post fader amplifier questions

[Analog_Fan]

I ended up getting some ADG419 and will use them in "series" configuration,
do you think I can use it before the fader, like shown here? I'd like to mute also the signal that goes to the pre fader auxes. In other examples I've seen, the switch was always after the fader and before the post fader amplifier. I'm wondering if placed before the fader would be a problem

View attachment 118718

C15 (51K) / C1 (100K)?
Why decouple twice?

You could remove C15 / R14 or move it to the pre fader aux (input).

 

[JohnRoberts]

C15 (51K) / C1 (100K)?
Why decouple twice?

You could remove C15 / R14 or move it to the pre fader aux (input).

You don't want DC across the fader because that causes scratchy noise when fading. Likewise DC at the input of a relay can cause clicks.

The cap feeding the op amp is to block DC Bias current from the bipolar op amp input devices from getting into the slider.

Customers don't like clicks or scratchy pots.

JR

 

[Analog_Fan]

You don't want DC across the fader because that causes scratchy noise when fading. Likewise DC at the input of a relay can cause clicks.

The cap feeding the op amp is to block DC Bias current from the bipolar op amp input devices from getting into the slider.

Customers don't like clicks or scratchy pots.

JR

Oh, yeah that opamp is a transistor based opamp and not a fet based from what i saw in the datasheet some time ago.

Maybe this is better choice: Opa1677 and is used a very famous voltage controlled filter: Mutable Instruments Ripples.
https://www.ti.com/lit/ds/symlink/opa1677.pdf
Also very small, SOT23-5

 

[Dimitree]

With a fet or cmos opamp I won’t need C1, because they have low input bias current?

 

[abbey road d enfer]

With a fet or cmos opamp I won’t need C1, because they have low input bias current?

You should not need the capacitor there.
Actually a FET or CMOS opamp is preferred here, not only because the DC current is very low, but also because the noise current is very low.
One of the problems with BJT opamps is that when the fader moves, the source impedance seen by the opamp varies and it creates noise modulation, which makes fast fader moves a source of noise.
In the last century, I had initialized an ECO for changing post fader opamps from 5532 to TL072, because although the 5532 was quieter statically, it was noisier dynamically.
Today, this is not an issue, with FET opamps having noise voltage performance equal to BJT.

 

[Dimitree]

removing C1, it means that also the bias resistor on the opamp input can/should be removed? and what about the large output capacitor? (C2 on my schematic here #57)

 

[abbey road d enfer]

removing C1, it means that also the bias resistor on the opamp input can/should be removed?

Yes.

and what about the large output capacitor? (C2 on my schematic here #57)

No, because the output has some DC offset, which would generate clicks when switching.