Beach 40 Build – Part 3: Audio Amplifier

This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.

Today I tackled the audio amp section of the Beach 40. And when I say tackled, I think perhaps the player was already down, and I just had to fall on top of him.

The audio amp configuration I’m currently using is the bog-standard, straight-from-the-datasheet LM386 20x or 200x configuration. A SPST switch puts the 10uF cap between pins 1 and 8 of the IC, or takes it out of the circuit.Currently, I don’t even have a volume control pot in place. Peter VK3YE later modified the LM386 configuration for greater gain and less hiss, which is something I’d like to tackle once I’ve gotten the rig working.

Beach 40 Audio Amp-05

Part of what made today so simple is that I’d already built this audio amp section for another project. I’d been using a bit of scrap copper-clad to experiment with coupling between an NE602 and an LM386, to see if passive filtering would be a useful option for future receivers without introducing too much loss. But since I haven’t touched that project in a couple months and this one’s exciting me now, that board hit the chopping block.

IMG_1266
The old receiver experimentation board. What a Frankenstein!

Extracting just the audio portion of the board was simple enough – now that I’m looking at the pictures and seeing the two big bypass capacitors on the red power line reminds me that I need to think about bypassing the power input at the source, probably with something beefy. I’ve had enough LM386’s turning into oscillators, and I’d rather not subject my ears to that too much if I can avoid it.

IMG_1267


With the VFO, mixer, and audio amp “completed,” I now have the bare bones of the receiver portion complete. Of course, there’s exactly zero filtering, either at the front or the audio portion, so it’s not like this would make a particularly reliable bit of kit. But it’s got enough parts that I can string them together and test.

IMG_1276.JPG

While I didn’t exactly succeed in keeping the leads short, using clip leads was just the fastest and simplest way to tie the modules together. RF from the ‘antenna’ (read: long bit of wire strung around the office) is brought into the mixer via the white clip lead at center. The VFO drives the mixer and converts the RF down to audio, where it’s amplified and sent out to the headphones.

To generate a test signal, I used my SI5351-board without an amplifier, with just a short bit of wire hooked directly to the output of the Si5351. I set the output somewhere around 7.150 MHz. Tuning the VFO across the band with the polyvaricon, I found my strong local source no problem. The receiver works!

I can see why Peter later added a fine tuning control to this project – with half a turn of the variable cap, I’m covering around 200Khz of the band, which makes tuning in to an individual station tricky. I do have a rather large panel knob in mind for the cap, which will help, but we’ll see if it’s enough. If I can’t get the VFO drift a little more under control, that fine tuning cap might be necessary to help stay on frequency.

Hear you on the air!

73

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