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!