This is an amp designed to deliver plenty of high-quality power to discerning headphone listeners. It can be used to boost the output level of a smartphone or digital music player suitable for studio-quality headphones, or to provide a headphone-level output from a line-level signal.

Caution: High sound levels can damage hearing! This applies to in-ear headphones with lower sensitivity (or higher impedance). Not recommended for use with standard smartphone earbuds. As a constructor and user, it is your responsibility to protect your ears from dangerous sound exposure levels.

Supplies

Circuit Board – I built this amplifier around two PCBs I got from eBay: JLH 1969

Printed Circuit Board Assemblies – Resistors, Capacitors and Transistors for Amplifier Circuit Boards can be accessed by clicking this spreadsheet (CSV) file

other components

24 V 1A power supply (eg CPC’s TIger TP1129)

Panel mount DC power input socket matching the above (eg 2.1mm)

10K or 22K dual log potentiometer (like this one)

Knob (RS 467-2479 is ridiculously expensive, but looks great)

2 phono sockets, panel mount (like this)

1/4″ (6.35mm) stereo jack, panel mount (eg RS 175-0155)

PCB interconnects (such as these headers and sockets)

Transistor Heatsink Insulation Pad

Shielded and Unshielded Wire

Housing material, nuts and bolts

tool

Soldering iron, solder

pliers; wire cutters; stripper

(Recommended) Adjustable workbench power supply

Drilling, hacksaw, etc.

Step 1: About the “JLH” Class A Amplifier

In 1969, John Linsley-Hood published an article in Wireless World magazine describing an audio power amplifier based on a four-transistor circuit. It’s a masterpiece of engineering – it uses a small number of off-the-shelf components and provides reliable and repeatable performance, but it’s not simple enough to require significant improvement. With modern components, the performance is at least as good as the original.

This is a “Class A” amplifier – a technical term that means its transistors are always on (passing current) even when there is no signal. The downside to this is that the amplifier uses much more power than the alternative “Class B” (or Class AB) types, but the distortion disappears as the output level is lowered.

The original JLH amplifier was designed to deliver 10 watts of output power into an 8 ohm speaker. For headphone applications, the power output may be much smaller (1 watt is sufficient), so I adjusted the supply voltage and operating current appropriately. This means that the amplifier produces a fraction of the heat of the 10 W version, and can use cheap off-the-shelf power supplies. A schematic diagram of the adjusted component values ​​is shown above.

Step 2: Match Power Transistors (Optional)

The JLH circuit achieves the best performance (lowest distortion) when both output transistors Q1 and Q2 have the same current gain (hFE). In this step, we batch measure the gain of each transistor and select the pair with the closest gain (one pair per channel).

This step is optional – to build the prototype amplifier I bought a set of 10 TIP3055 transistors and none of them differ by more than 5%. Modern builds are more consistent than they were in 1969, and it’s likely that any pair from the same batch will provide excellent performance.

You will need:

a 5V power supply

Meter for measuring current up to about 300mA (a bench power supply with an output ammeter is ideal)

a 2k2 resistor

Test leads (e.g. alligator clips)

A heat sink or a piece of flat metal

The process is to connect each transistor as shown, apply power and measure the current drawn by the transistors. (The gain is equal to the collector current divided by the base current – all we really need to match is the total current). For a typical TIP3055 with a gain of 100, the total current in the above circuit is about 200mA.

Note the current reading for each transistor, then you can select pairs with similar readings. Higher gain (higher current) would be better if given the choice.

Transistors heat up when powered, which affects gain! If you find it difficult to measure current within a few seconds of power-on, clip the transistor to a handy piece of metal to keep the temperature stable.

Step 3: Begin PCB Assembly

As always, it’s easiest to start with the lowest-height component first, in this case the resistors.

I’ve shown the component locations for the eBay PCB (see “Supplies” for a link). Note that some of these will be different from the values ​​printed on the board.

Notes on R2A and R2B

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

Step 4: Complete Circuit Board Assembly

The next step is to install the capacitors and transistors, and complete the board assembly.

Be careful with the two small (TO92 case) transistors: not all TO92 transistors have the emitter, base and collector leads arranged the same way. The eBay board uses Q4’s 2SA970 part, which doesn’t match my existing BC558 transistor. This isn’t a huge problem as you can carefully bend the leads (see picture) to fit. Alternatively, you can look for a 2SA970 or similar low noise, small signal PNP transistor with the correct pinout.

