Question - Potentiometer for LM3886

[aniruddha]

Is there any harm in having the potentiometer after DC blocking? as shown in the attached schematic.
Reason for this configuration is I don't want DC voltage to be applied to Potentiometer also.

Thanks,
Aniruddha

[GloBug]

Sounds like good practice to me. It keeps the pot a little quieter when operating too.

[djoffe]

Aniruddha...

your schematic is a little worrisome and unconventional...Perhaps though, the issue is just that the picture didn't come across too clearly...I couldn't see the 3 terminals of the pot...If indeed you planned to use it as a two terminal device as I think I see, it may not do what you'd like it to.

[aniruddha]

This is how i plan to connect potentiometer

[pacificblue]

It will lead to varying DC offset as you turn the potentiometer, and as a result you will probably get popping and cracking noises, when you change the volume.

If at all, replace R11 with the potentiometer, but it is better to have the potentiometer on the other side of the input cap.

[sofaspud]

Doesn't the blocked DC also need a path to ground on the input (left) side of the blocking cap?

[aniruddha]

Originally Posted by pacificblue

It will lead to varying DC offset as you turn the potentiometer, and as a result you will probably get popping and cracking noises, when you change the volume.

Varying DC offset is caused by variation in the value of resistor which is connecting input pin to GND? is my understanding correct?

Is there any other way to block DC to potentiometer as well as amplifier? (other than using 2 DC blocking cap)

Thanks all for your help

[AndrewT]

Move the pot to the input. Link across the two "pot pads".

Your circuit shows two RF attenuators.
You may find that the combined effect of these removes some of the treble audio information.

[djoffe]

the second cap...is part of the feedback frequency compensation system. Owing to opamp action, it has much less voltage across it than the input voltage. The result is that the input filter is, so far as the audio band is concerned, just a single pole.

Also...I agree with the other posters about DC in the control, quoting from Doug Self, on page 238 of "Small Signal Audio Design"

"Feeding a bias current through a wiper to the next stage tends to create more serious noise because the variations in wiper contact resistance are greater. This tends to get worse as the track surface becomes worn. This practice is often acceptable for FET input opamps like the TL072, but it is definitely not a good idea for bipolar opamps..."

[AndrewT]

Originally Posted by djoffe

the second cap.....Owing to opamp action, it has much less voltage across it than the input voltage. The result is that the input filter is, so far as the audio band is concerned, just a single pole.

No!
the opamp tries to maintain Zero Voltage between the +IN Pin and the -IN Pin when operating inside it's passband (operating bandwidth). Outside the passband, the gain drops like a stone (>=20dB/dec.) and without NFB the opamp can no longer achieve that zero voltage between the input pins.

These filters are RF filters. They are not intended to operate inside the Audio Passband. They are designed to attenuate outside the audio passband, precisely where the zero voltage between input pins does not exist.