Pure Class A Headphone Amplifier Schematic

400mW RMS into 32 Ohm load, Single-rail Supply - Optional Tilt Control

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This design is derived from the Portable Headphone Amplifierfeaturing 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.

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Tilt control:

The 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.

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Circuit diagram:

Class-A Headphone Amplifier Circuit Diagram

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Parts:

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 = 47K 1/4W Resistors
R9 = 47K 1/4W Resistors
R10 = 2R2 1/4W Resistors
R11 = 2R2 1/4W Resistors
R12 = 4K7 1/4W Resistor
R13 = 4R7 1/2W Resistor
R14 = 1K2 1/4W Resistor
R15 = 330K 1/4W Resistors (Optional)
R16 = 680K 1/4W Resistor (Optional)
R17 = 220K 1/4W Resistors (Optional)
R18 = 330K 1/4W Resistors (Optional)
R19 = 220K 1/4W Resistors (Optional)
R20 = 22K 1/4W Resistors (Optional)
R21 = 22K 1/4W Resistors (Optional)

C1 = 10µF 25V Electrolytic Capacitors
C2 = 10µF 25V Electrolytic Capacitors
C3 = 10µF 25V Electrolytic Capacitors
C4 = 10µF 25V Electrolytic Capacitors
C5 = 220µF 25V Electrolytic Capacitors
C6 = 100nF 63V Polyester Capacitors
C7 = 220µF 25V Electrolytic Capacitors
C8 = 2200µF 25V Electrolytic Capacitor
C9 = 1nF 63V Polyester Capacitors (Optional)
C10 = 470pF 63V Polystyrene or Ceramic Capacitor (Optional)
C13 = 15nF 63V Polyester Capacitor (Optional)
C11 = 1nF 63V Polyester Capacitors (Optional)
C12 = 1nF 63V Polyester Capacitors (Optional)

D1 = 5mm. or 3mm. LED
D2 = 1N4002 100V 1A Diodes
D3 = 1N4002 100V 1A Diodes

Q1 = BC550C 45V 100mA Low noise High gain NPN Transistors
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-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

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Notes:

• 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.

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Technical data:

• 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

Sensitivity:

• 140mV input for 1V RMS output into 32 Ohm load (31mW)
• 500mV input for 3.5V RMS output into 32 Ohm load (380mW)
• 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

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1V Low Power Stereo Headphone Amplifier with Shutdown MAX9725

The MAX9725A–MAX9725D fixed-gain, stereo headphone amplifiers are ideal for portable equipment where board space is at a premium. The MAX9725E offers the flexibility to adjust the gain with external input and feedback resistors. The MAX9725A–MAX9725E use a unique DirectDrive® architecture to produce a ground-referenced output from a single supply, eliminating the need for large DC-blocking capacitors, saving cost, board space, and component height. Fixed gains of -2V/V (MAX9725A), -1.5V/V (MAX9725B), -1V/V (MAX9725C), and -4V/V (MAX9725D) further reduce external component count. The adjustable gain of the MAX9725E DirectDrive headphone amplifier allows for any gain down to -1V/V using external resistors.

The MAX9725 delivers up to 20mW per channel into a 32Ω load and achieves 0.006% THD+N. An 80dB at 1kHz power-supply rejection ratio (PSRR) allows the MAX9725 to operate from noisy digital supplies without an additional linear regulator. The MAX9725 includes ±8kV ESD protection on the headphone output. Comprehensive click-and-pop circuitry suppresses audible clicks and pops at startup and shutdown. A low-power shutdown mode reduces supply current to 0.6µA (typ).

The MAX9725 operates from a single 0.9V to 1.8V supply, allowing the device to be powered directly from a single AA or AAA battery. The MAX9725 consumes only 2.1mA of supply current, provides short-circuit protection, and is specified over the extended -40°C to +85°C temperature range. The MAX9725 is available in a tiny (1.54mm x 2.02mm x 0.6mm) 12-bump chip-scale package (UCSP™) and a 12-pin thin QFN package (4mm x 4mm x 0.8mm). 

