• HiFi system too loud, LA4280 amplifier IC questions

    From Jeroni Paul@21:1/5 to All on Thu Jan 27 19:04:38 2022
    I have an HiFi system SABA CT9810 (also sold as THOMSON VTM 2800/2900, CRK 1207, BRANDT MID 3000, TELEFUNKEN POWER PACK 200) where the volume control does not allow low enough volume levels, the first step sounds normal volume, the second is quite loud
    and above too loud for domestic use. It was like this when new, not a fault but a design issue. It uses a pair of LC7533 for digital volume control and a LA4280 power amplifier.

    LA4280 circuit is exactly as depicted in its datasheet. Years ago I lowered its gain inserting 1K8 resistors in series with the capacitor at the feedback pins and now I decided to lower it even more with 4K7 resistors. That works and results in a much
    better volume control range but now I get strange squealings tuning AM radio.

    LA4280 datasheet explains nothing but LA4270 datasheet says to insert these resistors to lower the gain, but recommends to keep it above 30dB. That would be 700 ohm resistors, I have gone much lower gain and it is now prone to oscillate with the AM
    radio. The rest of the functions sound fine (FM, phono, tape, aux) but I wonder if it could be oscillating anyway.

    Of course I would like to find the schematic of this thing to know how it was supposed to work but couldn't find anything. This one could be some prototype unit or something. I don't find any fault or wrong/missing component, the circuits are built and
    work as per datasheet, it's just that seems incorrectly designed.

    The LC7533 is fed with 1Vpeak line-level audio from the selector and it adjusts that from an almost invisible scope trace to the full 1V input. Its output goes straight to the LA4280 and a led vu-meter that displays within reasonable range, this is why I
    think the amplifier input has the intended level. The LA4280 has a gain of 100 that to me is way too high, a gain of 10 or lower would seem more reasonable.

    So some questions. The LA4280 has two unused pins "Phase compensation", could these be used to make it stable at lower gains?
    Any other idea? Maybe a resistor divider at the input? (I would have to add an extra coupling capacitor for that).

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