The LUCAS Limiting Amplifier Operating Manual


The Lucas Limiting Amplifier is a single channel limiter featuring classic vacuum tube technology for both the gain reduction and gain make-up stages. Unlike most so-called tube limiters, which employ some device such as an optical circuit, a solid state VCA, etc. as the gain reduction element, with tubes present only in a preamp or gain make-up stage, The Lucas actually uses the tube itself as the volume controlling element; a two-section triode vacuum tube is utilised as a type of differential VCA, which, while more costly to implement, produces a warmth of sound otherwise heard only in a very few of the finest classic limiters of the past. Each unit is carefully custom built by hand. The Lucas Limiting Amplifier truly combines the best of modern technology with the best of the classic tube era.
While the limiter may be used well on any instrument one wants, it is especially useful on vocals and bass guitar, imparting a warmth, punch and roundness often difficult to otherwise achieve. It may also be employed as a linked stereo pair on drum overhead or room mics to achieve huge, soft, over-compressed cymbal sounds.


DESCRIPTION OF THE FRONT PANEL CONTROLS:
The POWER switch located in the lower right corner of the front panel turns the unit on and off. The adjacent LED indicates power on. Normal operation of The Lucas Limiting Amplifier is realised with the TEST/OPERATE toggle switch beneath the VU meter in the OPERATE position. In this mode, the following front panel controls are active:
VU meter - The operation of the VU meter is controlled by the toggle switch immediately to the right of the meter, labeled GR in the up position and OUTPUT in the down position. With this toggle switch in the OUTPUT position, the meter indicates output level: the level of the audio measured at the output jack. 0 VU corresponds to +4 dBu. With the toggle switch in the GR position, the meter indicates gain reduction in dB. ACTIVE/BYPASS switch - This toggle switch determines the mode of operation. In the BYPASS position, the tube limiting circuitry is bypassed, and the incoming signal is passed through by the input and output buffers. (The audio does not pass through any of the tubes in this position.) In the ACTIVE position, the signal is modified by the limiting section. With the limiter active, the following controls affect the limiting action: INPUT knob - The input attenuator controls the amount of limiting. The Lucas Limiting Amplifier is a fixed threshold limiterÑthe degree of limiting is set by the input gain. Clockwise rotation of the INPUT control will result in more limiting action; conversely, anticlockwise rotation will result in less limiting. The INPUT knob adjusts the pre-VCA/tube gain over a 26 dB range. OUTPUT knob - This is a volume control after the tube make-up gain stage. It controls the output level of the audio in the ACTIVE mode. RATIO knob - This five position rotary switch sets the compression ratio for the limiter. The ratio ranges from 1:1, with no limiting action taking place, to 12:1, resulting in the greatest compression. A setting of 1:1 may be used to pass audio through the tube gain stage without incurring limiting action. ATTACK + RELEASE knobs - These controls are used to adjust the attack and release times of the limiting action. Adjustment of these controls is entirely program dependent. Fastest attack/release times are achieved by turning the controls fully clockwise. While attack at 11 o-clock and release at 2 o-clock is a good starting point, experimentation is recommended. LINK switch - The link switch is used to link together two Lucas Limiting amplifiers for stereo operation. In the link mode, the two compressors will experience equal amounts of gain reduction, thereby preserving the stereo balance. In the link mode, the two limiters act as equal partners, so corresponding controls on the two linked units must be set to equal values. (Engage the switch on both units.) GR ZERO SET - The position of the VU meter corresponding to zero gain reduction is trimmed by the GR zero front panel locking potentiometer. To trim, first allow the unit to reach full operating temperature, then loosen the 1/2 in. locking nut (outermost) but do not remove it. Using a small flat blade screwdriver, adjust the meter to 0, then gently tighten the locking nut.

NB: Newer models with "times multiplier" switches are not referenced here...that will soon be added.


DESCRIPTION OF THE REAR PANEL CONNECTORS:
AC POWER JACK - 117 Vac 60 Hz power is supplied to the limiter via a standard power cord plugged into this jack. FEMALE XLR - The 3 pin female XLR jack is the audio input to the limiter. The input is electronically balanced with pin 2 hot. MALE XLR - The 3 pin male XLR jack is the audio output from the limiter. The output is electronically servo-balanced with pin 2 hot. 1/4 in. JACK - The 1/4 in. jack is the link interconnect point for linking two Lucas Limiting Amplifiers for stereo operation. A short shielded audio cable with mono 1/4 in. plugs on each end should be used for the interconnection.


BALANCING PROCEDURE:
The Lucas Limiting Amplifier utilises a two section triode vacuum tube as the gain reduction (or compressor) element. The two sections of the tube must be matched. Because of this, correct balancing of the internal circuitry is critical for proper operation. While not needed often, failure to properly balance the limiter when required will result in increased distortion and other undesireable side-effects, and extreme out-of-balance operation can result in damage to the internal components. Correct balance is achieved by carefully setting the two front panel balance trims. (The Lucas Limiting Amplifier is balanced when new as part of the factory testing procedure.) If gross distortion is noticed, or if the VCA tube (6BQ7) is changed, perform the following steps:
1. Preset the front panel controls as follows:

• TEST/OPERATE switch to OPERATE

• GR/OUTPUT switch to OUTPUT

• ACTIVE/BYPASS switch to BYPASS

• INPUT knob to 9 o-clock

• OUTPUT knob to 2 o-clock

• RATIO switch to 1

• both ATTACK + RELEASE to SLOW

SPECIFICATIONS:

• input impedence: 20 k ohms
• output impedence: 20 ohms
• frequency response:
• bypassed 20-20kHz +/- 1/4 dB
• active (with or without gain reduction) 20-20kHz +0/-2 dB
• noise floor -92 dBV (20-20k bandwidth)
• tube complement: 1 - 12AX7A, 1 - 12AU7A, 1 - 6BQ7
• fuse: AGC 1/2