This is the Transcendent Sound Grounded Grid Pre Amp design with a constant current source, a stepped attenuator, with dual outputs, 1 full range, one high pass, with redundant muting circuits, with additional power supply filtering and built with audio grade components.
The Muting Circuit
The muting circuit protects down stream components from the half second DC output that occurs when the pre amp is turned on. I built redundant circuits to protect against a transistor failure in the timing function of the muting circuit. The timing transistors are an eleven cent (USD) part with an indeterminate failure mode, meaning not fail safe. When you have thousands of dollars of down stream equipment connected to the pre amp it would be a shame to damage something due to a eleven cent part part failure.
The muting circuit also includes two green LEDs that monitor the muting circuit relay status. During normal operation both LEDs will light after 15 seconds. When the LEDs are lit the muting circuits have timed out and the relays have energized to remove the grounding of the pre amp outputs. If one of the LEDs lights up immediately after the power is switched on it's indicative of a "failed on" fault with the associated relay or the timing circuit. If this happens the down stream equipment are still protected from the half seocnd DC pulse by the other muting circuit. The likely hood that both muting circuits fail at the same time probably won’t happen during my life time, but if you want NASA type protection against this type of fault you can triplicate the muting circuits and add two out of three voting (2oo3). ;-)