Forgotten Gear - Shure M62V (Level-Loc)
The "Level-Loc" Audio Level Controller is basically a low-noise preamplifier with the capability of reducing its gain as the input signal increases, there by holding the output signal constant. After a predetermined input level threshold is reached the output level is "locked", that is, it remains constant.
Developed in the 1960's, the original Shure Level-Loc was a brick wall limiter designed to be used in public address systems.
It's distinctive, trashy and gritty character earned it a second life as a secret weapon in the racks of 90's/2000's mixing engineers. And once the word got out that the Level-loc was behind some famously flavorful drum sounds, it's used price ballooned over $500.
It's an extreme effect; it can create huge sounds with drums and room mics and, in small amounts, can add sizzle to things like vocals or acoustic guitar.
Level-Loc original schematic
The original schematic uses two input and output transformers for accept a balanced signal and bring it to the right impedance. The Impedance Selector allows to choose between line level (300 ohms in, 50 ohms out) and hi-z (50 kilohms in, 3.3 kilohms out). In addition, a high-impedance auxiliary output (10 kilohms) is provided capable of driving any high-impedance amplifier, mixer or tape recorder input requiring 1 volt or less.
Distance Selector switch determines the input level at which gain reduction (Level-Loc action) begins. It acts as a threshold by reducing the input voltage in Q3 using a variable voltage divider.
The output voltage (Vo) can be calculated by this formula: Vo=(R1+R2)/(Vi*R2)
- 18" position: from the 2µF cap straight to "D" (Vo=Vi)
- 12" position: from the 2µF cap in to 12k resistor and shunted by 22k to GND (Vo=1.5/Vi)
- 6" position: from the 2µF cap in to 12k resistor and shunted by 4.46k (22k + 5.6k in parallel) to GND (Vo=3.7/Vi)
The Input Level control can both be used as a vernier control to allow fine adjustment of input threshold and can act as an input attenuator for signals that are higher than normal microphone levels.