The goal I set for myself was to create, modify, or discover a high-quality TRF (Tuned Radio Frequency) radio circuit that could perform at least as well as the MK484 - TA7642 AM radio chip, while using as few parts as possible. My aim was to learn about the constraints of early tube designs from the twenties (1920s).
Back then, designing a TRF radio presented numerous challenges. The tubes of that era had limited gain, necessitating the use of multiple stages, which, in turn, led to additional issues requiring filters and workarounds. Moreover, these designs lacked sensitivity, paving the way for Regenerative Radios and the Superhets. Tube gains hovered around 10, whereas modern transistors, like the 2n3904, range from 200 to 800. However, even with this improvement, the gain remained insufficient without regeneration (superheterodyne).
I tried all sorts of designs, using FET's, Darlingtons, etc. They were all pretty awful.
This led me to think of the existence of a transistor with a gain of 5000 to 10000, a concept nonexistent in the 20s. Eventually, I stumbled upon a straightforward design employing multiple MPSA13 transistors (Darlington). I streamlined it to the detector stage, it worked really well! Adding a preamp stage on the output and connecting it to my headphone amp yielded good sound quality. By adjusting the voltage to the collector, you can enhance the gain, and it even performs OK up to about 17 MHz (Needs an antenna of a few feet at least). The preamp section using the 2N3904 is a great audio preamp design you can use anywhere.
Utilizing a ferrite core antenna wound with litz wire, I achieved satisfactory results on the AM broadcast band without an antenna, but you can use anything at hand. I am able to pick up AM Broadcast stations 600 miles away. It will even work great with an aircore winding on a 1 inch diameter core of plastic or cradboard, but you will need an antenna of some sort, even a 2 foot length of wire. Earth ground is not required if using a ferrite antenna core
I'm planning to explore the MPSA14 Darlington, which boasts twice the gain of the MPSA13, to further enhance sensitivity.
Despite its success, there are a few drawbacks:
1. Driving it too hard with higher voltage can induce oscillations.
2. The extreme high selectivity makes it challenging to use poly variable capacitors effectively.
3. It whistles and oscillates around a station until it is tuned. Going forward, maybe I can figure out an AGC circuit.
This is an easy project to build. It Works.
There's a two transistor one I did some years ago, if that's any help. I ac coupled the two transistors and followed with a biased diode detector.
Follow the link below.