Sparkos Labs SS3602 Dual Discrete Op Amp

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The DUAL SS3602 discrete op amp will outperform virtually all audio grade monolithic IC op amps in open loop gain, noise performance, output current, and magnitude of class A bias current. Even the coveted OPA627 monolithic op amp, with a price tag of over 25 dollars, has 30dB less gain and around twice the noise of these discrete op amps.

Op amps are a key component for processing and amplifying audio signals. They can be found in virtually all audio gear in the form of tiny integrated circuits (IC's). They also come with all of the pitfalls and shortcomings that IC op amps have, like limited power dissipation and crummy compensation capacitors. Discrete op amps do not have these limitations, and are a vastly superior op amp for amplifying audio signals than their IC counterparts are. They can run much higher power, have much deeper class A bias, and deliver a much more realistic and detailed sound. Discrete op-amps also permit the use of high-quality compensation capacitors and allow for two-pole compensation schemes which are impossible to implement in IC designs. All of this translates into a more detailed and engaging listening experience with better imaging and sound staging than IC op amps can deliver. In short, discrete designs are the best op amp for audio.

Overview:

All of Sparkos Labs discrete op amps are based on Lin 3 Stage topology consisting of an input stage differential pair, a gain (VAS) stage, and an output stage all biased in class A mode with two pole compensation. All active devices are Bipolar Junction Transistors (BJTs) for the greatest linearity and agility that any silicon device has to offer. The devices are fully protected from over current conditions by active current limit circuitry in the output and gain stages, as well as being protected from large differential input voltages by back to back high-speed schottky diodes across the inputs.

Input Stage:

The input stage of these devices are comprised of a dual matched pair of NPN BJTs. This means that the device’s inputs will pull a small input bias current (specified as Ib) that will flow into the device. The common mode input voltage range of the input stage can be as high as a few volts below the supply rails, however the best performance is obtained by minimizing this to a few volts above and below ground in a split supply application. Input offset voltage is factory trimmed and typically turns out to be better than 250uV @ ±12Vcc. The input stage is protected in the event that the inputs are driven apart, which usually happens during output clipping or rapid slewing. A cascode Wilson current mirror is utilized as the active load for the input differential pair for precise current matching between the input pair transistors.

Gain (VAS) Stage:

The gain stage of the device is a cascode loaded Darlington for the highest linearity and open loop gain possible. The cascode biasing voltage is derived from precision shunt references, which have a much lower dynamic impedance and lower noise than the low voltage zener diodes which are commonly used to derive this bias voltage. The gain stage is current limited by diode clamping action as opposed to a feedback action, which results in greater stability during clip.

Output Stage:

The output stage is a push-pull emitter follower biased in class A mode with 8mA of standing current. Due to push-pull action, the output stage can source or sink 16mA of current and still remain in class A mode. The output stage will automatically revert to class AB mode in the event that more output current is demanded by the load, however the best THD performance will be obtained by ensuring that the output stage stays in class A mode. Active current limiting is employed in the output stage to protect it from an over current condition. The output transistors are high gain (β) individual devices in a SOT23 package manufactured by Diodes, Inc. who have developed a special manufacturing and encapsulation process that allows their devices to dissipate two to three times the power of a typical SOT23 packaged device. Utilizing these output devices allows Sparkos Labs discrete op amps to have a high class A bias current and the ability to source or sink far more output current than comparable monolithic op amps in a DIP8 package.

Compensation:

All of Sparkos Labs discrete op amps employ a uniquely implemented 2 pole compensation scheme that is extremely tolerant of capacitive loading. This allows these discrete op amps to be dropped into virtually any circuit arrangement and work without any stability issues.

Two pole compensation, despite its superiority to single pole schemes, is not often used in monolithic op amps due to the difficulty in fabricating the 2 capacitors at minimum that are required to implement it. Capacitors inside of monolithics consume a large amount of the die area, and are therefore kept to a minimum in both capacitor value and quantity. The amount of capacitance required for at least one of the two capacitors in a 2 pole scheme tends be impossibly large for monolithic designs. Even if the die area were available for two capacitors. Beings how the Sparkos Labs discrete op amps employ 3 capacitors for compensation, they are impossible to fabricate as a monolithic, and are only possible as a discrete op amp. Such are the reasons that discrete op amps are the best op amp for amplifying audio signals.

Compensation in ICs:

Monolithic (IC) op amps mostly employ single pole compensation schemes. They pay for this with a reduction in open loop gain at audio frequencies, as well as a reduction in maximum open loop gain that they can have in the first place. Since compensation schemes burn off gain by nature, and since a single pole scheme burns it off at half of the rate of a two pole scheme, there is a limitation that exists in how much gain they can start out with in the first place. This is because they have to ensure they can burn it all off by the time the phase lag has shifted 180˚to maintain stability. The monolithic op amps that DO have a high open loop gain always wind up having an excessively high bandwidth in the 50MHz region or so, which tends to make for a finicky device prone to instability and oscillation. Such high bandwidth devices also suffer from more susceptibility to ill effects from layout parasitics, capacitive loading, and resistive feed back networks. Such limitations give most monolithic op amps, even the good ones, little chance of working as drop in replacements.

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