Help with R.G's Panning for Fun with my altered design.

[Se7en_Costanza]

Hello,

I have a small problem that id like to see if i could get fixed.

I'm making a blend control which i can blend 100% dry with 100% wet, no bleed, with a reversed phase output. Im using R.G's Panning for fun, but replaced some values to get the blend ratio how i wanted it, but im getting a crackle sound on the pot on either side of the turn.

On the panning for fun, he uses a 10k pot and 15k resistors which gives it a perfect volume glide from each input, but how i wanted my blend control to work was the first 50% of the pot i didnt want the volume to decrease (not too much anyway) so if the pot is set to the middle, both inputs would be almost at 100% each, but the next half i wanted one of the sides to glide down to 0, and vise versa when turned to the other side for the opposite input.
With the values i added in the schematic attached it does that fine, but when i use louder input signals it crackles on both sides of the pot. I made sure there wasnt DC on the pot, each lug reads 4.3v (referenced to ground).
Is this crackle due to the sudden decline in volume from the resistor configuration or could i add something to completely remove this crackle?

Panning for fun:
http://www.geofex.com/Article_Folders/panner.pdf

My Altered Schematic:


How i'd like the potentiometer to act in a graph form:

[Se7en_Costanza]

BTW:
That graph i drew is if nothing was connected to one of the inputs (just so you can see the curve), if the other input was connected then the graph would be the same but the curve will be the opposite way overlapping the curve in the drawing.

[thermionix]

(I didn't read the Geofex article and have no experience with panning, but...)  You do have DC on the pot.  You've got it coming out of both opamps, dropped a bit by series resistance, and coming in at the wiper at 4.5V.  Just a few mV is enough for a scratchy pot.  I would think that blocking caps and running the wiper to ground would work.

[Se7en_Costanza]

I've tried caps from the outputs of both op amps and to the input of the last op amp (after the resistors), with the wiper to ground and works exactly the same, still the scratchy pots when you turn it to either end.

[Se7en_Costanza]

Also its only scratchy with loud volume input signals. So hi output pickups, basses and line keyboard signals it crackles but a standard guitar its fine.

[marcelomd]

Is it scratchy when you play or only when you move?

[Se7en_Costanza]

Only when you move it, it crackles at either ends of the pot when audio is going through it.

[Se7en_Costanza]

It wont crackle if there is no audio going through it or if the input signal is low enough.

[R.G.]

You might try breaking the wiper of the pan pot loose from 4.5V and inserting a big capacitance between the wiper and ground. You'd make the cap be trivially small compared to that 15K from the output of a previous opamp at the lowest frequency you want. You have 10u's used in the circuit. 15K and 10u is a F = 1/(2*pi*15K*10E-6) = 1.06Hz, so 10uF is an OK place to start.

The split single-power-supply isn't really the same as a bipolar power supply because there is still a significant impedance in the +4.5 bias voltage that gets worse with lowering frequency. Caps to ground make this better for higher frequencies, but it's still a way for coupling remnant signal from the "ground" point back into the + biasing inputs of opamps.

You're using 10uF input caps with 1M bias resistors. We just saw that 10uF and 15K gives a 1Hz rolloff. Just juggling the decimal points, 1uF and 150K also gives a 1Hz rolloff, and so does 0.1uF and 1.5M.  So you could cut your input caps down to 0.1uF and still pass single-digit frequencies. That's probably not related to your current problem, but worth thinking about.

You could test the theory that you're having power supply range problems by temporarily running the thing from two 9'v batteries in series. That would move the power supply goalposts by another 4.5V and let you know whether the power supply liminations are contributing to your crackle.

Opamp voltage range is another item. The near-ubiquitous TL072 has some limitations with bigger signals in a 9V single supply world. Its output can only swing to about 1.5-2V of the power supply, and its input flat-lines when you get the + input too near a power supply.

[Se7en_Costanza]

Hey R.G. couldn't be more grateful you're helping me out,

Firstly sorry for my bad handwriting but that input capacitor is 100nf with the 1M to 4.5v.

I tried disconnecting the wiper and putting a large cap to ground, i tried from values from 100nf to 470uf and still that slight scratch with the pot is still there.

Also i tried the 2 battery trick and it still does the same, as well as a variety of op amps that were in my random op amp draw, i tried, TL072, TLC272, TLC2272, NE5532, OPA2134 and still the same problem.

i have to make a point that the pot isnt scratchy the whole rotation, only when you get to the ends of the pot, and only when a loud input signal is going through it, so if you were to just plug a guitar into this circuit in the schematic you wont hear a scratch, but if you have a boost pedal before or you plug a keyboard or bass into it you'll hear the scratch towards the ends of the pot.

Is this something to do with the resistor divider values or the sudden shunt to virtual ground on the rotation? or is it something that could be fixed.

[Se7en_Costanza]

also if you turn the pot reeeeallly slow it wont make the scratch sound