Passive A/B Switcher

[DMinor]

Hey all, newbie here so please bear with me. I stumbled upon Fulltone's "schematic" of an A/B switcher but it seems a little incomplete to me.  This will be my first attempt at building so if anyone has made this box I would be happy to hear any clarifications on this design.
I'm glad I found this site & hope to learn a whole bunch.
                                               Cheers, Paul

www.fulltone.com/PDfFiles/AB_switcher.pdf

[Mark Hammer]

Nope, it is as complete as it needs to be (maybe even a litle moreso), and while not exactly a Full-Tone "innovation" is probably one of the more perpetually useful builds, along with a loop selector.  Indeed, I would recommend a loop selector because it can also double as an A/B box.

Think of it like this.  The stompswitch takes its input from the input jack (goes to middle lug or common as it is called).  Step on the switch once, and the common is directed/connected to one of the side lugs.  Step on it again, and it is directed/connected to the other side lug.  If one of those side lugs goes to one output jack, and the other goes to a different output jack, you now have an A/B box.

What is shown in the Fulltone pdf is a step up in complexity, and uses a multi-pole stompswitch to do what it does.  What does it do?  First, it uses one pole to switch an indicator LED in case you want visual confirmation of which output you are activel using at the moment.  Some folks find this essential, some folks don't.  

The second thing the Fulltone example does is it grounds the unused output.  Why is this important or useful?  Well, imagine you have two long cords goin to two separate amps.  Both of those cords are capable of picking up hum and RF if they are just plugged into the amp and nothing else.  However, if you connect the hot and ground of the cord, presenting a load to the amp of what is probably less than 1 ohm, the amp behaves much better, noisewise.  

How necessary it is to ground the unused output would depend on your context.  For example, if your A/B fed two effects chains that BOTH went to a mixer which was followed by a common noise gate or some other noise reducer, then grounding the unused output may or may not be necessary.  If you used an A/B so that you could leave a *series* of effects "on", which went to a separate amp, then it might be a good idea to ground the unused output to keep noise at a minimum.

With a loop selector, there would be 4 jacks: an in-from-guitar, out-to-amp, off-to-pedal/s, and back-from-pedal/s.  Stepping on the switch essentially routes your signal directly to the output jack, or else sticks a bunch of other devices between the input and output jacks.  Here, you aren't quite as worried about grounding any inputs because when the loop is deselected, it is completely out of the picture.  My lunch period is quickly disappearing so I'll simply note that loop selectors are posted around so I'll leave my description at that.

[DMinor]

Now thats a reply! Thanks Mark. I guess I should have been more clear.
How could I run this box off a 9V? And I searched Mouser for the resistors but I came across a variety of choices for each specific value.
 Other questions are also of basic stompbox know-how so maybe I should quit being lazy and do a search of this forum.

[Mark Hammer]

You don't need power of any sort unless you want an indicator LED.  As noted earlier, there are plenty of situations where none is needed.  (I never put indicator LEDs on my fuzzes - if I can't tell a fuzz is "on" then I figure it's time to pack it in.)

If you DO use an LED, there are a few ways you can do it, none of them really requiring more than a few milliamps from a cheap battery.  The basic element is that the LED has to get enough current to light it up in a realistically visible manner, but needs to have that current limited so that it doesn't fry.  For most LEDs regularly available (and they vary in widely brightness), a resistor of somewhere between 1k and 18k will provide enough current and current limiting from/of a 9v battery (or external 9v source) to be visible.  The resistor doesn't need to be anything more than a garden variety 5% carbon film resistor.  You can probably cannibalize one from just about any trashed musical appliance sitting on the curb on garbage pickup day.

Your options consist of:
1) Using one LED and turning it on or off to indicate status
2) Using two LEDs and turning one or the other on, depending on status
3) Using a single bi-color LED and switching the color shown.  Bi-color LEDs come in a few flavours, but generally with a cloudy white dome.  One type has two leads and the colour depends on which way you orient the current flow (green in one direction, red in the other).  Another type has three leads, with current flow in the same direction for each colour.

