A Workable Solution?

Part of my own work, in response to customer demand, is to return PCB-built versions of classic amps to the sonic character of the hardwired originals. One popular modification is of a famous amp that was re-introduced in an updated version a few years ago. I normally remove its PCB-based preamp assembly and replace it with a hard-wired one, but an equally significant improvement is to disconnect from the power-stage PCB the resistors that couple the driver stage signal to the output valves and mount them on old-style tagstrips with flying wire connections. With this done, the amp 'comes alive', in the words of one customer.

Although hardwiring the preamp board makes a tonal difference, the amp only regains the touch response of the hardwired version with this modification. This is the part of the circuit where impedances and signal levels are at their highest and small changes could be expected to have a significant effect.

I wouldn't suggest that this is the only way of solving the problem. Recently, one or two manufacturers have reverted to an earlier type of PCB layout, using a 'trackless' PCB on which the components are fitted between turret tags whose connections are on common printed sections of board, avoiding interconnecting tracks (Pic 5).

While this is only practical with uncomplicated circuitry, it offers a solution for building older designs, which aren't usually densely populated, on PCBs, so most of their cost savings can be realised without sonic penalties. The claims of guitarists to hear differences that can't be effectively measured have traditionally been derided by the technical fraternity – but now they've become a starting point for any amp manufacturer who wants to be taken seriously.

Techie Sidebar: Transient Response Measurements

We were curious about how a subjective phenomenon like the 'touch response' of a guitar could be quantified, so we did some benchwork (Pic 6). A possible method of assessing this aspect of amplifiers is the square wave test. A perfect square wave has a practically non-existent rise time – it goes from zero level to max in a microsecond. Observing the output of an amp or circuit driven with a square wave reveals a lot about its transient response behaviour, which is an important element, if not the key, to touch response. No valve amp tracks a square wave perfectly, and the slight slope given to its leading edge by most analogue amp circuits is used as a measure of their success or otherwise in following its instantaneous rise-time. The difference between the vertical rise of the test-signal and the slightly less vertical version of the amp's output is measurable in µS (microseconds) on the horizontal ( X-) axis of an oscilloscope.

To test the effect of long parallel PCB tracks on transient response, two such tracks were isolated and a I kHz square wave applied to one end via a resistor, to simulate the high source impedance of a valve amplifying stage, the other track being used as the earth return. This resistor causes the squarewave to slope, even without connecting it to the PCB, because it interacts with the capacitance of the scope's test probe – but the slope increased when the end of the resistor touched the PCB track. The rise time reading the generator direct was measured at 1µS, via the resistor on its own 10µS, and 15µS with the resistor connected to the PCB (Pic 7). The tested track distance of 10cm is not uncommon in larger layouts. Often more than one such circuit would be found in an amp, and three such stages might add 15µS to the rise-time over the hardwired version. It's the difference between a high-quality guitar cable and an average one.