The Sound Barrier

Common sense tells us that such delay figures should be swamped by the time it takes for the sound to travel from the loudspeaker to the ear, but we must also consider the ability of the brain to compare the time taken between sending its impulses to the player's fingers and the time of arrival of the sound pressure wave at the eardrum.

Our brains are sophisticated things. A musician can detect a difference in pitch between two simultaneously played notes of less than 10 cents, although few can name with certainty a single note played in isolation. So it's easy to factor in constants like the speed of sound when assessing two amps played back to back. As in many other kinds of experience, we seem to be more alert to differences than to common ground.

PCBs OLD and NEW

You might argue that the increased complexity of modern amp circuits might account for this, but that doesn't explain the perceived difference between the same model built with the two methods. Considering the electrical character of a PCB made using modern techniques may help us find the explanation. In the early days of PCBs, artwork was laboriously laid out by hand using special drafting tape on clear film, a lengthy and expensive process that also required photography. This drove a preference to keep tracking as short as possible, orienting components to bridge the board with their own size and wire length, the tracks serving primarily as a method of making junctions (Pic 3). Larger boards used widely spaced tracks with lots of curvature (Pic 2). From an electronic viewpoint, this wasn't too different from the tagboards and three-dimensional birds-nest assemblies of hardwired amps. But as circuits became more complex and components smaller it became necessary to line them up in dense rows to squeeze enough of them on boards that got more crowded with added features, and link them with circuitous tracking made using software-driven drafting programmes. Closely spaced parallel tracks, the default preference of software-generated layouts, are a common feature of PCBs drafted like this (Pic 4).

Jumping The Tracks

This kind of tracking is used in computers, but computer circuits have a speed and bandwidth far greater than those of valve amps, so its effects are arguably negligible. Also, this takes no account of the much greater impedance and signal level differences in a valve circuit. In computers, impedances are generally no more than a few hundred ohms, with signal levels confined to 5 or 6 volts at most. In a valve amp they can range over several megohms, with signal levels up to 50 volts. A feasible intertrack capacitance of 10pF (closely spaced parallel tracks measure about 1pF per cm), equivalent to such a capacitor being connected between signal and earth in a valve circuit, would have little effect at the low impedances of digital switching circuitry, amounting to a resistance of 50K ohms. In the grid circuit of a valve long-tail pair driver stage, with working impedances of several megohms and gain about 50 times, it affects much lower frequencies, feasibly from 5Khz upwards. These frequencies are within the range of touch perception and have a measurable effect on transient response in high-gain valve stages (see our techie sidebar).