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-Tackô 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.

Speaker lid first cutslots in lid for speakerspeaker seal blu-tack

Speaker sealed into
          the generic starch based snack tube with generic poster fixing
          putty:)speaker wires go through a small hole in the cardboard
          tubeloose tissue in speaker

speaker in lid taped onto
          tubespeaker in transmission
          line tube connected up

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

Shortwave Coil for TRF
          Breadboard RadioSW Coil Detail

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