A/B box 2 in to 1

Phorhas · April 13, 2005, 02:43:01 AM

Ok, probably a really trivial question and I shuld know better, but just to clear things up. If I want to switch between 2 guitars going in to and amp, I just build the stadard design A/B box, with gounded unused input, and put pull down resistors on the inputs (10M - it's for bass)?

LivingDeadPunk · April 13, 2005, 05:01:06 AM

Yep, you got the answer in your question

robbiemcm · April 13, 2005, 08:27:41 AM

Woah... what are these resistors for? I thought you could just connect it all up?

DMinor · April 13, 2005, 09:58:00 AM

Yea, I just built an A/B box but didn't use any resistors in the signal path. Did I miss something?

LivingDeadPunk · April 13, 2005, 10:39:55 AM

Resistors are there to avoid pops when you switch the effect on/off.

Paul Marossy · April 13, 2005, 12:52:30 PM

This is what I built for myself: http://www.diyguitarist.com/PDF_Files/A-B_Box.pdf

mlabbee · April 13, 2005, 01:03:10 PM

You only need the pulldown resistors if you are using active circuitry (like Paul's buffered design) with coupling caps (the input and output caps that prevent DC from going in or out of the circuit through the audio path). The pull down resistors allow coupling caps to discharge to ground - this prevents a charge from building up on them, the discharge of whichis the source of the "pop" you hear when you switch an effect off or on (one wihtout pulldown resistors, that is).

TheBigMan · April 13, 2005, 06:32:28 PM

I just added some AB and ABY diagrams to my layouts gallery. Pulldown resistors aren't necessary in this application.

NaBo · April 13, 2005, 06:58:38 PM

Correct me if i'm wrong, but don't you only need pull-down resistors when signal and power grounds are shared? Paul states this in his "Solo Pro" volume-switcher-majigger... That's the principle i applied when drawing my feedback loop in my gallery...

If you keep the ground connections for the signal and the ground connections for active circuitry completely separated, there's no risk of popping, other than due to mechanical reasons... for most stompbox designs this would be impratical and huger than necessary so power and signal share the same ground path. Unfortunately this leaves the possibility for power to trickle up the signal ground wire. Pull-down resistors are in place to shunt that stray DC in the signal path back down to ground.

... Am i right? :?

Andi · April 14, 2005, 04:19:56 AM

If it's only going to be used as a 2-in 1-out, and never the other way, you don't need to ground the unused input, surely? So then you could use a DPDT instead of a 3PDT?

DMinor · April 14, 2005, 08:24:28 AM

As a noobie, I'm getting confused. This is the A/B box I just finished....

Its a pdf......
www.fulltone.com/PDfFiles/AB_switcher.pdf

Paul Marossy · April 14, 2005, 07:10:02 PM

QuoteCorrect me if i'm wrong, but don't you only need pull-down resistors when signal and power grounds are shared? Paul states this in his "Solo Pro" volume-switcher-majigger...

Not exactly. What I suggested was to keep the power supply ground seperate from the signal ground. That way, it eliminates any popping noises caused by the LED when it switches, because those grounds are not shared. This is a totally seperate issue from noise problems that you can get when using an A/B box with two different amps...

Phorhas · April 19, 2005, 03:47:30 AM

It's for a passive Bass, so It'll be wiser to use BIG pull down resistors, right?

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A/B box 2 in to 1