Strange Volume Boost on AionFX Luna build

[80k]

So I just finished building an AionFX Luna (based on the 4MS Tremulus Lune). The PCB is available here: https://aionfx.com/project/luna-optical-tremolo/

The schematic is available on page 6 here: https://aionfx.com/app/files/docs/luna_documentation.pdf

It sounds great and all the controls work well. Except the volume control starts with a small boost when all the way down, and then goes into a very high boost when cranked up.

On Page 5 of the documentation, I did the suggestion of using a common BJT (I used a 2N5088) instead of a JFET for the input buffer. That required me to omit R2, add RX1, and move the transistor over to the left.

Other than that, I assembled it as shown and triple-checked each value with a multimeter before soldering it.

Based on the schematic, is it expected to have a small volume boost? Any insight into what might be causing it?

Thanks!

[PRR]

> l starts with a small boost when all the way down, and then goes into a very high boost when cranked up.

The gain depends entirely on the LDR and the LEDs.

LDRs are wide-tolerance devices.

LEDs have got very much more efficient (brighter for the same current) since the Luna was developed.

I think you want less-bright (old) LEDs. Or increase the LED resistors, or the LED-LDR spacing. Finally that VOL pot should be an internal pre-set... tremolo should not have a different volume, it should be bench-set for how you think unity-gain should be.

[antonis]

Could you measure Vactrol resistance for dark and bright state..??

[80k]

Could you measure Vactrol resistance for dark and bright state..??

Unfortunately, I wasn't able to get the speed slow enough to measure the resistance. I set it to square wave and made it as slow as possible, but couldn't get a reading. My multimeter takes awhile to register.

[80k]

> l starts with a small boost when all the way down, and then goes into a very high boost when cranked up.

The gain depends entirely on the LDR and the LEDs.

LDRs are wide-tolerance devices.

LEDs have got very much more efficient (brighter for the same current) since the Luna was developed.

I think you want less-bright (old) LEDs. Or increase the LED resistors, or the LED-LDR spacing. Finally that VOL pot should be an internal pre-set... tremolo should not have a different volume, it should be bench-set for how you think unity-gain should be.

Hmmm, I suppose it is probably the Vactrol I selected. I used the part number (VTL5C3) that the build documentation suggested, but I got it on Amazon instead of Small Bear Electronics, and it's possible it's not to the same spec.

On Page 5 of the documentation, it does state that a low "on" resistance photocoupler would cause the gain ratio the be thrown off balance and that the volume control would be unusable.

I suppose a different photocoupler would be a good place to start? Maybe I should roll my own with a photocoupler and LED wrapped in tape, like the original Tremulus Lune.

Thanks!

[80k]

Looking at the schematic, I wonder if I could simply raise R15 from 1K to a higher value? Maybe try around 3.3K and keep going?

[PRR]

> raise R15

+1

[80k]

> raise R15

+1

Interesting how sensitive this circuit was, with regard to R15. At 4.7K, the pedal made no sound. I then changed it to 2.2K, and it seems just right. With the volume knob at minimum, it's below unity gain, and I get a good perceived unity gain when I turn it to around 9 o'clock and then gets louder after that. I think the photocoupler I got was just slightly different spec than the one that was recommended from SmallBear, but glad I got it working!