Liabilities of this pan circuit?

[bjosephs]

I'm working on a small mixer and feeling inclined to be minimalist on active devices/gain stages. This pan circuit seems common, referenced from the CAPI website, but It's meant to be driven by a low source impedance. What is the tradeoff of having, say, 250 ohms worst case? If the answer is crosstalk can someone give me an idea of how to quantify it?

 

[gyraf]

I don't see any possible source of crosstalk here, if it's feeding a standard virtual-ground mixbus?

The circuit's load, as presented to the stage that's supposed to drive it would be approximately

10K//10K//2K2+3K3//2K2+3K3//47K//47K - ca 1.65 K Ohm - this probably why they go with the 1K fader RV7/8, which is a really low resistance for here - so you'll need to make sure that your preceding stage can handle this while still sounding good.

/Jakob E.

 

[ruffrecords]

If the mix bus is virtual earth then there is no need to worry about crosstalk. The pan circuit itself looks a little odd to me. Usually the 3K3 resistors are from the wipers to the top of the pots rather than to ground unless the 10K pot has some strange taper or I have missed something.

Cheers

Ian

 

[gyraf]

Come to think of it, the 47K mix resistors kinda voids the low noise resistance of this whole circuit.

 

[bjosephs]

If the mix bus is virtual earth then there is no need to worry about crosstalk. The pan circuit itself looks a little odd to me. Usually the 3K3 resistors are from the wipers to the top of the pots rather than to ground unless the 10K pot has some strange taper or I have missed something.

Cheers

Ian

Different pan law maybe? For now I’m just copying this: https://www.capi-gear.com/catalog/images/gallery/pots/BI/Passive-pan-pot-network.pdf

 

[bjosephs]

Come to think of it, the 47K mix resistors kinda voids the low noise resistance of this whole circuit.

I agree, they are high. I linked the circuit I’m copying above. I guess they only need to be high enough to not load the pan circuit too badly and could come down to 22k. Maybe lower. It’s not a lot of channels so I can roll the dice on a little extra noise.

 

[MidnightArrakis]

I'm working on a small mixer and feeling inclined to be minimalist on active devices/gain stages. This pan circuit seems common, referenced from the CAPI website, but It's meant to be driven by a low source impedance. What is the tradeoff of having, say, 250 ohms worst case? If the answer is crosstalk can someone give me an idea of how to quantify it?

Is this what you meant and/or have "implied" as to the Pins 4 & 7 connections of your op-amps?

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I was just wondering, that's all.....

/

 

[bjosephs]

Is this what you meant and/or have "implied" as to the Pins 4 & 7 connections of your op-amps?

If you are referring to the power rail connection/labels I think these two should be equivalent. On my version I’m using a net label at the device with power rail symbol elsewhere on the page. I’m doing that because (and forgive me because this I’m my first time trying this) I intend to use hierarchical sheets for the channel schematic. I’m still working out how to set the labels up but as drawn the label does create a net when I go to PCB layout.

 

[MidnightArrakis]

If you are referring to the power rail connection/labels I think these two should be equivalent. On my version I’m using a net label at the device with power rail symbol elsewhere on the page. I’m doing that because (and forgive me because this I’m my first time trying this) I intend to use hierarchical sheets for the channel schematic. I’m still working out how to set the labels up but as drawn the label does create a net when I go to PCB layout.

[I’m still working out how to set the labels up] -- In assuming that you are using KiCAD, I believe the "Labels" in KiCAD are "Global" in their nature.

[as drawn the label does create a net when I go to PCB layout] -- While that may be.....visually, when casually reading your schematic, it is not readily apparent that the power-pins of your opamp are connected to anything. This becomes even more confusing should anyone be reviewing a schematic that has all manner of multiple voltages going to all sorts of IC's (i.e., 3.3V, 5V, +5V/-5V, 9V, +15V/-15V, etc.)!!! Using "Power Symbols" just makes those items more readily readable so someone else reading the schematic can more easily follow along with what's happening.

BUT!!!.....it's your schematic. Do what want, how you want.

/

 

[bjosephs]

[I’m still working out how to set the labels up] -- In assuming that you are using KiCAD, I believe the "Labels" in KiCAD are "Global" in their nature.

[as drawn the label does create a net when I go to PCB layout] -- While that may be.....visually, when casually reading your schematic, it is not readily apparent that the power-pins of your opamp are connected to anything. This becomes even more confusing should anyone be reviewing a schematic that has all manner of multiple voltages going to all sorts of IC's (i.e., 3.3V, 5V, +5V/-5V, 9V, +15V/-15V, etc.)!!! Using "Power Symbols" just makes those items more readily readable so someone else reading the schematic can more easily follow along with what's happening.

BUT!!!.....it's your schematic. Do what want, how you want.

/

I am using Kicad. These labels are global, and there are hierarchical labels and global labels to pick from, I just haven’t figured out have to apply them yet. So these are placeholders.

You are right that it isn’t very clear to a reviewer but they are going to look like whatever Kicad makes them look like when I am using the labels that function the way they need to function. It’s entirely possible that global labels are appropriate for these power rails any way and I can just use the traditional power net symbols. Suffice to say the feedback is noted.

 

[ruffrecords]

Different pan law maybe? For now I’m just copying this: https://www.capi-gear.com/catalog/images/gallery/pots/BI/Passive-pan-pot-network.pdf

My bad, I missed the additional 2K2 from wiper to the top of the pot.

Cheers

Ian