ZN414 IC

At this point it is worthwhile downloading the data sheets. Search for data on not just the ZN414, but the other clones as well.
One of the Ferranti sheets is available here on the Jaycar site.
Briefly, the tuned circuit is fed straight into the IC. There is no need for a tapped coil due to the 4M input impedance. Looking at the internal diagram of the TA7642, we can see this is a result of using an emitter follower at the input pin. Four stages of amplification and an active detector result in a gain of about 72dB which is high enough to give good results with no external aerial. In fact, gain is not too far off an average superhet using the same size aerial rod. The .1uF at the output pin is for RF bypassing, and in conjunction with Ragc, determines the audio bandwidth. The data sheet shows how this capacitor value is calculated. Apparently, the higher the value, the more gain can be had. This would make sense as RF bypassing would be improved with an increase in capacitance value.
The DC at the output pin varies with signal strength and this is used for AGC. The 100K and .01uF are the usual time constant to remove audio fluctuations. In addition, the .01uF is also the RF bypass for the earthy end of the aerial coil and tuning condenser. Gain of the IC is thus controlled by the DC at the input pin.
I have seen some circuits incorrectly drawn, particularly with the LMF501T clone, where the 100K has been connected directly to the input pin, along with the .01uF for DC isolation of the aerial coil. While it would work to a degree, and the DC conditions are correct, the problem is 100K is effectively shunted across the aerial coil and will result in loss of gain and selectivity.
It is important that the tuned circuit is the only thing connected to the input pin.

My interest had been rekindled of late as Dick Smith was selling MK484's at half price. Knowing that they are slowly getting rid of components, I went and stocked up with a lifetime supply. I have known about the ZN414 for years; for a long time the Dick Smith catalog had provided a circuit (EA May 1974) in the back of their catalog.
The ads for the device proclaimed such virtues as "equivalent to a ten transistor radio". Ten transistors it may have, but it actually has only four stages of RF amplification. This is about the practical limit before instability would set in. The other transistors are used for the detector, AGC, impedance matching, and stabilisation.

The first time I actually used the ZN414 was with the Funway kits back in 1981. In the Funway 1 volume was the "Beer Powered Radio" which was the standard ZN414 circuit driving a crystal earphone but powered from a homemade battery using beer as the electrolyte. A modification was shown to run the circuit from a 9V battery instead, which needless to say is the version I built. In Funway 2 was the "Pocket Transistor Radio" which drove a magnetic earphone and ran off a 9V battery. Its performance was very poor for two reasons. Firstly the method of obtaining the supply for the ZN414 was a bodge and resulted in instability, and secondly, the 9V battery pressed up against the ferrite rod ruined the signal pickup. This particular circuit was the same as EA's August 1979 design, except for the modification for 9V operation.

During the mid to late 80's when I was learning about solid state, I experimented with many different ZN414 circuits and associated audio amplifiers. I used to demonstrate to some of my fellow students a ZN414 receiver with a two transistor amplifier driving a speaker that I'd build on a breadboard from time to time. I recall listening to 2SM during one of our programming classes on such a set. Soon after, I went off listening to AM as a results of changes to formats and stations migrating to FM. So, the ZN414 became dormant in my designs until recently, now that I'm mainly listening to AM again.
Also was the fact I'd accumulated a few of the $2 shop radios using the ZN414 clones.

An AM portable radio receiver made from the ZN414 IC. The ZN414 ic has now been replaced by the MK484 which is identical in performance and pinout.

Notes:
Designed around the popular ZN414 IC this receiver covers the medium wave band band from approximately 550 to 1600 KHz with the values shown. The coil and tuning capacitor may be taken from an old MW radio to save time. The ZN414 IC, has now been replaced by the MK484. The integrated circuit is a 3 pin, tuned radio frequency circuit, and incorporates several RF stages, automatic gain control and an AM detector. It is easily overloaded and the operating voltage of th IC is somewhat critical to achieve good results.

In this circuit a small voltage regulator is built around the BC108B transistor, four 1N4148 diodes, the 2k7 and 10k preset resistor and the 820R resistor. The 10k pot acts as a selectivity control for the whole receiver, controlling the operating voltage for the ZN414 (or MK484). If you live in an area that is permeated with strong radio signals, then the voltage may need to be decreased. I found optimum performance with a supply of around 1.2 volts.

The audio amplifier is built about an inverting 741 op-amp amplifying circuit. Extra current boost is provided using the BC109C / BC179 complementary transistor pair to drive an 8 ohm loudspeaker. The voltage gain of the complete audio amplifier is around 15. The audio output of the complete receiver is really quite good and free from distortion. I may provide a sound sample later. Click here to see a picture of my prototype. I used a small wooden enclosure and the complete tuning assembly from an old radio.