ZL2PD No IC Audio Amplifier
A basic audio amplifier which uses a handful
of widely available discrete components at practically no cost and
avoids the need to rush out to purchase an integrated circuit such as
day, I needed to build an audio amplifier stage for a receiver. As
usual, I reached for my stockpile of LM386 chips…Hmmm….No chips in the
Well, I though, I'll just use one of those old LM380 chips instead. No,
I didn't have any of those in my junkbox either.
I went hunting for a couple of other alternatives. While I found a pair
of 50W audio amplifier ICs, and a 5W audio IC that needed close to a
dozen electrolytic capacitors and a bunch of resistors to make it go, I
reckoned those choices were a bit of an overkill for my little battery
So, I decided to build an audio amplifier from a few general purpose
transistors. I still wanted the benefits of the LM386-type amplifiers
including minimal parts and reasonably low current drain. This circuit
was the result.
It's hardly an original circuit, I know. Even so, I thought it worth
adding to my website because I've found it so useful over the last few
years. This doesn't mean I don't use IC amplifiers any more - I do!
Mostly low voltage chips recycled from AM/FM transistor radios. But I
continue to use this circuit from time to time, usually late at night
when I've run out of LM386 chips once more.
With a gain
of about 30dB, it can deliver more than 100mW into a small 8 ohm
speaker. It can drive a compact 50mm diameter speaker, like those used
in those cheap AM/FM transistor radios, to audio levels more than
adequate for room level listening.
Most receiver IF stages I build have audio outputs of between 50mVpp
(i.e. 50mV peak to peak) and 150mVpp, and this input level is enough
with this circuit to produce up to 1.5Vpp across an 8 ohm speaker.
this circuit is first powered up, adjust R3 (4k7) to give a voltage on
the emitter of Q2 close to half that of the supply rail. I seldom need
to change this value myself from that shown, but some transistors may
require a slight change. So, as you power this amplifier up, measure
the current drain. It should be about 3mA with no input signal and
about 20mA at full output. Currents as high as 50mA with the devices
shown are perfectly acceptable.
The reason for measuring the current drain is that some transistors
used at Q2 and Q3 can go into thermal runaway at high output volumes
with some values of R3. Thermal runaway occurs when the current through
Q2/3 just keeps on rising. Increased current results in a temperature
rise in Q2 and Q3, and this permits the current to rise still further.
This can continue until the devices fail. This is normally prevented by
adding a low value resistor (e.g. 3R3 to 10R ohms) between each emitter
and the common output on the positive side of C3. With these resistors
in place, increasing current will increase the voltage drop across
these resistors. This in turn starves the transistors of voltage, and
so prevents this thermal runaway effect.
I have not seen this runaway effect more than once in the dozen or so
times I've used this circuit with component values close to those shown
in the circuit diagram. I use this circuit due to the minimal number of
parts required, and because it's so cheap and easy to build. Adding
those extra parts also slightly reduces the output level. So, I tend
not to put them in, and I've not had any problems.
But if you feel a bit nervous about this, then by all means add those
two extra resistors. I won't complain.
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