John Curl is a master at eliminating undesirable electronic noise.
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For the Parasound JC 1+ he specified a pair of Bybee Music Rails, which are active high frequency noise filters that “scrub away†every vestige of noise on the input and driver stage B+ and B- rails. To optimize their effect, Carl Thompson designed Music Rail circuit boards exclusively for the JC 1+. The JC 1+ is the first commercial product to employ these remarkable devices. As in the JC 1, the JC 1+ input stage uses hand-matched pairs of Toshiba 2SJ74 P-channel and 2SK170 N-channel J-FETs. These coveted devices remain unmatched for their low noise.
Parasound is one of the few companies that has invested heavily to secure an ample stock of these rarest of all J-FETs. The JC 1+ employs FR408 circuit board substrate for the critical input and driver stages. FR408 was invented for ultra high-speed applications in super computers and aerospace. Curl predicted that a two-stage cascode driver would make a substantial performance improvement because a driver circuit with cascode topology has some of the favorable attributes of vacuum tubes, namely higher input-output isolation, higher input impedance and higher output impedance.
Its greater bandwidth and open loop gain were expected to improve linearity and a noticeable improvement in sound quality. This proved to be true. Driving low impedance speakers doesn’t necessarily require more watts, but higher current is imperative. We raised the JC 1+ peak current 33% to 180 amps with the addition of six Sanken 15A/230V/60Mz bipolar output transistors, for a total of 24. Accommodating six more output devices necessitated changing the output stage circuit board’s orientation from horizontal to vertical. This added welcome benefits of more effective heat dissipation and easier serviceability.
To conduct higher current flow in the JC 1+ we made the copper traces on its main circuit board and N and P channel output stage circuit boards twice as thick as on the JC 1. Supporting this substantial current required an even more potent power transformer. The massive 2.1 kVA transformer weighs 30 lb. (13.6 kg) - 30% more than its predecessor. Developing this power transformer was challenging and required 11 hand-made samples before we achieved the necessary power and total absence of mechanical vibration. The JC 1+ power transformer accomplished this with an especially potent and expensive steel core. ■
