This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.
Moving on to the transmit-only portions of the Beach 40, today I completed the microphone amplifier. Powered on transmit only, it’s responsible for both boosting the microphone input level to inject it into the mixer, and for switching the microphone out of the circuit on receive, as necessary.
One difference that I’m going to introduce into the original VK3YE design is that I’m going to use a dynamic mic instead of an electret mic, specifically one of the mics I picked up at the SMCC Hamfest back in June. This likely means I’ll need a bit more gain in the pre-amp than if I was using an electret element.
Peter mentions the possibility of using a dynamic microphone on his page on the Beach 40. He says:
“The circuit is suitable for an electret microphone. If using a dynamic unit leave out the 22k [bias] resistor and possibly raise the 100n [mic] coupling capacitor value if insufficient or thin audio.”
Leaving the bias resistor out of the original circuit, and replacing a couple of the electrolytic caps with ones from my stock, I ended up with this:
A quick spice simulation shows about 15 dB of gain through the audio spectrum with a slight high-pass characteristic. Note that I’m making a couple of approximations here. I don’t know exactly what the impedance of the dynamic mic is, but varying its input impedance between 100 and 500 ohms shows only a 2dB variation in gain, so I’m not too concerned. Similarly, I’m measuring power into a 50-ohm resistor as a representative of the mixer, instead of modeling the mixer directly.
This circuit came together very quickly on a piece of copper-clad, just five pads and a few minutes of soldering. The next step will be attaching the mic amp to the mixer and determining whether I do in fact need more gain. I’ll also need to experiment with the microphone input cap to see if I need more low-end response from the mic. But this is a start!
Hear you on the air.