Design Questions for Dual Tracking Power Supply Using 15 Volt 800mA Secondary Transformer, LM317, LM337, and Op Amp
Figure 1B (how does the tracking control loop work with LM337 in the feedback path?):

LM317 and LM337 are floating regulators (they keep the voltage between OUT and ADJ @ 1.25 V). If you set the voltage at the ADJ pin, OUT will follow (1.25 V above ADJ for the positive regulator or below for the negative one). The 741 (let's forget about the age...) samples the two outputs and controls the negative rail to keep both rails symmetric (opamp inputs @ ~0 V).

Figure 2 (how does this tracking control loop work derived from positive adjust pin?):

Same as above, but the voltage at the positive ADJ pin enters an inverting amplifier (gain = -1) and is applied to the negative ADJ pin. The positive regulated voltage will be 1.25 V above the ADJ+ pin and the negative regulated voltage will be 1.25 V below the ADJ- pin. ADJ+ and ADJ- will have voltages with practically the same module and opposite signs.


[...] Which design is likely to be more accurate or less expensive to use a pot for fine adjustments?


Both disregard the fact that + and - voltage inputs are available with a common low impedance 0 V reference and implement the tracking by trying to mirror the positive output in the negative one. I don't have a model for the LM337, so the following is just an example of the problem using a very poor &quot;voltage reference + opamp + bjt&quot; control:

The circuit above is just a &quot;split supply&quot; solution, similar to the one using the 741. Three load transients are tested: first a load between V+ and V-; second between V+ and 0V; finally between V- and 0 V. Each time the load is disconnected after 5 ms. As it's clear below, the load between V+ and 0V affects the V- voltage, as expected.

An option in which this effect would be considerably reduced is using a single voltage reference for both control loops (again, this is not a complete project but just a way to show how independent controls loops may give you better results). The following uses a fixed reference for simplicity. You'd just have to modify it to be an adjustable one (one adjust affects both outputs).


Obviously, the voltage reference is not completely immune to changes in the positive unregulated voltage, but the difference is clear when we zoom in the response to the second transient: