Headphone Amplifier

Headphone Amplifier

Circuit diagram:

Amplifier parts:

P1_____________22K   Log.Potentiometer (Dual-gang for stereo)
R1____________560R   1/4W Resistor
R2,R3__________10K   1/4W Resistors
R4_____________12K   1/4W Resistor
R5,R6___________2R2  1/4W Resistor
R7_____________22R   1/2W Resistor 

C1______________1µF   63V Polyester Capacitor
C2,C3,C4______100µF   25V Electrolytic Capacitors
C5_____________22pF   63V Polystyrene or Ceramic Capacitor
C6_____________22µF   25V Electrolytic Capacitor 

IC1___________LM833 or NE5532 Low noise Dual Op-amp
Q1,Q3_________BC337   45V 800mA NPN Transistors
Q2,Q4_________BC327   45V 800mA PNP Transistors 

J1______________RCA  audio input socket

Power supply parts:

R8______________2K2  1/4W Resistor
C7,C8________2200µF   25V Electrolytic Capacitors
D1____________100V 1A Diode bridge
D2____________5mm. or 3mm. Red LED

IC2___________7815  15V 1A Positive voltage regulator IC
IC3___________7915  15V 1A Negative voltage regulator IC 

T1____________220V Primary, 15 + 15V Secondary 5VA Mains transformer
PL1___________Male Mains plug
SW1___________SPST Mains switch

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Notes:

• Can be directly connected to CD players, tuners and tape recorders.
• Tested with several headphone models of different impedance: 32, 100, 245, 300, 600 & 2000 Ohm.
• Old 8 Ohm impedance headphones can be also driven, but these obsolete devices are not recommended.
• Schematic shows left channel and power supply (common to both channels).
• Numbers in parentheses show IC1 right channel pin connections.
• A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of J1, P1, C2, C3 & C4. Then connect separately the input and output grounds to the power supply ground.

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Technical data:

Output voltage: Well above 5V RMS into all loads

Sensitivity: 250mV input for 5V RMS output

Frequency response: Flat from 30Hz to 20KHz

Total harmonic distortion @ 1KHz & 10KHz:
Below 0.005% into 32 Ohm loads and up to 4V RMS output (typical 0.003%)

Total harmonic distortion @ 1KHz & 10KHz:
Below 0.005% into 100 to 2000 Ohm loads and up to 5V RMS output (typical 0.003%)

Unconditionally stable on capacitive loads

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Modular Audio Preamplifier

Modular Audio Preamplifier

Main Module circuit diagram:

Parts:

R1_______________1K5  1/4W Resistor
R2_____________220K   1/4W Resistor
R3______________18K   1/4W Resistor
R4_____________330R   1/4W Resistor
R5______________39K   1/4W Resistor
R6______________56R   1/4W Resistor
R7,R10__________10K   1/4W Resistors
R8______________33K   1/4W Resistor
R9_____________150R   1/4W Resistor
R11_____________ 6K8  1/4W Resistor
R12,R13________100R   1/4W Resistors
R14____________100K   1/4W Resistor

C1_____________220nF   63V Polyester Capacitor
C2_____________220pF   63V Polystyrene or ceramic Capacitor
C3_______________1nF   63V Polyester or ceramic Capacitor
C4,C7___________47µF   50V Electrolytic Capacitors
C5,C6__________100µF   50V Electrolytic Capacitors

Q1,Q2_________BC550C   45V 100mA Low noise High gain NPN Transistors
Q3____________BC556    65V 100mA PNP Transistor
Q4____________BC546    65V 100mA NPN Transistor

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Tone Control Module circuit diagram:

Parts:

R1,R7___________47K   1/4W Resistors
R2_____________220K   1/4W Resistor
R3______________18K   1/4W Resistor
R4_____________330R   1/4W Resistor
R5______________39K   1/4W Resistor
R6______________56R   1/4W Resistor
R8_____________150R   1/4W Resistor
R9______________10K   1/4W Resistor
R10,R16__________6K8  1/4W Resistors
R11,R12________100R   1/4W Resistors
R13____________100K   1/4W Resistor
R14______________1K5  1/4W Resistor
R15,R21,R22______4K7  1/4W Resistors
R17,R24,R26______8K2  1/4W Resistors
R18______________3K3  1/4W Resistor
R19______________1K   1/4W Resistor
R20____________470R   1/4W Resistor
R23,R25_________12K   1/4W Resistors
R27,R28__________4K7  1/4W Resistors

