Breadboard Transistor Radio
Modifications
Other More Advanced
Projects and Modifications
A Loudspeaker Enclosure
The sound output level and quality from the small speaker will be
substantially improved if it is fixed into
something as simple as small cardboard box with some holes cut in
the front. A much better option is to fit the speaker into
the lid of an empty Pringles tube with some loosely screwed-up
tissue inside it as shown in the following picture sequence.
You'll need some Blu-Tack™ or some other brand of poster-fixing
sticky putty to seal the speaker into the lid, as used by all good
acoustic prototyping engineers, and some sticky tape.








So, simply cut some neat slots in the lid and seal the speaker
into it using a blu-tack worm. It helps a great deal to keep
the material warm when doing this, and especially before finally
putting the lid on. Make a small hole in the tube for the
wires and feed them through. Put some loose tissues in the
tube, no more than three or four, then push the lid on and fix it
with some tape.
How It Works - If you're looking for science
project Brownie Points, you've just struck gold.
A speaker in free air produces an inverted sound signal from the
rear side. This tends to diffract back around to the front
of the speaker, like any sound can go round a corner to some
degree. As this rear wave is of opposite polarity to the
front one, it partly cancels the front sound wave. This
happens particularly for low frequencies which diffract more, so
you get poor bass response. Broadly speaking, a standard box
enclosure seals up the rear wave to prevent this. You can
demonstrate this by comparing the speaker performance with and
without the tube.
But, this is also a primitive transmission line speaker
enclosure. As well as generally trying to get rid of the
rear wave by sealing it in and absorbing it with tissue, the tube
acts a bit like an organ pipe closed at the opposite end to the
speaker, resonating at a certain frequency. The lowest
resonance of a tube closed at one end is at one quarter of the
wavelength of the sound pitch in question. What's the
pitch? At lowest resonance the tube is a quarter
wavelength, so the full wavelength (greek letter lambda, λ) is
four times the measured tube length, λ = 0.26 X 4 = 1.04m.
Now you have to turn the known resonant wavelength into a
frequency, and the physics equation for wave propagation is v = f
X λ, where f is the frequency and v is the speed of travel.
Re-arrange the equation to give f = v/λ, insert the standard value
for the speed of sound at standard temperature and pressure,
v=334m/s and the 1.04m that we already have, to get f = 334 / 1.04
= 321Hz.
The tissue tends to make that figure drop a little so let's call
it 300Hz. 300Hz isn't a particularly deep bass note, but
then we only have a small speaker and that is pretty much at the
bottom end of its frequency range in any case. Because the
tube resonates, the pitch around 300Hz is emphasised and so you
get more sound at the lower end of the speaker's range. This
is generally another design intent of speaker enclosures.
The tissue tends to damp the lowest and (even more) the higher
resonances so that the effect doesn't sound too artificial or
boxy, perhaps 'tubey' in this case. You can try using a
special-offer super-sized tube and compare the two for bass
tonality.
You won't win any hi-fi awards, but it's not difficult to beat the
tiny music player speakers found in mobile phones by experimenting
in this way.
Long Wave
There is nothing to stop you making a LW coil to make a long wave
radio, but you'll need about 180 turns of wire wound up in three
or more layers. This can be quite bulky when made with PVC
covered wire, so you might choose to use thinner enameled copper
wire or a ready-made LW coil. It's relatively easy to
connect one end of both coils together and arrange a switch to
connect either one of the other ends to the tuning capacitor.
Short Wave


If you remove the ferrite rod, make the main coil about 25 turns
and wind the coupling coils as shown in the picture, you will be
able to tune the shortwave bands from about 7 to 30MHz. At
this stage you will need to link in a few meters of aerial wire by
connecting it to one end of the short coil on the left and
connecting the other end of the short coil to a heating radiator
or earth connection. Don't connect to a mains power earth
pin. The tuning will be quite broad but you will be able to
hear several strong international broadcasters, especially after
dark. Sliding the ferrite rod back in will lower the
frequency range of the tuning.
More Signal
One way of sharpening up the tuning and getting more signal on the
station that you want is to use an external tuned loop
antenna. This requires no physical connection to the radio
and can be very effective. You can see how to make one
here: Tuned
MW Loop Antenna.
General Magnetic Pickup
I've not given this a
proper try yet, but because
our radio only has one tuned circuit, it is quite easy to change
it to pick up other magnetic signals that are around in the
environment, and we can choose to have a high-Q tuned circuit or
not. We can also choose whether to have an AM demodulator or
not. It's easier when playing around with magnetic pickup if
you use headphones instead of the loudspeaker, as the strong
signal from the coil in the speaker has an even greater tendency
to get back into the input. A good magnetic pickup for audio
frequencies is the primary of a small mains transformer with the
core removed, connected instead of the normal input coupling
coil. The demodulator will be fairly ineffective at audio
frequencies and if you want to remove any demodulation effect, you
could remove C6. If you wanted the AM demodulator to work
down to lower frequencies you may need to increase the value of
C6.
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© Henry J. Walmsley 2014