Series-tuned Air-core Coil Crystal Radio

A conventional parallel-tuned crystal radio was recently put together using a pair of sensitive DLR No.5 I.T.B.A.5 S balanced-armature headphones.

Conventional Parallel-tuned
Crystal Radio - Schematic

To further improve its performance, a germanium transistor (with its base and emitter interconnected) was used as a substitute for the OA5. This was based on the good results obtained earlier with my first series-tuned crystal radio.

Parallel-tuned Crystal Radio
with alternative detector - Schematic

But the results were disappointing.

However, when the circuit was reconfigured as series-tuned and shunt-fed, there was a dramatic improvement in performance.

Series-tuned, Shunt-fed  
Crystal Radio - Schematic

A 35mm film canister doubled as coil former and enclosure for the PVC variable capacitor and germanium transistor.

Series-tuned, Shunt-fed Air-core Coil
Crystal Radio

A screw-type terminal strip was used for connecting the antenna (yellow), earth (blue) and headphone (red & blue) leads.

The local 612 kHz, 200 kW AM broadcast station came in real loud with just a 60' wire antenna. Headphone current, measured using a 1mA FSD 60 Ω meter, was 700 μA.

Subsequently, an improved, link-coupled, fixed-tuned version was built. The 22 μH inductor, along with the 50 turn link winding, series-tunes the capacitive wire antenna to resonance at 612 kHz.

Link-coupled, fixed-tuned
version - Schematic

A cosmetic jar was used as the enclosure/base and a polythene can body as the coil former.

Link-coupled, fixed-tuned version

Performance was quite good with headphone current measuring 825 μA.  

Related post: Loudspeaker Crystal Radio
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Horn Speaker for a Crystal Radio

This horn speaker is an improvisation for my 'Loudspeaker Crystal Radio'.

Horn Speaker

It requires only 2 additional parts - a dustbin and a food container - with the crystal radio speaker itself serving as the driver.

In order to simplify the mounting, the speaker is made to drive from its rear while its front is sealed by the food container.

Horn Speaker details

Rubber based adhesive is used for assembly.

Audio output is considerably more than with the speaker in its normal enclosure.
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