Class-A Headphone Amplifier

This is an amplifier designed to provide plenty of high-quality power for discerning headphone listeners. It can be used to boost the output level from smartphones or digital music players suitable for studio-quality heaphones, or provide a headphone-level output from a line level signal.

CAUTION: high sound levels damage hearing! This is intended for use with lower-sensitivity (or higher impedance) on-ear headphones. It's not recommended for use with standard smartphone earbuds. As the constructor and user you are responsible for keeping your ears protected from dangerous sound exposure levels.

Circuit boards

I built this amplifier around two PCBs I got from eBay here: https://www.ebay.co.uk/itm/265295518352

This is a widely-copied circuit and many other alternative PCBs may be available - searching for 'JLH 1969' helps. Alternatively, it is simple enought to be built on prototyping board. Ready-built amplifier modules are not suitable without component changes - a bare PCB or unassembled kit is therefore recommended.

PCB components

Resistors, capacitors and transistors for the amplifier circuit boards are listed in the the spreadsheet (CSV) file attached below.

Other components

• 24 V 1A power supply (e.g. Tiger TP1129 from CPC)
• Panel mount DC power input socket to match the above (e.g. 2.1mm)
• 10K or 22K dual log potentiometer (e.g. this one)
• Knob (RS 467-2479 is ridiculously expensive but looks great)
• 2 x phono sockets, panel mount (like this)
• 1/4" (6.35mm) stereo jack socket, panel mount (e.g. RS 175-0155)
• PCB interconnectors (e.g. these headers and sockets)
• Transistor heatsink insulating pads (like this)
• Screened and unscreened wire
• Case materials, nuts and bolts

Tools

• Soldering iron, solder
• Pliers; wire-cutters; strippers
• (Recommended) adjustable bench power supply
• Drill, hacksaw, etc. if constructing case as illustrated

In 1969 John Linsley-Hood published an article in Wireless World magazine describing an audio power amplifier based around a four-transistor circuit. It is an engineering masterpiece - it uses a small number of readily-available components, gives dependable and repeatable performance, but it's not so simple as to need obvious improvements. With modern components the performance is at least as good as the original.

This is a 'Class-A' amplifier - that's a technical term, meaning that its transistors are always turned on (passing current) even when there is no signal. This has the drawback that the amplifier uses significantly more power than the alternative 'Class B' (or Class-AB) types, but the advantage that distortion vanishes away as the output level decreases.

The original JLH amplifier was designed to give an output power of 10 Watts into an 8 ohm loudspeaker. For a headphone application the power output can be much less (1 Watt is more than enough), so I've scaled the supply voltage and operating currents appropriately. This means the amplifier produces a fraction of the heat of the 10 W version, and can use an inexpensive, ready-made power supply. The schematic, with adjusted component values, is shown above.

There is plenty of information on this circuit on this website: https://sound-au.com/tcaas/index-1.htm

The JLH circuit achieves best performance (lowest distortion) when the two output transistors Q1 and Q2 have the same current gain (hFE). In this step we measure the gain for each transistor in a batch, and pick pairs (one pair for each channel) which have closest gain.

This step is optional - building the prototype amp I bought a set of 10 TIP3055 transistors, and none of them were more than 5% different from another. Modern manufacturing is much more consistent than in 1969, and it's very likely any pair from the same batch will give excellent performance.

You will need:

• A 5V power supply
• A meter for measuring current up to about 300mA (a bench power supply with an output current meter is ideal)
• A 2k2 resistor
• Test leads (e.g. crocodile clips)
• A heatsink or piece of flat metal

The procedure is to connect each transistor up as shown in the diagram, apply power and measure the current drawn by the transistor. (The gain is equal to the collector current divided by the base current - for matching all we really need is the total current). For a typical TIP3055, the gain is 100 and the total current in the circuit above will be about 200mA.

Make a note of the current reading for each transistor, and then you can pick pairs with similar readings. If you have a choice, higher gain (higher current) is better.

The transistors will heat up when power is applied, which will affect the gain! If you're finding it difficult to measure the current within a couple of seconds of applying power, clamp the transistor to handy piece of metal to keep the temperature stable.

As always, it easiest to begin assembling with the lowest-height components first, in this case the resistors.

I've shown the component positions for the eBay PCB (see 'Supplies' for the link). Note that some of these will be different from the values printed on the board itself.

Note regarding R2A and R2B

R2 is used to set the operating current of the circuit, which affects the maximum output power, and the heat produced. The eBay PCB has a trimmer in the R2 position. You can follow this approach (a 5K trimmer would be appropriate) but to keep 'moving parts' to a minimum I opted to use a fixed 2k7 resistor (R2A) in parallel with a second resistor (R2B) chosen during testing. See step 5 for more details.

Next step is to fit the capacitors and transistors, and complete the board assembly.

Be careful with the two small (TO92 case) transistors: not all TO92 transistors have the same arrangement of Emitter, Base and Collector leads. The eBay circuit board uses a 2SA970 part for Q4, which does not match the BC558 transistor which I had available. This is not a big problem, as with care you can bend the leads (see photo) to fit. Alternatively you can look for a 2SA970 or similar low-noise, small-signal PNP transistor with the right pinout.

For the output capacitor (C2) I used two 470uF capacitors in parallel, equivalent to a single roughly 1000uF component. This was mostly to make ordering the components easier (a pack of 10 is enough for both channels, with spares) but also helps to keep the ESR low.

