From Tim Smith's 1993-ish papers.
"The following discussion is provided, recognizing that not all measured performance behavior explains or correlates with listening tests by audio experts. The design of the OPA2604 included consideration of both objective performance measurements, as well as awareness of widely held theory on the success and failure of previous op amp designs.
The sound quality of an op amp is often the crucial selection criteria- even when a data sheet claims exceptional distortion performance. By its nature, sound quality is subjective. Furthermore, results of listening tests can vary depending on application and circuit configuration. Even experienced listeners in controlled tests often reach different conclusions.
Many audio experts believe that the sound quality of a high performance FET op amp is superior to that of bipolar op amps. A possible reason for this is that bipolar designs generate greater odd-order harmonics than FETs. To the human ear, odd-order harmonics have long been identified as sounding more unpleasant than even-order harmonics. FETs, like vacuum tubes, have a square-law I-V transfer function which is more linear than the exponential transfer function of a bipolar transistor. As a direct result of this square-law characteristic, FETs produce predominantly even-order harmonics. Figure 9 shows the transfer function of a bipolar transistor and FET. (Bipolar shows strong second, third, fourth and fifth content; the FET shows only a second harmonic, one-half the strength of the bipolar). Fourier transformation of both transfer functions reveals the lower odd-order harmonics of the FET amplifier stage.
THE OPA2604 DESIGN
TheOPA2604 uses FETs throughout the signal path, including the input stage, input-stage load, and the important phase-splitting section of the output stage. Bipolar transistors are used where their attributes, such as current capacity are important and where their transfer characteristics have minimal impact.
The topology consists of a single folded-cascode gain stage followed by a unity-gain output stage. Differential input transistors J1 and J2 are special large-geometry, P-channel JFETS. Input stage current is a relatively high 800uA, providing high transconductance and reducing voltage noise. Laser trimming of stage currents and careful attention to symmetry yields a nearly symmetrical slew rate of +/-25V/us.
The JFET input stage holds input bias current to approximately 100pA, or roughly 3000 times lower than common bipolar-input audio op amps. This dramatically reduces noise with high-impedance circuitry.
The drains of J1 and J2 are cascoded by Q1 and Q2, driving the input stage loads, FETS J3 and J4. Distortion reduction circuitry (patent pending) linearizes the open-loop response and increases voltage gain. The 20MHz bandwidth of the OPA2604 further reduces distortion through the user-connected feedback loop.
The output stage consists of a JFET phase-splitter loaded into a high speed all-NPN output drivers. Output transistors are biased by a special circuit to prevent cutoff, even with full output swing into 600 ohm loads.
The two channels of the OPA2604 are completely independent, including all bias circuitry. This eliminates any possibility of crosstalk through shared circuits- even when one channel is overdriven.