Originally Posted by infinitesymphony
Along those lines, what exactly are the differences between "discrete output," "class A discrete output," "passive I/V," "active I/V"? It's a little unclear in my mind which methods, if any, avoid the use of op-amps in or out of the signal path.
Passive I/V = a resistor is used to convert the output current from the DAC ("I") to a voltage ("V") that can be used to provide a line-level signal. This is sometimes a less-than optimum solution because every ohm in the resistor adds noise, but the output voltage is very low unless you raise the ohms. A happy medium is often selected, but is far less than optimum. This is the cheapest form of obtaining a usable output from a DAC.
Active I/V = same principle as above, only an active circuit is used. It's sort of analogous to an amplifier, but not really because the circuit is used to provide I to V conversion instead of gain and current buffering. IOW, it gets the very small current output from a DAC up to a signal voltage with a very low output impedance. This means the signal output can achieve line-level voltage with a very flat frequency response - best solution.
Discrete output - this is a form of Active I/V. Non-discrete is an opamp. Opamps are frowned upon by many audiophiles. They bring their own flavor to the circuit, adding some tizziness, filtering a lot of the detail with their protective circuitry, etc. A discrete output, Active I/V is analogous to a discrete buffer in an amp - a bunch of transistors everywhere with not an opamp in sight. It's very labor intensive to build and expensive to buy, but offers the best in detail and DAC output.
Class A Discrete output - this takes the Active I/V, discrete output one step further and adds a Class A bias - meaning an active current that always keeps the transistors turned on, removing any possibility of switching noise. This is the ultimate in solid-state, discrete audio output. (But it burns heat and usually comes with heat sinks and ventilation.)
There's probably some of the above that can be argued in terms of specificity, but I think it states the gist of the terms you asked about.