is adjusted so that the output level matches the sensitivSC ity of the following amplifier. MARCH 1999  61