Bob Gallien On Bass Power Amps What is the power amp, what makes it sound good?
Before we can talk about power amps we need to understand what the power amp does. Guitar signals are very weak, so upon entering the bass amp they get a little boost by the pre-amp (“pre” as in before). The pre-amp is also where controls like tone and volume live. Signals leave the pre-amp at line level which is suitable for driving a mixer, but not nearly strong enough to drive a speaker. This requires a power amp. Driving a speaker properly involves far too many issues for this article, so I will limit this discussion to a few of the more critical ones that are close to my heart.
Speakers are unwieldy things that fight the power amp’s every command. When the power amp has to reproduce a high-power transient like a string slap, it must be able to deliver a high current pulse to maintain cone control. If the amplifier can’t do this, it simply cuts the transient off producing an unresponsive less out front sound. Creating these high current pulses (three to four times the current required to deliver its rated power) requires extra power devices, and intelligent protection logic.
Additionally during live performances the power amp is often over-driven, and it must recover promptly to reduce the unmistakable ‘farting’ sound bassists abhor. Again the power amp must be able to deliver high currents and it must do it fast. Speed in this case means responding at several times the range of human hearing (at least 100 kHz bandwidth). If the power amp has sufficient high current capability and is fast enough, it can turn a potential distorted and weak situation into a strong silky growl. This is the GK growl!
Distortion is also an important attribute of a power amp, and it is measured in three ways Total Harmonic Distortion (THD), Inter Modulation Distortion (IMD) and Transient Inter-Modulation Distortion (TIM). Harmonic and Inter-Modulation Distortion are measures of how faithfully an amplifier reproduces the signals coming into it. These signal errors are very recognizable as distortion by most people and also are fairly easy to control by designers. On the other hand TIM is not easily recognizable as distortion, because it is only present when strong transients occur. Designing for low TIM is not easy and involves concepts beyond the scope of this article. Just let it be said an amplifier with low TIM will sound more articulate and responsive, again more out front.
These are the qualities a power amp must exhibit to separate its sound from the other chaos occurring on the bandstand, and are the qualities designed into every GK power amplifier.