Stereo Headphone Amplifier

Both halves of the circuit are identical. Both inputs have a dc path to ground via the input 47k control which should be a dual log type potentiometer. The balance control is a single 47k linear potentiometer, which at center adjustment prevents even attenuation to both left and right input signals. If the balance control is moved towards the left side, the left input track has less resistance than the right track and the left channel is reduced more than the right side and vice versa. The preceding 10k resitors ensure that neither input can be "shorted" to earth.

Source: http://english.cxem.net/amplifier/amplifier59.php

Class-A 100W Amplifier

A Class A // Class AB amplifier rated 100 Watts when driving a 4 ohm loudspeaker. This circuit developed out of my 30+ years of JLH class-A based investigations. The original 'simple' 1969 JLH class-A design provided excellent first cycle accuracy through mid and high frequencies (dynamic clarity) because there were no stabilisation components nor a series output choke, whilst the NFB error correction was established via the input emitter (some describe this as current feedback).

Source: http://english.cxem.net/amplifier/amplifier31.php

10 Watt Guitar Amplifier

The aim of this design is to reproduce a Combo amplifier of the type very common in the 'sixties and the 'seventies of the past century. It is well suited as a guitar amplifier but it will do a good job with any kind of electronic musical instrument or microphone. 5W power output was a common feature of these widespread devices due to the general adoption of a class A single-tube output stage (see the Vox AC-4 model). Furthermore, nowadays we can do without the old-fashioned Vib-Trem feature frequently included in those designs. The present circuit can deliver 10W of output power when driving an 8 Ohm load, or about 18W @ 4 Ohm.

Source: http://english.cxem.net/amplifier/amplifier16.php

Ultra-low quiescent current AF power amplifier

This is an audio power amplifier, similar to what is common in transistor radios and other portable audio equipment. This circuit however, has an ultra- low quiescent current of about 250uA, due to a unique bias circuit. I don’t even know of a chip that does that. This circuit also has much higher open-loop gain, due to the bootstrap effect that the circuit provides as a by-product of its operation.

Source: http://discovercircuits.com/Andy/Ultra.pdf (good site)

TBA820 Amplifier

A very useful audio amp in an 8-pin DIL package. The IC features a very low minimum working supply voltage of 3V, low quiescent current, good ripple rejection, no crossover distortion and low power dissipation. ...
Link : TBA820 Amplifier

Phono Preamps For All

This is a collection of phono (black vinyl for the youngsters) preamps and equalisation circuits, one of which is sure to meet your requirements. These are not my circuits, but were contributed by a reader (see above), so I am not really in a position to make any specific recommendations. They are provided as a service to the experimenters out there, and may be found useful for other applications as well.

Source: http://sound.westhost.com/project25.htm

Intercom preamp - Universal high quality preamp

The circuit is so simple that there is little to say about it, but it gives very good results. Tr1 is operated in grounded base mode with input to its emitter to give low impedance input. The values shown give correct operation from 9v. If operating from other voltages you may wish to alter the 1K8 resistor to give symmetrical clipping at high levels. The third circuit is one I was working on as I left the audio company but is seems to be potentially very useful. It should be simple to use it as a low impedance mic amp or as a high input impedance preamp for mic or, with a suitable RIAA equalization, for phono pickup or similar. It can even be used as a balanced input mic preamp.

Source:http://www.4qdtec.com/preamp.html