first ever project

[kanakanaka77]

Hi there,
Let me preface this with saying that I have never tried to create anything involving electronics.. I think it's fair to say that all of you were like me at one point so I won't feel too dumb in asking this. I really want to build a footswitch for my vox 15r pathfinder. I figure from what I've read that this should be a fairly simple task. Now seeing as I've never dealt with electronics does anyone have any beginners info. I realize that a  general knowledge of voltage, amps, transistors, resistors, and the like are going to be necessary in preparing this project and I've begun by getting quite a few books at the library to acquaint myself with the physics of it all. Aside from this I have no knowledge. Can anyone start pointing me in the right direction?

[punkin]

Welcome aboard!

I'd say have a long look here http://www.diystompboxes.com/smfforum/index.php?board=11.0 . Then, I'd have to ask, what sort of effect are you wanting?

Another link/post that comes to mind http://www.diystompboxes.com/smfforum/index.php?topic=77569.0 . The trotsky and sparkle boost are nice and have a low parts count.

HAVE FUN with it!

[kanakanaka77]

Thanks for the reply!! I appreciate the links and will clearly be pouring over them as they contain far too much information to read over and respond to. What I want to do is just build a footswitch for my vox 15r pathfinder that will allow me to switch between clean and boost channels and switch between clean and tremolo controls.

[kanakanaka77]

after doing a little research I've come to the conclusion that I just need a simple spst switch if I want to build a footswitch pedal for a vox pathfinder 15r. I imagine I would need 2 of them, one for the clean/boost channel and one for the tremolo channel? Am I on the right path? I'm sure this is an easy enough project for a newbie, just looking for some insight.

[JKowalski]

Here's the only schematic I could find

http://www.vintagedesign.halmstad.net/schematics/Pathfinder15.pdf

It looks like the foot switch is just two mechanical switches. You need a stereo jack, and two switches (SPST, DPST, DPDT, SPDT, 3PDT, they all will work fine)

Each of the switches connects one of the stereo tips to ground., turning on or off the effect. One turns on/off a Jfet switch, it looks like, in the feedback path of an op-amp stage, changing the gain.... The other grounds the output of a oscillator, turning on or off the oscillations.

EDIT: You posted first XD

Yes, you are correct! Two SPST switches will do nicely. Each one connects/disconnects the stereo tips to ground.  

If you feel at all iffy about the whole thing (not that you should) you can test it by plugging in a stereo jack and using a piece of wire to short the tips to ground

[kanakanaka77]

thanks for the response. I unfortunately am in the process of learning how to read a schematic, so I fear I have too much to learn before I can undertake this project. 

I shall attempt to isolate my misunderstanding and hope for the proper corrections:

I understand how, after reading up on trs jacks, a footswitch might change the clean /boost channels on the vox pathfinder, but I dont get the tremolo function? how does the wiring in the footswitch box tell the tremolo to turn on/off?

I realize how novice of a question this is, yet it's the lack of application that is getting in the way of me understanding the definitions.
thanks again

[petemoore]

  If it's a TRS footswitch jack, there can be no more than 2 functions triggered to be switched in the amp.
  IIUC this is 2 'short to ground' type switches, the sleeve being ground.
  When the ring connects to the sleeve [Gnd.] or not controls the 1rst function.
  When the tip connects to the Sleeve it controls the other function.
  Controlling the tremolo [if it's not one of these two functions] would take a bit more RnD to figure out.

[JKowalski]

Taking a look at the schematic, I posted earlier, and my post:  

Each of the switches connects one of the stereo tips to ground, turning on or off the effect. One turns on/off a Jfet switch, it looks like, in the feedback path of an op-amp stage, changing the gain.... The other grounds the output of a oscillator, turning on or off the oscillations. There you have your clean/boost effect and your tremolo effect.

thanks for the response. I unfortunately am in the process of learning how to read a schematic, so I fear I have too much to learn before I can undertake this project.  

I shall attempt to isolate my misunderstanding and hope for the proper corrections:

I understand how, after reading up on trs jacks, a footswitch might change the clean /boost channels on the vox pathfinder, but I dont get the tremolo function? how does the wiring in the footswitch box tell the tremolo to turn on/off?

I realize how novice of a question this is, yet it's the lack of application that is getting in the way of me understanding the definitions.
thanks again

Alright, I shall attempt to explain it to you, I hope you can make some sense of this:


Alright, you see the stereo jack marked FOOT SW in the  middle left? Thats our footswitch input. You see the sleeve (leftmost vertical line) goes to ground, which is the symbol that looks like a cloud with rain. (Please don't think I am trying to be condescending or anything, you probably know what I said already, but I just want to make sure we are both on the same page starting out)




The other two wires go to two different circuits, that have transistors in them.

