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SABA CT9810 volume too loud

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

Reply to
Jeroni Paul
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Jeroni Paul escribió:

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.

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.

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Regards
Miguel Giménez
Reply to
Miguel Gimenez

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.

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Saludos
Miguel Giménez
Reply to
Miguel Gimenez

r

Ostras t=FA por aqu=ED, que peque=F1o 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 as=ED siempre y parece que ya es por dise=F1o, pero la verdad me tiene extra=F1ado. El control de volumen seguramente sea logar=EDtmico, en el datasheet del LC7533 no dibuja ninguna curva pero dice Attenuation curve: Pseudo-curve A. No med=ED que el m=EDnimo fuera 1/16 u otra cosa, solo lo supuse cuando v=ED en el osciloscopio que la onda casi no se ve=EDa.

Tambi=E9n es curioso que esta marca y modelo no parecen estar muy documentados.

Saludos

Reply to
Jeroni Paul

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.

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

Reply to
Jeroni Paul

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.
Reply to
Jeroni Paul

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