ã 2002 ~ 2013 by Eddie Ciletti and Manhattan Sound Technicians, Inc. (MST)



Engineers know that the classic optical-based Limiters - like the Teletronics LA-2A (vacuum tube) and the LA-3A (solid-state) - are perfect for tracking Bass, Guitars and Vocals. Simplicity is part of the charm - just Gain and Gain Reduction controls to dial in a little opto-magic. As beloved as vintage compressor / limiters are, however, their controls are often not labeled or calibrated. The early digital plug-ins supplied all the parameters, but without presets, the learning curve was pretty steep. That said, there is a pot of audio gold at the end of the learning curve…

At the heart of the Gain Reduction or ‘opto-coupler’ module, sound is converted to light by an Electro-Luminescent "EL" panel (originally intended to illuminate aircraft control panels). A light-sensitive ‘photo-resistor’ determines the response time – Attack time: 6mS to 12ms – Release time: approximately 500mS. Considered "slow" relative to modern, dbx-style VCA processors, the photo-resistor has an ‘intelligent’ non-linear response time that varies with program material and the amount of gain reduction. Together, these two components require no side-chain – which is where - in more sophisticated processors - Ratio, Knee, Attack and Release parameters are generated.

Released in the mid-seventies, the LA-4 got a new optical transmitter (a red LED) and an IC quad-opamp, the RC-4136. A side-chain circuit drives the LED, "tickling" the photo-resistor into delivering five Ratio options - from 2:1 thru 20:1 - making the LA-4 a bit more versatile than its predecessors. However, the RC-4136 in the signal path (balanced input, gain make-up and output amps) ‘slowed’ down the signal when driven hard (regardless of the amount of Gain Reduction) making the sound ‘darker,’ smearing detail and adding non-euphonic harmonic distortion in the process. 

The EC-4136 opamp upgrade for the LA-4 preserves delicate signal details – like acoustic guitar strumming - no matter how hard it is driven. The sonic character is now solely from the opto and the output transformer.

If you are happy with the EC-4136 upgrade, know that three additional in-house mods are also available. "Dynamic Threshold" eliminates the ‘jump’ between 2:1 and 4:1 - each RATIO can be auditioned without the need to re-tweak Threshold for each step. A three-position Stereo Link switch adds a high-pass option to the side chain. On the rear-panel, a mod to the High-Gain Switch makes the LA-4 compatible with self-powered condenser and dynamic mics when capturing loud sources like drums, guitar and bass amps.

TIP-1: Judging Bass (compression and gentle RMS limiting @ 2:1, 4:1 and 8:1 ratios)

When recording Acoustic and Electric Bass, for example, it may be difficult to localize the source of note-to-note inconsistencies. The problem may be due to monitors, control room acoustics, the instrument or the player. VU meters are the perfect analysis tool for ruling out acoustic anomalies. So, before doing any processing, first set the Meter switch to read Output and monitor the bass for consistency. If the meter ‘suggests’ that the bass notes are inconsistent, adjust the Gain Reduction (GR) Threshold to "level" the bumps. Confirm with the Output Meter and use more GR if the notes are more than 3dB up or down. An aggressively played "slap" style bass may require higher ratios or be processed in two passes – with peak limiting and compression. 

TIP-2: Compression or Peak Limiting? (NOTE: Response Times are guidelines.)

Choose ratios below 8:1 for compression (Attack 10mS – Release 500mS) and ratios above 8:1 for limiting (Attack 1mS – Release 50mS). At 2:1 compression, GR well beyond 6dB can be applied. But at ratios of 20:1 and higher (Attack: 100uS – Release 5 mS), the peak limiting sweet spot is between 3dB and 4.5dB - the max GR is quite literally ‘limited’ to 6dB - beyond which lies the negative artifact zone. Sometimes it’s necessary to do both limiting and compression. 


Using a VU meter as intended - to judge perceived Volume (VU stands for Volume Units) - its slow 300mS response time is designed to average complex audio material so that an engineer can visually judge level discrepancies. (0VU = 1.23volts RMS in a +4dBu world). The VU meter is fine for assessing compression, but the most effective peak limiters have such super-fast Attack times (in micro-Seconds or uS) that a VU meter is useless. High resolution electronic bar-graph style metering have no mechanical inertia so they can accurately report transients (peak response).

TIP-4: Peak Limiting (with ‘magnetic’ insights)

Compression and Limiting are variations of a similar theme. The LA-4 for example, responds more quickly at 20:1 than at 2:1, but will never be so fast as to get ugly. The FET-based 1176 is significantly faster, crossing the threshold into Peak Limiting Territory (featuring an ultra fast attack time of 20 microseconds to 800 microseconds), but is still remarkably forgiving when ‘abused.’ Other ‘fast’ topologies include VCA (dbx, Distressor), Variable Mu (Altec, Fairchild, Manley, UA and Pendulum) and PWM (Crane Song, Great River, EMT). All can serve as effective Peak Limiters when understood and be most unforgiving if abused. 

That said, when a Peak Limiter has only a VU meter to display GR - as was the case when I reviewed the Pendulum 6386 - adjusting Threshold until the VU needle is just being ‘tickled’ can easily result in a magical amount of peak limiting (3dB to 6dB). Here, listening is more important than meter watching. Once the meter starting indicating GR, the magic was gone. (I had to remind myself that recording tambourine to analog tape required a similar consciousness.) Crane Song’s STC-8 and the Empirical Labs Distressor - with their high-resolution LED bar-graph displays - are great teaching tools because the display accurately relates to the processing being done – whether you hear it or not. You’ll be amazed at how effective and transparent 3db to 6dB of true peak limiting can ‘not sound.’ (PS: Audio Suite can serve as a digital confirmation.)

If you are struggling with dynamics processing when mixing, start with automation (clip-gain is great for this). Then try less aggressive processing on individual tracks along with simultaneously processing a sub- or main-mix (about 3dB of GR for each layer). Tweak each parameter one at a time (Attack, Release, Ratio, Knee) to see which yields the most dramatic results, keeping in mind that they interact. For example, slower Release times allow faster Attack times without artifacts, and vice versa. Release times for bass or kick drum could be as slow as 2 seconds! Response times for peak-limiting percussion will be fast. Fast attack times will bite into the peaks more - especially with higher ratios - allowing the Threshold to be desensitized for less GR. 

Thanks for reading! 

Eddie Ciletti