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It is a little collection of amplifiers in bridge connection. The output power is low and they are for general use. They can be used with small active loudspeakers, car stereos etc. The only restriction, is that we should use loudspeakers with resistance above 4 ohm.... [read more]

Having good contacts is important – not only in your daily life, but also in electronics. In contrast to social contacts, the reliability of electrical contacts can be checked quickly and easily. Various types of continuity testers are commercially available for this purpose. Most multimeters also have a continuity test function for electrical connections. A simple beep helps you tell good contacts from bad ones.... [read more]

Here's a top-class headphone amplifier that can drive high or low impedance 'phones to full power levels, with very low noise and distortion. For best performance, it can be teamed with the Stereo Preamplifier described last month. Alternatively, it can be used as a standalone unit, requiring only a power supply and a volume control pot for use with any line-level signal source (CD/MP3 player etc). It even includes dual outputs, so you can listen with a friend!... [read more]

This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise: in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply.... [read more]

There are not many AM transmitters that are easier to build than this one because the inductor is not tapped and has a single winding. There is no need to wind the inductor as it is a readily available RF choke (eg, Jaycar Cat LF-1536). To make the circuit as small as possible, the conventional tuning capacitor has been dispensed with and fixed 220pF capacitors used instead. To tune it to a particular frequency, reduce one or both of the 220pF capacitors to raise the frequency or add capacitance in parallel to lower the frequency. Q1 is biased with a 1MO resistor to give a high input impedance and this allows the use of a crystal ear piece as a low cost microphone.... [read more]

This circuit relies upon the extra high input impedance of a FET, and also demonstrates the gate terminals sensitivity to changes in voltage. The gate terminal here is left open circuit, connected only to the "probe" this being just a few inches of bare copper wire.... [read more]