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11-19-2018, 11:44 AM
Hi jmcd
If you look at TUT Fig.9-90 which shows A,B,A+B switching in the LPSP, the signal levels are shown throughout the preamp and specifically at the muting points. Everything is well within the capabilities of any jfet (at the mute points).
The jfets supplied in London Power's Electronic Relay Kit (ERK) are rated for 25V and the control BJTs for 40V. Provided the filter cap is 25V or more, the zener could be changed to 20-24V and signals of twice the standard amplitude can be handled. As my post above stated, the control voltage is the restriction to signal size in most cases.
If you have a preamp that you wish to add muting or switching to, measure the signals throughout the preamp prior to adding the switching. These measurements will guide you towards the correct switch element type and/or the voltage rating it must exceed.
Usually at the output of multiple preamps it is desirable to use series switching, which ERK is not designed for. You could add series resistors to allow shunt elements to control the selection, but this incurs a signal loss and increases the required impedance for the input of the following circuitry. The latter is not usually a problem in a tube amp.
If you look at TUT Fig.9-90 which shows A,B,A+B switching in the LPSP, the signal levels are shown throughout the preamp and specifically at the muting points. Everything is well within the capabilities of any jfet (at the mute points).
The jfets supplied in London Power's Electronic Relay Kit (ERK) are rated for 25V and the control BJTs for 40V. Provided the filter cap is 25V or more, the zener could be changed to 20-24V and signals of twice the standard amplitude can be handled. As my post above stated, the control voltage is the restriction to signal size in most cases.
If you have a preamp that you wish to add muting or switching to, measure the signals throughout the preamp prior to adding the switching. These measurements will guide you towards the correct switch element type and/or the voltage rating it must exceed.
Usually at the output of multiple preamps it is desirable to use series switching, which ERK is not designed for. You could add series resistors to allow shunt elements to control the selection, but this incurs a signal loss and increases the required impedance for the input of the following circuitry. The latter is not usually a problem in a tube amp.
