Permeability-tuned Crystal Radio

This simple permeability-tuned crystal radio is wired using a variable inductor, a germanium diode/transistor and a pair of sensitive DLR No.5 I.T.B.A.5 S balanced-armature headphones.  

A germanium diode is used as the detector in the series-fed version.

Permeability-tuned Crystal Radio - series-fed version
For the shunt-fed version it's a germanium transistor (with its base and emitter interconnected). 

Permeability-tuned Crystal Radio - shunt-fed version
An empty glue stick and a ferrite toroid are used for the tuning mechanism.

The toroid is fixed to the blue glue stick carrier using rubber adhesive. The coil is 60 turns of 30 SWG enamelled copper wire, close-wound on a homebrewed 1" diameter paper former which fits tightly on the glue stick body.

Tuning mechanism for Permeability-tuned Crystal Radio
Junk-box parts are used to assemble and wire the radio using a scrap wall wart enclosure as the base.

Permeability-tuned Crystal Radio Assembly
Reception of  the local 612 kHz, 200 kW AM broadcast station quite good with just a 60' random-wire antenna.

Unusual Crystal Radio Circuit

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 Circuit
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 good results obtained earlier in a series-tuned, shunt-fed crystal radio.

Parallel-tuned Crystal Radio Circuit with 
germanium transistor as detector
But the results were disappointing.

However, when the variable capacitor was wrongly connected across the headphones during trials, the germanium transistor worked as good as it did in the series-tuned circuit.

Unusual Parallel-tuned Crystal Radio Circuit
Hence this unusual circuit was adopted for the final version with a 35mm film canister serving as coil former as well as enclosure for the PVC variable capacitor and germanium transistor.

Crystal Radio in 35mm film canister
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' random-wire antenna.

Horn Speaker for a Crystal Radio

This horn speaker is an improvisation for my crystal radio through a speaker.

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.

Loudspeaking Crystal Radio

This is the speaker variant of my series-tuned crystal radio with an improved detector.

Loudspeaking Series-tuned Crystal Radio
Replacing balanced armature headphones (impedance around a few hundred ohms) with a low impedance speaker required the use of an output transformer.

A power transformer, with its inherent thick conductors, appeared to be an ideal substitute for a low-loss output transformer.

Trials with transformers rated 230V/24V-2A, 230V/24V-5A and 230V/24V-15A proved the last one to be the best.

Reception of the lone local 200kW 612kHz AM broadcast station, located 20km away, was quite good.

Antenna was a 60' random wire.

Performance was equally good in the parallel-tuned configuration also.

Loudspeaking Parallel-tuned Crystal Radio

Improved Detector for a Crystal Radio

After obtaining a significant improvement in performance by just adding an inductor to my crystal radio, the focus was on a replacement for the OA5 which would give a higher output.

OA5 diode
The base-emitter and base-collector junctions of germanium audio transistors like AC130, OC74, AC127, 2N61, AC188 and AD162 were tried out. However, they were all only as good as the OA5.

Germanium transistors
As luck would have it, when the 2N61 was being tried out, an accidental short between its base and emitter leads resulted in a tremendous increase in signal output. The other transistors too gave identical results with their base and emitter leads shorted.

Crystal Radio - Schematic
Shorting the base and collector leads gave similar, though not consistent, results.

With the OA5 in the crystal radio replaced by the base-emitter-shorted 2N61, the lone local broadcast station on 612 kHz is now loud enough to be heard faintly even at a distance from the headphones!

It is to be noted that germanium transistors, with base/emitter interconnection, outperform germanium diodes provided that the crystal radios are shunt-fed (series-tuned) and not series-fed (parallel-tuned).

Harmonic Interference

One day, in spite of my Minimalist Converter being disconnected, harmonics of the local 612 kHz broadcast station were still being received in the shack receiver.

That was indeed surprising. Then, after some thought, realisation dawned that the Crystal Radio on the shack table could be the cause.

This was confirmed by disappearance of signals when the the ground connection to the crystal radio was removed.
Crystal Radio - Schematic
Then, on a feeling that the minimalist converter would display identical behaviour, its output was next connected to ground.

Minimalist Converter - Output grounded
The signals returned.

It was my first-hand experience of harmonic interference!

Series-tuning for a Crystal Radio

Adding series-tuning was an afterthought to improve the performance of my first crystal radio.

Series-tuned Crystal Radio - Schematic
The loopstick (90 turns of Litz wire, close-wound on a 2" length of  ¼" ferrite rod) and variable (a 500+500 pF PVC gang condenser) were wired on a bread board and connected in series with the antenna lead-in.

Ferri loopstick & PVC gang condenser
With the plates of the condenser nearly fully meshed, the only local broadcast station on 612 kHz was received with a considerable increase in volume.

However, both the loopstick and variable were too big for the crystal radio enclosure.

Trials with a fixed 1000pf capacitor, in series with various small inductors, were unsuccessful.

Ultimately, a lone 200 μH moulded inductor was found to work quite well, with the short random wire antenna itself providing the capacitance.

Moulded inductor
It was easy to solder it right at the antenna socket.

Inductor-tuned Crystal Radio - Schematic
This minor change gave a significant improvement in performance.