Your 'which knobs matter' guide
Most players have a good idea of what instrument best suits them, but with amplification the choice can be less obvious.
When choosing an amplifier, what ever your choice of instrument, the first question must be 'how big?'. It was once the case that any self-respecting guitarist would need a vast wall of 100 watt amp heads and 4x12 cabinets in order to maintain credibility, and in fact some heavy metal acts still have a contingent of dummy cabs on stage simply for appearances. For most players though, the modern combo (combination amp and speaker) is capable of meeting all demands. This is not because demands have dropped, far from it, but rather that modern materials, components and system designs such as those used at Roland, are capable of far greater things. Hence, for most players a combo will probably be the answer.
Having said that, what sounds strong and punchy at the relatively low levels permissable in a showroom may pale to a thin and puny rendition when pitted against the vastness of a public auditorium. Many combos will have an amplifier with a power output capability that will allow a socket to be included for connection of an extension speaker thereby creating a powerful 'stack' effect. The greater volume derives from the fact that, with an extension speaker, the amp is connected to a lower impedance. Examples from the Roland catalogue include the top end of the Jazz Chorus range, the BN Series bass combos and almost all the Cube and Super Cube models.
An instrument amplifier can be split into two basic components: the preamp (the first stage), driving into the power amp (the second stage) which in turn develops the power to drive the speaker system. If the gain of the preamp is turned up high it can be made to cause the input of the power amp stage to distort giving that classic raunchy 'over-driven' edge to the sound. An amp using a 'Master Volume' system provides separate controls for the gain of both the preamp and the power amp, thereby allowing you to determine in some detail the amount of 'raunch' you want. The same facility is sometimes given a different name, for instance on Roland's Jazz Chorus amps you are actually given a 'Distortion' control and with the Spirit range the relevant knob is marked 'Overdrive'. With certain amps, such as the Super Cube Leads, it is possible to set two separate preamp levels, one which produces a clean sound and a second higher level which creates the distortion. In such cases a footswitch is generally provided so that you can switch quickly between these two states during a live performance. It's a very useful feature.
In terms of control, the tone or equalisation (eq) section, on an amp is also of great importance. Apart from very small practise amps any box worth its salt will have at least two eq controls: bass and treble. More professional amps, and indeed this includes the whole of the Roland range, will have a third knob labelled 'Mid', allowing control over the middle band of frequencies, which are very significant for creating full, rounded tones. More refined still is a sweepable eq section which allows you to alter the frequency bands effected. This gives greater scope to create precisely the tone colour you hear in your head. Because of the extra problems experienced with low frequency resonances, bass amps like those in the Super Cube Bass series, will be more likely to have this type of eq, although it isn't altogether unknown on guitar and keyboard amps.
Beyond its simple sound performance, however, a whole host of other facilities are available to allow an amp to interface with other vital equipment, such as mixing consoles or processing units. If you play bass or keyboards, it will often be the case that the engineer, whether in the studio or live, will ignore your amplifier's acoustic output in preference to a 'direct injection', or DI for short. This is where the instrument is electronically connected directly to the mixing console without the inconvenience of a speaker on the way. It is possible to achieve this by either simply plugging the instrument into a line input socket on the mixer (although this is generally a little unsatisfactory due to impedance and level mis-matching), or by using a purpose-built 'DI Box' which provides better matching. In each of these cases you have no control over the quality of the signal being sent, save for that offered by the instrument's tone and level controls. A better way is to use the 'Line Output' socket on the rear of your amplifier (if it's fitted with one). Naturally, Roland are not unaware of this fact and practically all of their amps provide some means of direct injection.
Effects units, whether in the form of pedals or sophisticated rack systems (did somebody mention the Boss Microrack?), are becoming an accepted part of a musician's equipment. In the case of pedals it is possible to simply string a series of effects between the instrument and the amplifier's input, but more professional units usually require a 'line level' feed in order to deliver optimum performance. Hence it is important to have an 'Effects Loop' facility on an amp, such as that found on the Super Cube range. Reverb is virtually indispensable for giving a sense of 'space' that no other effect can offer. In sizing up a reverb system, listen for a general bright tone, though with an evenness of response across the spectrum from high notes to low notes, and make sure that it isn't prone to 'wobbling' when subjected to sharp, short transient sounds. Ideally it will also have a footswitch to allow the effect to be quickly kicked in and out during live performances. The Roland JC series amps are interesting in that they are the only production amps to offer built-in stereo chorus. Next to reverb, chorus is the most widely used effect in modern music, and as Roland originally invented it you won't be surprised to find that the quality of the effect offered is quite excellent.
One last word for the sake of the neighbours. It's always worth making sure that an amp has a headphone output socket, so that you can go to town when the people next door want to go to bed—and sleep. You may quite possibly consider yourself 'One Mother of a Rock 'n' Roll Animal', but there's always room for a little civility, isn't there? Good night.