This simple bridge circuit can be used for impedance or resistance measurement using a high impedance earphone as an indicator. The circuit is from an American 80's documentation.
This amplifier has a few Watt output, using old transistors. The speaker is 45 ohms, but with...
This circuit is from an old American documentation, from 1977, but it can be implemented with more...
This circuit was obtained from the CA3140 datasheet. This component consists of an operational amplifier...