C1_____________220nF   63V Polyester Capacitor
C2_______________1nF   63V Polyester or ceramic Capacitor
C3,C6___________47µF   50V Electrolytic Capacitors
C4,C5__________100µF   50V Electrolytic Capacitors
C7______________10nF   63V Polyester Capacitor
C8,C9__________100nF   63V Polyester Capacitors

Q1,Q2_________BC550C   45V 100mA Low noise High gain NPN Transistors
Q3____________BC556    65V 100mA PNP Transistor
Q4____________BC546    65V 100mA NPN Transistor

SW1,SW2_______2 poles 6 ways Rotary Switches

Simpler, alternative Tone Control parts:

P1______________22K   Linear Potentiometer
P2______________47K   Linear Potentiometer
R29,R30________470R   1/4W Resistors
R31,R32__________4K7  1/4W Resistors

C10_____________10nF   63V Polyester Capacitor
C11,C12________100nF   63V Polyester Capacitors

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Comments:

To complement the 60 Watt MosFet Audio Amplifier a High Quality Preamplifier design was necessary. A discrete components topology, using + and - 24V supply rails was chosen, keeping the transistor count to the minimum, but still allowing low noise, very low distortion and high input overload margin. Obviously, the modules forming this preamplifier can be used in different combinations and drive different power amplifiers, provided the following stages present a reasonably high input impedance (i.e. higher than 10KOhm).

Main Module:

If a Tone Control facility is not needed, the Preamplifier will be formed by the Main Module only. Its input will be connected to some sort of changeover switch, in order to allow several audio reproduction devices to be connected, e.g. CD player, Tuner, Tape Recorder, iPod, MiniDisc etc. The total amount and type of inputs is left to the choice of the home constructor.

The output of the Main Module will be connected to a 22K Log. Potentiometer (dual gang if a stereo preamp was planned). The central and ground leads of this potentiometer must be connected to the power amplifier input.

Tone Control Module:

This Module employs an unusual topology, still maintaining the basic op-amp circuitry of the Main Module with a few changes in resistor values.

A special feature of this circuit is the use of six ways switches instead of the more common potentiometers: in this way, precise "tone flat" setting, or preset dB steps in bass and treble boost or cut can be obtained. Tone Control switches also allow a more precise channel matching when a stereo configuration is used, avoiding the frequent poor alignment accuracy presented by common ganged potentiometers.

Six ways (two poles for stereo) rotary switches were chosen for this purpose as easily available. This dictated the unusual "asymmetrical" configuration of three positions for boost, one for flat and two for cut. This choice was based on the fact that tone controls are used in practice more for frequency boosting than for cutting purposes. In any case, +5dB +10dB and +15dB of bass boost and -3dB and -10dB of bass cut were provided. Treble boost was also set to +5dB +10dB and +15dB and treble cut to -3.5dB and -9dB.

Those wishing to use common potentiometers in the usual way for Tone Controls may use the circuit shown enclosed in the dashed box (bottom-right of the Tone Control Module circuit diagram) to replace switched controls.

The Tone Control Module should usually be placed after the Main Input Module, and the volume control inserted between the Tone Control Module output and the power amplifier input. Alternatively, the volume control can also be placed between Main Input Module and Tone Control Module, at will. Furthermore, the position of these two modules can be also interchanged.

Power supply:

The preamplifier must be feed by a dual-rail, +24 and -24V 50mA dc power supply. This is easily achieved by using a 48V 3VA center-tapped mains transformer, a 100V 1A bridge rectifier and a couple of 2200µF 50V smoothing capacitors. To these components two 24V IC regulators must be added: a 7824 (or 78L24) for the positive rail and a 7924 (or 79L24) for the negative one.

