Audio Amplifiers

Modest power audio amplifiers for driving small speakers or
other light loads can be constructed in a number of ways. The first choice is
usually an integrated circuit designed for the purpose such as the LM386 or
newer class D switching types that often accept digital data instead of simple
audio voltage. Discrete designs can also be built with readily available transistors or op-amps and
many designs are featured in manufacturers’ application notes. Older designs employed
audio interstage and output transformers but the cost and size of these parts
has made them all but disappear. Here are a few easy-to-build analog audio amplifier circuits for a variety of
hobby applications:

Simple LM386 Audio Amplifier

This simple amplifier shows the LM386 in a high-gain
configuration (A = 200). For a maximum gain of only 20, leave out the 10 uF
connected from pin 1 to pin 8. Maximum gains between 20 and 200 may be realized
by adding a selected resistor in series with the same 10 uF capacitor. The 10k
potentiometer will give the amplifier a variable gain from zero up to that
maximum.

High Gain and Fidelity Audio Amplifier

I’ve moved this circuit to
Area 50 as it’s a bit
experimental.

Curiously Low Noise
Amplifier

The Curiously Low Noise Amplifier takes advantage of the
wonderful noise characteristics of the 2SK117 JFET that boasts a noise voltage
below 1 nV/root-Hz and virtually no noise current. The noise voltage of the
amplifier is only 1.4 nV/root-Hz at 1 kHz, increasing to only 2.7 nV/root-Hz at
10 Hz. The noise current is difficult to measure, so this simple utility
amplifier can see the noise from a 50 ohm resistor and a 100k resistor, too.
(The 1.4 nV input-referred noise will increase to about 1.7 nV with a 50 ohm
resistor, instead of a short, and a 100k resistor will give an input-referred
noise near 40 nV, with very little contribution from the amplifier.)

This amplifier is a “utility” amplifier with a gain of
100, that would typically be
used in a lab setting to boost tiny signals for measurement or further
processing. It isn’t intended to drive a speaker or headphones directly. (It
could drive the LM386 quite nicely.) The circuit is a simple discrete transistor
feedback circuit with two gain stages and a unique class-A output buffer:

• The 2sk117 is from the “BL” Idss current range and is
  selected for an Idss near 7 mA. The drain resistor is adjusted to achieve
  about 4 volts on the drain and the value depends on the Idss of the JFET.

• Most of the resistors aren’t critical, but precision values are
  shown because the resistors should be metal film types for best noise
  performance. Approximate DC voltages are shown for helping with resistor
  selection. Deviating from the shown voltages will reduce the available output
  voltage swing, but the amplifier might work fine for smaller signals. Unloaded
  swing should be about 6 volts, p-p with about 60 mV p-p input, before distortion
  is observed.

• The MPSA18 acts as a noise filter. High gain is desirable here
  to keep the value of the base filter capacitor reasonable, but a 2N4401 could be
  substituted by reducing the 10k and 120k by a factor of 5. The filter will still
  be rolling off the noise voltage from the 15 volts supply above about 0.2 Hz.
  But some power supplies can be really noisy!

• The 0.1 uF capacitors serve as bypass capacitors but mainly as
  terminals for holding the components. These are the white rectangles seen in the
  photo.

• The feedback resistor is selected for a gain of exactly 100 and
  the value is well above the expected 1k, due to the limited open-loop gain of
  the simple circuit.

• A small resistor is included in series with the output for
  stability and that resistor can reduce the gain a bit when driving a lower
  resistance load. The designer may choose to set the gain for that particular
  load, say 75 ohms, or for a high impedance load. The circuit can drive a lower
  resistance than 100 ohms,  but the swing will be somewhat limited. It may
  be possible to leave out the 33 ohm resistor without stability issues. (Usually,
  such a utility amplifier is driving a much higher resistance load, typically
  600 ohms or above.) Note: To give you an idea of how you
  can play with the output resistance, I just changed my unit’s series output
  resistor to 55 ohms and adjusted the gain for 35 dB when driving 75 ohm loads.
  Unloaded the gain is exactly 5 dB higher at 40 dB. This way I have even number
  gains whether driving a 75 ohm instrument or a high-Z device. The output buffer
  has no trouble driving the total 125 ohm load, with a swing limit of about 3.5
  volts, p-p.

