500 Series Engineering Constraints
The 500 series has become a popular modular format for customisable, compact and affordable studio setups. However, the format also introduces a number of constraints that are less than ideal for precision audio design. Compromise is always present in design be it a building material for a boat or a size restriction for a transformer but the key to good design is to get the best result despite the given restrictions. With the 500 series, the limiting factor that would not be a problem in a fully integrated rack unit design, is the power supply. The 500 series is limited (with the exception of a 48V rail for the phantom power) to a single bipolar rail being +/-16V. Normally in electronics design, areas of circuitry would be separated with different supplies, for example the 5V for logic, switching and relays would be isolated from the supply used for audio signals to make sure there is no noise from switching or processing induced onto the audio signal.
As there is no separate power present in the 500 series and that the quality of the power supplies are an unknown in whichever frame the module will be plugged into. So we must start by making a well designed local power supply for each module to remove this problem.
Output Stages
Balanced connections are very important for signal integrity but there is a trade-off when choosing an output topology.
Simple op-amp outputs based on dual or differential types can offer the most transparent options but lack the level compensation and the galvanic isolation that transformers offer.
Monolithic line drivers such as the THAT 1606 are very very good, giving the best you can achieve on a practical level in place of a cross coupled output circuits that are well known to be fussy. It is however limited in THD+N characteristics when compared to op-amps available today.
Transformer balanced circuits with mixed feedback and high current buffers offer an excellent option for balanced outputs that can deliver low distortion (though not as low as an op-amp) but also with the characteristics that are hard to achieve with electronically balanced outputs, such as galvanic isolation, CMRR and level compensation. However, they are expensive, large and heavy compared to the alternatives.
Much thought must be given to the specific device’s application before choosing which to use. For example, a transformer balanced output on a monitoring controller would be counter productive as, in general the signals are only traveling a short distance and in the same room and the added distortion is not favourable in this area.