Macheen external trim pot mod discussion

[Om_Audio]

Hey guys- I asked Madbean about the Macheen external trimpot mod in a PM- I was unsure of the correct/recommended value. I think the info is worth starting a thread for others to read (and does not seem to be an inappropriate PM-->public posting!):

I asked:
I have searched the forums and the build sheet regarding the mod to put trim on external. I cannot find anything that states what the value of the external pot should be. I used a 2k.

Anyway- I guess I could assume a 5k would be the right value- no idea why I ordered a 1k and 2k to try- maybe they didn't have 5k at the time- I seem to remember for some reason.

Would just like to clarify if using 5k is best or if 2k or other would work fine too.

Madbean replied:
"It can actually be any of those values, but 5k is stated as the value of the trimmer on the schem, so the same value applies as external.

Take another look at the schematic: the trimmer is set up as a variable resistor. When it's turned all the way up, the Source of the BS170 is connected directly to ground. This means that you get the most effect when the trimmer is essentially "removed" (all the way up) from the path to ground. That's why the actual value doesn't matter too much. If you used a 100k, you wouldn't notice any change when turning it until about the last 10% of the rotation. But, between 5k and 2k, you just don't get a whole lot of difference."

Link to build sheet for ref:
http://www.madbeanpedals.com/projects/Macheen/docs/Macheen.pdf

[Om_Audio]

Is there more to the trim than in this photo? (red lines area) In other words if I study this area (tranny/diode/trim/loop?) will I be able to get how it works essentially or are there more things on the board that are directly involved?

[jkokura]

Nope, that's it, and really it's just the trim pot you need to be interested in.

Essentially the way it works is that it's a variable resistor. What's happening is that an amount of DC is on the trace that connects to pin 3 of the trim pot. When you turn the trimpot one way, you vary the resistance between the BS170/9.1V zener and ground. Full resistance gives you 5K between there and ground, and no resistance is the opposite. Set it in the middle and you've got 2.5K resistance. This will affect the way your signal on the other side of the Mosfet behaves because you'll affect the amount of DC leaking to ground across that variable resistor. Are you with me here?

So, when you make that external, you'll be able to play with it a little bit. However, you may find yourself with a very small amount of useable range on that pot. What I would do would look like this:

Setup the PCB with the 5K trim pot on board. Find the sweet spot you like, Now measure the Resistance between pin 3 and pin 2 of the trim pot when it's set at the sweet spot. Could be anything between .001 ohms and 5K ohms, but we'll say for arguments sake that it's ~2.5K. Now, slowly turn it one way to find the max usable setting that direction. It could be as high as 5K for example. Mark it down, and we'll say for in this case it's 3.5K. Next, find your other limit by turning the opposite direction. Find the limit of it's useable sound, and mark that down. We're going to argue that it's 1.5K in this case.

So the sweet spot is 2.5K, and your range is 2K from upper to lower, starting at 1.5K.

What I would do is then remove the trimpot and set up a B2K pot on the enclosure. I woud run a 1.5K resistor from where the trimmer's pin 3 would normally connect on the PCB to lug 3 of your pot. Now your base will always be 1.5K, so you even at 0 it's useable. Now connect lug 2 and 3 of your pot to the other two spots on the PCB for the trimmer. You can also just connect them together and run one wire to where the trimmer pin 1 would go, up to you. Now you have just your useable range on your pot control, 1.5K to 3.5K, and the sweet spot will be in the middle at about 2.5K resistance.

Jacob

[DutchMF]

Jacob, I'm a bit suprised, but I totally understand what you just said! This works the same for al trimmers wired like variable resistors, right? Let me guess, you work as a teacher? 

Paul

[jkokura]

Great Paul,

It works for almost all variable resistor situations, but not all of them. Some times pots are used as variable resistors, and other times they get used in different ways. If it's variable resistance, yes this method should work.

Sort of. I don't want to get all weird on people here, because spirituality is very personal and also very much a 'hot topic' for people, but my day job is that I'm a Pastor and my specialty is teaching young people.

Jacob

[DutchMF]

Hey Jacob,

I think we're going a bit off topic here, but I keep meeting people here that I wouldn't expect being here, which says more about me than about any of you. Just goes to show what a cool place this forum is, and how pedal-building, and music in general, brings people together. Thanks for taking the time to teach us!

Paul

[bigmufffuzzwizz]

That is a good suggestion Jacob. That would set your pot limits exactly where you like them.
I feel like he's asking something different, and I'm curious about it too. What would be the difference between using a 5K, 2K or 1K here? Increasing to 10k would push the range to the second half of the pots turn right? So then decreasing to 2k would spread the range out, but what part of it? Would it be 0-2K or 3-5K?
Something tells me it really depends on the circuit at hand and what your trying to use it for.

[Om_Audio]

Thanks Jacob!
So I understand the testing/2nd half of your post more than the first. I have a conceptual dilemma:
-when I look at the circuit I cant help but wonder about the electron's movement- people talk of "flow" but I wonder if it is more like it "appears" =all over= and is sort of channeled and contained and controlled by the components. I guess it is drawn towards ground- but the idea of flow to me infers gravity and electrons seem weightless. I just had a thought- that I could imagine the signal being pulled and sucked to all the ground points- like magnets- vacuum holes- not flowing but being pulled through- and if there were no ground the electrons would just populate the circuit and not do anything- like birds on a wire- or static electricity for that matter- it does not feel like anything or hurt until you ground out on a door knob etc.

So you have all these vacuum holes (ground points) and you can adjust the aperture of the holes with resistors or variable resistors/pots in front/on top of the holes- which retards or lets free the "suction".
I still don't quite get the tranny above and the diode symbol is annoying bc I just want to feel like it is pointing in direction current is allowed to pass through it. I will get used to it- just think of it like an < symbol which is smallest value moving to larger value.

I could ramble on- it seems yes, i can just focus on the trim pot- which with your help i know understand as a valve restricting flow ,or suction of current being drawn through the zener diode, but I still wonder how the tranny comes into play because it is there too and the 3 items in question are not simply in series.

I am also drawn to the trace after the pot from before ground which loops around to pole 2 on the pot, and the transistor above the pot, and then the items after those. My curiosity and study make me appreciate there is a quite interesting and complex design

Thx again-
I think I will def try the method you suggest at some point in the near future! The 2k pot I have there now has a dramatic change in the last small bit of travel as does the gain pot. Not sure if the gain pot is worth experimenting with but worth considering.
Clifford