JHL 1969 Class A Headphone Amplifier Circuit

The JHL 1969 Class A headphone amplifier is a fully transistorized stereo headphone driver that operates with a ±12V symmetrical supply and draws continuous current. It is suitable for those who want to drive 32 ohm and higher headphones with low distortion and try a warmer, more controlled class A output stage than simple opamp-based solutions.

General Structure of the Circuit

JLH 1969 Class A headphone amplifier schematic

The schematic shows one audio channel and the common symmetrical power supply. The second channel is repeated with the same structure.

The circuit consists of input level adjustment, a low-level voltage amplifier, driver section, class A output stage, and ±12V regulated power supply.

This structure should not be considered directly as a speaker amplifier. Although it can drive low-impedance loads, the main target is headphones.

Due to class A operation, which draws continuous current, the output transistors heat up even when there is no signal; this is not a design fault, but a natural result of its operating method.

Signal Flow from Input to Headphone Output

The audio signal comes from the input on the left side to the PR1 10K potentiometer. PR1 determines the volume level and transfers the input signal to the C1 1uF coupling capacitor.

C1 blocks the DC component that may come from the source device.

After C1, the signal is applied to the input amplifier section around T1 2N2907.

This section brings the small signal to a usable level and provides a suitable operating point for the following driver stages.

The network around R2, R3, R4, PR2, and D3 is involved in setting the DC operating point.

T2 2N2907 and T3 2N2222 operate as the intermediate stage that drives the TIP41 output transistors. T4 and T5 TIP41 transistors are the main current stage that feeds the headphones.

At the output, the R13 2R2 series resistor makes the output more stable against headphone cable and capacitive load effects.

The C12 and R12 network is also a Zobel-like damping section used for high-frequency stability.

Why Is Class A Operation Important?

In a class A output stage, the transistors are kept in the conduction region continuously, not only during half of the signal period.

This reduces crossover distortion that may occur at the zero crossing. In headphone amplifiers, this provides a clean and smooth character, especially during low-level listening.

The cost is efficiency. The circuit draws current even when there is no signal. According to the operating information given in the source, the quiescent current of each channel is approximately 125mA.

Since the stereo circuit will draw more than 250mA in total, the power supply, regulators, and output transistors must be selected carefully in terms of heat.

To compare different designs in a similar class, class A headphone amplifier examples can also be examined. Unlike simple opamp-output circuits, the jlh 1969 structure provides the output current completely with discrete transistors.

Input and Adjustment Section

PR1 Volume Level Adjustment

The PR1 10K potentiometer adjusts the level of the input signal.

If the output level of the source device is high, when PR1 is opened too much, the input stage or the following driver section may start clipping.

In the first test, the potentiometer should be kept at the middle-low level, and the audio signal should be applied after measuring DC at the output.

PR2 Operating Point Adjustment

The PR2 10K trimpot is the critical component that adjusts the DC operating balance of the circuit.

Before connecting headphones, the DC voltage at the output point should be measured and adjusted as close to zero as possible.

If there is high DC at the output, the headphone coil draws unnecessary current and sensitive drivers may be damaged.

The D3 2V7 zener diode acts as a reference in this adjustment section. The C2 and C3 capacitors keep the reference line clean in terms of AC.

If the polarity or connection of these components is incorrect, the output offset cannot be adjusted or the circuit operates noisily.

Driver and Output Stage

The 2N2907 transistors used as T1 and T2 are PNP type. T3 2N2222 is an NPN transistor.

These three form the voltage and current drive section that controls the TIP41 transistors at the output.

Since the TIP41 transistors carry most of the class A current, they must be mounted on a heatsink.

Although it looks like a small headphone amplifier, continuous heat is generated in each channel.

If a heatsink is not used, the transistor temperature rises, the operating point shifts, and reliability decreases during long-term use.

For T4 and T5, the pinout and metal tab connection should be checked according to the manufacturer.

In TO-220 packaged transistors, the metal surface is usually connected to the collector.

If more than one transistor will be mounted on the same heatsink, an insulating washer and insulated bushing must be used.

Feedback and Stability Components

The network around R8, R9, and C9 creates a feedback effect to the stages before the output.

Feedback keeps the gain under control, reduces distortion, and corrects the frequency response.

The C9 0.33nF small-value capacitor reduces the risk of oscillation by limiting high-frequency behavior.

R13 2R2 is connected in series with the output. It prevents the amplifier from becoming unstable when the headphone cable is long or when a capacitive load occurs at the output.

The R12 22R and C12 33nF network helps the output behave in a more controlled way at high frequencies.

Special Note for the C10 Capacitor

C10 is shown as 100uF/25V in the schematic. Since the DC voltage at the point where this capacitor operates can be very low or close to zero, care must be taken when using a polarized electrolytic capacitor.

The safe solution is to use a bipolar electrolytic capacitor or to obtain a bipolar equivalent by connecting two polarized electrolytics in reverse series.

If a polarized capacitor is used, the polarity should not be trusted without measuring the actual DC voltage on the circuit.

When the output offset is adjusted, the DC direction at this point may change or remain very small.

Therefore, C10 should not be treated as an ordinary bypass capacitor during assembly.

Symmetrical Power Supply

7812 7912 symmetrical power supply schematic

The power supply in the lower section rectifies the approximately 12V transformer supply coming from the AC input and converts it into a ±12V regulated voltage.

The D4 bridge diode rectifies the AC voltage. Large electrolytic capacitors reduce the ripple after rectification. The 7812 regulates the positive line, and the 7912 regulates the negative line.

