Jeroni Paul escribió:
Hello

I've a stereo receiver / cassette / turntable SABA model CT-9810. It
always had a problem with its digital volume control not allowing low
volume levels. It has 16 levels of volume and the first level, the
lowest one sounds like normal volume, second is quite high and higher
levels too loud for any use.

The problem was "fixed" many years ago by adding a 56 ohm resistor in
series with each loudspeaker however I am not really sure this is the
right solution. Does anyone have its manual or service manual, does it
mention something about that? Any similar experience?

I've gone inside and took some measures. Output amplifier is LA4280
which according to datasheet has an inbuilt gain of 100 and is
supplied with 32V. Implemented circuit is identical to datasheet.
Sound comes from a pair of LC7533 digital volume controls which are
feed with a measured 1Vpp audio signal from the equalizer and they
correctly adjust from 1/16Vpp to 1Vpp.

Are you sure? For a volume pot the scale should be logarithmic. I
mounted a linear pot in my first amplifier, and the effect was the same
you observed: the volume rised quickly, and only the first quarter of
the pot was practical.

The output amplifier gain of 100 seems excessive to me, a 1Vpp signal
at full volume becomes 100Vpp, which is obviously badly clipped at
32V.
1/16Vpp (min volume) is 6Vpp output to the speakers.

For now I reduced LA4280 gain to 30 by adding 680 ohm resistors in
series with the Feedback pins. But I still wonder if it rings a bell
to someone.

This solution looks far better than the first; the 56 Ohm resistor
reduced the volume but didn't solve the clipping, and you were wasting a
lot of energy.

Thank you


--
Regards
Miguel Giménez