Hum Free ABY help needed.

[Jimi W]

Evening gents!

I built an ABY for a friend using this layout:



I added a 3rd output that could be bypassed by simply adding a switch just after the input jack that always allowed the signal to flow through but would either send the extra signal to the 3rd amp or to ground. I warned him there would probably be ground issues but he was really pleased with it, said there were no noise issues etc. I popped round to watch his band rehearse the other day and guess what....humming like a *^%&.

I've decided to try the GEOFEX Hum Free ABY (schem not showing on GEOFEX at the mo) and ordered the parts from Smallbear and etched the board from GGG.

http://www.generalguitargadgets.com/pdf/ggg_amp_aby.pdf

After some browsing I came across the Transformer Isolated Splitter ABY box at GEOFEX



I prefer the switching on the Splitter aby as I like the additional led for the 'Both' switch because, for example, if you are on amp A and then press Both, the A LED stays on and therefore you know what amp will be selected when you press Both again. I also like the direct out as I could use this for the 3rd amp.
However, I want to use a charge pump as I have ordered one now and it seems logical rather than running 2 batteries.

If I was to add the charge pump would it just be a case of using the Hum Free ABY charge pump section and inserting it in the Splitter schem?
Also, is there any benefit in using opamps for each output as the Hum Free does or can I just use an opamp on the input before the signal is switched like the splitter does?

Sorry if these are dumb questions, I'm still on a very steep learning curve!

Cheers,
Jimi

[familyortiz]

Here's more info and Auke has done a lot of the charge pump stuff:
http://www.diystompboxes.com/smfforum/index.php?topic=64924.0

I like the way RG has morphed and simplified the design with one opamp driving the 2 channels. You're gonna like the performance of these designs.

[Jimi W]

Thanks for the reply.

If its ok to use just one opamp to drive 2 channels I think I have my plan.

I'm going to take the charge pump section from below to get my +9Vdc and -9Vdc



and then use the following schematic.



I'll ommit the 2.2uf cap to ground to keep the gain closer to unity.

I'll then place a switch on the direct out to send the signal to enclosure ground for bypassing the 3rd amp. I'll keep the AB/Y switch wiring as it is (bigman layout above) but I'll take the signal from after the 100uf/0.1uf caps and use that as the input signal for the switching section. I'll then take the A and B outputs from the switches and take them to their own transformers and keep the A and B output jacks isolated from the enclosure.

Will this work or am I being dumb and missing something obvious?

Cheers,
Jimi

[familyortiz]

It sounds like it should work. That's not a bad idea switching at the outputs as opposed to the opamp inputs. Let us know how it goes. I did not entirely like some of the designs that switch at the inputs as I like as much channel isolation as possible.