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Crystal Detector Radio Receiver One Transistor Audio Amplifier Headphone Set

OddMix.com - Technical Note - Crystal Radio - TN080821 - Karl Nagy

A major problem in crystal detector radios is that selectivity and available sound volume can't have together. A high Q (quality) tuned circuit has a narrow pass-band. This high Q makes it possible to pick out one radio station of many that is equally strong. A lower Q tuned circuit has a much wider pass band. It allows more than one station through and makes listening to either station's frustrating. Higher Q resonant circuit can be made with a low resistance coil that is made with short and thick copper wire and a low loss capacitor. The capacitor is always the better of the two circuit elements (for Q), as it usually has air for dielectric with good insulation.

Coil resistance for tuned circuit's can be further reduced if a ferrite core is used. The increase in inductance allows using a shorter copper wire with less resistance resulting in a higher Q coil. When a tuned circuit is in resonance the higher the circuit's Q the higher is the available RF voltage for the diode detector. The problem is that the diode is loading the tuned circuit, degrading its Q and reduces this small available voltage. For this reason, the diode is connected to the lower tap on the coil. Many taps are better because each tap allows a selection of slightly different loading, but too many are difficult to handle. Two taps in a third of the coil works out just right.

Figure 1. One Transistor Crystal Detector Radio [6 KB]
Figure 1. One Transistor Crystal Detector Radio
To restore some of this lost audio volume this crystal detector radio uses a single transistor audio stage that amplifies the input signal by at least a factor of ten. The exact amount of amplification is mostly dependent of the transistor gain. The resulting amplification is also a function of the type of headphone set being used. A one or two thousand K headphone in this set must works fine. This amplifier does not work with crystal headphones. Remember that when trying it out.

This radio allows higher volume reception even in areas with weak stations without using much power. The point contact diode D1 is germanium. Detector radios produce louder sound, using a germanium or silicon transistor amplified audio stage than what is obtainable by the single diode crystal sets. The higher audio volume requires some extra work and components.

The schematic diagram for the basic crystal detector radio is on Figure 1. This set uses a 200 uH coil wound on a 1/4-inch coil form with a tunable ferrite screw in it. Ferrite has higher magnetizing losses than air, but an air core coil would use a lot longer wire that has a higher resistance. Using ferrite in the coil will increase the overall receiver efficiency. C1 is a good quality, air dielectric rotary capacitor.

               Parts list for this one transistor crystal detector radio:

                 B1 - Two AA 1.5 Volt cell -  any kind - Carbon Zinc is fine
                 C1 - Rotary Capacitor 15-500 pF
                 C2 - Antenna Capacitor, 100 pF, mica, 200 VAC - or better
                 C3 - Electrolytic Capacitor - 1 uF, 10 VDC, Tantalum
                 C4 - RF Filter Capacitor 1 nF, Disc 6-10 VDC
                 D1 - Diode - Germanium, point contact
                 L1 - Coil 1 - 200 uH - taps at 1/3 and 2/3 turns
                 Q1 - Transistor -  germanium or silicon, high gain, small signal
                 R1 - Resistor, 20K, 1/4 Watt, CC
                 R2 - Resistor, 270K, 1/4 Watt, CC
                 Magnetic headphone, or a small high impedance speaker

Use long wire, high antenna, tune in a station and set a tap to position "B" to produce a stronger sound. If another strong station is interfering, use tap A. This radio may be connected to a two-transistor amplifier described by another OddMix Crystal Technology article.


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