The first thing that we noticed upon evaluating some devices in this class was that most of them are manufactured with archaic, decades old through-hole components.  This is not so bad in and by its self, aside from component lead inductance.  But the problem with them is that most of these through-hole components (especially the transistors) are old, slow, and pale in comparison to modern devices.  The SS2590 discrete audio op amp makes use of the highest speed and highest gain transistors available.  We use 300MHz output transistors while everyone else is using devices in the 20 to 50 MHz region.

We’re the only ones doing this.  Two Pole Compensation is the ultimate compensation technique for maximizing open loop gain within the audio bandwidth.  High open loop gain makes for a more precise amplifier, lower THD performance, and keeps the slew rates high, which makes for more natural attacks and transients.  A white paper on two pole compensation can be found here. 

We found the noise performance of the majority of the op amps in this class that we evaluated to be abysmal. Most measured in at several nv√Hz in most designs. Some of the better devices specified in the 1 to 1.5 nv√Hz range, but they all seemed to achieve this noise performance by putting shunt inductors across the RE resistors in the input stage. This technique allows the inductors to “short out” the RE resistors and eliminate their noise contribution. At least at low frequencies.  While this technique does yield lower noise, it does so at the expense of input stage linearity. This is because the linearizing affect of the RE resistors are lost since the shunt inductors essentially "short them out".   

RE resistors in the input stage are a good thing, and will linearize the input stage by swamping the non-linear Rbb of the input stage transistors.   On the SS2590 discrete audio op amp, we opted to keep the input stage RE resistors in order to reap their associated linearity benefit, and we reduced the noise back down by paralleling the input transistors.  It’s the best of both worlds, with the only drawback being a slight increase in input stage complexity.

The DC precision of the devices that we evaluated in this class left something to be desired as well.  We saw offsets in the double digit mV range with warm-up drifts of several mV more.  The SS2590 has a built in POT to allow precise offset adjustment and it has a warm up drift of less than 1mV.  Input bias current induced offsets are greatly reduced by using on-board current sources to supply the input bias current for the device.  This keeps the SS2590 discrete audio op amp from pulling input bias current from the outside world and creating offsets against the resistances seen by the input pins.

Features:

• 165 dB Open Loop Gain to 100 Hz
• Two Pole Compensation
• 10MHz Unity Gain Bandwidth
• Class A Output Current Of +/- 32mA
• Maximum Output Current Of +/- 250mA
• 1.5nV√Hz noise in a 20KHz bandwidth
• Less than 1mV Offset
• On-Board Supply Decoupling Caps
• Able to drive 75 ohm loads at full power
• +/-9V to +/-24V supply voltage range
• All BJT design with input bias current cancellation
• Unity gain stable
• Fully Discrete Design
• PRO op amp package

Compatible with:

• Automated Processes API2520, API2525
• Lunch Box Mic Preamps
• Jensen and John Hardy 990 series
• Sound Skulptor SK series
• Purple Audio KDJ series
• Five Fish Studio Discrete series
• Avedis Audio 1122
• Seventh Circle Audio SC series
• Yamaha NE series
• Rogue 5 Series Discrete Op Amp
• Whistle Rock Audio series
• Warm Audio Discrete Series