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Keyed Noise Gate

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A versatile design, by David Strange, with a host of creative studio applications.


The noise gate is really a sophisticated sound operated switch that requires only a very low signal to make it turn on. The threshold of its operation is variable so that different noise and signal separations can be accommodated, also the speed at which operation takes place, both on and off, are independently variable. This is to cope with the differing natures of sound, from percussive, across to speech and to gentle rolling sounds. As well as being opened by its own signal it is also possible to switch the noise gate so that it is operated by a completely independent sound.

The noise gate can be used for artistic effect and in this area its use is going to be defined on each occasion it is used, but there are many good technical applications that are a little more definable. For instance, correctly adjusted, the unit may be applied to the output of a tape channel on mixdowns so that the desk fader can be left open without overhearing tape hiss from unrecorded sections of tape. Similarly from open microphone channels incorporating the gate, silence will prevail until the piano, singer or whatever sound begins. A sequencer or rhythm unit could be used to open the gate to allow through a guitar or synthesizer chord.

Fig 1. Keyed Noise Gate circuit diagram
(Click image for higher resolution version)


Construction



A printed circuit board has been designed for the project and it is shown with component overlay in Fig 2. Start the construction by first inserting printed circuit pins for external connections to input/output sockets, switches etc. This having been accomplished insert the IC sockets, or if these are not used (although they are recommended) the IC's. If the IC's are directly soldered, solder just one pin until all are verified in their correct places. A good frame of reference should by now have unfolded for the positions of all the other components, which should be inserted starting with those of lowest profile such as links, resistors and diodes building up to the higher components like capacitors.

Fig 2. Component overlay
(Click image for higher resolution version)


The input and output sockets etc. should be soldered to flying leads and then the leads soldered to the printed circuit board pins. The board can now be examined for building errors, and don't forget to check the printed circuit track for shorting blobs of solder. (Note C7 must be a low leakage capacitor — a polycarbonate type is specified.)

The logic IC's are at their supply voltage limit working on 18V and so two diodes, one in each rail, are used to give a 1.2V voltage reduction. The diode arrangement additionally gives protection against reversed batteries. The same diodes should also be kept if the noise gate is powered by a mains supply using 9V regulators. Capacitors C8 to C15 are used to prevent switching spikes either interfering with the logic or becoming imposed onto the audio.

Testing And Alignment



For aligning and testing the prototype, a FM radio tuned to an 'all talk' station like Radio 4 was found best. Once powered up, the noise gate output should be connected to the line input of an amplifier feeding into a loudspeaker. SW1 must be in the internal position and the headphone output from the radio connected into the noise gate input. Before turning on the radio VR1 and VR3 should be wound to minimum resistance. With the radio on, VR4 can be used to set the point where the gate allows the signal, in relation to its volume, to come through.

It will be found that the gate turns off almost as soon as the signal ceases, but by increasing the resistance of VR3 the recovery time can be extended to last many seconds if required. If the turn on is too abrupt or slow VR2 can be adjusted.

Summary



When testing is complete a suitable box for the noise gate should be chosen. All controls should be easily accessible on the box and VR1 and VR4 are best as multiturn potentiometers. The noise gate is designed to operate on high level signals and so to be most effective an 0dB input is needed, however if this is not available up to 20dB of gain can be made up by adjusting VR1. The minimum turn on threshold is approximately -55dB referred to the output of IC1 being at 0dB.


The Keyed Noise Gate foil.
(Click image for higher resolution version)


Parts List

Resistors
R1,R2,R3,R6,R7,R8 10k
R4 100k
R5,R14,R15 120k
R9 100R
R10 82k
R11 33k
R12,R13 3k3
R16 470R
R17 15k

Potentiometers
VR1 100k log
VR2 5k Lin Multiturn
VR3 1M Lin Multiturn
VR4 20k Lin Multiturn

Capacitors
C1,C2,C4 1u metalised layer
C3 150p polystyrene
C5,C6 100 nmetalised layer
C7 1u polycarbonate
C8, C9, C10, C11, C12, C13, C14, C15 100n disc ceramic

Semiconductors
Q1 BC337 or similar
D1,D2,D3,D4 1N914 or 1N4148
IC1, IC2, IC6, IC7, IC8, IC9 LF351
IC3, IC4 4011
IC5 4016


Miscellaneous

Battery clips for PP3, Batteries (PP3), Double pole double throw switch SW2, Single pole changeover switch SW1, PC pins, Wire, Input and output sockets, 24 SWG link wire.


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Electronic Soundmaker & Computer Music - Copyright: Cover Publications Ltd, Northern & Shell Ltd.

 

Electronic Soundmaker - Dec 1983

Donated & scanned by: Mike Gorman

Feature by David Strange

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> Innovators

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