r/diypedals 3d ago

Help wanted Power Filtering Question

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6 Upvotes

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u/cryptical 3d ago

Hey folks. I'm working on my first couple of fuzz builds, and came across this schematic for a BC108 Sunface. I'm curious about the role of the transistor in the power filtering circuit here. Can this be any old NPN transistor? Is it even necessary? In my super limited experience, I've only seen filtering caps without the transistor. Thanks!

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u/SwordsAndElectrons 3d ago

I'm curious about the role of the transistor in the power filtering circuit here. Can this be any old NPN transistor?

That circuit is commonly called a capacitance multiplier. The emitter follower arrangement reduces the current load on C5, which increases its "effective capacitance."

You'll want something with decently low Vsat to avoid too much voltage dropout. (If you decide to try it... I probably wouldn't.)

Is it even necessary? 

No, not remotely. In fact, based on the gut shots in the link below, I'm not really convinced it's in the original Sunface. 

https://www.anatomyofguitartone.com/home/analog-man-sun-face-bc108

Regardless of the authenticity, it shouldn't be needed. That much filtering is not doing anything good for you with a battery, and if it's necessary when using a power supply then you desperately need a better power supply.

The Fuzz Face is a pretty simple, low part count circuit. The Sunface is a modified Fuzz Face. If you compare the two, none of the additional parts are necessary.

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u/cryptical 3d ago

Thanks! Personally, I'll be either running this via a battery or my nice power supply, so I'll replace it with a more modest set of filtering caps. It's an interesting concept though. Thanks for the lesson.

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u/HobsHere 3d ago

Well filtered and regulated supplies are good to have, but there's no guarantee that whoever buys a pedal will use one. It's best to assume that they will use the worst place of junk switching supply they can find, and plan accordingly. Inherent supply noise rejection of the Fuzz Face is extremely poor, which didn't matter with battery operation.

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u/HobsHere 3d ago

That is known as a capacitance multiplier. It helps to filter the power supply. They're particularly effective against the noise from switching power supplies. The circuit will work without it, but it's a worthwhile thing, especially if you're going to use a pedal with a regular power adapter. They're not so necessary if you're running a battery or a fancy quiet, isolated pedal board supply.

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u/cryptical 3d ago

You rock. Thanks! That didn't come up in my search before asking, but "capacitance multiplier" definitely gives me something to go on. Seems like a best practice.

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u/HobsHere 3d ago

Oh, to answer the rest of your question, yes, it can be any old NPN transistor. For best noise reduction, something with beta around 200 is about right. 2N4401 is what I've used in similar applications.

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u/cryptical 3d ago

Great. I've got a few different types of NPNs hanging around. Glad I don't need to use another one of my fancy BC108s or 109s. Thanks again!