geo a/b/y box

[m_charles]

feel kinda dumb. researched the heck outta this before building, finally ready to rock, got all the parts together and am left with 2 questions:
1) on the transformer is the primary facing in toward the buffer or primary faces out?
2) The isolation thing has me confused. do I isolate all jacks? I think I wire the IN as normal then isolate the A and B outs. right?
any help much appreciated!

[R.G.]

In this setup, the nominal primary winding faces the buffer, the secondary faces out. However, it doesn't matter a huge amount since both primary and secondary are nominally the same voltage/impedance. In this circuit, it will probably work OK either way.

The problem that isolation solves is hum cause by "grounds" that carry different AC voltages. You usually don't see a problem when you have one guitar/effect chain driving one amplifier. When you add a second amplifier, you also add a second AC power cord to the second amp, and therefore the possibility of hum caused by two different AD "ground" voltages.

So the input and the NON-transformer output have to share a common ground. They can be connected to the box and each other. The output from the transformer needs to be isolated from the ground shared by the other two.

Note that ground hum is very devious. In some situations, you will need to run a resistor of 100K to as little as 1K between the "isolated" ground output and the Input+Output ground. In other situations, no connection at all. This depends on the AC wiring the amps are connected to.

[m_charles]

thanks RG. maybe if I have time I'll post a little project up with all the help you've given me for the a/b/y in one place for others.

It seems with all things electronic, you finally open one door, only to find three more doors, and a box full of keys that may or may not be the ones you need...         

[m_charles]

still confused on one thing though. Where does the a and b outs (if not connected to the general ground) get their ground connection?
How do the outs "talk" to the guitar if the guitar and jack don't share a ground?

[R.G.]

It seems with all things electronic, you finally open one door, only to find three more doors, and a box full of keys that may or may not be the ones you need...   

Now you're beginning to understand! 

still confused on one thing though. Where does the a and b outs (if not connected to the general ground) get their ground connection?
How do the outs "talk" to the guitar if the guitar and jack don't share a ground?

Well, for one thing, there is no "general ground" unless something, somewhere, is connected to a metal rod driven into the earth. That happens to be true for the amps which connect to this thing through the AC power safety ground, but they may each see a slightly different voltage because of ground currents and wiring issues.

Electricity does not require a ground; it require a circuit. A flashlight is not grounded, but it spits out light. A guitar which is not grounded still makes a signal. The signal from the guitar on the input jack makes the signal line wiggle above and below the sleeve contact of the input jack. This is amplified and used to drive the primary of the transformers, relative only to that "sleeve of the input jack". The transformer primaries don't care that one side of them is not connected to earth ground. All they know is that there is a voltage across them, and when that happens, the transformers make a voltage across their secondaries. These secondaries may not be "grounded" to whatever is considered "ground" by the amps they drive. The secondaries float, and can be connected to another ground either way. In fact, that is the whole point of using transformers. It cuts any dependency on a "ground" and lets you connect it up as you want.

So I guess the answer is that if the A and B outputs are not connected to the guitar "ground", they talk to it through the magnetic fields of the transformer core.

Notice that you may still need to connect the sleeve contact of either or both of the A and B outputs to the guitar ground with a 1K or larger resistor to be completely hum free. This is because there are many ways for hum to be transferred. The resistors instead of a solid ground connection take care of some cases which can't be handled by complete isolation.