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"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. RBX may need to be added for proper functioning.
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, or a fixed-power amp that has TBS installed.
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.
For non-Power Scaled amplifiers with TBS installed:
Use SUS-3 and RBX
For non-Power Scaled amplifiers without TBS:
Use SUS-1 and RBX.
Note: Proper functioning of the SUS kit with the fixed-bias Power Scale kits or TBS may require the addition of the RBX to assure function of SUS all the way to full output power. RBX provides a much higher raw bias voltage than most stock bias supplies, and at a much lower impedance. Both attributes are important for proper functioning of the SB-1, SF-1 and TBS, but becomes more important with the addition of sustain.
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