17 februarie 2021

Xiegu G-90 Audio Gain for quiet nights

 I use to listen to CW to train the reception but not always I am not so willing to use the headphones. Sometime, late at night when I am working at some projects the sound can be heard in the bedrooms and I already had some complaints about this.

On Xiegu G90, the minimum audio level is still too loud for this and some faint signals need to keep the RF Gain at higher settings so something about the audio has to be done.

The audio volume is digitally defined into the audio codec in the Main unit and even I sent the issue to Xiegu, nothing has been done yet (FW 1.77).

Therefore, I felt I must do it myself...

The audio amplifier is made around a wellknown LM386 in SOIC package, not the best for thermal dissipation, if you ask me...

LM386 is a very versatile audio power amplifier designed to be used in portable applications. 

It feature a wide supply voltage range, between 4V and 12V (some versions from 5V to 18V), and can deliver 125 mW on 8 Ohm speaker with 0.2% distorsion.

Because it is designed for minimum external parts, it has an internal fixed value resistor (1.35 kOhm) for an amplification factor of 20.

Internal schematic of LM386:

This internal resistor can be bypassed with an external network to achieve higher gain (up to 200 or 46db) and Xiegu put there a 10uF capacitor and a 100 Ohm in series.

Sidenote: I found a nice online gain calculator for LM386.

The values used by Xiegu in G90:

According to the formula, the Gain is 124.5.

After tweaking with some Rext values, I replaced the original 100 Ohm with a 300 Ohm for a Gain of 76.

Now, the audio is pretty nice, the first step of volume settings is just where it should be for a quiet night.

Audio circuit, PCB layout in Xiegu G90:

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