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Using Amps As Effects

Tips & Techniques By Paul White
Published September 1994

We've got so used to DI'ing keyboards that miking up an amp is something that never occurs to some people. Paul White explores the benefits of getting out the mics and plugging in the amp.

All naturally occurring sounds are coloured by their environment, and we associate certain types of sound with specific acoustic spaces. For example, a church choir only sounds right within the acoustic confines of a church or cathedral, an underground busker desperately needs the acoustic of a Bakerloo Line tube station — and the speaking clock would sound totally out of place on a full‑bandwidth hi‑fi. In the studio, we usually use effects to simulate a plausible environment for DI'd sounds, the most obvious choice of effect being reverberation. Even so, the result often turns out to be more impressive as a spectacle in its own right than an accurate emulation of nature. Perhaps it's this lack of a breathing, organic environment that has sent people chasing old technology, such as tape echo units or valve processors, to try to put some of the character back. The philosophical implications of electronic instruments provide enough material to fuel many a closing‑time conversation, but the purpose of this article is not to bury Caesar — it's simply to borrow a few ears.

The key to the 'organicness' (or lack thereof) of electronic instruments is largely down to the amplifier and loudspeaker system used to reproduce them, and to the acoustic environment in which that amplifier is placed. If the instrument is DI'd, then the performance loudspeaker is the studio monitor or end‑user's hi‑fi system (which are both designed to deliver a nominally uncolored sound), and the acoustic environment can be anything from a studio control room to a bedsit in Putney. In other words, by DI'ing the instrument, you completely bypass the organic quality that comes from live performance in a specific acoustic environment. Trying to put back those missing components using effects can only be partially successful, because the fractal nature of real life is infinitely more complex than the algorithmic nature of digital effects. It's hard enough trying to make a recorded acoustic sound appear convincing when the holophonic soundfield of reality has to be replaced by the dual point‑source compromise of stereo loudspeakers, but it's even harder when the sound source you're reproducing never existed in the real world at all.

To be fair, there are occasions when a DI'd keyboard works on an artistic level; we're used to hearing music made that way, so our frame of reference is already based on artificial values, and because we've been listening to digital reverberation for the past 15 years or so, that also forms a part of the listening experience against which we tend to judge new work. Having made that point, all but the most ardent electronic music protagonists seem to agree that a piece of music sounds far more human or organic if at least some of the instruments are real rather than being all synthesized. It doesn't take much — just one electric or acoustic guitar added to a totally synthesized, sequenced composition can make a huge change to the way that the music is perceived.

Virtual Or Real

It could be argued that a DI'd keyboard isn't a real instrument at all, because the sound does not exist until it reaches the listener's loudspeaker system; it might be more appropriate to call it a virtual instrument. The line isn't quite so clear‑cut with a sampler, because it is possible to sample an acoustic instrument along with the ambience of its environment, but unless each note is separately sampled, the perceived environment will change as the pitch of the sample is changed. For example, if you take one sample with reverb already present, and use it over one octave, then the lowest note will have twice the reverb time of the highest note, and will appear to have been recorded in a room of twice the size. This being the case, you can see the logic in sampling sounds fairly dry and then adding sound processing afterwards; the effects may be artificial, but at least they'll be consistent, regardless of the note being played.

But there is a simple way to change a virtual instrument into a real one; plug it into an instrument amp and mic it up in a sympathetic acoustic environment. To the purists who would claim that an electronic instrument is basically a machine and so can never be classified as a 'natural' instrument, no matter what you plug it into, my response would have to be that all music instruments are the product of the technology prevailing at the time they were invented. If the digital synth doesn't qualify, then neither does the violin or the piano. In fact, the only 'natural' musical instrument, by definition, must be the human voice. My own view is that a synth plugged into an amp qualifies as a performance instrument, so I'll continue on that basis.


The wonderful thing about instrument amplifiers is that they do so much more than simply make a sound louder. All the best‑loved instrument amps, whether for guitar or keyboard, introduce their own subtle (or not so subtle) distortions and colorations. They all have their own distinctive tone circuits, may use valves which exhibit interesting non‑linearities, and may include transformers, which can do wonderfully constructive things to a sound. All these 'attributes' have the hi‑fi purists in tears, but the whole point is that amps don't just reproduce a sound, they help to create one!

Perhaps even more significant is the loudspeaker system used, and while the better keyboard amps are now a cross between a big hi‑fi speaker and a small PA system, it is possible to use different speaker systems to create specific tonalities. Before modern full‑range keyboard amps were developed, keyboard amps were more like guitar amps, often fitted with 12‑inch speakers and no tweeters. This limited the high frequency response of the systems quite severely, and had the effect of rounding off harmonically rich tones. At the same time, the open‑backed speaker cabinets so popular at the time behaved most irrationally at low frequencies, and created a deep, pleasing — and totally inaccurate — bass end.

Ambience always sounds more satisfying in stereo than in mono, so try to mic up your amp in stereo wherever possible.

Such keyboard amps tend not to be made any more, because there are occasions where you want to hear the fine edge of a bowed string, the rasp of a sax, or the breathy chiff of a piece of hollow tree with an oriental gentleman blowing meaningfully into the end of it, but other sounds cry out for the 'old amp ' treatment.

While keyboard amps have changed dramatically over the years, with a host of technological improvements, guitar amp manufacturers panic every time technology accidentally improves the sound, and then spend fortunes trying to get the new technology to sound the way the old valve circuits sounded in the '50s. Ironically, the valve amps of the '50s and '60s only sounded the way they did because the technology wasn't then available to make them sound any more accurate, especially when it came to loudspeakers. Designers probably stayed awake at night wondering how to reduce the horrendous level of distortion their circuits generated and improve the bandwidth and power handling of their loudspeakers! I wonder if future generations will modify their high‑definition TVs to give that slightly fuzzy, 'painting by numbers' look that you get from an early video machine with worn heads?

Fortunately, because there are so many technically awful, sonically wonderful guitar amps around, you can have a lot of fun by plugging a keyboard into one. For example, if you have a digital keyboard pad sound that seems a bit too thin and is a little too gritty around the edges, simply plugging it into a guitar combo will filter all the edge out of the sound without actually making it seem dull, and the uncontrolled speaker response at the lower‑mid and bass end will fatten the sound up quite nicely. You could of course do a similar thing using a guitar preamp and a speaker simulator, but then you'd lose the opportunity to mic it up and add a little real‑world ambience.

Mic Placement

So far, I haven't mentioned the mics or where you might want to put them. Ambience always sounds more satisfying in stereo than in mono, so try to mic up your amp in stereo wherever possible. An ordinary domestic room with the carpet rolled up and the major soft furnishings removed will produce plenty of ambience, while a completely empty room, concrete stairwell or glass conservatory can sustain several seconds of reverb.

Ideally, set up the room and the mics so that you get slightly less reverb than you need, because you can always lengthen the reverb time by adding a little extra artificial reverb. Whilst you might think that using this contradicts everything I've said in this article, artificial reverb added to natural ambience tends to sound much more natural than artificial reverb on its own. Additionally, if you can only mic up the amp in mono, then a touch of artificial reverb will help restore the illusion of space, though it's seldom as convincing as starting with a true stereo source.

On the subject of the mics themselves, if you're miking a small combo, a modest dynamic microphone should work fine, because it will have a significantly greater bandwidth than the loudspeaker it's 'listening' to. Even so, every mic sounds different, so the characteristics of the microphone become an integral part of the instrument. If you have several mics to choose from, try as many as you can to see which gives the best subjective sound, and though 'serious' stereo miking demands that you use an identical pair of microphones, in practice, you can use quite different mics and still get an artistically valid result. You have to remember that we're not so much interested in accuracy — just in getting a sound we like.

Placing the mic(s) close to the speaker grill will exclude most of the room ambience, but it will produce a focused, punchy sound which will cut through a mix without sounding edgy. Moving the mic back four or five feet will yield a softer, less upfront sound with the room ambience making a greater contribution. In a room with lots of hard, reflective surfaces, it's even worth pointing the mic at these surfaces, and not at the amplifier at all. Experimentation is the key to success here, and the best way to get results quickly is to put on a pair of high‑quality sealed headphones to monitor the output from the mic(s), and then wander around the room with the mics until you find the magic spots that deliver the sound you want. You don't even have to obey the rules of symmetry in stereo miking; you can use one close mic and one distant mic, or two mics pointing in quite different directions. All that matters is that the result works, though it is a good idea to press the mono button occasionally, just to make sure that phase cancellation doesn't screw up your sound.

Further Possibilities

One of the great things about recording sound is that absolute sound levels have very little meaning. A tiny amplifier can be made to sound huge simply by winding up the level in the mix, while a steaming Marshall stack can be pushed right to the back simply by pulling down a fader. Small practice amps often sound wonderful when miked up, and because they're not as loud as a performance amplifier, you don't have as many problems with isolation if the amp is running in the next room while you're trying to mix. Stories abound of famous musicians using the Tandy Microamp (which is little more than half an intercom in a plastic box) to record everything from guitar to harmonica. And while we're on the subject of guitars, if you need to create an over‑driven guitar effect from a synth, how better to do it than plug it through a guitar amp and turn up the overdrive? This invariably sounds better than the digital distortion effects built into synths.

If you have a small combo or practice amp, you can also experiment by connecting it to different speakers — even the most unlikely combinations can work extremely well. For example, an old TV or car radio speaker might distort in a particularly vigorous and interesting way when driven hard. Similarly, if you're after a boxy sound, don't resort to EQ straight away — stick a small speaker inside a tea‑chest or large cardboard box with a mic, and go for the real thing!

I'll finish off where I came in, by saying that the amp isn't just something to make the sound louder, it's a significant part of the instrument, and if you DI everything as a matter of course, you could be throwing away the best part of your sound without even knowing it.

A Subtle Point

Contrary to what you might think, you don't need lashings of cavernous acoustic reverb to give your sound that stamp of authenticity. Indeed, the room ambience may be so subtle that you don't realise there's any reverb there at all, but your brain will recognise it and respond to it. To illustrate this point, think about how the human voice sounds in a typical living room. It isn't obviously reverberant, but it sounds right. If the same voice were to be heard in an anechoic chamber, where all sound is absorbed, it would sound completely different and disturbingly unnatural. And DI'ing an instrument is, of course, exactly the same as listening to its acoustic counterpart in an anechoic room.

Fixed In The Mix

It's all very well trying to create the perfect sound in isolation, but there's always a danger that when it's heard in the context of a mix, it won't quite work and it will be beyond the capabilities of mere EQ to remedy matters. One way round this is to DI the basic sound (or sequence if synchronised to tape), but when you come to mix, feed the keyboard sound directly to your guitar amp (set up in another room), set up the mics and feed what they pick up back into the mix. Now, if the sound doesn't work, you have the opportunity to move the mics around, try different ones altogether, or make the room more or less ambient by changing the amount of soft furnishings, or by introducing reflective surfaces such as hardboard. Another benefit of working this way is that you don't have to record the sound to tape at all (if you're using a sequencer), or you only need to record it in mono if you want to play the part live to tape. If you have a limited number of tape tracks to play with, this can be a major consideration.