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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