Pace, Rhythm, & Dynamics Page 6

Amplifiers & dynamics
It's hard to extrapolate from mechanical and acoustic concepts, whose effects may be directly appreciated in physical terms expressed in the movement of sound-reproducing structures, to electronics. In an amplifier, the sound pressure is represented by its electrical voltage analog due to currents flowing in wires and electronic components. Yet there is now the beginnings of a dossier containing data on electronic parallels with the acoustic rhythm experience. There are related types of uncertainty or randomness in electrical behavior which can disturb the internal equilibrium of an electronic component. Such disturbances seem to relate well to subjective weaknesses in rhythm and dynamics.

One area increasingly familiar to amplifier designers is the effect of large transients on conventional amplifier power supplies. A fast crescendo momentarily drains the reservoir capacitors faster than they can be refilled by the charging diodes, causing the main voltage rails to sag. Heavy ground currents also flow, which may inject additional disturbances according to the specific circuit design. Output tubes may suffer partial saturation effects, a loss of space charge, or heating of the plate/anode structure. In a solid-state unit, MOSFET or bipolar junctions will heat up, the rise in temperature being correlated with the signal waveform. All of these effects result in altered operating characteristics. Now we have the recipe for a degree of chaos, the kind of rhythm-disturbing randomness discussed earlier with loudspeakers. (Footnote 5)

Leaving aside the music-related waveform decay, consider the internal behavior of an amplifier following a heavy transient. The temperatures of the output stage devices stabilize over their time constants, these changes also associated with changes in device gain and bandwidth, operating point, and quiescent bias. The main supply rails have dipped and are now recovering to their mean voltages, the speed of this process largely dependent on the surge capacity of the supply lines and the regulation of the mains transformer. The recovery proceeds in bites of rectified 120Hz supply-line current in a semi-exponential form, these bites possessing a wide noise bandwidth.

During the initial dip and subsequent recovery, the amplifier circuit may fail to fully reject the supply-rail imbalances, and thus may show perturbations in its DC output level, this a particularly insidious error for the bass/mid drivers of two-way speaker systems due to the resulting voice-coil offset. The recovery characteristic of the main supply has a primitive time constant, but so do many other parts of the amplifier: for example, the negative feedback decoupling section; or, if fitted, the DC servo. Simpler amplifiers often have weak decoupling for their lower-level power lines serving earlier sections. These contribute their own recovery delays.

Thus, following a big transient, both voltages and currents inside an amplifier are on the move, none directly related to each other or, more important, to the envelope of the music.

The amplifier's overall negative feedback loop will largely mask this internal behavior from the traditional tests, which are essentially confined to the steady-state domain. However, the ear is certainly aware that all is not right. Some measure of proof that these internal effects are responsible have resulted from interactive listening tests in which some of these problems were specifically addressed by the designer. The results were well correlated with critical auditioning.

One such change examined was the size of the amplifier's power transformer. A clear correlation emerged between improved dynamics and rhythm and increased transformer VI rating. One explanation is that the larger transformer provides superior regulation, hence more stable internal power rails. Even more important, a larger transformer helps the reservoirs recover their equilibrium more quickly after a transient. Consequently, the amplifier spends more of its time in equilibrium.

Likewise for an output stage: Increasing its capacity and/or the number of power output devices has the benefit of reducing the thermal and current effects in each one. The result is more stable output behavior. Once again, the payoff is improved rhythm and dynamics.

Footnote 5: See the second half of Ben Duncan's "Harmonic Convergence," Stereophile, Vol.15 No.10, October 1992.---JA