Fader Unity Bypass

[a soBer Newt]

Hello All looking for some recommendations on bypassing a fader. I am working on a console project and want a way to bypass the fader so that if you are using automation in the DAW and do not want the faders to affect the summing you can bypass the fader. Here is what I came up with but wanted to know if there was an gotchas with this? Falstad Simulation

Edit, I did just notice that if the fader was all the way down it would short the audio to ground.

Edit Edit... Changed the switching arrangement.

 

[ruffrecords]

The two circuits are different. The second one will work, the other will not.

Cheers

Ian

 

[Brian Roth]

In many (most?) cases there is gain in the fader buffer stage. 10 dB is common. Reason? The unity gain point of the fader is physically below the wide open position of the fader. This gives you the ability to "bump" the level if needed.

Bri

 

[kags]

In many (most?) cases there is gain in the fader buffer stage. 10 dB is common. Reason? The unity gain point of the fader is physically below the wide open position of the fader. This gives you the ability to "bump" the level if needed.

Bri

Exactly, so a fader bypass must include a pad that takes into account the “gain in hand” of the fader.

 

[Brian Roth]

Exactly, so a fader bypass must include a pad that takes into account the “gain in hand” of the fader.

I haven't "sharpened my pencil yet" <g> but it seems the second diagram in the original post could also adjust the gain around the buffer to handle the two situations. But I'm just "spit balling" at this moment.

One BIG issue in that second schemo just hit me. There needs to be a resistor to ground of the non inverting input of the buffer. Otherwise, the buffer will go "ape sh!t" as the switch is activated, momentarily leaving the + input floating.

Bri

 

[a soBer Newt]

So probably the easiest would be to have a 10K trim pot and a relay that looks at the fader or the trimmer? This way I can align the unity position exactly on each channel.

 

[john12ax7]

Low tolerance resistors are accurate enough, not sure a trim pot would even be necessary. You can just switch between the pot wiper and appropriate resistive divider to feed the next stage.

 

[JohnRoberts]

I am not making an actual circuit suggestion but I think I have seen an approach to do this with only one switch pole that simply lifts the ground leg from the bottom of the fader, and the bottom of the gain resistor from - input to ground. This turns it into unity gain, more or less.

I am NOT recommending this topology.

JR

 

[RSRecords]

could just swap fader for a calibrated trim. Kinda like the mci jh110 record/repro uncalibrated front panel controls. Switch to bypass fader and use unity cal’d pot.

 

[FETlife]

Wouldn't this be easier?