• The output stage is an unusual self-biasing arrangement where
  the PNP holds the gate-source voltage near 0.6 volts, running the JFET somewhat
  below its Idss. The 2N5486 was chosen to not waste too much current, but a
  higher Idss JFET will give more drive capability, if desired.

• Input Impedance: 47 megohm (set by bias resistor), shunted by
  20 pF

• Output Impedance: 36 ohms, set by series resistor plus about 3
  ohms from the circuit. My 55 ohm resistor mentioned above
  gives an output Z of about 58 ohms and exactly 5 dB of gain loss from no load to
  75 ohms.

• Output voltage swing: 6 volts p-p into a high impedance load.

• Gain: 100 (40 dB) set by feedback resistor. Lower gain could be
  selected for wider bandwidth.

• Frequency Response: flat from below 1 Hz to above 2 MHz.

• Input Noise: 1.4 nV, rising to 2.7 nV at 10 Hz. Noise current has
  eluded measurement so far, but it’s really low. With a 97.3 k resistor (100k in
  parallel with 3.6 meg) connected across the input, the noise voltage measures
  within a tiny fraction of a dB of 40 nV, so little to no noise current is seen.
  In fact, this amp and a selected resistor make an inherently accurate noise
  source. Connect a 152k across the input (in a shielded box), and you have a
  precise 5 uV/root-Hz noise source throughout the audio spectrum (50 nV times
  100). A quick measurement at 40 Hz gives 770 nV/root-Hz with nothing connected;
  the 47 megohm is expected to contribute 867 nV. That’s pretty close and still
  little noise current from the FET.

For even better performance, the bipolar stages could be
replaced with a low noise op-amp. The input noise would drop a little, perhaps
to 1 nV, as would the input capacitance, perhaps below 10 pf. Compensating the
op-amp might be a bit of a challenge.

Here’s another version with some
interesting features. There’s a two-transistor “brute-force” noise shunt that
cleans up the power supply quite effectively and it will work well with a series
resistor as low as 1 ohm. But the required DC current goes up if there’s lots of
noise to shunt. As with the “finesse” circuit, it’s only good for removing
random noise, say from a three-terminal regulator, and will be overloaded by
large spurs or hum. Here’s what it does to a test source (red) inserted in
series with the power supply:

Use high-gain transistors for best results. Ordinary
transistors will give about 30 dB of rejection but the bias resistor values may
need to be modified to increase the current back up to around 30 mA, depending
on how much noise needs to be shunted. The circuit has an excellent noise floor
so start with a good supply and the noise will be in the single-digit nanovolts.
(Spice thinks less than 1!)

For a special application needing minimal loading, the amplifier
includes feedback to bootstrap the input capacitance to a low value (about 4
pF). That technique combined with the source feedback usually leads to terrible
ringing for some source impedance but this amplifier has only 1dB of peaking at
the worst value (around 30k). Run the
LTSpice simulation to see the response curves for various values of input R
(change the list as desired). Right-click on the .step param command to comment
it out and change {R} to a fixed value, say 1 ohm, for testing the amp at a
single source impedance. The noise is just below 1 nV/root-hertz. This amplifier
works up to the bottom of the BCB for source impedance as high as 30k. This is
just a spice implementation at this point – stay tuned.

Note: I connected the top of the 220 ohm resistor
directly to the power source to reduce the drop across the 4.7 ohm resistor. If
the extra drop is not a problem the circuit works slightly better with the top
of the 220 ohm connected to the right side of the 4.7 ohm resistor.

