Circuit Diagram Knowledge

This is a Super Hi-Fi Headphone Amplifier  Circuit Diagram.

Super Hi-Fi Headphone Amplifier Circuit Diagram

Many modern high quality amplifiers have loudness controls built in. In most instances they are manually switched into circuit when required  in a few amplifiers the circuit is switched in at all times.
Nevertheless there are innumerable older or present-day low-priced amplifiers that are not fitted with loudness compensation - and it is for units such as these that this simple project has been designed. The device shown is for a mono amplifier two are required for stereo amplifiers. lt can be very simply assembled on tag strips or matrix board, and, when completed connected between your preamplifier and main amplifier. lf yours is an integrated unit it should be readily possible to break into the volume control circuit just connect the unit in series with the slider terminal of the potentiometer. Screened leads may be necessary of long lengths are required.  We would like to emphasize that this is a c0mpr0mise circuit. Ideally a loudness control must be designed specifically to suit the amplifier for which it is intended. Also the degree of Loudness compensation should be related to the volume control setting. This latter requirement involves replacing the existing volume control by a suitably tapped potentiometer a device that is not readily available  "off the shelf" - so the circuit shown here introduces a fixed amount or compensation that is adequate for moderate listening levels. This circuit will suit most amplifiers quite well ·- and in any case can be adjusted by minor variation of component values if required. `Switch SW1 should be a double·p0le double-throw type if stereo operation is required. 

Some lovers of High Fidelity headphone listening prefer the use of battery powered headphone amplifiers, not only for portable units but also for home "table" applications. This design is intended to fulfil their needs and its topology is derived from the Portable Headphone Amplifier featuring an NPN/PNP compound pair emitter follower output stage. An improved output driving capability is gained by making this a push-pull Class-B arrangement. Output power can reach 100mW RMS into a 16 Ohm load at 6V supply with low standing and mean current consumption, allowing long battery duration. The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.

Battery-powered Headphone Amplifier Circuit diagram

Notes:

• For a Stereo version of this circuit, all parts must be doubled except P1, SW1, J2 and B1.
• Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to maximum resistance and Trimmer R3 to about the middle of its travel.
• Connect a suitable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output.
• Switch on the supply and measure the battery voltage with a Multimeter set to about 10Vdc fsd.
• Connect the Multimeter across the positive end of C4 and the negative ground.
• Rotate R3 in order to read on the Multimeter display exactly half of the battery voltage previously measured.
• Switch off the supply, disconnect the Multimeter and reconnect it, set to measure about 10mA fsd, in series to the positive supply of the amplifier.
• Switch on the supply and rotate R6 slowly until a reading of about 3mA is displayed.
• Check again the voltage at the positive end of C4 and readjust R3 if necessary.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.
• Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.

Technical data:
Output power (1KHz sinewave):
    16 Ohm: 100mW RMS
    32 Ohm: 60mW RMS
    64 Ohm: 35mW RMS
    100 Ohm: 22.5mW RMS
    300 Ohm: 8.5mW RMS
Sensitivity:
    160mV input for 1V RMS output into 32 Ohm load (31mW)
    200mV input for 1.27V RMS output into 32 Ohm load (50mW)
Frequency response @ 1V RMS:
    flat from 45Hz to 20KHz, -1dB @ 35Hz, -2dB @ 24Hz
Total harmonic distortion into 16 Ohm load @ 1KHz:
    1V RMS (62mW) 0.015% 1.27V RMS (onset of clipping, 100mW) 0.04%
Total harmonic distortion into 16 Ohm load @ 10KHz:
    1V RMS (62mW) 0.05% 1.27V RMS (onset of clipping, 100mW) 0.1%
Unconditionally stable on capacitive loads