DIY Stompboxes > Building your own stompbox

Boss FA-1 Circuit Analysis

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redbagy:
Hi, with reference to this schematic (https://aionfx.com/app/files/docs/prism_documentation.pdf) - can anyone let me know the purpose of C4 please?

FSFX:

--- Quote from: redbagy on February 06, 2023, 03:26:20 PM ---Hi, with reference to this schematic (https://aionfx.com/app/files/docs/prism_documentation.pdf) - can anyone let me know the purpose of C4 please?

--- End quote ---
It is not really needed - then also theoretically C5 isn't really needed either as both op amps are biased to the same reference voltage.

PRR:
Both these caps reduce "scratch" from the pots. Omit them if you wish.

FSFX:

--- Quote from: PRR on February 06, 2023, 05:12:04 PM ---Both these caps reduce "scratch" from the pots. Omit them if you wish.

--- End quote ---
That is not likely to be the case. As long as the offset voltage of the op amps is very small, there should be no DC across the pots to create a 'scratch'. The feedback pot even has the wiper connected to one end which is good practice when pots are used as variable resistors. The tone pot should not be affected by any 'scratch' unless the track is worn out and the wiper is dirty.

Rob Strand:

--- Quote ---That is not likely to be the case.
--- End quote ---
It's highly likely the intent is to prevent scratch - whatever the cause.   Some designs throw everything they have at a problem other draw the line a little below.   The downside of lots of AC coupling caps is more caps to fail.

What's a little obscure is taking the output from C4 instead of the opamp, since C5 is present.

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