Tube amplifiers designed for headphones have the principal property that they can be used as preamplifiers too. In most cases, the output impedance of a tube headphone amplifier is (or should be) less than the output impedance of a tube preamp. This amplifier features relay-based input and power switching. The stepped and slow turn-on for the power supplies result in less stress for the tubes and other components and reduces turn-on thumps that could damage headphones. Most tubes fail at turn-on. Without the slow turn-on, if the tube heater filament is cold, the resistance is lower and the in-rush current of a cold filament could be very high and cause the filament to break. And if the filament is not hot, it is better that the plate voltage is not applied. Applying the plate voltage with cold filaments can reduce the lifetime of the tubes.
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Figure 1 shows the circuit of the tube amplifier designed without global feedback. It is a pure Class A OTL design with a triode-connected pentode output section. The input section was built with the double triode E83CC as a differential amplifier. The big advantage of this circuit is that the output signal can be taken from the first or from the second triode. The output at the plate of the second triode isn't phase inverting providing that the whole amplifier isn't phase inverting. The E83CC is very often mentioned as a tube with excellent audio properties. There are selected types and different brands on the market. The E83CC is the high-grade type of the ECC83. It has a very high gain (µ = 100) and is therefore well suited for building up an amplifier with only one amplifying device.
To circumvent having a high voltage negative power supply for the current source (simply R4) of the differential amplifier and for improving the linearity, it is necessary to lift the cathode voltage and the grid voltage of V1 above zero volts. This is done by R7 and R8 acting as a voltage divider. This voltage divider could be built two-fold, each for one of the triodes, but since tubes have a negligible input current (gate current), the same divider gives the gate voltage for the first triode too through R6. With a gate-cathode voltage of about -1.2V and R4 = 4.7K Ohm, the plate current for each triode is about 1mA. This current...
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In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....
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