The dynamic range of an audio signal is the difference between the quietest discernible part of the signal and the loudest undistorted part of the signal. It is normally expressed in decibels (dB).
Dynamic Range Processor
The Dynamic Range Processor presented here allows the dynamic range of a signal to be increased (expansion) or decreased (compression) by a variable amount up to 3:1. The unit was developed for use when recording music from various sources.
For example, when recording CD’s onto cassette for use in a car it is helpful to use some compression so that the quieter sections are not drowned out by the engine noise. Also it is useful to employ some expansion when recording tracks from poor quality records or pre-recorded cassettes. Suggestions for using this unit and processing music from various sources are given later.
Obviously a system such as this could never be truthfully described as Hi-Fi, since any form of audio effect unit, by definition, “distorts” the original signal. However the unit has been subjected to extended listening tests and the design has optimised to produce the best possible results on a wide range of music.
The circuit is based on the SSM2120 dynamic range processor IC. This is available from various suppliers including Maplin and RS, and costs about £10. The IC contains two level detectors and two voltage controlled amplifiers, making it ideal for processing stereo signals. The circuit diagram for the two channels is virtually identical, so this discussion will concentrate on the left channel.
The level detection circuits contain a wide dynamic full-wave rectifier, logging circuit and a unipolar drive amplifier. These circuits will accurately detect the input signal level over a 100dB range from 30nA to 3mA peak-to-peak.
Referring to the block diagram of the level detector, the REC-IN input is an AC virtual ground. When applying signals a DC blocking capacitor (C-IN) is used since REC-IN has a DC potential of about 2.1V above ground. The value of the input resistor is set to give a +/- 1.5mA peak signal. For +/- 15V operation this corresponds to 10K.
Audio Signal Path
The audio signal into the unit is buffered by U1:A. This is preceded by a DC blocking capacitor (C1) to remove any DC offset on the input. The value of C1 together with R1 is set to give a low frequency roll-off, with a -3dB point at about 30Hz. Allowing lower frequency signals through at a high level can cause an unpleasant pumping effect on signals with a high bass content.
The input to the voltage controlled amplifier (VCA) section of the SSM2120 (U2:A) is a virtual earth. The audio voltage signal is converted to a current by R15, while R11 and C4 ensure stability. The current output from U2:A is converted back to a voltage signal by U1:B. C5 ensures stability, while C6 is a DC blocking component.
For clarity the power connections to U2 are shown separately on the power supply circuit diagram. R37 is a biasing component for the VCA sections, and sets the output current.
The circuit requires a supply of +/-15V at about 50mA. This is derived from a 15V transformer with the usual rectification and smoothing components. The voltage is regulated by a pair of standard three-pin regulators (U4 and U5). Although the 78L15 and 79L15 TO92 cased 100mA devices would be adequate, it was felt that standard TO220 cased 1A devices would be better able to dissipate 600mW of heat. Indeed the power supply could power two circuits if separate compression and expansion arrangements were required for a record/playback system. No heatsinking is required.
The whole circuit with the exception of the transformer and switches is constructed on a single sided PCB. The dual track pot (RV1/RV3) should have PCB mounting pins otherwise it will need to be mounted with short pieces of stiff wire.
Due to the cost, a socket is recommended for U2. If you do not have a 22 pin socket, use a 14 pin socket and an 8 pin socket next to each other. Do not fit U2 into the socket until the power supply has been checked. The completed PCB is then fitted into a suitable case and wired up as shown. A single DPDT toggle switch is used for SW1 and SW3, and a single 4P3W rotary switch is used for SW2 and SW4.
Ensure the mains connections are adequately insulated – if necessary add some insulation tape while carrying out these tests. With U2 removed, connect the unit to the mains and switch on. Connect the negative lead of a meter to a suitable ground point such as the wire form J15. With the positive meter lead connected to pin 4 of U1 the reading should be 15V +/- 0.5V, and on pin 11 there should be -15V +/- 0.5V.
If this is OK, switch off and insert U2. Set presets RV2 and RV4 to the centre position. Switch on again and check the voltage at pins 1, 7, 8 and 14 of U1. These should all be between +0.5V and -0.5V. With RV1/RV3 set fully anti-clockwise the voltages on pins 1 and 7 of U3 should be within the same range. If these voltage checks are OK, the unit can be tested with an audio signal.
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