the lo-fi pedal on geofex

[hobageeba]

hello.  i'm really new to this whole stompbox building thing but i found a schematic on geofex.com for a lo-fi/telephone/noise pedal.  it seems like a really cool idea but i was wondering if any of you have successfully built this pedal.  i also don't totally understand the schematics.  it seems like the bypass switch isn't wired to the input or output jacks.....is that wierd??  is that even a bypass switch??(it's in the noise source section).  and, it seems like there's only one pot ( in the panner section).  if any of you can help me out that'd be awesome.  thanks!!

[R.G.]

i was wondering if any of you have successfully built this pedal

I have built both the "telephone filter" and noise source separately, and both work well. The rest is a mixer to mix the two. I have not built the whole mess as a single pedal.

it seems like the bypass switch isn't wired to the input or output jacks.....is that wierd?? is that even a bypass switch??(it's in the noise source section).

There isn't any bypass switch shown, only the effect circuit. The circuit needs a "wrapper" with bypassing, indicator, etc. See the article on wrappers for some more info. Most effects circuits, like this one, can be extracted out separate from the wrapper that does the in/out/bypass/indicator/battery, etc.

it seems like there's only one pot ( in the panner section).

There is indeed only one pot. That pot pans the signal from pure noise to pure telephone signal, with a mixture in between.

[brett]

For those interested in Lo Fi, there's also Tim Escobedo's LofoMofo, which gives a very thin, crappy noise like a cheap transistor radio.  To get a better (medium fidelity?) sound, I changed the 500pF input cap in the LofoMofo to 1500pF (1nF+470pF).  That lets just a bit more bass through, so that it sounds like a very cheap solid state amp with a 4inch speaker.

[mattv]

Regarding the Lo-Fi pedal at Geofex:

It's a very cool effect indeed!

One thing Tim Escobedo helped me figure out was that the wiper of the panner pot needs to go to Vb, not ground.

Also, I liked the noise so much that I amplified it quite a bit more than shown in the schematic.

There are threads about this effect in the archives I'm sure.

[hobageeba]

hey thanks everyone.  but, like i said, i'm very new to this and i've been trying to read up on everything so i don't have to bug all you knowledgable people    but i still have more questions.  i figured out that the things i thought were bypass switches on the schematics were actually just transistors....ooops.  one of the transistors is wired to ground, a resistor, and then "NC"??  what's that?  also mattv said that the "wiper of the panner pot needs to go to Vb, not ground".  ok, what's the wiper of the panner, and what's Vb?   where can i find Tim Escobedo's lofomofo?  finally, i looked around for an article of a "wrapper," but i can't find one.  i figure that maybe it's a term for everything but the cirucit board like the dc jack, bypass switch, jacks, etc.  is there an article out there that i can read that'll tell me how to add all that stuff onto this pedal?  thanks for your patience and i'm sorry for the tedious questions.

[brett]

Hi.  It seems to me that the Lo-Fi pedal at geofex.com will take a bit of extra reading, etc for you to make.  The LoFoMoFo might be a fair bit easier.  Definately have a look at it before deciding on the more complex option - it's quite good:
http://www.geocities.com/tpe123/folkurban/fuzz/snippets.html

Also, I said above that I changed the 500pF input cap to 1500pF.  As you can see on the schematic, it's a 220pF cap (and I changed it to 1220pF by putting a 1nF cap in parallel with it).

Concerning your other questions:
Vb is the "DC bias voltage", usually about 1/2 way between the battery+ (usually 9V for pedals) and earth (or 0V).  So it's usually about 4.5V, produced from a voltage divider consisting of two resistors of the same value (e.g. 10k ohms).  The two resistors connect, in series, from the 9V to earth, and Vb is obtained from the connection between the resistors.
The "wiper" is the moving contact in the pot, which is connected to the middle lug (often called No 2).

Hope this helps
Cheers
PS Don't worry about asking lots of questions.  None of us were born with electronics knowledge.

[hobageeba]

awesome!  thanks a lot brett.  that looks a lot more simple.  i'll try it out and i think that i might be able to figure out how to throw in a bypass switch.  thanks again everyone.

[Travis]

NC generally means no contact.  The collector on that transistor is not attached to anything.

RG's wrapper article is here:
http://www.geofex.com/Article_Folders/wrapper/wrapper.htm

[hobageeba]

thanks travis for the wrapper article.  i have another question i thought of.  what kind of caps should i get for the lofomofo?  film, ceramic, aluminum electrolytic? also, what kind of watt rating should i use for the caps and resistors?  thanks again.

[hobageeba]

crap, i forgot to ask this too:  in the wrapper article, it says that you need to put a cap inbetween the jack and the board to use as a hum filter.  what kind of cap do i need?  the article also mentioned puting a diode in series or in shunt with a battery to prevent reverse voltage.  do i still need to do this if i'm not using a battery?  and if so, then what kind of diode?

[Travis]

I would imagine that the lo-fi/telephone pedal would eat through batteries like crazy.  An outside power source is definitely a good idea.

The lofo mofo, on the other hand, is probably pretty miserly.  You could certainly use a battery with this.  

This comes into play because it is very difficult to insert a 9V battery into the incorrect slots in the clip.  Batteries also ten dot produce very smooth power, wiht not much chance for powe supply hum.  Reverse voltage protection/hum filter caps really only become necessary when you use a wall wart.  

I tend to add these things as needed. If you use power supplies with different polarities (if you own both negative AND positive tip adapters), drop a protective diode in the power supply.

The same with the hum reducing cap.  Use a large value cap (100uF on up, electrolytics will be the only thing small enough to use) between your positive and negative rails to help kill any hum problems.



edit:  To answer your question about ratings:  1/4 watt resistors will be plenty.  In these 9 volt circuits, make sure that you caps are rated at at least 12-16 volts.  Anything higher will work the same, but will also be more expensive.  Types of caps are always a big issue with audiophiles of any type; for a lo-fi circuit, just get the cheapest components that meet your specs.  Here, they might make some difference in the sound, but not too much.