The diagram of such a power supply is the same of that used in the Headphone Amplifier, but the voltages of the secondary winding of the transformer, smoothing capacitors and IC regulators must be updated. Alternatively, the dc voltage can be directly derived from the dc supply rails of the power amplifier, provided that both 24V regulators are added.

Note:

• If this preamplifier is used as a separate, stand-alone device, thus requiring a cable connection to the power amplifier, some kind of output short-circuit protection is needed, due to possible shorts caused by incorrect plugging. The simplest solution is to wire a 3K3 1/4W resistor in series to the output capacitor of the last module (i.e. the module having its output connected to the preamp main output socket).

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Technical data:

Main Module Input sensitivity:
250mV RMS for 1V RMS output

Tone Control Module Input sensitivity:
1V RMS for 1V RMS output

Maximum output voltage:
13.4V RMS into 100K load, 11.3V RMS into 22K load, 8.8V RMS into 10K load

Frequency response:
flat from 20Hz to 20KHz

Total harmonic distortion @ 1KHz:
1V RMS 0.002% 5V RMS 0.003% 7V RMS 0.003%

Total harmonic distortion @10KHz:
1V RMS 0.003% 5V RMS 0.008% 7V RMS 0.01%

3 - 5 Watt Class-A Audio Amplifier

3 - 5 Watt Class-A Audio Amplifier

Circuit diagram:Parts:

P1_____________47K  Log. Potentiometer (Dual-gang for stereo)
R1____________100K  1/4W Resistor
R2_____________12K  1/4W Resistor (See Notes)
R3_____________47K  1/4W Resistor
R4______________8K2 1/4W Resistor
R5______________1K5 1/4W Resistor (Optional, see Notes)
R6______________2K7 1/4W Resistor
R7,R9_________100R  1/4W Resistors
R8____________560R  1/2W Resistor (See Notes)
R10_____________1R  1/2W Resistor 

C1,C2__________10µF  63V Electrolytic Capacitors
C3_____________47µF  25V Electrolytic Capacitor
C4____________100µF  35V Electrolytic Capacitor
C5____________150nF  63V Polyester Capacitor (Optional, see Notes)
C6,C7_________220µF  25V Electrolytic Capacitors
C8___________1000µF  25V Electrolytic Capacitor 

Q1___________BC560C  45V 100mA Low noise High gain PNP Transistor
Q2,Q3________BD439   60V 4A NPN Transistors 

SPKR___________One or more speakers wired in series or in parallel
Total resulting impedance: 8 Ohm
Minimum power handling: 5W

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Comments:

In the old valve days, most commercial audio amplifiers suited for compact integrated mono or stereo record players used a one-valve amplifier topology. The circuit was usually implemented by means of a multiple type valve, e.g. a triode pentode ECL86.

Common features for those amplifiers were: Class A operation, output power in the 3 - 5W range, input sensitivity of about 600mV for full output power, THD of about 3% @ 3W and 1KHz.

Best types showed THD figures of 1.8% @ 3W and 0.8% @ 2W.

This solid-state push-pull single-ended Class A circuit is capable of providing a sound comparable to those valve amplifiers, delivering more output power (6.9W measured across a 8 Ohm loudspeaker cabinet load), less THD, higher input sensitivity and better linearity.

Voltage and current required for this circuit are 24V and 700mA respectively, compared to 250V HT rail and 1A @ 6.3V filament heating for valve-operated amplifiers.

The only penalty for the transistor operated circuit is the necessity of using a rather large heatsink for Q2 and Q3 (compared to the maximum power delivered).

In any case, the amount of heat generated by this circuit can be comparable to that of a one-valve amplifier.

An optional bass-boost facility can be added, by means of R5 and C5.

This circuit was built and compared with a one-valve box gramophone circuit of the late 1950s by , a Dutch biochemist working in the field of medical imaging (PET) with a strong interest in audio and valve amplifiers.

A thorough description of both circuits and the results of subjective test comparisons made by this distinguished Author appeared on AudioExpress magazine: February, March and April 2005 issues.

