I/V Basics
Well basically the purpose of an I/V stage is to turn a current into a voltage.
I am not really interested in opamp stages as I like the sound of my current setup which is an openloop transistor I/V.
This is pretty simple, the transistor holds the DAC output voltage while passing the current to a resistor. However, it is a bit too simple, because the output is floating a few volts above ground so you need a large capacitor, which is expensive and sucks.
Some also suggest doing away with the upper current source and replacing it with a resistor. This means either you have to connect the V+ power rail to the preamp ground, or you get back to the capacitor, but as a bonus, the PSRR is so low you're really listening to your power supply.
Adding a transistor and a current source references the output to ground, which is a lot more useful. This looks funk because a cascode is not usually drawn this way but it is actually pretty simple. Just replace the transistors by wires and remove the current sources, and you'll get it.
This is a simple simulation, the lower curve shows output voltage, horizontal axis in input current. The top curve shows d(Vout)/d(Iin), which is a useful linearity measure. Ideally, it should be a straight horizontal line.
Note that the simpler circuit is more linear, but since it sucks, that doesn't matter.
The second circuit also has a hidden problem, that upper and lower current sources need to be well balanced or else there will be an offset at the output. A DC servo will take care of this.