Firstly, I’d like to
stress that the intended use of this circuit is only one of many
possible applications. Apart from the obvious usage as a headphone
amplifier, the circuit can be used for a range of applications where a
wide bandwidth low power amplifier is needed. Some of the options
- Reverb drive amplifier – ideal for low and medium impedance reverb tanks
- High current line driver – suitable for very long balanced lines
- Low power speaker amplifier – better performance than small integrated amps
- … and of course, a headphone amp.
In short, the amp can be used anywhere that you need an opamp with
more output current than normally available. Since most are rated for
around ±20-50mA, general purpose opamps are not suitable for driving
long cables or anywhere else that a relatively high output current is
As a headphone amplifier, this design is very similar to others on the ESP
site, but the main difference is that this one (and P70) has been built
and fully tested. The design is fairly standard, and every variation
was checked out before arriving at the final circuit. A photo of the
prototype is shown below, and at only 64 × 38mm (2.5 × 1.5 inches) it is
very small – naturally, the heatsink is not included in the dimensions.
The amplifier is capable of delivering around 1.5W into 8 ohm
headphones, and 2.2W into 32 ohms – this is vastly more than will ever
be needed in practice. The use of a 120 Ohm output resistor is
recommended, as this is supposed to be the standard source impedance for
headphones. Unfortunately, many users have found that their ‘phones
perform better when driven from a low impedance source.
The circuit is based on an opamp, with its output current boosted by
a pair of transistors. Distortion is well below my measurement
threshold at all levels below clipping into any impedance. Noise is
virtually non-existent – even with a compression driver held to my ear, I
could barely hear any, and I couldn’t hear any with headphones.
Headphones are rated in dB SPL at 1mW, and this amplifier (like many other similar headphone amps) is capable of producing extreme SPLs.
The levels obtainable are sufficient to cause almost instantaneous
permanent hearing damage! Never operate the amp at very high levels, and
never switch the amplifier on with signal while wearing you headphones.
Always start with the volume control at minimum, and gradually
increase the level until it is comfortable, but not too loud. Because of
the very low distortion, it is easy to increase the level too far
without noticing. Your ears are precious – safeguard them at all times.
Note the warning above – this is serious. Most headphones are capable of at least 94dB SPL at 1 mW, with some as high as 107dB SPL.
Even 10mW is enough to create sound levels capable of causing hearing
damage, so you must be very careful to avoid damaging levels.
Table 1 – Maximum Exposure to SPL
Continuous dB SPL Maximum Exposure Time
85 8 hours
88 4 hours
91 2 hours
94 1 hour
97 30 minutes
100 15 minutes
103 7.5 minutes
106 < 4 minutes
109 < 2minutes
112 ~ 1 minute
115 ~ 30 seconds
Note that the exposure time is for any 24 hour period, and is halved for each 3dB SPL
above 85dB. The above shows the accepted standards for recommended
permissible exposure time for continuous time weighted average noise,
according to NIOSH (National Institute for Occupational Safety and Health) and CDC
(Centers for Disease Control). Although these standards are US based,
they apply pretty much equally in most countries – hearing loss does not
respect national boundaries.
The amplifier itself is fairly conventional, and is very similar to
another shown on this site (see Project 24). This amplifier does not
include the active volume control, because in general it is far easier
to get a good log pot (or simply ‘fake’ the pot’s law as described in
Project 01). Likewise, it does not include the cross-feed described in
Project 109. If this is desired, it is very easy to implement on a small
piece of tag board, or even ‘sky hook’ the few components off the
bypass switch. Full details of how to do this will be included in the
construction guide when PCBs are available.
The output transistors are biased using only resistors, rather than
constant current sources. Extensive testing showed that using current
sources made no discernible difference to performance, but increased the
complexity and PCB size. Using separate caps
for each biasing diode does make a difference though – and although it
is relatively minor, the use of the two caps is justified IMHO.
The bias diodes should be 1N4148 or similar – power diodes are not
recommended, as their forward voltage is too low. This may result in
distortion around the crossover region, where one transistor turns off
and the other on. As shown, crossover distortion is absolutely
unmeasurable with the equipment I have available.
