Audio compressor is the main device, used in the processing and recording of audio signals. In general, audio compressors have a lot of manipulation of elements, through which sound can strongly interfere with the despread signal. In many situations, these functions are not needed. Often need a simple compressor with a control, which can only change the "depth" of compression. There is a diagram of such a device.
Described audio compressor based on ASIC THAT4301 on That Corporation. This device contains all the blocks, necessary for implementation of the audio compressor. The present device is modeled with additions, necessary to comply with home recording studio. Audio compressor amplifier circuits based on a controlled voltage feedforward topology (rice. 1 (a)), t. it is. VCA control signal, prepared in side chain, is taken directly from the input audio compressor, in contrast to the topology of the feedback (rice. 1 b) where it is taken from the outlet of the compressor.
norms, responsible for the "level" in a typical audio signal compressor, linked. for example, increasing the compression ratio (Ratio) or threshold reduction causes a reduction of the output level. In order to maintain the output level, after compression is necessary each time to align its (MakeUp). This procedure can be uncomfortable, when you want to quickly change settings. This configuration requires several operations to produce the desired effect. Application Note dn115 is a system application, wherein the control minimized, and to operate the compressor only one potentiometer. The circuit automatically adjusts the level changes. surge characteristic, depending on the setting «COMP» potentiometer, shown in Figure 4.
Schematic diagram of the audio compressor module is shown in Figure 2.
The input signal of the «IN» jacks supplied to a cascade of the variable gain amplifier based on U2-1. The system allows you to adjust «INP» potentiometer input sensitivity. During operation, the input signal (sensitivity) is set once for each value of the signal at the input of the compressor to about VCAIN -10 dB. The relay RL1 can activate «BYPASS» compressor function by transmitting a signal from input to output without any change, when the compressor is deenergized (This facilitates the construction of series circuits) or when it is switched off «BYPASS» switch, connected to the BYP. Compressor operation is signaled by diode «INL».
The signal after amplification in coordination U2-1 is fed through the input capacitor CE6 VCAIN U1 and CE5 through the detector RMS U1. The time constant of the detector was set about 30 ms (C6 = 10 uF). The signal from the «COMP» potentiometer (compression) is added to the output of RMS detector. Transistor Q2 is responsible for generating the threshold and "soft" compressor refractive characteristics of (Soft-Knee) around the compression threshold. Transistor Q2 is used to correct the control signal CTRL, when set to greater compression. C «VCAOUT» outlet of the compressor after buffering OA3O signal is supplied to an output stage with amplification, adjustable potentiometer «OUP». The compressor is powered symmetrical voltage ± 15 AT (200 mA), connected to «PWR» connector. It has a connector “S.C.”, which is used to connect an external equalizer. CMPIN – output, SCIN – input signal the compressor control circuit. In the normal mode contacts 1-2 connectors “SC” must be closed. As an external circuit SC, e.g. lowpass filter, You can use the tone corrector, which when properly shaping SC characteristics will allocate corresponding bandwidth during compression. Make, that the external circuit SC corresponds to the phase.
Audio compressor mounted on a small printed circuit board shown in Figure 3. Switch «BYPASS» installed outside board. The author recommends the use of a socket for a rather expensive chip U1.
After proper assembly device to be adjusted. If there is no assembly error, sinusoidal signal 1 Vrms must be connected to input devices, and the output should be loaded with a resistor 10 ohms and connected to the input of the oscilloscope. Set potentiometers «INP» and «OUP» in the middle position, а «COMP» – to the left. After switching on the system using the «BYPASS» Switch LED should INL. Potentiometer «LEV0» should be set at the same level input and output signal (static compressor gain = 1 AT / AT), switching to «BYPASS» signal level is not changed.
After adjustment for the compressor to be supplied music signal (For example, drums or bass guitar) and checked for proper operation of potentiometers «INP», «OUP» и «COMP». After adjusting the input level via «INP» change in a wide range of compression «COMP» potentiometer settings does not change the output level. Potentiometer «OUP» adjusts the output level, to avoid overloading the next stage.