P1_____________22K Dual gang Log Potentiometer (ready for Stereo) R1_____________15K 1/4W Resistor R2____________220K 1/4W Resistor R3____________100K 1/2W Trimmer Cermet R4_____________33K 1/4W Resistor R5_____________68K 1/4W Resistor R6_____________50K 1/2W Trimmer Cermet R7_____________10K 1/4W Resistor R8,R9__________47K 1/4W Resistors R10,R11_________2R2 1/4W Resistors R12_____________4K7 1/4W Resistor R13_____________4R7 1/2W Resistor R14_____________1K2 1/4W Resistor R15,R18_______330K 1/4W Resistors (Optional) R16___________680K 1/4W Resistor (Optional) R17,R19_______220K 1/4W Resistors (Optional) R20,R21________22K 1/4W Resistors (Optional) C1,C2,C3,C4____10µF 25V Electrolytic Capacitors C5,C7_________220µF 25V Electrolytic Capacitors C6,C11________100nF 63V Polyester Capacitors C8___________2200µF 25V Electrolytic Capacitor C9,C12__________1nF 63V Polyester Capacitors (Optional) C10___________470pF 63V Polystyrene or Ceramic Capacitor (Optional) C13____________15nF 63V Polyester Capacitor (Optional) D1_____________5mm. or 3mm. LED D2,D3________1N4002 100V 1A Diodes Q1,Q2_________BC550C 45V 100mA Low noise High gain NPN Transistors Q3____________BC560C 45V 100mA Low noise High gain PNP Transistor Q4____________BD136 45V 1.5A PNP Transistor Q5____________BD135 45V 1.5A NPN Transistor IC1____________7815 15V 1A Positive voltage regulator IC T1_____________220V Primary, 15 + 15V Secondary (30V center-tapped) 5VA Mains transformer SW1____________4 poles 3 ways rotary Switch (ready for Stereo) SW2____________SPST slide or toggle Switch J1_____________RCA audio input socket J2_____________6mm. or 3mm. Stereo Jack socket PL1____________Male Mains plug
This design 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-A arrangement. Output power can reach 427mW RMS into a 32 Ohm load at a fixed standing current of 100mA.
The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.
The above mentioned shunt-feedback configuration also allows the easy addition of frequency dependent networks in order to obtain an useful, unobtrusive, switchable Tilt control (optional).
When SW1 is set in the first position a gentle, shelving bass lift and treble cut is obtained. The central position of SW1 allows a flat frequency response, whereas the third position of this switch enables a shelving treble lift and bass cut. See the graph below:
- Q4, Q5 and IC1 must be fitted with a small U-shaped heatsink.
- For a Stereo version of this circuit, all parts must be doubled except P1, IC1, R14, D1, D2, D3, C8, T1, SW1, SW2, J2 and PL1.
- If the Tilt Control is not needed, omit SW1, all resistors from R15 onwards and all capacitors from C9 onwards. Connect the rightmost terminal of R1 to the Base of Q1.
- Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to zero 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.
- Connect a Multimeter, set to measure about 10Vdc fsd, across the positive end of C5 and the negative ground.
- Switch on the supply and rotate R3 in order to read about 7.7-7.8V on the Multimeter display.
- Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 200mA fsd, in series to the positive supply of the amplifier.
- Switch on the supply and rotate R6 slowly until a reading of about 100mA is displayed.
- Check again the voltage at the positive end of C5 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.
- Output power (1KHz sinewave):
- 32 Ohm: 427mW RMS
- 64 Ohm: 262mW RMS
- 100 Ohm: 176mW RMS
- 300 Ohm: 64mW RMS
- 600 Ohm: 35mW RMS
- 2000 Ohm: 10mW RMS
- 140mV input for 1V RMS output into 32 Ohm load (31mW)
- 500mV input for 3.5V RMS output into 32 Ohm load (380mW)
- Frequency response @ 2V RMS:
- See the above graph
- Total harmonic distortion into 32 Ohm load @ 1KHz:
- 1V RMS 0.005% 3V RMS 0.015% 3.65V RMS (onset of clipping) 0.018%
- Total harmonic distortion into 32 Ohm load @ 10KHz:
- 1V RMS 0.02% 3V RMS 0.055% 3.65V RMS (onset of clipping) 0.1%
- Unconditionally stable on capacitive loads