If you use two separate different-colored LEDs (e.g., green and red, or blue and red), try to make sure they have the same luminance.  LEDs are rated in terms of millicandles (mcd).  "Garden variety" would be around 200mcd or less, and "superbright" ones would be in the 1000-1500mcd-or-greater range.  If you end up with two LEDs of different brightness, you can always match them by using a 10k trimpot instead of a fixed resistor, and adjusting the current-limiting resistance (more-resistance in series = less current flow = less brightness) for each LED.  Certainly a bi-color LED will come matched for brightness but I realize they aren't always available everywhere.

Finally, the standard trick used for stompboxes is to use a stereo input jack to turn the battery on and off.  You can find out more about this in the FAQs here.  There is a pdf document posted on my site (http:/hammer.ampage.org) about how to make your own custom extra-strength battery snaps if it should happen you don't knw where to get some or if you don't feel like springing for the pack of 25 they sell or some such nonsense.

[gtrmac]

I have two of those A/B boxes in my pedalboard. One is at the front of the chain so that I can route the guitar to a tuner. This way the sound is muted while I tune up. the other, at the end of the chain switches betwee my two amps. I did them just like the diagram on th fulltone site with two LEDs and a 3PDT switch. I also used Cliff jacks which islote the grounds from the case to prevent a ground loop.

They're very handy. I was even going to make a third one so that I can keep two guirs plugged in and switch them with my foot.

[DMinor]

Well, I got my parts from Small Bear yesterday and last night I drilled the box for the jacks/ switch (just a dry run). Easy enough, things are clean, symmetrical and honestly its a hoot just looking at this alone. Hey, this is my first time.  Now, the wiring. The Fulltone PDF above "says" to add resistors to the LEDs. Do I just solder one leg to another? Also, since I'd rather just use a 9V to power the LEDs, where should each wire from the batt. terminal go? Finally, the "stop piece" for the LED holders designate " + or - for each hole. Is the round side of the LED itself + or-. Does it matter? Elementary questions for you guys but a whole new world for me. Thanks, Paul

[robotboy]

Well, I got my parts from Small Bear yesterday and last night I drilled the box for the jacks/ switch (just a dry run). Easy enough, things are clean, symmetrical and honestly its a hoot just looking at this alone. Hey, this is my first time.  Now, the wiring. The Fulltone PDF above "says" to add resistors to the LEDs. Do I just solder one leg to another? Also, since I'd rather just use a 9V to power the LEDs, where should each wire from the batt. terminal go? Finally, the "stop piece" for the LED holders designate " + or - for each hole. Is the round side of the LED itself + or-. Does it matter? Elementary questions for you guys but a whole new world for me. Thanks, Paul

I'm a noob myself, but I think I can help you with these questions :wink: LED's are diodes which means that the current only flows through them in one direction and is blocked going the opposite direction. You want to connect the resistor in series to the anode of the LED, which is going to be the round or non-striped end. Anode=positive Cathode=negative. Resistors don't have a polarity (positive or negative), so you can orient them either way with no problem. Also, the value of the resistor will determine the brightness of the LED as well as the battery life, so you can adjust the value as you see fit. For 9V power, just use a battery snap instead of the DC jack and orient positive and negative the same way. Hope that helps, and good luck with your first build!

Read these too:

http://www.answers.com/topic/light-emitting-diode

http://www.kpsec.freeuk.com/components/led.htm

[DMinor]

Hey robotboy, thanks. Geez, if you're a noob then I must be .....an idiot. Thanks again.

[Andi]

I can never remember which is the flat side, but it's the short leg that's -ve.

[DMinor]

Wired it all up last night and, voila! Success. I finally popped my cherry. A beginner build but still quite gratifying. The Fulltone diagram left out wiring for the 9V but was simple enough to figure out. Also, I found the trickiest part was soldering the little "jumpers" on the switch. Is there a trick to this?   Cheers, Paul