Having 0.1uF ceramic capacitors at the input and output of the regulators is important for stability.

The 2200uF electrolytic capacitors reduce supply line sag during sudden current demands. LED1 and the 2K2 resistor indicate the presence of the supply.

If a more general background about symmetrical supply is needed, symmetrical adjustable power supply circuits provide a useful comparison. In this headphone amplifier, the current capacity of the regulators is especially important; instead of low-current 78L12/79L12 types, regulators suitable for the actual current requirement should be selected.

Technical Specification Summary

Feature	Value
Circuit type	Transistorized jlh 1969 Class A headphone amplifier
Channel structure	Stereo, two channels are built with the same circuit structure
Supply	±12V regulated symmetrical supply
Transformer input	Approximately 12V AC secondary
Recommended headphone impedance	32 ohm and above; heat control is required at low impedances
Minimum load information	It is stated that it can operate down to 8 ohm level; in practice, 32 ohm and above is safer for headphones
Frequency range	12Hz-50kHz, 0/-1dB; 12Hz-120kHz, 0/-3dB
Gain	Approximately 21dB
Signal / noise ratio	Approximately 101dB
Slew rate	Above 35V/us
Total current consumption	Approximately 260mA
Quiescent current per channel	Approximately 125mA
PCB size	Approximately 124 x 111mm / 124 x 112mm

Component Functions

Component	Value / Model	Function
PR1	10K	Adjusts the input volume level.
PR2	10K	Used for the operating point and output DC balance.
T1, T2	2N2907	Works as PNP transistors in the input and driver section.
T3	2N2222	Operates as an NPN transistor in the driver section.
T4, T5	TIP41	Power transistors that provide the class A output current.
D1, D2	1N4148	Used for biasing and fast signal line functions.
D3	2V7 zener	Creates a reference for the operating point.
D4	1N4007 bridge	Rectifies the AC transformer voltage.
7812 / 7912	Regulator	Provides the +12V and -12V regulated supply.
R13	2R2	Isolates the headphone output against capacitive load effects.
R12 + C12	22R + 33nF	Creates an output damping network for high-frequency stability.
C1	1uF	Blocks the DC component at the input.
C10	100uF	A critical coupling / balancing capacitor at the operating point; a bipolar type is preferred.

Equivalent Component Notes

Instead of 2N2907, PNP small-signal transistors such as 2N3906 or 2N4403 can be tried in some applications.

A similar NPN small-signal transistor can be used instead of 2N2222; however, the pinout, maximum current, and frequency characteristics must be checked.

If a different power transistor will be selected instead of TIP41, looking only at the current and voltage values is not sufficient.

Transition frequency, gain, package structure, and thermal resistance are also important. Since the transistor continuously generates heat in class A operation, using a fake or low-quality power transistor makes the circuit adjustment more difficult.

For those looking for a simpler opamp-based headphone driver, the NE5532 Hi-Fi headphone amplifier offers a different approach. The jlh 1969, on the other hand, targets a discrete-transistor class A output character at the cost of low efficiency.

Points to Consider During Assembly

• Since the two channels will be built with the same circuit, the left and right channel components should be placed symmetrically.
• The TIP41 transistors must definitely be mounted on a suitable heatsink.
• If there is more than one transistor on the same heatsink, an insulating washer should be used.
• Headphones should not be connected before PR2 adjustment is performed.
• The input cable should be shielded, and the chassis connection should be arranged close to a single point.
• The heating of the 7812 and 7912 regulators should be checked during the first startup.
• A bipolar electrolytic should be preferred for C10, or a suitable series connection method should be applied.
• Testing should not be done with expensive headphones without measuring the DC voltage at the output.

First Startup Sequence

• Before applying power, check for a short circuit between the +12V, -12V, and chassis lines.
• Measure the regulator outputs without connecting a load; +12V should be seen on the positive line and -12V on the negative line.
• Measure the DC voltage at the output of each channel without plugging in headphones.
• With PR2, bring the output DC level to the point closest to zero.
• Monitor the quiescent current and heating of the output transistors.
• Perform the first audio test with a low input level and a cheap test headphone.
• If the heatsink temperature reaches a level that cannot be touched by hand during long-term operation, the assembly should be reviewed.

Common Mistakes

Mistake	Possible Result
Connecting headphones without measuring the output DC voltage	DC current flows through the headphone coil and the driver may be damaged.
Using the TIP41 transistors without a heatsink	Heat rise, operating point shift, and failure occur.
Using low-current regulators such as 78L12 / 79L12	The regulator may overheat or the supply may collapse.
Using an electrolytic with the wrong polarity for C10	Distortion, noise, or capacitor failure may be observed.
Routing input and output cables side by side	The risk of hum, noise, or oscillation increases.
Not checking the transistor pinout	The circuit will not work or the transistors may fail at first power-up.

The JLH 1969 Class A headphone amplifier can be used to drive quality headphones with a desktop DAC output, low-power preamp output, or sound card output.

Rather than output power, current capacity, low distortion, and stable drive are targeted.

It is not a suitable circuit for portable battery-powered use. Energy consumption is high due to the continuous quiescent current.

The most logical usage method is to build it as a desktop headphone amplifier inside a small metal enclosure, with a properly grounded transformer power supply and well-ventilated heatsinks.

Source: tutlay.ru/radioshemy/r2/85-usilitel-dlya-naushnikov-jlh-class-a.html