Above is the schematic of one channel. Resistors and caps use the
suffix ‘R’ for the right channel. The second half of the dual opamp
powers the right channel. Note that the volume control shown is
optional, and is not on the PCB. If needed, it
may be mounted in a convenient location and the output connected to the
inputs of the board as shown. D1 and D2 (L and R) are 1N4148 or
One of the reasons the amp is so quiet is that the entire board runs
from a regulated supply, so hum (in particular) is eliminated. Although
an unregulated supply can be used, this is not recommended. The supply
should be separate from that used for your preamp, because of the
relatively high current drawn by the amplifier (at least with low
impedance ‘phones). A P05 preamp supply can be used, and will ensure
The prototype amplifier has flat frequency response from 10Hz to
over 100kHz. Distortion is below my measurement threshold with any level
or load impedance, and output impedance is almost immeasurably low.
Your headphones may be designed to operate from a 120Ω source impedance
(many are), so this may be added if it improves sound quality. Adding
any series resistance will reduce the available power, but it is already
far greater than you can use. Without series resistance, the minimum
power into various load impedances is given below (based on ±15V
Table 2 – Output Power Vs. Impedance
Impedance Power (Direct) 120 Ohm Feed
8 Ohms 1.5 W 35 mW
32 Ohms 2.2 W 99 mW
65 Ohms 1.1 W 136 mW
120 Ohms 595 mW 149 mW
300 Ohms 238 mW 121 mW
600 Ohms 119 mW 82 mW
This is not especially comprehensive, but will cover the majority of
headphones in common use. In all cases, the available power is more
than needed … not so you can damage your hearing, but to allow adequate
headroom for transients.
While it may be possible to build it using Veroboard or similar,
there is a high risk that it will oscillate because of the very wide
bandwidth of the amplifier. A capacitor may be added in parallel with R4
(L and R) to reduce the bandwidth if stability problems are
encountered. Although I used an NE5532 opamp for the prototype, the
circuit will also work with a TL072, but at reduced power. You may also
substitute an OPA2134 or your favorite device, taking note of the
The standard pinout for a dual opamp is shown on the left. If the
opamps are installed backwards, they will almost certainly fail, so be
The suggested NE5532 opamp was used for the prototype, and
performance is exemplary. Devices such as the TL072 will be quite
satisfactory for most work, but if you prefer to use ultra low noise or
wide bandwidth devices, that choice is yours.
Construction is fairly critical. Because of the wide bandwidth of
the NE5532 and many other audio grade opamps, the amplifier may
oscillate (the prototype initially had an oscillation at almost 500kHz),
so care is needed to ensure there is adequate separation between inputs
and outputs. Even a small capacitive coupling between the two may be
enough to cause problems.
As shown in the photo, this amplifier needs a heatsink. While it can
operate without one at low power using high impedance headphones, you
need to plan for all possibilities (after all, you may purchase low
impedance ‘phones sometime in the future). The heatsink does not need to
be massive, and the one shown above is fine for normal listening
levels. An aluminium bracket may be used to attach to the chassis – I
recommend 3mm material. Note that the heatsink should always be earthed
The output transistors must be insulated from the heatsink.
Sil-Pads™ are quite suitable because of the relatively low dissipation,
but greased mica or Kapton can be used if you prefer. If you use the
suggested 3mm aluminium, you can drill and tap threads into the
heatsink, removing the need for nuts.
Connect to a suitable power supply – remember that the supply earth
(ground) must be connected! When powering up for the first time, use 56
ohm “safety” resistors in series with each supply to limit the current
in case you have made a mistake in the wiring. These will reduce the
supply voltage considerably because of the bias current of the output
If the voltage at the amplifier supply pins is greater than ±6V and
the output voltage is close to zero, then the amplifier is probably
working fine. If you have an oscilloscope, check for oscillation at the
outputs … at all volume control settings. Do this without connecting
your headphones – if the amp oscillates, it may damage them.
Once you are sure that all is well, you may remove the safety resistors and permanently wire the amplifier into your chassis.