After the undeniable success of the SS3601 and SS3602 discrete OPA, SparkoS Labs focused on discrete OPA that can be found in studio equipment or recording devices, such as the API2520 operational amplifier or the Jensen 990. After numerous tests and measurements, SparkoS Labs decided to offer a much more efficient OPA.
The brand's first observation when studying these OPA models is the presence of old components, particularly transistors, which are often not very reactive and offer a tasteless reproduction. The SS2590 uses the transistors with the best reactivity and highest gain available today. Where other brands mostly use models between 20 and 50MHz, SparkoS Labs uses 300MHz transistors.
In addition, the brand uses a unique and exclusive bipolar compensation technique to maximize open loop gain over the entire bandwidth. This technique increases amplifier accuracy and lower THD while maintaining a high scanning speed, ensuring a better transient response and a more natural dynamics.
SparkoS also worked on the noise level, which is too high on many discrete OPA. On the SS2590, RE resistors are used at the input stage to benefit from their linearity. Noise has been reduced with a parallel circuit of the input transistors.
SparkoS Labs Technology
The SS2590 discrete OPA is based on the Lin 3 Stage topology which consists of a pair of differential input stages, a gain stage (VAS) and a Class A output stage with two pole compensation. All active components are bipolar junction transistors (BJTs) offering the best linearity that silicon components can offer. In addition, the components are fully protected against overcurrents by active current limiting circuits in the ain and output stages, as well as against large input voltage differences using Schottky diodes. In short, the OPA consists of 35 BJTs, 37 resistors, 7 capacitors and a few diodes.
Input Stage :
The input stage of the OPA integrates a double pair of NPN bipolar junction transistors (BJTs) paired and cascoded. This arrangement using several parallel input transistors reduces the noise of the input stage to 1.5nV√Hz over the entire bandwidth. The input offset voltage is set at the factory to exceed 300µV @ +/-18Vdc. A Wilson cascoded current mirror is used as an active load to obtain a precise current correspondence between the input transistors.
Gain Stage (VAS) :
The SS2590's gain stage is a Darlington cascode providing very high linearity and high open loop gain. The polarization voltage of the cascode is derived from precision shunt references, having a dynamic impedance and a much lower noise than the low voltage Zener diodes used to derive this polarization voltage. In addition, the current of the gain stage is limited, allowing a much better stability.
The SS2590 uses a 2-pole component architecture that provides the best possible open loop gain while providing high resistance to capacitive loads. A high loop gain corrects distortion when the loop is closed. This 2-pole compensation, although more efficient than 1-pole diagrams, is not often used in integrated circuit OPA due to the difficulty of manufacturing.
Output Stage :
The output stage is a common Darlington push-pull collector (emitter-follower) polarized in class A with a stable current of 16mA. Due to the push-pull action, the output stage can supply or absorb a current of 32mA while remaining in Class A. It will automatically switch to Class AB if more power is required. A limitation system is used in the output stage to protect it from possible overload. Large copper areas under the PCB are used as heat sinks in order to properly dissipate heat.