When I say “supply can be terrible if this circuit is used” I
mean in terms of random noise, say from an LM7815 three-terminal regulator
(hundreds of nV/root-Hz). This noise shunt can’t handle unregulated sources or
lots of huge spikes on the power. Whatever noise is present will generate
current in the small resistor (assuming the circuit is working) and the circuit
needs to be able to shunt that current. With 30 mA bias the circuit can handle
about +- 25 mA signals so the above circuit with the 4.7 ohm resistor can handle
just under 250 mV p-p. Drop the resistor to 1 ohm and the limit is more like 50
mV p-p, well within expectations for a three-terminal regulator but not capable
of removing much ripple or large transients.

White Noise Source

Here’s a 1uV/root-Hz
noise voltage source that will drive a 50 ohm load from below 10 Hz to
over 500 kHz. The noise shunt actually stabilizes the circuit against battery
resistance, a common feedback path in such simple circuits. Current consumption
is under 20 mA.

Note: I connected the top of the 220 ohm resistor
directly to the power source to reduce the drop across the 3.3 ohm resistor. If
the extra drop is not a problem the circuit works slightly better with the top
of the 220 ohm connected to the right side of the 4.7 ohm resistor.

Computer Audio Booster

Here is a simple amplifier for boosting the audio level from low-power sound cards or
other audio sources driving small speakers like toys or small transistor radios. The
circuit will deliver about 2 watts as shown.  The parts are not critical and
substitutions will usually work.  The two 2.2 ohm resistors may be replaced with one
3.9 ohm resistor in either emitter.

The circuit above shows a 4-transistor utility amplifier suitable for a variety
of projects including receivers, intercoms, microphones, telephone pick-up coils, and
general audio monitoring. The amplifier has a power isolation circuit and bandwidth
limiting to reduce oscillations and “motorboating”. The values are not
particularly critical and modest deviations from the indicated values will not
significantly degrade the performance.

Three cell battery packs giving about 4.5 volts are recommended for most
transformerless audio amplifiers driving small 8 ohm speakers. The battery life will be
considerably longer than a 9 volt rectangular battery and the cell resistance will remain
lower over the life of the battery resulting in less distortion and stability problems.

The amplifier may be modified to work with a 9 volt battery if desired by moving the
output transistors’ bias point. Lowering the 33k resistor connected from the second
transistor’s base to ground to about 10k will move the voltage on the output electrolytic
capacitor to about 1/2 the supply voltage. This bias change gives more signal swing before
clipping occurs and this change is not necessary if the volume is adequate.

As before, the two 4.7 ohm
resistors may be replaced with a single 10 ohm resistor in series with either emitter.

Op-Amp Audio Amplifier

The above circuit is a versatile audio amplifier employing a low cost
LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode
signals which are a common cause of amplifier instability. The dotted ground connection
represents the wiring in a typical project illustrating how the ground sensing input can
be connected to the ground at the source of the audio instead of at the amplifier where
high currents are present. If the source is a power supply referenced signal then one of
the amplifier inputs is connected to the positive supply. For example, an NPN
common-emitter preamplifier may be added for very high gain and by connecting the
differential inputs across the collector resistor instead of from collector to ground,
destabilizing feedback via the power supply is greatly reduced. By the way, the
LM358 is a fairly poor audio amplifier and you may wish to switch to a better
part for reduced distortion. Frankly, for a little bench amplifier, you’ll never
notice the distortion.

My utility amplifier was built into an aluminum Bud box and eventually
ended up bolted to the bottom of a shelf as shown. The well-behaved and ready-to-go
amplifier is really handy.

Crystal Radio (and other purpose) Audio Amplifier

Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier
will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire
antenna and good ground. The circuitry of such a radio is similar in complexity to a simple one-transistor
radio but the performance is superior (with the exception of the amazing
one-transistor reflex ). The TL431 is available in a TO-92 package and
it looks like an ordinary transistor so your hobbyist friends will be impressed by the
volume you are getting with only one transistor and the amplifier may be used for other
projects, too. Higher impedance headphones and speakers may also be used. An earphone from
an old telephone will give ear-splitting volume and great sensitivity! The 68 ohm resistor
may be increased to several hundred ohms when using high impedance earphones to save
battery power.

Here is the amplifier used to boost the output from a simple crystal
radio. The volume control is at the bottom left and the other components are on the
terminal strip at the bottom of the picture. This is a really quick and easy audio
amplifier!

