Author Topic: Setback / FX90 volume drop  (Read 603 times)

Aentons

  • Electron Doctor
  • *****
  • Posts: 1090
  • but..but.. why?
    • View Profile
Setback / FX90 volume drop
« on: May 23, 2021, 11:38:55 AM »
Im finishing up a Setback build and yes the thru volume drop is a bit annoying. Is it something that was solved for in later revisions?

I used trimpots for R4 (200k) and R48 (100k) and even maxed out they don't seem to help. Any Ideas?

madbean

  • Administrator
  • Electron Doctor
  • *****
  • Posts: 10002
  • Brian
    • View Profile
    • madbeanpedals
Re: Setback / FX90 volume drop
« Reply #1 on: May 23, 2021, 12:16:04 PM »
Aside from increasing that 18k resistor to 47k as suggested, anything further and you will probably need to modify the output section a bit. One thing you could try - jumper R9 and use a 100pF cap for C4. Then use a 68k resistor for R18. If that's too much output reduce R18 until you get the volume you want.

Aentons

  • Electron Doctor
  • *****
  • Posts: 1090
  • but..but.. why?
    • View Profile
Re: Setback / FX90 volume drop
« Reply #2 on: May 23, 2021, 04:21:03 PM »
Thanks for the tips!

I tried your suggestions and used a 100k trimpot for R18 and it does not seem to do much if anything at any position.

The R9 and C4 changes made everything really bright and noisey(hiss). Turning R48 back down to around 18k clears the noise but anything above that is pushing it and the volume drop persists. The brightness stays too.

I'll fiddle around with it some more and see if I can maybe find some values that balance it out.

madbean

  • Administrator
  • Electron Doctor
  • *****
  • Posts: 10002
  • Brian
    • View Profile
    • madbeanpedals
Re: Setback / FX90 volume drop
« Reply #3 on: May 23, 2021, 05:51:00 PM »
Thanks for the tips!

I tried your suggestions and used a 100k trimpot for R18 and it does not seem to do much if anything at any position.

The R9 and C4 changes made everything really bright and noisey(hiss). Turning R48 back down to around 18k clears the noise but anything above that is pushing it and the volume drop persists. The brightness stays too.

I'll fiddle around with it some more and see if I can maybe find some values that balance it out.

Right, well you have two resistances in parallel there so just adding lots of resistance to just one of them only gets you so far. Changing R18 to 47k while leaving the 3k3/33n in parallel was what I found to be the best compromise without really getting into altering the circuit.

The trick is to first set the output volume where you want then choose the appropriate value cap that reduces the hiss to a minimum without rolling off too much high end. There may be some perfect combination of values that does this, it's just a matter of finding them. Or, maybe leave that as is and reduce R2 to 390k. A little more at the front end without lowering the input impedance of IC1A too much.

Aentons

  • Electron Doctor
  • *****
  • Posts: 1090
  • but..but.. why?
    • View Profile
Re: Setback / FX90 volume drop
« Reply #4 on: June 14, 2021, 03:49:17 PM »
I got a chance to open up my buddy's FX90 to see if it is a later version and the component placement doesn't seem to match up with the revA schematic & layout diagram. He modded it for true bypass but that was all. Anybody know what revision this might be?

My goal in lookin at it is to see if there changes for how they may have solved the volume drop because this one doesnt have it. It actually has a slight volume boost.

 I will try to start tracing down around the TL062.



Also, I did notice a cap on the bottom that is on pin2 of the 1M trim pot.


And here is the front if it helps to identify:


Here is the revA diagram im referencing:
« Last Edit: June 14, 2021, 08:03:18 PM by Aentons »

tcpoint

  • Solder Ninja
  • ****
  • Posts: 326
    • View Profile
Re: Setback / FX90 volume drop
« Reply #5 on: June 16, 2021, 11:58:57 AM »
Here's a link to the Rev H schematic.  There's a lot of differences.  https://www.madbeanpedals.com/forum/index.php?topic=29170.msg308109#msg308109

Aentons

  • Electron Doctor
  • *****
  • Posts: 1090
  • but..but.. why?
    • View Profile
Re: Setback / FX90 volume drop
« Reply #6 on: June 16, 2021, 01:00:07 PM »
Here's a link to the Rev H schematic.  There's a lot of differences.  https://www.madbeanpedals.com/forum/index.php?topic=29170.msg308109#msg308109
Awesome! Thanks for that.

This is much closer to the one I have.

It looks like the layout diagram is rev H, but the schematic is rev I. (If you look at the IN jack label, the uppercase I matches what is over in the bottom right rev box)
« Last Edit: June 16, 2021, 01:21:55 PM by Aentons »

Aentons

  • Electron Doctor
  • *****
  • Posts: 1090
  • but..but.. why?
    • View Profile
Re: Setback / FX90 volume drop
« Reply #7 on: June 16, 2021, 05:19:35 PM »
Here is the modern art masterpiece I managed to scrawl out. It's not perfect but it confirms the the pedal I have does indeed match the input and output sections on the rev I schematic.

The differences from revA are almost all in the feedback loop for the IO opamp
« Last Edit: June 16, 2021, 05:24:02 PM by Aentons »