metal detector apparatus includes an increased pulse transmit frequency and reduced integration time constant for rapid target response and improved sensitivity to low conductivity metals , including cupro - nickel alloys , platinum and gold . resistor - capacitor coupling totally eliminates the need to reset the controls due to changes in temperature or mineralization in the area being searched . the short time constant used in the rc coupling circuit improves the overall stability while reducing noise , allowing for greater amplification in the audio stages . the time constant of the rc coupling stage is equal or similar to the time constant of the integrator . the audio tone is derived from the pulse transmit clock which is synchronous with the power supply inverter to reduce noise and instability in the system .