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Old 6th March 2008, 10:13 AM   #1
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Default 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.
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Old 6th March 2008, 10:27 AM   #2
AndrewT is offline AndrewT  Scotland
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gross distortion if the amp operates on the ramp?
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Old 6th March 2008, 11:20 AM   #3
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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.
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Old 6th March 2008, 11:42 AM   #4
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Default 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 ?
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Old 6th March 2008, 11:47 AM   #5
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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?
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Old 6th March 2008, 03:09 PM   #6
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Default 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.
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Old 7th March 2008, 06:28 AM   #7
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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.
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Old 7th March 2008, 06:33 AM   #8
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Default 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.
I have looked deeper in the schematic. As noted by cabbagerat and AndrewT, the mute will not do the job. Sadly.
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Old 7th March 2008, 08:39 AM   #9
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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.
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Old 7th March 2008, 09:02 AM   #10
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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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