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SABA CT9810 volume too loud

J

Jeroni Paul

Jan 1, 1970
0
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.

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.

Thank you
 
M

Miguel Gimenez

Jan 1, 1970
0
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.
 
M

Miguel Gimenez

Jan 1, 1970
0
Hola, Jeroni, no me he dado cuenta de que eras tú hasta que el corrector
ortográfico me ha sugerido cambiar "Jeroni" por "Neroniana", y ya le
estaba dando a Enviar. Es una alegría volver a leerte, después de la
pseudo-defunción de es.ciencia.electronica.
 
J

Jeroni Paul

Jan 1, 1970
0
Hola, Jeroni, no me he dado cuenta de que eras tú hasta que el corrector
ortográfico me ha sugerido cambiar "Jeroni" por "Neroniana", y ya le
estaba dando a Enviar. Es una alegría volver a leerte, después de la
pseudo-defunción de es.ciencia.electronica.

Ostras tú por aquí, que pequeño es el mundo ;)
Vaya, a mi el corrector no me sugiere nada con mi nombre, jeje.
Siempre me acuerdo de ti cuando veo el reloj con el nixie.

Pues esta minicadena ha funcionado así siempre y parece que ya es por
diseño, pero la verdad me tiene extrañado. El control de volumen
seguramente sea logarítmico, en el datasheet del LC7533 no dibuja
ninguna curva pero dice Attenuation curve: Pseudo-curve A. No medí que
el mínimo fuera 1/16 u otra cosa, solo lo supuse cuando ví en el
osciloscopio que la onda casi no se veía.

También es curioso que esta marca y modelo no parecen estar muy
documentados.

Saludos
 
J

Jeroni Paul

Jan 1, 1970
0
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.

Well, I did not check if it was indeed logarithmic or if the minimum
volume was actually 1/16Vpp but the audio wave was almost invisible on
the scope, so it was at least under 0,1Vpp. In the LC7533 datasheet
says: Attenuation curve: Pseudo-curve A.
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.

Yes, I calculated since it runs on 32V a 1Vpp signal would be
amplified to 30Vpp at highest volume thus getting the maximum
undistorted swing at the output. I wonder why the designers didn't
make this simple calculation.

Thanks
 
J

Jeroni Paul

Jan 1, 1970
0
Yes, I calculated since it runs on 32V a 1Vpp signal would be
amplified to 30Vpp at highest volume thus getting the maximum
undistorted swing at the output. I wonder why the designers didn't
make this simple calculation.

Actually that would be for a symmetric supply, but in this case it has
single supply so the maximum swing is half (+/- 16V). I used a pair of
1K8 resistors placed in series with capacitors at feedback pins 2 and
7 to set amplifier gain to 15. Volume regulation is now much better.
 
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