Hum problems with GEO ABY box....any ideas?

[bryantabuteau]

I just finished rebuilding this thing last night to your (Mr Keen) new swtiching with the extra buffers.   Very neat switching, very simple.  I changed the circuit a bit, so I could have a 'Y' LED, cause I need to know what side I'll be coming out on when i switch the Y off, which i can't do with only two LEDs.   I'll be trying it out tonight when I get home from work, but it looks all good so far.  I'll let you know what happens.

[bryantabuteau]

After a little bit of troubleshooting, the ABY is working sweet with no hum.  Thanks everybody for your help.  I've learnt a lot in the process.  

Now my next objective is to find a viable way to put +9 and -9v supply in the box and have it switch off (I don't mind using, say, a slider switch instead of the input jack method)

The switching method I originally used was one from the web somewhere attached to the original GEO ABY box, but Its no longer out there.

I think the method I'm using, from looking at it, is not going to work, and i'm pretty sure one of my batteries is heating up  still puts out the required voltage, but not a long term solution.

GND----- +|9v|-  ->-9
|
|
-|9v|+  ->+9

I've seen the fancy switching solution on the GEO schematic, but is there another way round this?

[Ben N]

I have read this thread with great interest.  I have always wondered about this in amps, too: doesn't shunt switching in the buffers leave a tiny bit of residual noise from the buffer in the "off" channel?  And that is added to the noise that the "off" amp is producing on its own.  For lowest noise, wouldn't it be best to do the shunting on the secondaries--that way, not only is there no buffer noise, but the input of the amp is grounded, same as if there was nothing plugged into the (closed circuit) input jack?  
'Course, then you are back to 3PDTs for LED indicators...
Ben

[bryantabuteau]

not that i'm the knowledgeable person round here, but wouldn't grounding the secondaries remove any isolation that the outputs would have had? They'd now be tied to the box's ground which would be different to the amps ground?

[Ben N]

Yeah, if they were grounded to common ground that would be true.  That is why you need the 3PDT--because you would have to ground them to their own, isolated amp grounds, NOT common ground, so you need separate poles for each, unlike the single pole in RG's scheme above.  I.e. you would connect the "ground" throw of the switch to the ring on the isolated jack of that channel; same for the other channel.  This is also why I don't think you could use the other throw of the same pole to ground an LED.
Ben

[bryantabuteau]

Coolness!!    I decided to bite the bullet and make up the little power supply from the GEO splitter box and add it to the build that i've been doing.  Worked sweet straight off, so now i have a nice quiet active ABY switching box with both batteries switching on the input plug.  A little bit of heatshrink, and it'll be all wrapped up, tidy(ish) and screwed down ready to gig.  Thanks all...