Fostex 3070 Compressor/ Limiter

A Compressor/Limiter is a device for reducing or altering the dynamic range of signals put into it. In our case this is likely to be music programmes. The dynamic range of our material is the difference between the quietest sound experienced and the loudest crescendo. The Compressor tends to make the quieter sounds louder and to hold back the peaks of the stronger signals. The Fostex Model 3070 is a cost-effective Compressor/Limiter which has the parameter adjustment facilities of many devices several times its price. It can operate in the stereo mode or as two mono units. Principle A Compressor only operates on signals that are above its threshold level which is usually set by the sensitivity control which, in the case of the Fostex Compressor, is the input control to the device. Compressors work on a ratio of input signal to output signal. For instance, working at a ratio of 4:1, for every 4dBs of input level gain experienced by the Compressor above the threshold setting, the Compressor will give 1dB of gain at its output. The ratio control on the Fostex 3070 changes the compression ratio continuously from a 1:1 setting (no compression) through to 20:1 (hard limiting) and then on to infinity. The Compressor/Limiter controls must also have the ability to adjust for music programmes of differing attack and decay times (the envelope of the sound). On the model 3070 the attack times are continuously variable from 0.2 to 20 msecs. The attack time is the time that it takes the Compressor to react to the signal exceeding the threshold. The release time which is the time it takes the Compressor to recover to its unity or 1:1 position, is continuously variable from 50 msec to 2 seconds. We shall discuss attack and release settings later in this article. The input control of the 3070, which is a rotary control, performs two functions simultaneously. By increasing the input level, one is effectively increasing the input experienced by the Compressor above the threshold level. The output control is designed into the circuitry before the by-pass in/out switch which, on this device, is labelled COMP for compression in/compression out. While this, to some people, may seem to be a small point, this configuration is, in fact, very convenient, as it enables the user to make a true comparison between input and output signals without having to re-adjust the output controls. Features The dimensions of the device are 17" wide x 1¾" high x 8¼" deep and I understand attachments are available to prepare it for 19" rack mounting. This Compressor also incorporates an expander gate which is useful for eliminating unwanted noise that can sometimes occur by over-use of compression which may be necessary on certain types of programme material. On each channel of the Compressor there is a meter in the form of a strip of light emitting diodes (LEDs) which indicate the amount of gain reduction being applied to our programme material. On the rear of the unit are the input and output connections to the device which are on either ¼" jack or phono connectors. The inputs have been, in my view, cleverly arranged with 10dB pads behind the jack sockets so that, in conjunction with the input gain control, the Compressor's threshold can be matched to different pieces of equipment with varying operating levels. I found that the manual gave a very good indication of the operation of the input control but, unless one is aware of these different operating levels in terms of decibels of voltage (dBV) then the interconnections could become a little confusing. On the rear of the unit there are also insert points for the patching of effects devices into the side-chain of the Compressor. These are arranged as phono jack sockets with a plastic-coated bridging pin

Condition

Used (normal wear)

Brand

Fostex