Passive A/B/Y...I've got A and B but no A&B

[patrick398]

Idiot check complete; here comes the question.

Struggling with this GGG Passive A/B/Y box tonight. More pedal based problems but this time it's switch issues so i at least thank the pedal gods for variety.

It was working when i built it a couple of weeks ago but came back to it today to wire the DC socket and now the A&B switch isn't working. I'm pretty rubbish when it comes to switches, they confuse me and i can't seem to work out what's going where half the time.

I'm feeding my guitar into 'B' and my radio into 'A'. I can switch between the two no problem using the 'A or B' switch. When i hit the 'A&B switch i lose the guitar and i can't figure out where i should be looking on the switches. Everything looks in order, can't see any shorts or cold joints. Anyone care to localise this for me?

Oh and i have about 3 inches of solder left so i need to use it wisely ha!
Thanks!

[PRR]

> feeding my guitar into 'B' and my radio into 'A'.

A guitarist's "A/B/Y" is designed so the one guitar goes to the Champ or the MesaBoogie, or both. One input, two outputs.

I'm guessing that when you try to "Y" the radio and guitar, they fight each other. The radio is maybe 100 Ohms and the guitar may be 100,000 Ohms. The radio wins 1000:1. (The guitar may be back there, but not so you can hear.)

[patrick398]

It worked fine with my other radio, the TDA7000 one. This one is a bit fancier though so maybe it's causing issues the other didn't. I'll try A/B/Y two guitars into the amp in the morning just to verify the wiring is sound. Can't believe i didn't think to try that earlier. If it does work i guess i'll have to start thinking about matching the impedance...is it impedance we're dealing with here? I guess some kind of buffer?

With the A&B selected, and with the radios volume all the way down i still don't get any guitar. I assumed that if it was just a lot quieter it would come through when the radio was silent but i guess if it's a choice between 100,000ohms and 100ohms the latter is still the path of least resistance irregardless of set volume?

Thanks for the reply, gives me something to work with tomorrow

[patrick398]

Good news: wiring is fine, both guitars are audible when in A&B.
Bad news: I misplaced that three inches of remaining solder. Off to maplins to buy some of their crappy lead free stuff.
So this things needs a buffer now right? To match the impedances of guitar and radio?



Something like this but this assumes two outs whereas i need two ins

[patrick398]

Just drawing something up now

[patrick398]

Something like this...but i'm missing something, some resistors and maybe some jumpers?



Thanks!

[patrick398]

Turns out it was a simple as adding a 330k  resistor between input and switch. I'm sure this is not a technically or theoretically good approach but i don't need this to be performing amazingly well, just performing. Having said that, i can't hear any problems with this approach. Happy to be told otherwise though