Based on information supplied by a reader, I have learned that my
recollection of the El Cheapo design was rather badly flawed. It seems
I had mixed up recollections, and merged a couple of completely
different designs into one - none of which resemble the original El
Cheapo in any way.
Originally designed by R.R. Moore and published in Audio Magazine
(November 1964 edition), the design is presented more for historical
interest than as a recommended design. While not a bad amp by the
standards of the day, it will be found lacking if compared to any more
modern offerings. This will apply even using the latest transistors,
because the amp has very limited open loop gain. To some people, this
is seen as a benefit rather than a limitation, but it does mean that
low-level non-linear distortion will be higher than you are used to.
The amp was called "El Cheapo 2-30", and was rated at a maximum of
30W per channel into 16 Ohms (which used to be a very common speaker
impedance). It used a single regulated power supply and capacitor
coupled speaker. Having a scan of the original, I can now reproduce the
exact circuit details. It was a very simple amp, and used
quasi-complementary symmetry for the output stage. For those younger
than I who have no idea what I'm talking about, quasi-complementary
symmetry was a scheme used in the days when PNP power transistors were
expensive and silicon devices were pretty much useless. If you wanted
any sort of voltage and current rating, you had to use NPN devices. The
quasi-complementary output stage used a (discrete) Darlington for the
positive side, and a complementary pair for the negative (i.e. a PNP
driver coupled to an NPN power transistor).
Meanwhile, in those days, if you wanted high gain and reasonable
current capacity, germanium transistors still ruled supreme. Provided
they were used in applications where leakage was not a major problem,
germanium devices worked very well - this did not really include the
output stages of power amps though. Back then the majority of
loudspeakers were 16 Ohms, with only a few venturing down to 8 Ohms.
Anything lower than that was almost unheard of in 1964.
Figure 1 shows the circuit - it was a cheap amp compared to
most offerings of the day. It also managed to sound respectable - again
by comparison - and I and many of my friends of the day built these
amps with abandon - guitar amps, hi-fi, you name it, El-Cheapo was in
Note that the transistor types are the original specified
devices. Most are obsolete now, but a list is shown below of suitable
Figure 1 - Original El-Cheapo Circuit
These were the days when the 2N3055 was the pre-eminent power
transistor (NPN of course), and there were no vaguely equivalent PNP
devices for less than about 5 times the price, and even these were
highly inferior. As a result, the quasi-complementary output was very
common, and indeed this is still the case with most IC power amps. The
quasi-complementary output stage was the most popular until relatively
recently, until decent PNP power devices became more readily available.
Immediately, just about everyone started using NPN and PNP Darlington
coupled devices for the output stages (as shown for Q3 and Q4) - the
funny part is that it was demonstrated back in the mid 1970's that the
full Darlington connection actually sounds (or at least measures) worse
than quasi-complementary stages. Is not progress a wonderful thing?
The input stage of the El-Cheapo is not subject to the phase
problems of the long tailed pair, since the Class-A driver (or VAS -
Voltage Amplification Stage) is used as the input. Amps driven in this
manner tend to be inherently stable. There is a major problem
with DC offset of course - the input is referenced to the negative
supply. If this is earth (ground) then it's not an issue, but it
precludes using this design with a dual supply. The DC was not a
problem with capacitor coupled speakers.
As shown, the gain for audio frequencies is 18 (25dB), which means
an input sensitivity of 1V for an output of 40W. The closed loop gain
is set by R4 and R7. Since the feedback is taken after the output
coupling cap, the latter has no influence on the low frequency response
- however, this arrangement creates an underdamped filter network that
causes a 4.5dB peak at about 5Hz. Increasing C7 to 4700uF eliminates
this problem for all intents and purposes.
In the original article, there were several variations of the
design, however I will only present the amp in basic 40W form here. The
variants were mainly based on using lower supply voltages, but included
a dual (parallel) output stage for the odd low impedance load.
