Aion Prism - FA-1 FET Amplifier Question ( Fixed)

[AllenM]

I just completed my build and was testing it before boxing it up noticed the volume control works in reverse. Trying to figure out what part of the build could cause that. The pot is PCB mount so I know it is correct.

Any ideas on what I should be looking at? I thought maybe the transistor as I had to substitute it, but the one I am using is only reversed orientation of the one specd.





Here is a link to the project page

https://aionelectronics.com/project/prism-boss-fa-1-fet-amp-pcb/

Just looking for som insight on what part or parts could cause the volume pot to act in reverse.

Thanks

AllenM

[thesmokingman]

which side of the board do you have the pot wired to? (solder side or component side)

[AllenM]

Solder side along with the other pots, this why I am wondering what other part of the pedal could cause this action to happen.

AllenM

[EBK]

I'm betting that you have the pot on the correct side of the board, but 180° rotated from its intended orientation.

[AllenM]

added a pic of the pot installation.. they should be good to go, hard to mess them up since they can only really go in one place. There are even little outlines on the PCB showing the pot outlines.

I have also added a picture of the component side, this was before I installed the ICs and Transistor.

Maybe my description of the problem is incorrect, I have volume counter clockwise and as I turn the pot clockwise the volume fades. Maybe my issue is the volume is not being amplified? So possibly an issue with the ICs or Transistor?

AllenM

[EBK]

Weird.

[AllenM]

I update my last post. Tonight I will look into all my solder joints and also double check my ICs. I am now thinking the circuit is not amplifying the signal, I have used the FA-1 before and if normally has a lot of boost. The volume I have counter clockwise is not very loud and then fades as I turn clockwise.

AllenM

[EBK]

Maybe C4 isn't working, in which case your feedback path is broken and the volume knob would act as a series attenuator instead?

[Boba7]

Such a weird issue.
C4, that's smart, it would make sense!

[reddesert]

Check the values and connections of R5, R6, and C4.

In the first op-amp stage, the DC gain is set by the ratio of R6 to R5, as usual for a inverting op-amp. However, there is a parallel feedback path through C4 and the volume pot. C4 blocks DC, so this adjusts the gain for AC (audio) signals. As you turn the volume down, the resistance of the volume pot is lower, and if everything is wired correctly, that's in parallel with R6, so it lowers the gain of the stage.

If C4 were fully open circuit, the volume pot would not be connected to the feedback loop and would just be in series attenuating your signal as EBK said. However, I think you might actually get zero signal then, because signal would be trying to get from the - input of the opamp through the volume pot to C5 and the rest of the circuit. But since the + input is wired to Vref, the - input would also be at a constant voltage, and have no signal.  (As has been discussed elsewhere, you hear no signal if you audio probe the - input of an inverting op-amp, since it's "virtual ground.")

Testing continuity of the components around that feedback loop and audio-probing from Q1 through the op-amp and to C5, the connection to the next stage, will probably reveal what is going on.

[AllenM]

So it turns out one of the ICs was bad. When I swapped them I lost the EQ and the volume started working. Removed them and installed the optional IC3 slot with a TL072 and everything is working!

Thanks to everyone one who chimed in!

AllenM