Technical data:

(measured on 8 Ohm resistive load unless otherwise specified)

Sensitivity:

230mV input for 1.5W output
380mV input for 3.5W output
560mV input for 5.6W output

Sensitivity with bass-boost:

400mV input for 1.5W output
630mV input for 3.5W output
850mV input for 5.6W output

Sensitivity with 8 Ohm nominal, loudspeaker cabinet load:

210mV input for 1.5W output
325mV input for 3.5W output
477mV input for 6.9W output

Frequency response:

100Hz to 20KHz 0dB; -3dB @ 40Hz

Frequency response with bass-boost:

+5dB @ 100Hz; +3.9dB @ 200Hz; +2.5dB @ 400Hz; -1dB @ 10KHz and 20KHz

Total harmonic distortion @ 1KHz:

0.3% @ 0.5W; 0.45% @ 1W; 1% @ 5.6W

Unconditionally stable on capacitive loads

Notes:

• If necessary, R2 can be adjusted to obtain 13V across C8 positive lead and negative ground.
• Total current drawing of the circuit, best measured by inserting the probes of an Avo-meter across the positive output of the power supply and the positive rail input of the amplifier, must be 700mA. Adjust R8 to obtain this value if necessary.
• Q2 and Q3 must be mounted on a finned heatsink of 120x50x25mm. minimum dimensions.
• Add R5 and C5 if the bass-boost facility is required.

Mini-box 2W Amplifier

Mini-box 2W Amplifier

Circuit diagram:

Parts:

P1_____________10K   Log.Potentiometer
R1,R2__________33K   1/4W Resistors
R3_____________33R   1/4W Resistor
R4_____________15K   1/4W Resistor
R5,R6___________1K   1/4W Resistors
R7____________680R   1/4W Resistor
R8____________120R   1/2W Resistor
R9____________100R   1/2W Trimmer Cermet 

C1,C2__________10µF   63V Electrolytic Capacitors
C3____________100µF   25V Electrolytic Capacitor
C4,C7_________470µF   25V Electrolytic Capacitors
C5_____________47pF   63V Ceramic Capacitor
C6____________220nF   63V Polyester Capacitor
C8___________1000µF   25V Electrolytic Capacitor

D1___________1N4148   75V 150mA Diode
Q1____________BC560C  45V 100mA PNP Low noise High gain Transistor
Q2____________BC337   45V 800mA NPN Transistor
Q3____________TIP31A  60V 4A    NPN Transistor
Q4 ___________TIP32A  60V 4A    PNP Transistor 

SW1___________SPST switch
SPKR__________3-5 Watt Loudspeaker, 8, 4 or 2 Ohm impedance

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Device purpose:

This amplifier was designed to be self-contained in a small loudspeaker box. It can be feed by Walkman, Mini-Disc, iPod and CD players, computers and similar devices fitted with line or headphone output. Of course, in most cases you will have to make two boxes to obtain stereo.

The circuit was deliberately designed using no ICs and in a rather old-fashioned manner in order to obtain good harmonic distortion behavior and to avoid hard to find components. The amplifier(s) can be conveniently supplied by a 12V wall plug-in transformer.

Closing SW1 a bass-boost is provided but, at the same time, volume control must be increased to compensate for power loss at higher frequencies.

In use, R9 should be carefully adjusted to provide minimal audible signal cross-over distortion consistent with minimal measured quiescent current consumption; a good compromise is to set the quiescent current at about 10-15 mA.

To measure this current, wire a DC current meter temporarily in series with the collector of Q3.

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Technical data:

Output power:
1.5 Watt RMS into 8 Ohm, 2.5 Watt into 4 Ohm, 3.5 Watt into 2 Ohm (1KHz sinewave)

Sensitivity:100mV input for 1.5W output @ 8 Ohm
Frequency response:30Hz to 20KHz -1dB
Total harmonic distortion @ 1KHz & 10KHz:<0.2% @ 8 Ohm 1W, <0.3% @ 4 Ohm 2W, <0.5% @ 2 Ohm 2W

60W MosFet Audio Amplifier

60W MosFet Audio Amplifier

Circuit diagram:

Parts:

