Fatpants 17v Tap and SPDT C3 Questions (Two In One)

[Roctopus]

Hello again,

I'm working on another "Two In One" project. It's going to be a Fatpants and Quasar (RC) in one enclosure. Brian mentions on the Fatpants that "There is an extra pad on the board that you can use as a ~17v tap. This is helpful if you are building the FatPants in the same enclosure as another effect and want to run the second
effect at ~17v." Could someone please explain to me what this means, how this works, and how it's wired?

I'm most interested in knowing if this would be recomended to partner the FP with the Quasar using the 17v Tap?

Brian also mentions" You can sub higher values for C3 if you want a little more low-end out of the effect. A 47n or 68n is suggested. Or, you could use a switch to add another cap in parallel to C3 (untested)." I also saw a FP mod for a SPDT center off switch at C3. Does anyone know who this would be wired? http://www.madbeanpedals.com/forum/index.php?topic=686.0

Thanks!

[Fastocker]

Hey Roctopus, here's a crude pic of how I wired C3 for the 'presence' switch in my Fatpants build.  I called it a presence switch because that's what I'm hearing with different values of C3 --- I wasn't really hearing more bottom end as I increased C3 but actually more high end.  I have a real EP-3 Echoplex tape echo unit here to compare to and, honestly, eliminating C3 altogether sounds the closest to the real thing as the EP-3 does kill some high end.  So I wired mine for 22n as a 'high' setting, 10n as a 'low' setting and no cap as the 'off' setting.  The 10n is a nice compromise . . . less sizzle than the 22n but a bit more bite and presence than no cap at all.

This was all done on a version 1 board --- not sure if the newer V2 boards would be the same.  Hope this helps!

[stecykmi]

Hello again,

I'm working on another "Two In One" project. It's going to be a Fatpants and Quasar (RC) in one enclosure. Brian mentions on the Fatpants that "There is an extra pad on the board that you can use as a ~17v tap. This is helpful if you are building the FatPants in the same enclosure as another effect and want to run the second
effect at ~17v." Could someone please explain to me what this means, how this works, and how it's wired?

I'm most interested in knowing if this would be recomended to partner the FP with the Quasar using the 17v Tap?

basically you can use the voltage converter on the fatpants board to power another circuit. on the second PCB, connect the 18v tap on the fatpants to the +9v input. just keep in mind you'll have to adjust the current limiting resistor (ie make it larger).

as for whether it's worth doing or not, i can't say. theoretically, it should be easy to switch between the two supply voltages since all you have to do is switch +9v input.

[Roctopus]

Hey Roctopus, here's a crude pic of how I wired C3 for the 'presence' switch in my Fatpants build.  I called it a presence switch because that's what I'm hearing with different values of C3 --- I wasn't really hearing more bottom end as I increased C3 but actually more high end.  I have a real EP-3 Echoplex tape echo unit here to compare to and, honestly, eliminating C3 altogether sounds the closest to the real thing as the EP-3 does kill some high end.  So I wired mine for 22n as a 'high' setting, 10n as a 'low' setting and no cap as the 'off' setting.  The 10n is a nice compromise . . . less sizzle than the 22n but a bit more bite and presence than no cap at all.

This was all done on a version 1 board --- not sure if the newer V2 boards would be the same.  Hope this helps!

Thank you so much! This was extremely helpful! I ordered another 22n and 10n box cap and look forward to adding this mod!

[Roctopus]

I appreciate your help so much. I think I'm just going to wire the 9v power from the DC in parallel like I did with my last "two in one" since I'm not sure how the 17v tap will affect the other pedal and I'm also not sure what value of the limiting resistor should be.

[maysink]

FWIW, I've never bothered with special diode CLRs for pedals that run above 9v. I usually pop a 10K for clear LEDs (20K for clear blue or green LEDs) and call it good.

If you *DO* use the ~17v tap off the fabb'd FatPants board to power a second pedal just be sure to use caps rated for the higher current. As mentioned before, you power the second pedal with a wire from the ~17v tap off the FatPants to the second board's 9v pad instead of running a wire from the DC jack (which it sounds like you've done before).

[bigmufffuzzwizz]

I appreciate your help so much. I think I'm just going to wire the 9v power from the DC in parallel like I did with my last "two in one" since I'm not sure how the 17v tap will affect the other pedal and I'm also not sure what value of the limiting resistor should be.

To calculate the CLR you would use Ohm's law. V=IxE. Your looking for the current limiting resistor so I(current) is gonna be isolated. I=V/E is your working formula, now just plug in the values. If your using a typical setup it would look something like 9 volts w/ a 4.7k CLR.

So I=9V/4.7k which equals 1.91 mA. If you were to increase the voltage..17V/4.7K=3.62 mA. So you can see by increasing the voltage, current increased too. Now to counter act this rise you would increase the CLR. Since your doubling the voltage, try doubling the CLR first. Since 9.4K isn't so common I would say 8.2K-10K would work fine.
17V/8.2K=2.07 mA now your current is close enough to the original value!

Thanks Fastocker for posting your mod. I for some reason have a hard time understanding this easy concept so I must learn it through repetition. So for the middle option with the switch off, its basically eliminating the cap from the signal flow?
Well if your doubling the voltage

[stecykmi]

Actually to calculate the current limiting resistor, you have to account for the voltage drop across the LED. That's usually about 2.4V or so (it depends highly on the type, 2.4v is typical for red coloured lense). If you're not sure about the voltage drop (usually it's symbolized by Vf - for forward voltage), look up the datasheet on google. They're usually the same for any manufacturer as long as you get the right type.

Take the supply voltage and subtract that from the LED voltage:

9 - 2.4 = 6.6v

Then choose a current, we can use 2mA because it's a nice number, but you can look this up on the data sheet. They typically have a graph that shows brightness vs current.

Then use Ohm's law to figure out the resistor:

V = I * R

6.6 v = 2mA * R
R = 6.6/2m
R = 3.3Kohms

The wikipedia article is pretty good if you forget how this works: http://en.wikipedia.org/wiki/LED_circuit

[Roctopus]

wow...you guys are awesome! thanks for all the info

[bigmufffuzzwizz]

Right I always forget about the forward voltage drop. Thats what I get for falling asleep in class. I didn't realize the data sheets showed that information for LED's.

[Myramyd]

Just want to chime in and say the "Presence" mod on the Fatpants is an absolute must IMHO. Especially if you use different amp or pedal setups. It makes things more flexible.

I did the 3 options as Fastocker mentions and it really works well.

J