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A Headphone Monitoring Switch

In any recording situation, monitoring is critical to make sure youre getting what you want on tape. This is just as true in field recording, but in most cases, ones monitoring options are severely limited--stereo headphone is the only choice.

Headphone Monitoring Switch  :

Since I often use dual-mono mics, hearing a stereo feed of the two is not always convenient. I wanted the option to hear JUST the left mic in BOTH ears, or just the right mic in both ears, as well as a normal stereo signal. This is simple enough to do with a big rotary switch. When completed, you can create a little box that your headphones plug into, which in turn is plugged into the stereo phone output of your deck. Then, by turning the knob on the switch box, you can hear normal stereo, left-only mono, right-only mono, left+right mono and even left-right reversed stereo (or normal stereo again). 

Note the use of summing resistors in the left+right mono section. This was an attempt to prevent the two outputs from "fighting" each other if there were very different voltages in left and right outputs. I used 8 ohm resistors here, but a higher value might be better. Maybe ~20 ohms? Also, I initially decided to put normal stereo on both ends of the switchs travel so Id always be able to find it without looking. However, I sometimes wish to have left-right reversed. If youd like to try this, simply swap the leads on one of the "normal stereo" connections. 

One final caveat: The left only/right-only mono positions are -6dB down, since only one half of the decks headphone amp is driving your phones when the switch is in those positions. 

Source by : Ediagramlab

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Battery powered Headphone Amplifier

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.









Parts:

P1_____________22K Dual gang Log Potentiometer (ready for Stereo)

R1_____________15K 1/4W Resistor
R2____________100K 1/4W Resistor
R3____________100K 1/2W Trimmer Cermet
R4_____________47K 1/4W Resistor
R5____________470R 1/4W Resistor
R6____________500R 1/2W Trimmer Cermet
R7______________1K 1/4W Resistor
R8,R9__________18K 1/4W Resistors
R10,R11_________2R2 1/4W Resistors
R12____________33R 1/4W Resistor
R13_____________4K7 1/4W Resistor

C1,C2__________10µF 25V Electrolytic Capacitors
C3,C5_________100nF 63V Polyester Capacitors
C4,C6_________220µF 25V Electrolytic Capacitors

Q1,Q2,Q5______BC560C 45V 100mA Low noise High gain PNP Transistors
Q3,Q4_________BC550C 45V 100mA Low noise High gain NPN Transistor
Q6____________BC327 45V 800mA PNP Transistor
Q7____________BC337 45V 800mA NPN Transistor

SW1____________SPST slide or toggle Switch

J1_____________RCA audio input socket
J2_____________6mm. or 3mm. Stereo Jack socket

B1_____________6V Battery (4xAA or AAA Alkaline or rechargeable cells, etc.)



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 schemary giving excellent frequency stability.



Notes:

* For a Stereo version of this schema, 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



Read More

Class A Headphone Amplifier

     This circuit 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.

     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.

P1 : 22K Dual gang Log Potentiometer 
R1 : 15K 
R2 : 220K
R3 : 100K
R4 : 33K 
R5 : 68K 
R6 : 50K 
R7 : 10K 
R8,R9 : 47K 
R10,R11 : 2R2 
R12 : 4K7
R13 : 4R7
R14 : 1K2
R15,R18 : 330K
R16 : 680K
R17,R19 : 220K
R20,R21 : 22K
C1,C2,C3,C4 : 10µF/25V 
C5,C7 : 220µF/25V
C6,C11 : 100nF
C8 : 2200µF/25V
C9,C12 : 1nF
C10 : 470pF
C13 : 15nF
D1 : LED
D2,D3 : 1N4002 
Q1,Q2 : BC550C 
Q3 : BC560C 
Q4 : BD136 
Q5 : BD135 
IC1 : 7815
T1 : 15CT/5VA Mains transformer
SW1 : 4 poles 3 ways rotary Switch 
SW2 : SPST slide or toggle Switch

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