IF amps for a superhet AM receiver

[vk6kro]

If you can find them, try to get the 14 pin LM380. This uses the centre 6 pins as a connection to a heat sink.

The output of these is much greater than most opamps. They also give very undistorted output.

A trick to make them stable is to connect an 82pF capacitor between the two inputs.

Also don't omit the series R/C network across the speaker. (12 ohms / 0.1uF)

You need the type of IF transformer used in transistor radios. The ones used in vacuum tube radios are not suitable.

Most of the transformers will have tappings on the windings, but you don't need them in this circuit.

You only need one transformer between stages. They should cost less than a dollar each on EBay.

 

[cabrera]

Hi, first of all I have been following this topic as I always wanted to built a transceiver. I don't know much about this type of transformer but I guess that you have to check the impedance ratio between the primary and secondary windings. The data sheet only shows a single impedance ratio (between primary and secondary) therefore I guess all are centred tapped.

 

[vk6kro]

The IF transformers used in older transistor radios have a capacitor across the larger winding and also have a ferrite slug which moves into both windings, changing their inductance.

Used with the input on the lower inductance winding, the secondary is almost an unloaded tuned circuit.

Viewed from the primary, the impedance rises at resonance and there is a large step-up of voltage from primary to secondary at resonance.
This is ideal for FET RF amplifiers which have a low impedance output and a high impedance input.

There is usually a tapping on the larger winding, but this is not needed in this circuit.

So, you can see that the action of the transformer is not just impedance transformation.