R1______________47K   1/4W Resistor
R2_______________4K7  1/4W Resistor
R3______________22K   1/4W Resistor
R4_______________1K   1/4W Resistor
R5,R12,R13_____330R   1/4W Resistors
R6_______________1K5  1/4W Resistor
R7______________15K   1/4W Resistor
R8______________33K   1/4W Resistor
R9_____________150K   1/4W Resistor
R10____________500R   1/2W Trimmer Cermet
R11_____________39R   1/4W Resistor
R14,R15___________R33 2.5W Resistors
R16_____________10R   2.5W Resistor
R17_______________R22   5W Resistor (wirewound) 

C1_____________470nF   63V Polyester Capacitor
C2_____________470pF   63V Polystyrene or ceramic Capacitor
C3______________47µF   63V Electrolytic Capacitor
C4,C8,C9,C11___100nF   63V Polyester Capacitors
C5______________10pF   63V Polystyrene or ceramic Capacitor
C6_______________1µF   63V Polyester Capacitor
C7,C10_________100µF   63V Electrolytic Capacitors 

D1___________1N4002   100V 1A Diode
D2_____________5mm. Red LED 

Q1,Q2,Q4_____MPSA43   200V 500mA NPN Transistors
Q3,Q5________BC546     65V 100mA NPN Transistors
Q6___________MJE340   200V 500mA NPN Transistor
Q7___________MJE350   200V 500mA PNP Transistor
Q8___________IRFP240  200V 20A N-Channel Hexfet Transistor
Q9___________IRFP9240 200V 12A P-Channel Hexfet Transistor

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Power supply circuit diagram:

Parts:

R1_______________3K9   1W Resistor
C1,C2_________4700µF  63V Electrolytic Capacitors (See Notes)
C3,C4__________100nF  63V Polyester Capacitors 

D1_____________400V 8A Diode bridge
D2_____________5mm. Red LED
F1,F2__________4A Fuses with sockets
T1_____________230V or 115V Primary, 30+30V Secondary 160VA Mains transformer 

PL1____________Male Mains plug
SW1____________SPST Mains switch

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Comments:

To celebrate the hundredth design posted to this website, and to fulfil the requests of many correspondents wanting an amplifier more powerful than the 25W MosFet, a 60 - 90W High Quality power amplifier design is presented here.

Circuit topology is about the same of the above mentioned amplifier, but the extremely rugged IRFP240 and IRFP9240 MosFet devices are used as the output pair, and well renowned high voltage Motorola's transistors are employed in the preceding stages.

The supply rails voltage was kept prudentially at the rather low value of + and - 40V. For those wishing to experiment, the supply rails voltage could be raised to + and - 50V maximum, allowing the amplifier to approach the 100W into 8 Ohm target: enjoy!

Matching, discrete components, Modular Preamplifier design are available here: Modular Audio Preamplifier.

Notes:

• In the original circuit, a three-diode string was wired in series to R10. Two of these diodes are now replaced by a red LED in order to achieve improved quiescent current stability over a larger temperature range. Thanks to David Edwards of LedeAudio for this suggestion.
• A small, U-shaped heat sink must be fitted to Q6 & Q7.
• Q8 & Q9 must be mounted on large heat sinks.
• Quiescent current can be measured by means of an Avo-meter wired in series to the positive supply rail and no input signal.
• Set the Trimmer R10 to its minimum resistance.
• Power-on the amplifier and adjust R10 to read a current drawing of about 120 - 130mA.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.
• The value suggested for C1 and C2 in the Power Supply Parts List is the minimum required for a mono amplifier. For optimum performance and in stereo configurations, this value should be increased: 10000µF is a good compromise.
• A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of R1, R3, C2, C3 and C4 and the ground input wire. Connect R7 and C7 to C11 to output ground. Then connect separately the input and output grounds to the power supply ground.

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Technical data:

Output power:
60 Watt RMS @ 8 Ohm (1KHz sinewave) - 90W RMS @ 4 Ohm
Sensitivity:
1V RMS input for 58W output
Frequency response:
30Hz to 20KHz -1dB
Total harmonic distortion @ 1KHz:
1W 0.003% 10W 0.006% 20W 0.01% 40W 0.013% 60W 0.018%
Total harmonic distortion @10KHz:
1W 0.005% 10W 0.02% 20W 0.03% 40W 0.06% 60W 0.09%

Unconditionally stable on capacitive loads