Note that the amp is inverting, and the input impedance of the power
amp itself is 1k (R4). Because of this, there is an emitter follower
(Q1) before the amp proper to convert the impedance to something
usable. The arrangement shown is less than ideal though - a better
solution would be to delete everything to the left of C2, and drive the
circuit from an opamp (C2 would have to be reversed if the opamp uses
Suggested Modern Transistors
Regulated Power Supply
The power supply was specified as regulated, and for a single supply
amp with low open-loop gain this is a good idea to maintain low hum
levels. Using a regulated supply is not normally desirable, but in this
case was probably warranted. Germanium transistors were used as shown
in Figure 2 (all medium and high power germanium transistors are/were
PNP). Since the amp will have a PSRR (Power Supply Rejection Ratio) of
only about 36dB, supply noise will be a problem with an unregulated
supply. It is worth noting that the emitter follower stage (Q1)
contributes most of the supply noise - another good case for driving
the amp from an opamp stage.
The regulator is only simple, but would have worked well enough as
shown. The Zener diode would normally be a 62V 1W unit, to obtain a 60V
supply voltage for the amp (equivalent to using ±30V with a more
conventional split supply). By the standards of today, the filter caps
are probably too small (as is the speaker coupling cap), but I am
presenting this as it was originally described.
Figure 2 - The Original Power Supply (Using Germanium Transistors)
I have spared readers the potential agony of the pilot lamp and its
ballast resistor (LEDs? In 1964? The LED was only invented in 1962, and
was still a curiosity when this design was published). If anyone really
wants to build an original El-Cheapo then I'm sure you can work the
details out for yourself. In case you may consider asking ... No, I
will not assist. Same applies for the germanium transistors originally
used - they are obsolete, and there is simply no point specifying the
All diodes should be rated at 5A minimum, and 400V types should be
used. The benefit and usefulness of D5 and D6 is rather suspect. I
don't recall the reason they were included originally, but modern
practice would be to leave them out altogether. Today it would also be
normal practice to use a reasonable sized electrolytic capacitor
(1,000uF or so) directly across each amplifier's supply (after the
fuse, and close to the power amp itself).
Figure 3 - Power Supply Modified to Use Silicon Transistors
The regulator shown in Figure 3 is a better proposition these days,
and silicon NPN transistors can be used for Q1 and Q2. A single
Darlington device could also be used. It may be necessary to reduce R1
from the 820 Ohms shown, depending on the gain of the transistors.
Again, I will leave that to prospective constructors (assuming that
anyone wants to build one ).
Despite initial appearances, the two regulator circuits are
functionally identical. Since the circuit of Figure 3 is likely to be
the most commonly used (if it is ever used), and suitable
transistors would be BD139 for Q1, and 2N3055 for Q2. The transformer
secondary should be 55V RMS, and this will give an unregulated voltage
of around 77V. That means that only 17V will be dropped across the
regulator, and with a worst case current of well under 4A (both
channels, and assuming 8 Ohm speakers!) the peak dissipation will be
around 38W - the average will be a lot less. Transformer rating is
around 250VA - a lot higher than you'd expect because of the regulator.
Needless to say, the regulator requires a heatsink as well as the
amplifier output transistors, so you will spend a lot more on heatsinks
The mains fuse will need to be a slow-blow type, and the amp supply fuses must be fast blow.
There's not a great deal to say about the amp. It is simple, and as
such may appeal to some readers. The three diode string sets the bias,
and it will be found to be quite variable in real life. Ideally, the
diodes should be mounted in contact with the heatsink, but if the sink
is adequately sized this should not be necessary.
The input capacitor is much larger than it needs to be, and there is
a real benefit if it is reduced to 1uF. Since input impedance is about
50k at the base of the emitter follower (Q1), a 1uF input cap will
still provide a -3dB frequency of just under 4Hz - provided the output
cap is increased to 4700uF.
With an open loop gain of only 180 (45dB), there is not as much
feedback as we are used to with modern amps. However, the amp remains
essentially flat to 10kHz open loop, and this is dramatically better
than most amps having a massively high open-loop gain. Because of the
small amount of feedback, it may be thought that output impedance would
be somewhat higher than we have come to expect. This is not the case
though, and Zoutwill normally be less than 100 milliohms (a damping factor of >80 for an 8Ω load).
Speaking of feedback - because the input stage creates an inherently
stable amp, there is no reason to expect that TIM (Transient
Intermodulation Distortion - assuming that you believe it exists of
course *) will be a problem, since feedback is simply applied to the
base of the input amp, and very little frequency "compensation" is
needed. Although a 68pF cap was specified for the original, it should
be possible to reduce this if wider open loop bandwidth is your goal.