For the second module in the 'build-it-yourself' modular effects system introduced last month, Paul Williams describes the attributes and construction of an attractive, high quality stereo Compressor-Limiter which features variable slope, attack, release and includes a switched key input.

There will be few readers who do not appreciate the value of a good compressor in the home studio. Compressors are used every day in professional studios for creative effects such as mix thickening, adding percussiveness and extending decay times on strings or reverb, as well as the 'necessary evil' of trying to squeeze a wide dynamic range programme into a medium with not such a wide dynamic range such as the vinyl disc. It is also desirable sometimes to compress signals such as vocals or bass so that they 'sit' better in the mix, without bobbing up and down in level. This technique also allows higher average levels to be maintained, making the compressed sound more prominent in the mix.

Principles

Compression is a means of reducing the dynamic range of a programme signal by progressively reducing the gain at higher levels. This means that a 3dB increase in signal level at the input might for instance only result in an output level increase of 1dB. The 'compression slope' in this case would be 3:1. No compression takes place, however, below a predetermined threshold level.

The limiter is an equally valuable device for both the home recordist and gigging band. It is used for curtailing high signal levels applied to, for instance, a tape recorder to prevent overmodulation of the tape, or to a PA amplifier to prevent output saturation, either of which situations would otherwise lead to harmonic distortion.

Vocals, for instance, are notorious for their unpredictable level when miked up, and are ideally suited to the application of a limiter, which would allow a higher system gain than normal to be set, where the occasional high level peak would be taken care of by the limiter. This allows an improvement in signal-to-noise ratio to be achieved in the case of the tape recorder, or a higher average output power in the case of the PA amplifier. Limiters typically exhibit a compression slope of up to 20:1; that is a 20dB increase in the input level results in a 1 dB increase in output level. There is also, of course, a threshold level below which no limiting takes place.

The Module

HSR's Comp-Lim easily fulfills both of these functions by incorporating a slope control, adjustable from a gentle 2:1 to a sharp 20:1. Figure 1 shows the typical transfer characteristic from input to output, with the input threshold set at — 30dBm, and the output threshold at -10dBm. The input threshold is set by varying the input 'drive' using the input control. This allows the depth of compression to be set in the easiest and most direct way. To make compression indication straightforward and uncluttered without compromising usefulness, a colour-changing LED is used. This changes gradually from dark with no compression through green at 10dB, yellow at 20dB to red at 30dB (just like traffic lights). The inclusion of attack and release controls and a key input further widens the range of applications and allows some spectacular effects to be produced.

As with the Pro-Gate module described last month, the accent is very much on quality of performance and appearance, uncompromised facilities and ease of construction.

Circuit

The circuit diagram for the Comp-Lim is shown in Figure 2. It can be seen that although the schematic seems to be littered with op-amps, the audio signal only passes through two stages for each of the stereo channels.

IC1 a buffers the left channel input presenting a nice high input impedance, and a low output impedance to the next stage, IC1b. The gain of IC1b, prior to compression, is determined by VR5a and R5. At the onset of compression however, IC3a, an Operational Transconductance Amplifier (OTA), is forced by the sidechain to raise its transconductance such that it will increase negative feedback around IC1b, reducing its gain until the output level is low enough to satisfy the sidechain.

The sidechain is driven by the combined outputs of IC1b and IC2b via R19 and R20, into IC4a. The gain of this stage can be trimmed by VR1 to vary the output threshold level. D1, along with D2 driven by the inverter IC4b, perform full-wave rectification and provide a threshold voltage below which no current is supplied to the following stage. IC5a amplifies this rectified signal with a gain determined by the slope control, VR2.

When the sidechain input is sufficient to overcome the forward voltage of D1 and D2, the output of IC5a goes negative, charging C10 negatively via R29 and VR3, the attack control. When the sidechain drive signal drops in level, the output of the slope amplifier stage around IC5a goes high impedance, allowing C10 to discharge via D5, R28 and VR4, the release control.

IC5b, along with TR1 form a current generator driving a control current into the OTAs which is proportional to the negative voltage on C10.

The OTAs thus keep the gain of the audio stages IC1b and IC2b sufficiently low that the output voltage matches the input threshold of the sidechain.

The voltage on pin 6 of IC5b also follows the negative voltage on C10, which is proportional to the depth of compression. This is monitored by the LED compression indicator circuit built around IC4c and IC4d. The current driven via R35 into IC4d causes pin 8 to go positive, making the green half of the LED, D7b, to glow. At about 15dB of compression, the gain of IC4a, set by R31 and R32, is such that the voltage at IC4 pin 14 is sufficient to overcome the forward voltage of the red half of the LED, D7a, which, when combined with the green produces a yellow glow. At 30dB of compression, the output of IC4c saturates, allowing pin 13 to go negative. This voltage, applied to pin 10 of IC4d, causes the green half of the LED to progressively turn off, leaving the red half alone to glow. TR2 provides the LED with a synthetic ground preventing these 'noisy' currents from getting onto the OV line.

Construction

By extensive use of PC mounting connectors and potentiometers, the thoughtful design of the PCB eliminates the need for any interwiring.

The first step in constructing this useful project using the high quality kit is to locate the resistors into the PCB, according to the parts list and the overlay printed on the PCB itself. These are then soldered and cropped. Only insert ten or so resistors at a time to prevent crowding. Solder the eleven wire links in place using resistor lead off-cuts, then the diodes D1-6 and transistors.

