How important is the 4.5v decoupling cap

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themaninthesuitcase
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Post by themaninthesuitcase »

I’m building a pedal from a schematic, but it was missing the power section so I did my own.

I have diode polarity protection, a 47uF, a 1uF (suggested in one of Brian Wamplers books) for the 9v supply.

I also needed a 4.5v feed so added a 10k based divider called it good and ordered my boards. I dislike vero and PCBs are cheap now, though a little slow.

After ordering I was looking at other opamp circuits and noticed they all had a 47uF decoupling cap to ground on the 4.5v line. :slap:

How important is this? And how will I be able to tell if there is a problem?

If needed I can graft on a cap after the fact, I’ve identified pads I can attach leads too and a space I can hot glue a cap. Saves reordering boards that are other wise fine.

Thanks.

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FiveseveN
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Post by FiveseveN »

themaninthesuitcase wrote: 18 Oct 2020, 19:34 How important is this?
Crucial if it needs to function as a proper, low impedance supply node. But depending on the circuit, you might not even notice its absence. For all we know it could be feeding a voltage follower. So which pedal and which schematic is it?
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themaninthesuitcase
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Post by themaninthesuitcase »

The JHS 3 series overdrive.

https://imgur.com/a/hQo4ayk

I suppose this is my lack of proper electronics knowledge showing. But looks like a feed for biasing the opamps, a feed into the drive circuit and also a few other drops I admit to not understanding properly. This is my first opamp build.

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deltafred
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Post by deltafred »

It's as important as power supply decoupling because it is part of the power supply. It is false economy to leave any decoupling off (and shows a lack of basic understanding of electronics).

As FiveseveN states, it might work without it, but it might not or it might seem ok now then give trouble later. For the sake of a 10uF capacitor it isn't worth the risk.
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FiveseveN
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Post by FiveseveN »

themaninthesuitcase wrote: 18 Oct 2020, 20:47 looks like a feed for biasing the opamps
You got it. If it were only used for biasing, through those big 470K resistors, another 10 or 5K might not be an issue. But for things like the volume control to work properly, you need lower impedance, which the cap provides.
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themaninthesuitcase
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Post by themaninthesuitcase »

FiveseveN wrote: 19 Oct 2020, 08:15
themaninthesuitcase wrote: 18 Oct 2020, 20:47 looks like a feed for biasing the opamps
You got it. If it were only used for biasing, through those big 470K resistors, another 10 or 5K might not be an issue. But for things like the volume control to work properly, you need lower impedance, which the cap provides.
I will add the cap in off the bat then. That said I might build one with out and see what the effect of it missing is just to see/hear it.

Thanks.

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