Before you solder the TIP3055 power transistors in, figure out how you will mount them in whichever case you're using. The leads will need bending accurately so both PCBs have the same dimensions.

In this step, we will check each circuit board for correct operation, and adjust the R2 resistor to set the operating current.

When the boards are assembled it is important to ensure the power transistors are mounted on a heatsink of some sort before applying power. The picture shows the two boards mounted on the aluminium channels that form the case described in the next step. The TIP3055 transistors need insulating pads between the cases and the heatsink - the case is connected to the collector terminal (see picture for details). Don't over-tighten the nut, or it will damage the transistor case.

Initial power-up

If you have a bench power supply (with variable output voltage, and current limiting), it is highly recommended to perform a 'smoke test' to check the PCB for faults.

If you have used a trimmer for R2, set it to half-way. Set the power supply to 12V, with a current limit of 250mA, and use a multimeter to monitor the voltage between the positive end of C2A/C2B and ground. When you apply power this should slowly (over 10 seconds or so) increase from zero to roughly half the supply voltage (i.e. 6V). If this works you're good to go to the next stage.

If this doesn't happen, check for the following common faults:

• Component leads which you forgot to solder
• Short circuits caused by solder blobs, or from Q1 or Q2's cases to the heatsink
• Transistors incorrectly inserted
• Incorrect resistor values

Setting the operating current

In this step we choose a value for R2B to set the operating current for each amplifier channel. The optimum current is around 180mA, which allows full power to be delivered to a 32 ohm load.

You'll need to apply 24V power to the amplifier, while measuring the supply current (again - a bench power supply is ideal). When you apply power there will be a high current draw (up to 400mA) until the DC voltages stabilise, and then it should eventually settle to some stable value (perhaps 100-150mA). When this is stable, record the value.

You can then pick a value for R2B in order to reach 180mA (or just over) operating current, according to the following guide:

• 10K - increases current by 25%
• 8K2 - increase by 33%
• 6k8 - increase by 40%
• 4k7 - increase by 60%
• 3k3 - increase by 80%

You can quickly tack the resistor on to the PCB, then apply power and measure the current when stable. When you're happy, trim & bend the leads and attach it properly (see picture).

If you're using a trimmer for R2, start with it in mid-position, and slowly adjust it to reach 180mA. A lower resistance will increase the current.

Setting the current precisely is optional - it mostly affects performance at maximum output power, which hopefully you won't need very often. If you are unable to perform this step, use a fixed 2K2 resistor for R2, or 2K7 in parallel with 6K8 (=1.93K).

Obviously there's no reason you couldn't use a ready-made case here - the only important thing is to ensure all the power transistors have adequate cooling: each transistor dissipates approximately 2 Watts, so use a 10 °C/W heatsink per transistor, or 5°C/W per pair of transistors.

My case was built around two 200mm lengths of 50 x 25mm aluminium U channel, 3.2mm thick, which form the sides. The PCBs are mounted between them by bolting the power transistors to the channels. This is somewhat flimsy, so the next step is to mount them to a base plate.

The base plate here is 200mm long by 135mm wide, and is bolted to the sides with four M4 Allen bolts, one at each corner. It's important not to stress the transistor leads when the bolts are tightened, so ensure there is enough clearance in the bolt holes to allow adjustment.

One the base is attached to the sides, the rest of the case can be built up. The front and rear panels shown were made from a thick brushed aluminium plate I salvaged from an old 19 inch rack case, each 135mm wide x 63mm high. They are attached to the case sides using short lengths of 20 x 20mm aluminium L-section (see photo), one at each corner. With this arrangement it's quite easy to remove and replace the panels for drilling and wiring up.

The top of the case is the same size as the base plate, and fits between the front and rear panels. I made the top from a thin piece of aliuminium sheet with a piece of 6mm plywood cut to size on top. I applied a couple of coats of Danish oil, to complete the overall "1970's hi-fi" vibe of the finished case.

Wiring up the amplifier is relatively straightforward: we have the signal input and volume control, the power supply input, and the headphone output socket.

The signal input wiring uses thin (e.g. 3mm diameter) screened cable from the input sockets to the volume potentiometer, and from the potentiometer to the amplifier input. See the photo for the wiring. I used isolated sockets here (i.e. the 'ground' side of the socket is not connected to the case).

The power supply input jack has a 220nF capacitor directly attached directly to help reduce any high-frequency noise from the power supply - although I didn't find this to be an issue with the PSU I used.

The headphone output socket is wired as shown in the photos - the left ground and right ground are connected together. The socket I used was a non-isolated type, so the case is connected to ground through this point.

When all is done, you're ready for a first listen. Start with the volume right down, and increase slowly!

One final note: this amplifier uses roughly 10W of power when operating. Please don't leave it permanently switched on! Five minutes' warmup time is plenty for it to reach stable operating conditions.

Here is the measured performance of the ampifier as built.The frequency analyser plots were made with a Focusrite Clarett audio interface, and NAK T-100 analyser software.

Maximum output power

Measured at 1Khz, 0.5% THD

1.8W rms (21.6V pk-pk) into 32 Ω

1.0W rms (23.2V pk-pk) into 64 Ω

Gain/frequency response

x13 (+22dB) at 1KHz

-0.7dB at 20Hz, 32 Ω load

< -0.1dB at 50KHz

Distortion

32 Ω load

0.015%, 1Vrms out (31mW)

0.025%, 100mW out

64 Ω load

0.005%, 1Vrms out (15mW)

0.011%, 100mW out