Wiring Diagram

This circuit can be used for a headphone amplifier, arose after the purchase of commercial equipment with separate pre and power amplifiers without a headphone output. The intention is to permanently insert the headphone amp between pre and power amps. The gain control being used to pre-set the gain so that the pre-amp's gain control is normally used for setting the listening level.

Its advantages are :

    * low output impedance to drive several pairs of phones
    * The active gain stage is, almost, perfectly logarithmic and ...
    * Independent of the absolute value of the pot
    * Excellent channel tracking
    * O/P noise reduces with gain reduction.

The first stage is a conventional series feedback circuit using the ubiquitous NE5534, the gain being set by the feedback AOT (adjust on test) resistor to suit individual needs, this stage provides the necessary low impedance output for the variable gain stage. The resistor/ capacitor networks around the input stage may seem a little extravagant, but are necessary to reduce any possible RF pickup especially the 470 pF between the two IC + and - inputs.

The voltage drop across the resistors R2 and R3, are used as the input of the Darlington pairs.
 As the input current to the op-amp depends on the level of the signal op amp is amplifying
the voltage drop across the resistors R2 and R3 will be proportional to the input signal.
These voltage drops are given to the base of Darlington pairs. The amplification is stabilized
as a result of the negative feedback from the junction of collectors of Q2 and Q4.
The theory may seem little awkward for you.But its working good.Such a simple but stable
circuit as this can produce a reasonable output of 12W on a 4 Ohm speaker.