Wired For Sound: The Great PCB Debate
If you use a valve amp, you’ll know how much importance is attached to hardwiring. Hi-tech manufacturing is a must when it comes to keeping costs down… but why is it that our ears can hear the difference? Dave Petersen investigates.
During the 1950s and ’60s, when valve technology was standard in consumer goods, the cost of making a hand-wired amplifier was kept affordable by the low prices of valves and other components needed for the large-volume production of radios and televisions. Since then valves have disappeared from consumer goods, leaving music products particularly guitar amps as the main market.
Suddenly, good valves are expensive. In recent years prices have risen to several times their former level, and although this can be said of many other kinds of product, few can match the steep upward price curve of valves and their supporting components. The truth is this: with labour-intensive assembly procedures factored in, valve amp manufacturers are having difficulty keeping their products within reach of guitarists. There are solutions, and in this article we’re going to examine them while keeping an eye on the most important thing of all the way amps sound.
PCBs: The answer?
One way makers keep costs down is by using the assembly speed advantages of printed circuit boards (PCBs). PCBs are essential to all modern consumer electronics, and a valve amp made with them is significantly lower in price than an equivalent hardwired one (Pic 1). This is because it takes about a quarter of the time to build, and there’s another bonus you get a more consistent product that’s not likely to need much postassembly quality control. For every 10 handwired wiring workers, there’s a skilled technician whose job is to check wiring and correct errors, and this adds further to the cost of hardwired products.
Why then do hardwired amps still have a market, rather than simply leaving it to their more reasonably-priced PCB counterparts? Is there a discernible difference in sound quality? Many guitarists say there is, and this may account for the survival and the recent growth in sales of old-fashioned hardwired amps.
Taste The Difference
From an engineer’s viewpoint, using conventional test methods, there’s no measurable change between PCB and hardwired versions of the same model of amp in standard parameters like output, frequency response, noise and distortion. Induced hum the bugbear of hardwired assemblies is often lower in the PCB version, although this can vary with the PCB layout (indeed, some respected old valve amps used PCBs very early on without noticeable ill-effects). In others that transferred later to this technology the difference is apparent to the player, and that’s useful in helping to determine what causes this impression.
The words ‘to the player’ are significant, because it’s doubtful if it would be noticeable to the average listener. But ask a guitarist to comment on a hardwired amplifier and a later version of it made with PCBs and the answer will probably be that the amplifier ‘feels’ different.
Touch response or how a guitar’s strings respond to the player through an amp is a difficult aspect of an amp to assess technically. It’s governed by more than one factor, an important one being the amp’s input impedance, which determines how much it loads the guitar’s pickup. Equally important is group delay, the time taken for the magnified incoming signal to appear at the amp’s output. Group delay is the sum of the transient responses (also called rise time or slew rate) of all the stages in the amp’s signal chain. A difference of more than 10 microseconds is perceptible as a change in the touch response. A frequent comment of players checking out early digital modelling amps, whose group delay is inherently big because of the time needed to encode and decode the signal, was that their strings felt a gauge heavier. This is a worst-case example, and this type of amp has greatly improved recently, but it identifies the area we should search to find the causes.