Class-A Audio Amplifiers 

A class-A audio amplifier is pretty wasteful of power but when plenty of
power is available the simplicity is attractive. Here is a simple darlington transistor
example intended for use with a 5 volt power supply:

This circuit and the following aren’t
for beginners; they are of limited usefulness and require an understanding of
the underlying principles and potential applications. They all pass DC through
the speaker which is wasteful and can cause problems for the inexperienced
builder. If built without variation, they should perform as described but make
sure to read the text.

The 5 volts should be provided by a regulated power supply. The efficiency
is below 25% and significant DC current flows in the speaker and that additional power
should be figured in to the power rating of the speaker. But look how simple it is! The
voltage gain is only about 20 and the input impedance is about 12k. The schematic shows
two values of bias resistor to be used with the corresponding speaker impedance. With the
150k bias resistor and 8 ohm speaker, the circuit draws about 210mA (1 watt) and can
deliver about 250 mW to the speaker which is plenty of volume for most small projects.
  The speaker should be rated at 500 mW or more and should exhibit a DC resistance
near 8 ohms (perhaps 7 ohms). Check the candidate speaker with an ohmmeter; much below 7
ohms will cause excessive current draw. With the 220k resistor and 16 ohm speaker, the
circuit draws about 100 mA (500 mW) and delivers about 125 mW to the speaker. The 16 ohms
speaker should be rated at 200 mW or more and exhibit nearly 16 ohms of DC resistance.
(Most small speakers have a DC resistance near the rated impedance and that resistance is
used to set the quiescent current level in this circuit.) Other NPN darlington transistors
will work but choose one that can dissipate 1 watt minimum. Most power types don’t need a
heatsink but tiny TO92’s might overheat.

If the inefficiency of the class-A hasn’t dissuaded you yet, here is a
4-transistor amplifier suitable for small signals:

The input impedance is about 5000 ohms and the frequency response is flat
from 30 Hz to over 20,000 Hz. With the 8 ohm speaker the current drain is about 215 mA and
the gain is about 1700 (64 dB). With the 16 ohm speaker the current gain is about 110 mA
and the gain is about 2500 (68 dB).  A volume control may be added by connecting one
end of a 5k potentiometer to ground, the wiper to the amplifier input. The other end of
the pot becomes the input.

Lets face it; just about any of the various IC audio amplifiers make more
sense than this inefficient design. But, this circuit uses parts with only 3 legs. Umm, it
doesn’t use large capacitors except for the power supply bypassing. Lets see, its more
fun-ariffic.  Well, lets see if we can come up with a project that takes advantage of
the inefficiency:

So, what is it?

It is a modulated light sender! Connect the input to an audio source or
microphone (a speaker will work) and the audio will amplitude modulate the light
intensity. The inefficiency of the class-A works in our favor now, lighting the lamp to
mid-brightness with no audio present. Actually, with a 4.7 volt bulb, the lamp
will be near full brightness and will be “overdriven” on sound peaks.
A higher voltage bulb will last longer but will be dimmer. Try a 6.8 volt bulb
as a compromise. With a sensitive detector like a phototransistor,
this communicator will work several hundred feet (at night). Best range is realized if the
bulb is mounted in a typical flashlight reflector and the detector is similarly mounted.
The input capacitor is reduced to .01 uF to give the amplifier a high-pass character to
compensate for the slow response of the bulb. The audio will sound a bit muffled, anyway.
The clever designer could use this amplifier for the receiver, too, switching the speaker
to the input for transmitting and to the output for listening. If you choose a detector
with good infrared response, like a pin photo diode, you can add plastic IR filters to
block out ambient light and make the communicator harder to see at night.

Increasing the voltage to 12 VDC, replacing the bulb with a  3 watt,
16 ohm speaker and replacing the .01uF with a 1uF gives an audio amp that will deliver
nearly 1 watt of audio power. The speaker will get warm, however! (Due to the nearly 2
watts of DC power in the speaker coil.)