Yaesu radios' audio output stages

[Daniel Wee]

Most of Yaesu radios appear to employ the same kind of design:-

1. Various audio signals AFOUT, AFOUT2, TONE comes into a 8-bit digital volume control chip M62364FP chip.
2. The output of this chip is mixed without gain using a single NJM2737 op-amp stage.
3. The output of this mix is fed into the LA4425A gain-less power amplifier stage.

[Daniel Wee]

The noise in the output of the amplifier does not change with volume, so it must be inherent to one of these three parts - the M62364FP, NJM2737 or the LA4425A. The NJM2737 is specifically a low noise part so I'm not suspecting it as a noise source at the moment. The M62364 has only 8-bits so it is possible that we are hearing the 1-bit noise dither.

The datasheet of the LA4425A specifies the output noise at 40dB for zero input. It is unclear what this is in reference to. This should translate to 0.5mW or about 1% of the output voltage - 132mV. It is possible that this is what we're hearing - the inherent noise of the power amplifier stage. The Sanyo datasheet (attached) for the part specifies the typical noise to be around 0.150mV which is much better.

This power amplifier has a typical voltage gain of 45dB (around 300x amplitude) meaning that the noise could be 300 * 0.15mV = 45mV?

[Daniel Wee]

Interestingly, ICOM also uses the LA4425A in the output stages, for example in the IC-718 (pg.16 of attached file). This has been confirmed to exhibit that hiss at zero volume as well.

The FT-DX101MPD, on the other hand, uses the TDA2003 power amplifier chip.