Hardwiring vs. Printed Circuit

Some amplifier gurus extol the virtues of hardwiring while warning against the use of printed circuits. In reality, hardwiring and printed circuits share a problem that may be exaggerated by the printed circuit board material.

AMPLIFIER LAYOUT

The shared problem is layout, or as it is known in hardwiring, lead dress. There are two types of lead dress problems, capacitive and inductive, and both can cause the amplifier to behave strangely even if it does not oscillate from the unwanted feedback. Capacitive coupling occurs when the grid wires are too close to plate wires of later stages, i.e. ones with much higher gain relative to the grid. Inductive coupling occurs when the grid wires and the high current wires form a virtual transformer. (Inductive coupling of the grid wiring with the filament wiring is one source of amplifier hum.)

The vintage amplifier layouts generally avoid this problem with their straight line input-to-output layout. The straight line layout keeps the sensitive input well away from the high-gain, high-voltage, and high-current signals. This distance significantly reduces the possibility of unwanted feedback. More complex and really high gain amplifiers simply have more opportunities for problems. However, they can be managed by carefully taking into consideration the direction, distances, and layout between suceptable circuitsts. Unfortunately, the ubiquitous digital technologies have not given printed circuit board designers of today the experience needed to avoid poor layout of high-gain analog circuits.

PRINTED CIRCUIT BOARD LAYOUT

Printed circuit boards have three distinct advantages over hardwiring. First, the wiring is fixed photographically and thereby removes a potential manufacturing error source. Second, the printed circuit board mechanically retains the part, like the various types of vintage terminal strips. Finally, the printed circuit board is amenable to modern manufacturing methods, either lead insertions in plated through holes or the still later and faster surface mount technology. All these in turn allow the amplifier to have tremendously fewer variations from amp to amp.

PRITCHARD AMPS

The printed circuit board material increases capacitive coupling because the material, like all nonconductors, has dielectric properties. Because of this dielectric property conductive surfaces can be closer to each other than a air capacitor would allow thus increassing their wire-to-wire capacitances by about three.

The capacitive disadvantages of printed circuit boards found in vintage amplifiers has been eliminated by the XGPA technology. This technology recreates the grid-plate action with roughly the same currents, but about one-tenth the voltages. Thus, the translation of vintage circuits to the XGPA technology requires reducing all of the resistors by a factor of 10 and increasing all of the capacitances by the same factor of 10. As a consequence, XGPA technology reduces the effect of any particular source of capacitive coupling by a factor the same factor of 10 and neatly overcoming the printed circuit board’s factor of three increase. This and substantial analog layout experience and techniques (also used by the military to avoid interference from RF and EMF in weapons guidance systems) removes typical printed circuit board problems. Pritchard Amps are truly above and beyond.