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Ashly Noise Gate SC-33

A dual channel noise gate with, truly professional specifications designed for studio or PA applications. Paul White examines the SC-33 and comments on its performance and applications.

There are, as you are probably aware, several methods of noise available which can reduce the effective noise level produced by your tape machine and its electronics, but these do nothing to remove noise that you feed into the system along with your programme material.

To explain: an electric guitar usually picks up some mains hum which you don't want, its amplifier adds hiss which you don't want, and the guitarist may mutter and curse at the end of a 'take' which you also don't want. These accidental noises are passed through the noise reduction system unscathed and so to remove them, you need to use a noise gate.

Sadly, a noise gate can't remove noise during useful programme material but it can, if properly set up, turn off the offending channel in silent passages or even between drum beats when the residual noise would normally be most noticeable.

The basic gate works by constantly sampling the mean value of the incoming signal and comparing it with a DC threshold within the machine. Any signal above the threshold is allowed to pass, whilst any signal below is rejected. If you, the user, set this threshold so that it rejects only the background noise, then only useful sounds will operate the gate.

Most sounds produced by natural means start off loud and then gradually fade away as the vibrational energy is dissipated, and with a simple gate, there is a danger of chopping off this natural decay, and so more sophisticated equipment control is required.

The Ashly SC-33 under review has five user variable controls on each of the two gates so I'll examine them in turn and try to explain their finer points.


This is the level that the circuitry uses as its reference to accept or reject the input signal.

The input signal is monitored by the circuitry as a DC level, obtained by rectifying and smoothing a sample of the input signal; if the raw input was used, the gate would switch on and off each cycle and produce awful distortion. A yellow LED lights when the gate is open which is essential for setting up.

The SC-33 allows the threshold to be set anywhere between -40dB and +20dB so the full dynamic range of line level signals can be processed.


If a gate were to be allowed to switch on instantaneously, the resulting transient would produce harmonics which would manifest themselves as an audible click. This is not really noticeable on percussive sounds such as drums but could ruin, say, a violin track where the natural attack of the instrument is fairly slow.

By setting an attack time to match the type of programme material, this problem is eliminated, and the SC-33 has a range from 10μs to 150ms which is more than adequate for all normal uses, allowing the gate to be used for special effects; for example, to impart a slow attack to a guitar or piano.


This dictates how long the gate will stay open before the release phase is started and is useful for special effects such as when gating reverb.

In this instance, the gate would be triggered via its external key input and the reverb signal passed through the gate; by setting a short but noticeable hold time (50-500ms), and a very fast delay time, the signal is followed by a burst of reverb followed by silence. This is a popular technique for enhancing drum sounds, much used by Phil Collins and Peter Gabriel. The SC-33 may be varied between 0.04 secs and 12 secs hold time.


This sets the time for the signal to fade after the hold phase is complete, and it is this control that enables you to leave the end of decaying notes intact. The fade time is variable in the range 0.015 secs to 30 secs.


It is not always desirable for a gate to shut down entirely when the threshold is not reached and so the floor control was devised. This sets the reduction in gain when the gate closes and may be set from -1.5dB to -75dB, enabling signals to be reduced rather than completely removed.

Once again, this facility may be used to reduce gating side effects or for special effects.

Gates are often used to keep drum sounds separate, resulting in a clinical sound, but if some ambience is necessary, the floor control may be adjusted to pass the required amount of crosstalk during gate-off periods.

Figure 1 SC-33 front and back panels

Other Controls

The SC-33 is fitted with an external key input and this is sensibly selected by a front panel pushbutton and not by a break jack on the rear panel. This means that the inputs, outputs and keys may be permanently connected to a patchbay, no further plugging and unplugging being necessary.

Each gate is fitted with a bypass switch and on the rear panel there are two link sockets which, when patched together, allow the unit to track as a stereo pair. This would be useful if you were gating the outputs of a stereo reverb or miking up a piano in stereo for example.


What can you say about a black 19 inch case, one unit deep, that has clear white legending? It's heavier than you would expect and the top panel is secured with special star headed screws which are there to prevent me from getting inside and telling you what's in it!

Five minutes later, after filing down my favourite Allen key, I discovered four fibre glass PCBs, one for each gate, one for the power supply and one for the rear panel sockets. Extensive use is made of ICs and all parts are well secured mechanically so that no problems should be encountered during on-the-road use.

The whole presentation of this Ashly unit is smart and clearly labelled and the front panel corners have been removed to make the unit safe for use by clumsy Americans.

In Use

Most professional noise gates work well so the selling point becomes the facilities on offer. The area in which many gates fail is that the voltage controlled amplifiers that they are based on, introduce distortion during the release phase of operation, but none was evident on this Ashly unit.

The only thing I did miss was a filter in the key circuit and, although one can be patched in, it is so much more convenient in a professional 'I want it now' environment to have it built-in ready for instant use.

Of all the gates that I have used, this one seems to have the widest range of controls and the long attack and release times can be used to generate some novel stereo effects.

One trick is to set one gate with a long attack time and one with a short attack time but long fade time. Processing a signal through both gates and panning the outputs left and right can generate a novel autopan effect and, of course, the sound of an instrument can be modified by using the gate as an envelope shaper, possibly triggered by a snare drum mic or whatever.


This is a reasonably priced dual gate with truly professional specifications, the only adverse comment probably being the omission of a built-in key filter.

As with all such processors, it must be used intelligently to obtain good results and here Ashly score with a handbook so explicit that even drummers could understand it.

This is an easy to operate and very useful piece of gear, and if Atlantex would like to let me have a couple for indefinite field trials, I'd be happy to oblige!

The Ashly SC-33 retails for £427.80 inc VAT.

Details from Atlantex, (Contact Details).

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Cutec Microphones

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Sub Woofers

Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Nov 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Studio FX > Ashly > SC-33 Noise Gate

Gear Tags:


Review by Paul White

Previous article in this issue:

> Cutec Microphones

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> Sub Woofers

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