Let’s Talk Discrete
By Paul Wolff Ex-President of API Audio Products, Inc
The basic design philosophy of the entire API product line is the all discrete amplifier.
Ever wonder why “discrete” just sounds better? Let’s look at the pros and cons of the IC chip as well as discrete technology to discover what makes the difference and how it impacts what a console can produce in quality and purity.
Perhaps it would be easiest to start with the ever popular IC chip which is used in all module and console amplifiers except API.
IC Based Amplifiers
IC based amplifiers start with the monolithic die process which creates either NPN or PNP transistors.
The diemaker must choose which will be the primary die type. Usually the NPN type transistor is chosen because it utilizes a process called “buried layer,” which makes it possible to attain higher current levels than with PNP transistors. When an NPN buried layer is chosen, the PNP transistor is created in a way that causes slower response, less available gain, and thermal expansion problems.
Through the diemaking procedure, the isolation process of making an actual transistor adds parasitic shunt capacitance (increasing as the area increases) and a current leakage path within the transistor.
Further, the actual layout of the die causes a higher resistance within the collector portion of the transistor (the collector is what basically puts out the power), thus limiting the current capability, i.e. output drive, and adding to the instability of the transistor. This instability can be increased by putting long cables, etc. on the outputs of certain IC type amplifiers, causing ringing and distortion.
To go one step further, the big difference, aside from the inherent design flaws of the IC amplifier, as stated above, is most console manufacturers add an additional pair of external power transistors to get the needed power output to drive a lo-z or transformer output. This creates a potential problem in stability, because unless carefully chosen, larger transistors are generally slower than the small signal transistors found in the IC and could lead to stability problems i.e. ringing and oscillations.
Discrete epitaxial transistors
None of the problems associated with IC chips are present in high power discrete epitaxial transistors, since the metal case of the transistor is the collector contact, which is located in the opposite end of bottom of the die. An IC transistor must access the collector from the top.
The discrete amplifier allows the designer to not only decide what design criteria to use for each stage of amplifier, but also allows him to select special components (i.e. capacitors and resistors) to enhance the performance of the circuit.
The discrete 2520, in conjunction with the AP2503 or AP2623 output transformer has a well-known and desired audio “signature.” It has a unique tone that most engineers, producers, and artists prefer over most other professional quality amplifiers. This sound is the legend of API and has been preserved in the current API designs.
In summary, the only advantage to monolithic transistors (IC based amplifiers) is that they are closely matched, because they are cut from the same die. However, API precisely matches all the critical transistors manually to accomplish the same effect.
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