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Helios F760
Helios F760-Limiter/Comp-SVS

This Helios F760 module was originally designed to the original specification which was originally in the Island Studio Basing St console. The unit is a 40x190mm module for insertion in a custom-built Helios mixing desk, or certain 19” Racks and Lunchboxes, i.e. Helios Electronics Ltd, UK, Vintage King Audio USA, Morphon Studiotechnik Germany. The unit comprises. a variloss FET controlled amplifier with associated side-chains and line-drive amplifiers. Basically the unit has unity gain with a ''make-up facility of 30dB, the input and output are symmetrical and floating, it is designed for operation in a +4 to 8dBM line and operating below this level will increasingly impair the signal to noise performance. A gains reduction meter indicates 0 - 20dB Gain Reduction.
The unit should ideally be operated so that the, signal is reaching +4 to 8dBm at the output; in this way optimum signal-noise ratio is maintained. The Threshold and Make-up Gain will therefore be adjusted so that- for the
amount of gain reduction required the signal 'still peaks to +8dBm at the output. The signal-noise ratio is nearly -80dB but falls to -70 odd when operating at a peak output level of around 0dBm. Should it ever be required to work in line levels of -10dBm, then the +8dBm threshold should be still used, but the output attenuated to -10dBm or whatever; the make-up gain, being used to bring the signal level up internally.

The threshold switch is calibrated in steps of 1dB between +8 and 0dBm; thereafter in 2dB steps down to -2OdBm. This is so arranged since the tighter slopes (3:1 to 20: 1 ) will only be used down to round 0dBm with the lower threshold levels used on 2:1 only, which will give a 1dB change at the output per step.

The ratio switch selects a: range of compression slopes (1:1, 2:1, 3:1, 5:1, 10:1 and 20:1). The slopes are set-up relative to the 2:1 ratio which is established for 20:10 (i.e. 10dB gain reduction), below that amount of Gain Reduction the slope will appear slightly less than 2:1 or 3:1 etc.

Peak Limiter
In addition to the compressor side-chain the facility of an overload limiter is available.
This may be made operative by the switch on the front of the module. As supplied this has been pre-set to operate at a threshold level of +10dBm. Thus on tighter ratios (5:1, 10:1, 20:1) it will never operate, but is there to provide some overload protection on the 2:1 and 3:1 slopes. Release The recovery control provides a range of release positions from 25mS to 3.2 secs. (these are calculated on the CR time - for 12dB Gain Reduction to recover within 4dB of the unattenuated position). In addition an 'Auto' position provides multiple network that has
proved to be a good general purpose position. It allows fast gain change to occur over 5 or
6dB whereupon the release characteristic changes, becoming very slow. Thus. Modulation effects are reduced even when considerable overall gain reduction is being used. It is an effective mean level control.

The attack time is more extensive on this model, -25, .5, 1, 2.5, 10 & 25, normally there are only 3 settings
Stereo operation. Stereo operation is effected by linking the control volts between pairs of modules, gain reduction
will be identical on both channels and will be controlled by the channel causing greatest gain reduction. Ratios and Threshold levels need not be identical, in fact one could set-up a 2:1 slope and threshold in one module with a higher limit (20:1) slope in the other: The two would combine to give 2:1 up to the 20:1 and thereafter they both would limit. There are balancing transformers available as an accessory for both the output & the input
The input is a floating Sowter 10kohm impedance transformer mounted in the desk this obtains a floating symmetrical output. The output 3 position Toggle Switch mounted in the front panel can trim the output level. approx minus or plus 6dB to give a maximum output threshold level of +14dBm if necessary.
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Additional information
For more information about the F760, please visit the website of Helios
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