Q-SF – Single Jfet Control Interface for Multi-Voice Control

CAD $23.00


Q-SF, a single jfet control interface for multi-voice control, allows multiple voice control inputs to interface with a set of jfet control lines, linked to series- and shunt-connected jfet audio switches. Q-SF is equivalent to one quarter of our Q-FET quad jfet controller. It is a member of London Power’s Q-system of switching kits.

  • controls shunt-connected n-jfets
  • controls series-connected n-jfets
  • function LED driver
  • requires only -12Vdc if shunt-only is used
  • requires +/-12V if series is used
  • requires <3mA if no LED is connected
  • requires <12mA if LED is used
  • accommodates easy programming DIP switches (order Q-DIP)


Jfets can be used to switch signals up to a line level in amplitude, and can do so with very little power consumption compared to mechanical relays. Shunt-connected jfets can mute audio or enable cathode-bypass caps for gain boosts, requiring just -12Vdc to control them. Series-connected jfets can switch audio but require +/-12Vdc for proper control. Q-SF provides proper interface with the jfet gates and the “voice” selection for the amp.

Shunt-wired jfet gates can tie directly to the control line. Series-connected jfet gates require diode isolation, so add a diode in series with the gate of each jfet.

Q-SF will produce symmetric control lines for shunt wired jfets while using just -12Vdc, by linking VP to V0.

With +/-12Vdc supplies, the series output can be used to control grounded-source n-channel power mosfets to switch high-power and/or high-voltage loads, such as cathode selection of power tubes, and Power Scale control selection.

Q-SF is useful where the number of elements to be switched does not require the four-times capability of Q-FET. Q-SF is also handy when more than the capability of Q-FET is needed but not that of two.

The statuses of the jfet output control lines are set so that if both types of audio mute are used within a circuit, the shunt element and series element will be in the correct state to allow signal through.

DIP switches provide easy programming to designate which voice control lines will interface with the jfet controller. Add Q-DIP for this option. If this easy programming capability is not required, solder in wire jumpers where the DIP switches would be.


Q-SF comes with a standard-brightness 3mm round green LED. The onboard LED driver can drive any LED the builder wishes to use.

The PCB makes assembly fast and simple.

Assembly is required.

PCB size: 1.8″ x 1.35″ (48 x 35 mm)

See also:
Q-DIP – 4-way DIP switches

For more information:
– see our page, About Our Switching Kits
– download our informative PDF: London Power Switching Kits Selection & Application (135 kB, 22 pages)