The IC sockets come next, making sure that they are pressed firmly down onto the PCB whilst soldering. Leave the ICs out till later though. Now insert and solder the capacitors, taking care with the polarity of the electrolytic types. Mount and solder the presets VR1, 6 and 7 so that they lean back slightly to ease adjustment later. The bus connector and the five jack sockets can then be soldered whilst holding them firmly down onto the PCB. A piece of foam rubber laid on the workbench comes in handy for holding connectors and the like in place on up-turned PCBs during the soldering process.

Trim the pot shafts to 8mm from the bush, fit a PC bracket to each one then locate them in the appropriate PCB positions, but don't solder at this point. After determining the correct orientation of the LED, bend its leads down at right angles 4mm from its body and locate into the PCB, but again don't solder. Screw one nut onto each of the two toggle switches, then locate them into their respective PCB positions.

Place shakeproof washers on the pots and switches, then offer the front panel up, feeding the pot and switch bushes and LED dome into the appropriate panel apertures. The panel is then fixed in place by means of the pot nuts, which should be fully tightened. The toggle switch front nuts are, however, only finger-tightened, after which the backing nuts are fully tightened onto the back of the panel. The pots, brackets, switches and LED can now all be soldered, after making sure that they are all positioned correctly.

Although the excitement of it all may tempt you to shove the ICs in and plug the module into the rack, its advisable to have a break for a while then check the assembly very carefully, especially on the track side where solder splashes and dry joints are all too common. A little time spent now could save you hours later on trying to locate 'fried' ICs.

Once you are happy with the assembly, plug the ICs into their sockets, being careful with orientation. Make a special point of noting that IC4 is mounted the opposite way around from all the others - you have been warned! Fit the knobs with their caps so that the marker line of each covers the entire scale evenly, then fit the switch lever covers.

Having now completed the module, slide it into place in the Sub-Rack and secure with black M2.5 screws. Although not essential, you can screw the jack socket nuts onto the backplane if the module is not to be removed frequently.

Testing

Remove the module in the Sub-Rack to the right of the Comp-Lim, turn the input control anti-clockwise and slope control to 10. Set the other controls and presets to mid-way and switch on the rack power. With both toggle switches set to the left, a signal applied to the inputs will be passed to the outputs unaffected. You should also notice little difference in the output signal when the unit is switched 'in'.

Advancing the input control will increase the signal level until the LED starts to glow first green, then yellow, and finally red, when the increase in output level should be less significant. You will find that silent passages in your music are now not so silent, and loud passages not so loud. Advancing the attack control while reducing the setting on the release control will make the sound more 'punchy', whereas advancing the release control will cause 'pumping' effects as the compressor takes time to recover after signal peaks.

Having now confirmed that the unit is operating correctly, disconnect the inputs and instead, feed a nice 'buzzy' bass note from a synth, amplified bass guitar or whatever into the key input. Switch the 'key' toggle on and monitor the left output with plenty of gain. Set the slope control to '8', and all the others fully anti-clockwise, then adjust the signal level to the key input so that the LED just turns green. The low frequency tone that now breaks through to the output should be minimised by adjusting VR6, the front-most preset. The top preset, VR7 is used to null the breakthrough on the right channel.

In Use

Your Comp-Lim is now fully operational and will work over a wide range of signal levels, even directly from a guitar to give you lots of clean sustain. Depending on your standard system operating level (-10dBV, 0dBm or whatever), the compressed output level can be set using the bottom preset, VR1. The rack's linking system will route signals via the left channel, which is switched in/out using the 'in' toggle. The right channel, which shares a common control sidechain with the left channel, is accessible via the rear jack sockets only, and is permanently 'in'.

For use as a protection limiter, set the controls as follows: slope 8 to 10, attack 0 to 1, release 3. The input control is then adjusted so that the loudest passages cause the LED to glow green to yellow depending on the size of the signal peaks. The input sensitivity of your amplifier or tape recorder can then be adjusted to give the desired maximum level when limiting takes place.

For mix thickening, use fairly low attack and release settings, say 1 and 2 respectively, but beware of very low settings on both simultaneously, especially when a high slope setting is used, since this can lead to modulation of the signal by the low frequency notes, and even distortion. The input control will then determine the amount of thickening and generally, the slope control will affect the 'obviousness' of the treatment. Remember that by the time the LED turns yellow, not only are you compressing by 20dB, you will also be knocking 20dB off your signal-to-noise ratio. So if you want lots of compression, you should have a low noise source, preferably not from tape unless you have a very good noise reduction system.

Advancing the attack control to 6 or so accents the percussiveness of the sound, resulting in a tight 'punchy' effect. Individual instruments processed in this way will also have a much stronger attack characteristic; an effect particularly noticeable on basses and guitars.

The key input can be used to reduce the level of the main programme dependent on the level of the control signal, as for 'voice overs' or 'ducking', where part of the amplified microphone signal is tapped off and applied to the key input. Alternatively, a particular instrument could be lifted out of a mix by allowing the signal from the track of that instrument to control the level of the remainder of the mix. This will result in a very powerful, dynamic sound when used on a lead guitar track. If an equaliser is patched into the key input, this can be used to selectively compress particular frequencies, such as to reduce sibilance on vocals, or to limit the level of a frequency band occupied by a particular instrument.

A very informative article on compression and limiting techniques by Dave Lockwood appeared in the HSR August 1984 issue, which makes a useful reference for the Comp-Lim owner.

No problems should be experienced in building the kit for this project but just in case, Tantek provide a 'get-it-going' service for all their kits, for a standard charge of 20% of the kit price.

Next month: Dynamic Noise Filter 2.

A complete kit of parts for the Comp-Lim is available from Tantek, (Contact Details) at a fully inclusive price of £36.95. Allow 28 days for delivery.