First the clean/boost:

The top footswitch circuit outputs 5 volts on it's collector when the base of the transistor is not connected to anything, and outputs -5 volts when the base is grounded (this is what the footswitch does, it grounds or floats the base of the transistor)

Now, you see the output of the circuit, which is either 5 volts or -5 volts depending on the position of the footswitch, goes to that Jfet transistor. The Jfet is set up as a switch. The 5 volts and -5 volts are the controls to this switch. So the JFET either is conducting or not conducting.


Now, the op amp. An op amp inverting gain stage looks like this



The gain of the signal is determined by the ratio between the two resistors:



Now, when the JFET is not conducting, you ignore the effects of the 22k resistor, because it is blocked by the JFET. For reasons of simplicity inthis explaination, I am going to ignore all the capacitors (They have a resistance based on frequency) So when the JFET is off, the only resistors conducting are the 150k and 470k in series. Add them together, and that is 620K, and since it is in the feedback loop of the op amp, we call that 620K Rf (in the case of the formula above)

Rin to the circuit is 22k (I am ignoring the resistance introduced by the circuit preceding it). So the gain is:

-620k/22k = -28

So the output is 28 times as big as the input, and flipped around (negative, thats why they call it an inverting op amp stage)

Now when the JFET is conducting, the 22k resistor we ignored earlier is now conudcting. It is in parallel with the 620k resistance we used earlier, so use the formulas for finding resistances in parallel, and you get ~21k = Rf

Try the gain calculation again:

-21k/22k = -0.95

So you almost have unity gain (almost the same as the input signal)

So you see, when the JFET is off, you get a high gain signal, or a boost. When the JFET is off, you get almost a unity gain signal, much lower. Clean/boost.




Now the tremolo,
(which is mainly what you were asking about....)

That whole circuit with the two op amps in it with SPEED and DEPTH controls is an oscillator, it provides a low frequency oscillation. The LFO signal controls a LED/resistor combo (when LED is lit, resistance goes down) which (by the changing resistance) controls the volume of the signal somewhere else: tremolo).


Now, before the LFO signal gets to the LED/LDR (marked as opto-module in the schematic), there is a transistor whose collector is connected to the LFO output and emitter to ground. Depending on the poisition of the footswitch, this transistor is either conducting or not. If it IS conducting, then the LFO signal is connected to ground, and cannot reach the LED/LDR because it is all being dissipated to ground before it gets there. If the transistor is NOT conducting, however, then the signal has a higher resistance to ground, is not dissipated, and reaches the LED just fine,letting it wobble away and change the volume of the signal.

The OP amp after the transistor part is just a unity-gain amplifier, if buffers the signal (makes it so you can output more power without distorting the input signal)

[kanakanaka77]

wow!!! That was really cool of you to do all that!! I don't understand it all in it's entirety right now but it's given me loads of other terms (jfet, lfo,etc) that I really need to know. So it's given me quite a bit to research, and for that nugget of info I truly thank you. I'm going to pick up craig anderton's electonic projects for musicians today as the other books I have deal so much in theory that I get lost in possible application. Any other ideas for books?

[JKowalski]

Um, I would recommend as I always do The Art of Electronics. The way it starts out is friendly to beginners, it makes a point of lightly going over the complex math so you get an idea of it but don't necessarily have to pour over it to understand what they are saying, and after you get through the basics it gives you a very comprehensive guide.

The internet is a wonderful resource too.

Here's a site I like, it doesn't explain theory or terms that much but when you learn about a circuit you can find it here and VISUALLY see how it works! This guy has a ton of circuit examples and they cover the whole range of basic circuit building blocks:

http://falstad.com/circuit/e-index.html

And you know the inverting op amp amplifier i was talking about? He has that right here:

http://falstad.com/circuit/e-amp-invert.html

Wikipedia is pretty useful, when you have a term or common circuit you want to learn more about.

When I first started out, I went through about 4 different electronics books, all aimed at beginners. I found that every book has it's own ways of explaining things, it's own analogies, etc., and its alot easier when you find one that just "clicks". TAOE did it for me.




And, well, theres the forum! Anything that you are confused about and can't find with basic resources (or if you dont know how to phrase your idea or problem) people will gladly help you out here.

[kanakanaka77]

thanks again... The site's you posted have been so useful in providing a visual guide to my confusion thanks again