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-   -   LM3876 MUTE as volume control (http://www.diyaudio.com/forums/chip-amps/118832-lm3876-mute-volume-control.html)

Tcheko 6th March 2008 10:13 AM

LM3876 MUTE as volume control
 
Hello,

I was wondering it the mute pin could be of any use for controling volume. The documentation shows a nice ramp from 0dB/0.5mA to -100dB/0.01mA. The ramp is not really linear but a tweaked log pot may do the trick.

Any insightful though about this?

Czk.

AndrewT 6th March 2008 10:27 AM

gross distortion if the amp operates on the ramp?

cabbagerat 6th March 2008 11:20 AM

Quote:

Originally posted by AndrewT
gross distortion if the amp operates on the ramp?
AndrewT is right. Look at the equivalent schematic on page 7 of the National dataheet. Opening the mute transistor slightly isn't going to have a linear effect on the volume of the output signal - you are going to see big (and asymmetrical) distortion.

Tcheko 6th March 2008 11:42 AM

Volume control
 
Could we change the gain of the opamp to adjust volume ? I was told it would change the bandwidth.

Would it be possible to replace20k feedback resistor by a potentiometer to adjust gain ?

Dxvideo 6th March 2008 11:47 AM

No...
Because the op-amp becomes unstable under 10 V/V gain. Just a question;
Why do you need a volume control except "normal" ways? :)

peranders 6th March 2008 03:09 PM

Re: LM3876 MUTE as volume control
 
Quote:

Originally posted by Tcheko
Hello,

I was wondering it the mute pin could be of any use for controling volume. The documentation shows a nice ramp from 0dB/0.5mA to -100dB/0.01mA. The ramp is not really linear but a tweaked log pot may do the trick.

Any insightful though about this?

Czk.

The mute function is intended as mute and my gut feeling says that this is not a good idea. :no:

Tcheko 7th March 2008 06:28 AM

Quote:

Originally posted by Dxvideo
No...
Because the op-amp becomes unstable under 10 V/V gain. Just a question;
Why do you need a volume control except "normal" ways? :)

Ordinary ways are not fun. ;)

I started electronic hobby one month ago and I am still in the learning curve. I am used to look at odd ways to do things. That's why. :)

Ordinary ways seems to me a bit odd too. Dividing the voltage with a stepped attenuator or potentiometer means for low level a bad SNR.

Would it be more efficient for adjusting volume to change the gain ?

LM3876 is not designed to do such thing. Is it possible to design such volume control? I guess it is possible but there is certainly some tradeoff.

Tcheko 7th March 2008 06:33 AM

Re: Re: LM3876 MUTE as volume control
 
Quote:

Originally posted by peranders

The mute function is intended as mute and my gut feeling says that this is not a good idea. :no:

I have looked deeper in the schematic. As noted by cabbagerat and AndrewT, the mute will not do the job. Sadly.

peranders 7th March 2008 08:39 AM

Quote:

Originally posted by Tcheko
Would it be more efficient for adjusting volume to change the gain ?
A good pot will do and let the gain be fixed, my recomendation.

Tcheko 7th March 2008 09:02 AM

Quote:

Originally posted by peranders

A good pot will do and let the gain be fixed, my recomendation.

For the LM3876, I agree. But for a design from scratch with discrete component, will it be possible ? Anyone have heard about such way of adjusting volume ?


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