Geofex A/B/Y question

[Fleetdog]

For the hum free A/B/Y design found here http://www.geofex.com/FX_images/humfree2.gif, isn't it really the transformers that are isolating the grounds and preventing ground loops?  It looks to me like all the opamps are there just to boost the impedance but I'm not totally sure.  I was thinking of making a passive A/B/Y using the 1:1 transformers on one or both outputs to prevent loops.

[zpyder]

The transformers are isolating the ground, yes.  And the opamps do not have anything to do with isolation... The opamps here are being used as buffers - if you removed them from the circuit and ran it passive, then you will have a noticable drop in volume level when you are running your pedal in "Y" mode.  This is essentially two parallel buffers each with their own outputs, all in one box - the gain of each opamp buffer should be 1 (unity - no gain - the same as your original signal).  They are both always running - when you select "A", then "B" just gets dumped to ground, and vice-versa.  When the "Both" switch is on, then the path to ground is eliminated, so neither is dumped to ground, and you get two outputs with a gain of 1 relative to your original signal each.  The overall effect is that you split your signal into two outs, but you've buffered the levels so they're not quieter than the original signal.

See ROG's Splitter-Blend: http://www.runoffgroove.com/splitter-blend.html.  These circuits are essentially the same, except the splitter-blend has no isolating transformers and it also includes another TL072 dual opamp which is running inverse the first, bringing the levels back down - called a "blend" - so you can put the two signals back together and once again have the same volume level...  You  could build the splitter-blend without the second TL072, and it is essentially an A/B/Y without the switches to select the channels..

cheers,
zpyder

[Fleetdog]

That certainly makes sense, but what about the amp on the input?

[zpyder]

That certainly makes sense, but what about the amp on the input?

For that I'm not sure... my only guess would be that he's driving the second opamp with the first.  Why...? I don't know...

cheers
zpyder