I built the Aion FX Refractor Klon kit. I did a full rundown of my build. It occurs specifically when the volume reaches a certain threshold between the volume and the gain. I can filter the frequency out by lowering the treble pot, which makes sense to me to cut high frequencies. For what it's worth, pedal sounds exactly as expected otherwise. I could simply never turn the knobs up that high and use it as is.
Annotated voltages here: https://imgur.com/a/klon-voltage-annotations-8FMCdgH
Photos of the build: https://imgur.com/a/klon-build-nt4C12P
I've been trying to figure out where it's introduced with an audio probe setup, but because it's introduced somewhere after the buffer from the charge bump all I know is it's present after the first buffer in the opamp. Therefore the squeal is present somewhere in the feedback loop.
Notably, I don't believe this is similar to the high frequency oscillations happen due to clock mismatch in a pedal chain. This occurs with or without bypass true or buffered, and it's not from the power supply because it has the same issue with a 9V battery.
Here's what I've done so far, based on my research:
- replaced the charge pump from a TC1044 to a LT1054 (tried 3 different TC1044 chips)
- cleared off all flux
- made sure pins 1 and 8 of the charge pump had connectivity
- checked reference voltages for the charge pump as well as all 3 reference voltages entering the circuit
- triple checked all power section voltages and component values.
- noted that regardless of charge pump, 16.80 is the doubled Vb reference voltage. Only pins 6 and 7 on the charge pump differ between chips
Audio probing has been tough because after pin 3 of the first op amp stage, the whine is introduced. It's present as well on pin 2 of the feedback loop. So effectively I have only been able to isolate that it occurs after the first opamp stage and that's it.
My charge pump (IC3) looks correct on output voltages except the unused pins 6 and 7
1: 9.29 (same as input voltage)
2: 4.84
3: 0
4: -4.19
5. -8.92
6: 2.53
7: 1.43
8: 9.29
The fact that the whine engages with bypass engages has me fairly stumped. I don't want to bail out and say it's a bad PCB/poorly isolated, but what else could it really be at this point?