How to trace signal path?

[jtn191]

Hi, I've got 4 builds under my belt so far...
but in preparation for my next 2 or so, I've decided to use metal film resistors--when they're "in the signal path" so I'll get less noise and use carbon film in other areas.

Is there a quick 'n easy way to find where these metal film resistors would be necessary?

Right now I'm looking at the boards for a Fatpants (simple) and Q-Vibe (more complex) and my stock of mostly carbon film Rs and a few of the metal film variety.

I'm thinking (for a Fatpants) R1, R3, & R2+R4(?) are "in the signal path"

Thanks

[k.rock!]

Just out of curiosity, do you have a reason not to replace all with metal film?

As far as how to tell if a resistor is in the signal path, I can give you an educated guess, or a SWAG (Scientific Wild-Ass Guess)  ...

Looking at the Fatpants, I would think the R1, R3 and R6 are directly in your signal path, since the rest seem to be for creating potentials along the signal. Now, I reached to this SWAG by thinking that the current likes to follow the path with least resistance, so following the signal, R1 is a given. R2 goes to ground but on the same node, there's a connection from C2 to Q1, so I would think the signal wants to go that way (even though there is some current going down into R2 as well)...etc...

Honestly, I would just replace all with metal film, but that's just me (probably cause I haven't given any thought to signal path 'till now haha)


-Kaleb

[jkokura]

The best place to trace the signal path isn't the board, but rather the Schematic. If you pull up the schematic for the fatpant's you'll see how the method below works.

1. Obviously you start with the input.
2. From the input, the first part is R1. It's a 100K resistor inline with your signal. Obviously, there is nowhere for your signal to go except THROUGH R1.
3. Next we hit both C1 and C2. C1 goes to ground, C2 is inline. In this case, signal goes through C2, but C1 leaks some to ground before getting there.
4. R2 and Q1 are up next. R2 is connected to ground, so just like with C1 it's leaking something to ground but most of the signal is headed to Q1.
5. South out of Q1 we have a network of C3, R3 and the Fat pot. The only other connection is between Q1, those 3 parts, and ground. In this case, we can see that no signal is going THROUGH the parts and on, it's only a complicated system for leaking some signal to ground. This produces a filter of sorts, just as C1 and R2 are doing.
6. North out of Q1 we have a T1 and C4. T1 is connected the system providing power to the circuit, so we can imagine that instead of signal traveling through it, it's introducing power to the signal, so it's something ADDING to the signal path, not leaking as we've seen so far. But C4 is inline with our signal, so we can assume it's an important part just like R1, and C2.
7. After C4 we hit both R4 and the level pot. Again, R4 connects to ground so it's leaking something. The pot is inline with the signal, and the output connects to lug 2.
8. We hit the output, we're all done!

So if you were following, the only parts that are inline with out signal path are R1, C2, Q1, C4 and the Level potentiometer. Everything else either connects to ground or to power.

If I were you, I'd just go for using ALL metal film in your pedals. That's what I do. However, if you were to really care which parts are quality, the ones that are inline are important, but in some cases the other parts matter too. For instance, the potentiometer's usually have 20% tolerance, so they can really, really affect the signal path simply because your range may be extended too far or diminished. Sometimes having very accurate parts in your power path is important if you need a precise divided voltage amount (half of supply usually).

Anyway, that's a bit of an idea of how you'd figure things out.

Jacob

[keysandguitars]

Jacob,

Awesome post! Thank you for taking the time to write that out.

After having built a bunch of pedals I've decided to stop and start to go back through them to try and at least understand the basics of what is happening in the circuit. I've read through Brian Wampler's books that were posted in another thread and would recommend them to fellow noobs. He walks through a number of circuits like you just did. I know it's child's play to a lot of you guys, but your explanation really helped further my knowledge, so thanks again.

I'm now going through the online books Brian posted in the extra projects section to see which one might be best to start with. I don't have time to get deep into the science, but I'd like to have some understanding as to what I'm doing as I build so that I can start tweaking things with some anticipated results.

Brandon

[jtn191]

cool, mucho thanks Jacob

Just out of curiosity, do you have a reason not to replace all with metal film?

-Kaleb

Well, I have a bunch of odd value carbons film leftover when I bought 10x everything for my sunking builds. So now I'm building the FP and when I see R2 is 1M, I reach for a carbon comp rather than send in another order...but then the perfectionist/ocd part of me winces haha

@Brandon: yeah man, me too. check out http://www.kitrae.net/music/BIG%20MUFF%20CIRCUIT%20GUIDE.jpg
and http://www.beavisaudio.com/techpages/HIW/hiw1.gif

[TNblueshawk]

Oh man, I'm diggin' this thread. I've used my audio probe on 3 builds and sort of took a stab at the signal path not knowing if I was right. I did fix the builds though.

Ok, questions for a better understanding.

- Using C1 and R2 as an example; so any time you see a component that at least a little of the signal is bled to ground you rule that out as being in the signal path?
- Which leg(s) of the trannie on Q1 might you touch the audio probe to?
- For the Vol pot you would touch lug 3 and 2 to make sure you have signal correct?

Thanks Jacob.

[keysandguitars]

Thanks for those links jtn, very helpful info.