Homemade ABY Switchbox

[DeFrag]

I've modified TonePad's A/B Selector by mixing the channels by way of a 2nd 3PDT switch, adding a couple level pots & a dedicated tuner output. I'd like to add a few more features such as pulldown resistors, TB loops, a stereo patchboard for combining or passing multiple signals from stereo stompboxes etc. Here is my circuit http://gamefreaks.net/defrag/personal/graphics/baby.gif. I have a few questions regarding the basic design as listed below:

So I guess audio-taper log pots should be used as opposed to linear pots for level control of the channels. I've seen other circuits use 100K, 500K, & even 1M pots. How do I choose the value of my pots? I've read about muddiness the higher up we use & it just seems for a passive box 100K should do nicely but I don't exactly know why. Should I incorporate .001uF caps to help pass the high frequencies at lower output settings? (http://www.diyguitarist.com/PDF_Files/SoloPro.pdf)

Is my circuit good for both low-level & line-level use? Any considerations here? I'm not sure what to ask but there must be advantages as well as disadvantages in supporting both.

In all other circuits but one (http://www.fulltone.com/PDfFiles/AB_switcher.pdf) that I've seen, the signal-ground & LED circuit ground is shared. Then I run across a reference to an RC solution to minimize the "pop" (http://www.muzique.com/lab/led.htm) in higher gain circuits when the LEDs are switched. Is there a way I can isolate my existing circuit grounds & keep the same functionality. While my circuit isn't necessarily designed for high gain applications, I'd like the pedal to be as versatile as possible & wouldn't mind incorporating the RC solution to support higher gain inputs (but not necessarily).

I've read that #22 wire is the standard gauge to use. I'd like to fully shield all the runs & isolate it from the box itself but am a bit confused. I think to fully isolate the wiring, I need plastic jacks. But then I read that to properly ground everything & have the box itself provide shielding from interference, I need to ground the circuit to the box. Can you clarify this for me please?

I'd like to have current-limiting pulldown resistors on the inputs like the big-boys' pedals to maintain high impedance on the input(s) & low impedance on the output(s). But it looks like I'd need a DPDT switch to choose between 1M on the single input & 10K on the two outputs... or have the 1M on the two inputs & the 10K on the single output, depending on how I use the splitter functionality. Is there a better way or being mainly a passive box, not worth the extra switch(es) necessary?

Apparently I didn't understand shielding very well 20 years ago where only one end of a given wire is shielded. So does this imply that if I were to use shielded wires throughout the project, I simple ground out one end of each & never tie them together. I suppose its starting to sound a bit impractical to shield everything eh?

Perhaps a gated IC or a multi-transistor flip-flop of sorts would be a better solution to having a 2nd 3PDT switch. I don't know for sure as I'm juust getting back into electronics & have been reading everything I can get my hands on. It would be great to take all the best ideas & stuff them into one powerhouse do-all stomper.

TIA

[Austin73]

Hey Defraq,

Not sure if this is any help but I've made plenty of AB boxes and Loop select boxes and to be honest haven't had many problems just using normal wire (non shielded)
Also On my pedals I put 10M resistors across the inputs and outputs as das musique schematics too prevent popping which seems to work very nicely. Idon't get any problems with external noise I use Hammond enclosures for these pedals and I suggest that you don't go the IC switching route as it seems rather heavy handed for a simple idea.

Just my thoughts anyway

Good Luck

Aus

[DeFrag]

I have a few more questions regarding my ABY switchbox if you please

1. Considering that a 500K pot will cut high frequencies more than a 1M pot & there is always a compromise, what disadvantage is there in choosing the 1M pot over the 500K? Would using 500K pots with .001mF caps be better or worse than using 1M pots?

2. Checking my schematic http://gamefreaks.net/defrag/personal/graphics/baby.gif, do I have the diode facing the right way & is it of proper specification to protect the 9v output jack from reverse-polarity transformers that are standard in BOSS pedals that require center negative?

3. The specified Mouser LEDs have been discontinued. Can you recommend new LEDs that are a compromise between brightness & low-current draw? What do you guys normally recommend?

4. I've read about ABY switchboxes & ground-loops caused by two amps. How can I minimize or eliminate this condition in my circuit?

Couple 3PDT TB switches with a Switchcraft #13 "make" input jack allows the LED circuit to be totally isolated from the signal path. No pops, no RC solutions necessary, no way! Why doesn't the major manufacturers incorporate the #13 in THEIR pedals? It can't be but a couple cents difference.

Btw, I'm thinking about making a TB looper pedal out of my Oakley X Metal Vault which you can get yourself one here: http://oakley.com/o/o1999d for a cool $45. Its about 3"x8"x2.5" & opens like a clamshell with spring-loaded hinges... fairly large for a stompbox but awesome!