"Sag" is a well-known power supply effect caused by resistance in the supply and variable current loading. As an amplifier pulls more current from the supply, the supply voltage drops, or sags, and this can cause a compression effect. The transient or attack portion of a note is "squashed" or limited by the falling supply voltage, and thus the attack part of the signal envelope blends with the sustained portion. Sonically, we perceive this as an increase of sustain.

London Power's SCK Sag Control Kit provides true variable sag effects. The SUS-kits provide an identical sound but uses a compression side-chain circuit that samples voltage instead of current.

SCK

The SCK provides a true sustain effect based on dynamic current drawn by the amplifier. A 28mm mil-spec pot allows continuously variable sustain and accommodates supply voltages up to 540Vdc. The amplifier must have an idle current that is much lower than the peak audio current, otherwise there will be little current change to respond to and thus minimal compression.

The SCK was brought out with the original PSK-series Power Scale kits, all of which used switching methods and relied on mil-spec pots. SCK responds to current change, so cannot be used with high-bias amplifiers.

SUS 1, 2 & 3

These sustain-enhancing kits use non-critical 16mm pots and other components to achieve the desired compression effect. An amplifier with any power rating can be fitted with one of the SUS kits.

SUS-1 can be used with any non-Power Scaled fixed-biased amplifier.

SUS-2 can be used with any cathode-biased amplifier, regardless of whether the amp is Power Scaled.

SUS-3 is for fixed-biased amplifiers fitted with Power Scaling.

For Power Scaled amplifiers:
If fitted with SB-1 or SF-1, use SUS-3 to add sustain.
If fitted with SB-2 or SF-2, use SUS-2 to add sustain.
If switchable between fixed-bias and cathode bias, use SB-1 or SF-1, plus SUS-3 for sustain.