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

Before soldering TIP3055 power transistors, figure out how to mount them in whatever situation you use. The leads need to be bent precisely, so both PCBs are the same size.

Step 5: Test and Setup

In this step we will check that each board is functioning correctly and adjust the R2 resistor to set the operating current.

When assembling the board, it’s important to make sure the power transistors are mounted on some sort of heatsink before powering up. The picture shows the two boards mounted on the aluminum channel, forming the enclosure described in the next step. The TIP3055 transistor requires an insulating pad between the case and the heat sink – the case is connected to the collector terminal (see picture for details). Do not overtighten the nut or you will damage the transistor case.

1. Initial power-on

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

If you used a spinner for R2, set it to halfway. Set the power supply to 12V, limit the current to 250mA, and monitor the voltage between the positive terminal of C2A/C2B and the ground with a multimeter. This should slowly (over 10 seconds or so) increase from zero to about half the supply voltage (ie 6V) when you power up. If this works, you can move on to the next stage.

If this does not happen, check for the following common faults:

Component leads you forgot to solder

Short circuit caused by solder joint or case to heat sink of Q1 or Q2

Transistor insertion error

Incorrect resistance value

2. Set the working current

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

You need to apply 24V 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 voltage stabilizes, then it should eventually settle to some stable value (maybe 100-150mA). When this stabilizes, record the value.

You can then choose a value for R2B to achieve 180mA (or just over) operating current according to the following guidelines:

10K – 25% increase in current

8K2 – 33% increase

6k8 – 40% increase

4k7 – 60% increase

3k3 – 80% increase

You can quickly attach the resistor to the PCB, then power up and measure the current when stable. When you are satisfied, trim and bend the leads and connect them correctly (see picture).

If you’re using a trimmer for R2, start in the middle and slowly adjust to 180mA. Lower resistance increases current flow.

Precisely setting the current is optional – it mostly affects the performance at maximum output power and hopefully you won’t need it very often. If you can’t do this step, use a fixed 2K2 resistor for R2, or 2K7 in parallel with 6K8 (=1.93K).

Step 6: Build the Shell

Obviously, there’s no reason you can’t use an off-the-shelf case here – the only important thing is to ensure that all power transistors have adequate cooling: each transistor dissipates around 2 watts, so each transistor uses 10 °C/W of heat dissipation , or 5°C/W per pair of transistors.

My case is built around two 200mm long 50 x 25mm aluminum U-slots, 3.2mm thick, that form the sides. By bolting the power transistors to the channels, the PCB is mounted between them. This is a bit fragile, so the next step is to mount them to the baseplate.

The base plate here is 200mm long and 135mm wide and is fastened to the sides with four M4 socket head cap screws, one at each corner. It’s important that you don’t put pressure on the transistor leads when you tighten the bolts, so make sure you have enough clearance in the bolt holes to make adjustments.

A base is attached to the side and the rest can be assembled. The front and rear panels shown are made from thick brushed aluminum sheets I recycled from an old 19″ rack box, each 135mm wide x 63mm high. They are attached to the sides of the housing using short lengths of 20 x 20mm aluminium L-section (see photo), one at each corner. With this arrangement, the panels for drilling and wiring can be easily removed and replaced.

The top of the chassis is the same size as the bottom plate and fits between the front and rear panels. I made the top out of a thin sheet of aluminium with a 6mm piece of plywood cut to size. I applied several coats of Danish oil to complete the overall “1970s hi-fi” vibe of the finished case.

Step 7: Wiring

The wiring of the amplifier is relatively simple: we have signal input and volume control, power input and headphone output sockets.

Signal input wiring uses thin (eg 3mm diameter) shielded cables from the input socket to the volume potentiometer and from the potentiometer to the amplifier input. See photo for wiring. I used isolated receptacles here (i.e. the “ground” side of the receptacle is not connected to the case).

A 220nF capacitor is connected directly to the power input jack to help reduce any high frequency noise from the power supply – although I found this was not an issue with the PSU I used.

The headphone output jacks are wired as shown – left ground and right ground connected together. The receptacle I’m using is non-isolated, so the case is grounded through this point.

Once everything is done, you are ready for your first use. Try increasing the volume slowly.

One final note: this amplifier uses about 10W of power when running. Please don’t leave it on forever! A five-minute warm-up time is enough to allow it to reach stable operating conditions.

Step 8: Measure Performance

Below is the measured performance of the built-in amplifier. Frequency analyzer curves were created using a Focusrite Clarett audio interface and NAK T-100 analyzer software.

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