What's causing the "pop". Mixer input mutes.

[Buffalo Tom]

Hi,

Im having trouble with switch pop noice when I active channels. Its a big mixer with 20 stereo inputs and 12 mono.. The schematic only shows the four first inputs on the left side but you get the idea. I wanted to leave out all the 20K resistors when they are not in use to minimize the noice from them.. (there is a lot of them with 32 inputs..) therefore I placed the switch after them. The pop noice is only present first time I activate the channel.. If I mute the channel then active it again the switch pop is gone... Same goes for all channels the pop is only there the first time I switch.

Any ideas how the make the "pop" go away?

The mixer is used with keyboards/synthesizers.

[antonis]

You can see that both legs of each input ca[acitor are "floating" when channels are off..

[Buffalo Tom]

You can see that both legs of each input ca[acitor are "floating" when channels are off..

Hi antonis!

Yes I posted this over at diyaudio forum and got the suggestions 200K or 1M resistor to VB after the input cap...

https://www.diyaudio.com/community/threads/whats-causing-the-pop-mixer-input-mutes.384281/

Will try tomorrow 

[antonis]

Make pull down resistors 560k to shake hands both with PRR and Rayma..

[Buffalo Tom]

Make pull down resistors 560k to shake hands both with PRR and Rayma..

So the pull down resistor will not have any effect on the input impedance? I will still have 20K input Z regardless the value of the  pull down resistor?

[antonis]

So the pull down resistor will not have any effect on the input impedance? I will still have 20K input Z regardless the value of the  pull down resistor?

Unfortunately, No..!!

Pull down resistor is effectively set in parallel with 20k resistor, despite they have different apparent terminations, 'cause GND (for pull down resistor) and Virtual Ground (op-amp inverting input for 20k resistor) should be considered identical from signal point of view..



The reason PRR suggested you 200k pull down resistor is a compromise between the limit of 1/10th (rule of thumb) lowering equivalent input impedance and better RC time constant for C1 discharge..

P.S.
From another point of view, you can consider pull down resistor forming a voltage divider with INPUT output impedance, with same results as above..
(signal income rather than signal leak amounts..)

[ElectricDruid]

What Antonis says is true, but he's making it sound worse than it is: the value of a small resistor in parallel with a large resistor is approximately the value of the small resistor. (Run a few numbers through a calculator online - this is easy to confirm)

This is where the "x10" rule he's talking about comes from. If it's "x10" or more, you can ignore it. So if you're willing to accept 20K input impedance, anything over 200K for the pulldown will be fine, basically. 1M doesn't really change anything in your situation. 470K would work too, 330K, whatever. Since the input impedance is low (20K input resistor) then you have a lot of options.

HTH

[antonis]

What Antonis says is true, but he's making it sound worse than it is

Thank you very much Sir for your kind fussing..

[pacealot]

If the input impedence were truly a life-or-death, 20K-or-fight! issue, you could raise the input resistor to 22K and put 220K to virtual ground, and voila! you get 20K. But as the pros have already noted, it shouldn't make any functional difference....