Simple DCR: Assembling a 7 MHz (40m) Direct Conversion Radio Receiver – Part 1

RE: Simple DCR: Assembling a 7 MHz (40m) Direct Conversion Radio Receiver – Part 1

shabaz — Fri, 13 Mar 2026 02:49:39 GMT

I took another look at the construction, and realized that for the transistor biasing, I had set R12 to 56k, and R13 to 10k (I don’t know why; I have forgotten, since I soldered those resistors almost a year ago!). This resulted in very little current through the transistor, just a few milliamps, which is great for battery life, but not so great for generating more output.

I’ve experimented a bit, and now replaced with R12 = 18k, and R13 = 8.2k, and that results in a value closer to about 15 mA. Now with a 64 ohm load, I can drive the input to about 40 mV p-p, and see 1.35 V p-p output.
However, with an 8 ohm load, I can drive the input higher, to about 80 mV p-p, and see about 830 mV p-p output (these values are with a 1 kHz signal). Even at 100 mV p-p input, the distortion would be tolerable.

So, the conclusion now is, that the 8 ohm speaker would be better – the amplifier stage can be driven with more input, and the power output should be higher. I didn’t have an 8-ohm speaker to listen to it, but I’m going to go with those resistor values, and use an 8-ohm speaker speaker.

For anyone curious about the transformer, it has about 89 ohm DC resistance at the primary, and 3.3 ohm on the secondary. I put some impedance measurements in the table below, but I’m not sure how useful they are, I don’t think there’s much value trying to improve this circuit much further, since it is based on an AliExpress transformer which could be very different when ordered a year later that the one I measured.

RE: Simple DCR: Assembling a 7 MHz (40m) Direct Conversion Radio Receiver – Part 1

DAB — Thu, 12 Mar 2026 19:15:52 GMT

Nice post.

I never got into the amateur radio hobby, but I did have a 2nd Class FCC license that would allow me to work with commercial grade devices.

I never used it, but it did get me an A for my radio communications course.

RE: Simple DCR: Assembling a 7 MHz (40m) Direct Conversion Radio Receiver – Part 1

michaelkellett — Thu, 12 Mar 2026 11:06:04 GMT

This article:

https://www.vintage-radio.com/repair-restore-information/transistor_output-stages1.html

suggests that you are getting about what you might expect from a low bias current single transformer power amplifier.

Time to go fully complementary capacitor coupled I think !

RE: Simple DCR: Assembling a 7 MHz (40m) Direct Conversion Radio Receiver – Part 1

BigG — Thu, 12 Mar 2026 10:46:48 GMT

Wow, yet another excellent post. Not to nit pick, but it should really be saying "this is a popular licensed band for amateur radio enthusiasts."