Electronics

truSDX Audio Amplifier

Markus Kolb

• 4 min read
truSDX Audio Amplifier
Photo by ANDY ZHANG / Unsplash

The (tr)uSDX is a 5-Band / Mulitmode QRP Transceiver in Pocket Format (90x60x30mm – 140g). It features a highly efficient Class E PA and Supports CW/LSB/USB and AM/FM. It covers by default 80/60/40/30/20m.

Have a look at https://dl2man.de/ for more information.

The audio output via internal mini speaker and the audio jack on the right top side is quite quiet. With headphones it is nice.

The preferred way to get some more audio power is to use an active speaker. But these speakers need additional power. And also you have a small radio and would have to take some bigger thing with you on your portable journeys.

My Idea

I've had a speaker-microphone combination with a K-plug and loop in a small audio amplifier, powered by the same source like the radio, between radio and speaker-microphone combi.

Adapter Cable for K-Plug

I've created 2 short shielded audio adapter cables.

  1. TRS 3.5mm jack to 3.5mm plug for microphone. On radio side T is PTT and goes to microphone side S. Radio side R is microphone audio and is also microphone side R. Radio side S is ground and is internally connected to radio speaker ground. So radio side S is not connected to K plug. And microphone side K plug T is also not connected to the radio.

  2. TRS 2.5mm jack via adapter to 3.5mm plug. TRS 3.5mm jack to 3.5mm plug for speaker adapter cable. T is connected to T. R is unconnected on both sides. S is ground and connected to S.

truSDX radio with speaker-micro and connected audio amp

Selfmade LM386N-4 Low Voltage Audio Power Amplifier

It is important to use the LM386N-4 (N-4) with the supply voltage up to 18V. So the same 13.8 voltage source of the radio is usable for the audio amp.

I've soldered the parts on a stripboard with 15x15 holes size.

Audio Amplifier Circuit

Audio Amplifier Circuit Description

I plugin in the speaker cable from the speaker-micro to the jack J1 (audio output).

The audio plug J2 (audio input) is soldered with 10-15cm cable to the board. T more or less directly to pin 1 of potentiometer RV1. S is on my board connected to the ground near RV1 pin 3.

The LM386N-4 is used with 26dB gain, which is enough. There will be around 60-70 mA additional current for the audio amplifier. Which is roughly the same the truSDX receiver needs.

Look at the description boxes of the circuit for the used values and setting of RV1.

The jack J3 is the same jack used in the truSDX for power. So instead of plugin the 3.5mm-1.3mm hole plug into truSDX you plugin in the audio amplifier.
And from the audio amplifier I've soldered one end of short power cable 10-15cm with contact to pins 1 (-) and 2 (+) and on the other end I soldered a 3.5mm-1.3mm hole plug which goes into the truSDX radio.
The amplifier should work with 5V-13.8V like the truSDX radio.

PCB layout also usable on stripboard

Stripboard positioning (PCB)

The jacks on this PCB have different connections to my used jacks. But there should be enough space around to wire the contacts to the right places. On my stripboard only the bottom side is used for wiring.

The left contact of RV1 is GND. The right contact of RV1 is the audio input from the truSDX audio output. So to these 2 contacts the audio cable with TRS-plug is soldered.

I've fixed the cable with a cable tie through 2 holes in the stripboard.

Audio Amplifier for truSDX (in blue shrinking hose)

Parts used