A Direct Amplifying AM Receiver

The first receiver for a beginner, with the non-traditional use of LED for bias voltage detector diodes,
and a television module as an amplifier

The direct-amplifying receiver for amplitude module transmitting using medium waves is suitable for beginners because it is simple and with its external antenna makes it quite efficient. Not many receivers then would have had hard to control feed-back but would have had “a crystal with an amplifier”. These kinds of receivers were built back in the 1920’s and 30’s and had as many as 4 tuning circuits separated by electronically amplified stages. It was suitable for long and medium-wave transmitting and simpler to build than the superhet.

Our receiver contains only one tuning circuit. So that we could get good function quality, the coils were wound around a larger diameter air frame. For the same reasons, an air tuning condenser is used. It is fundamentally important to have a correctly positioned antenna coupling with which you can set the ratio between sensitivity and selectivity depending on the antenna used and on the time of day. A long-wired antenna should be connected to the A3 port. Short antennas should be connected to either the A1 or A2 port. We can measure the sensitivity of the crystal radio using a high-ohm headset plugged into the Audio 1 port.

So that it can be used as a detection germanium and silicone high-frequency diode, a low voltage instrument has been constructed. The light diode is advantageous because it can be used as a voltage stabilizer too. For tuning on to weak stations we can set the R2 capacitor to maximum volume. The regular capacitor will then have a voltage between 0.1 and 0.5 V, according to the type of diodes. The difference in the sensitivity is really noticeable and we can receive several more stations.

For high-volume listening we have installed an efficient low-frequency amplifier. The easiest way to do this is to use a low-frequency module from a marked TV receiver. The integrated circuit MBA810 adds up to 5 W of efficiency depending on the voltage supply. The excess high tones can be removed with the C5 condenser with a low-frequency value of 5 to 10. We can connect the speaker to the Audio 2 output using the 3.5 mm tri-pole jack. It is possible to hear it on a walkman headset.

The receiver can be used as an amplifier by connecting to the Audio 1output. A high-ohm headset can be used with the microphone function. This can be demonstrated, for example, by moving the headset closer to the speakers to produce acoustic feed back.

The mechanical construction is completed with pieces of double-sided cuprextite, some basic tools and a soldering gun. The front panel has the dimensions of 90x105 mm and is 150 mm deep. If you use a tuning button with a diameter of at least 40 mm, it is not necessary to have a mechanical gear. It is useful to place a simple indicator on the button.

This connection is suitable for beginner interest courses in radio techniques and for construction courses at summer camps. Out in nature you only need a flat battery for receiving and the sensitivity will be satisfactory too.

By listening to this old-fashioned machine you will note how during the day and night the receiving conditions change, how in the evening far away stations become clearer, what fading is, the efficiency of different antennas, how you must lessen the noise so you can distinguish between individual stations, and why it is important to have a loose coupling for receiving short end band. When listening with the amplifier, notice that the efficiency drops when you lessen the load resistance by connecting the detector to a high-ohm headset plugged into Audio 1. All of these things are useful for the basic study of radio techniques and spreading radio waves.

This simple construction can give you a surprising and almost unknown experience which your FM receiver or Internet cannot give you.