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Page 2: ARP Quadra: Part 2

Keyboard Synthesizer (Retrozone) By Gordon Reid
Published May 2002

The Polyphonic Synth

Finally, we reach the Polyphonic Synthesizer. In Part 1 I suggested that, if this had been a true polysynth, ARP would have had another winner on its hands. But rather than provide independent voices for each of the notes you play, the Quadra uses the paraphonic architecture of an earlier generation of string machines, see the Polyphonic and Paraphonic box. To be specific, it uses the guts of an ARP Omni II, meaning there's only one envelope, VCF, and VCA for the whole polyphonic section. In my opinion, this above all else killed the Quadra's chances of commercial success.

Like the Strings section, whose divide-down oscillators it shares, the Polysynth covers the top four octaves of the keyboard, unless the Bass is active, in which case it restricts itself to the top three. This bass/poly split overcomes some of the limitations of the paraphonic architecture because you can trigger bass notes, even when holding a sustained polyphonic chord. But it's still no substitute for true polyphony.

FOUR IN ONEPhoto: Richard Ecclestone

It's now time I mentioned the Hollow Waveform switch lurking halfway between the Strings and the Polysynth, which toggles the wave-shaper I mentioned in the Strings section, changing the source waveform from a sawtooth wave to a square wave. Unfortunately, because there's only one master oscillator for the Strings and the Polysynth, the switch acts upon all footages of the Strings and Polysynth simultaneously, so you can't mix the waveforms.

Returning to the polysynth itself, there are two octave settings (8-foot and 4-foot) and, as in the Strings section, you can select these individually or in unison. As already noted, there's only a single filter for the whole section, although you can modulate this using the dedicated LFO and envelope. Strangely, the Release setting in the Polysynth ADSR is dependent upon the Release setting in the Strings section, so you can't obtain a slow release from the former without checking the setting in the latter. Weird! In addition, the maximum envelope times are rather stingy (in the range of one to three seconds) so, even with the single and multi-triggering options it shares with the Lead synth section, the ADSR and VCA aren't as flexible as they could have been. On the other hand, the filter is suitably aggressive and the envelope is nice and snappy, so the Poly section can be quite punchy when required.

If the description of this section appears to be very brief, it's because there's nothing more to say. As facilities go, the Polysynth has the bare minimum of features needed to justify its name, but you can still coax a fair range of sounds from it. I use mine to recreate electric pianos, other percussive sounds, and a range of fine brassy timbres. But as for the warm, thick and evolving textures that pour almost unbidden from a Prophet 5, an OB-X, or a Jupiter 8 — forget it! The Quadra is a string machine on steroids, not a real polysynth.

Polyphonic & Paraphonic

You should never call a synthesizer 'polyphonic' simply because it can play several notes simultaneously. To earn this description, it must be able to shape each note using dedicated contour generators, VCFs and VCAs, and without deference to any notes that may already be sounding.

There are two ways to accomplish this: firstly, you can use one or more envelope generators and filters for each note on the keyboard, which is the approach used in the Korg PS32xx series; secondly, you can restrict the number of notes that will sound simultaneously, and assign a more limited number of oscillators, filters, amplifiers, and envelopes to them as the notes are played, which is the approach used in almost every other polysynth. So, while the Quadra's polysynth section is polyphonic in the sense that you can press every note simultaneously, its single envelope generator, VCF, and VCA means that it can't articulate them correctly.

Roland coined the name 'Paraphonic' for this architecture, and it was used by many famous keyboards, including the Solina, the ARP Omni and Omni II, the Roland RS202 and RS505, the Korg Trident and Poly 800, the Moog Opus III, and the filter section on the Polymoog.

The Phaser, Mixer Section & Rear Panel

The outputs from all four sections travel to the Phase Shifter/Output Mixer section that dominates the centre of the Quadra. This is where you mix their relative amplitudes and, if you wish, mix in an external signal. Unfortunately, the external signal passes only through the mixer, so you can't filter or otherwise modify it, which is a shame.

But the phaser is perhaps the most well-known feature of the Quadra, and has been the subject of much hushed reverence over the years. Technically speaking, it's an inverted 14-stage phaser generating six notches and seven peaks in the spectrum. It also incorporates a compressor and two expanders that maximise the signal-to-noise ratio in the affected signal, allowing much deeper regeneration than would otherwise be possible.

You can apply the phaser to any combination of the sections, and it will add anything from an ethereal stereo swirl to a rasping flanger-like sweep, courtesy of its dedicated LFO and self-oscillating resonance. But this isn't the end of its abilities: you can control the sweep using a dedicated sample & hold (sampled from VCO2 in the Lead section), from a dedicated AD envelope generator triggered by playing the Bass or Lead sections, or by using a foot pedal. Ah yes, a foot-pedal! The mere mention of this takes us round the back to find no fewer than 26 inputs and outputs — what the Quadra lacks elsewhere, it makes up for in interfaces.

Firstly, there are the seven signal outputs on quarter-inch jacks: one for each of the four sections, a left/right stereo pair (each side of which carries one or more sections depending on the internal routing of each), and a mono sum carried on both quarter-inch and XLR sockets. Plugging a cable into any of the individual outputs removes that signal from the mono, stereo, and phaser busses, which is useful in both live and studio situations. It does mean, however, that you have to think about what you want to direct where, before you start connecting plugs into the back of the synth.

Next, there are six footswitch and pedal controller inputs on quarter-inch jacks. The first three of these are the Lead synth filter cutoff frequency, the Poly filter cut-off frequency (the same pedal also controls the phaser shift, depending upon the status of a front panel control) and the master volume. The others are the Poly and Strings sustain, a Lead synth 'hold' that allows you to vamp over a held note or trill, and semitone pitchbend (another dual function pedal whose action is determined by a front panel switch), plus the Lead synth portamento on/off.

To the right of these, you'll find 11 CV and gate inputs and outputs on 3.5mm jacks. The first four are dedicated to the Bass section: CV and gate in, plus CV and gate out. The next three are CV in, gate in, and trig in for the Lead synth; and the last four comprise two CV outs (one for each oscillator in two-voice mode) gate out and trig out for the Lead synth. These make the Quadra an interesting, if somewhat unwieldy addition to a 1V/octave modular synth set-up.

The final item on the back panel is a switch that enables and disables memory programming.

More Hidden Tricks

I'm indebted to New York composer and engineer Bill Lacey for reminding me that each of the Quadra's individual outputs is a tip/ring/sleeve socket, capable of sending and receiving an audio signal. This means that, with a number of suitable 'Y' cables, you can direct each of the Quadra's four sections to the external effects units of your choice, and then return the resulting sounds back to the Quadra's 4-channel mixer and phaser.

The possibilities are astonishing and Bill uses a Line 6 distortion modeller on each of the Lead and Bass sections, which apparently provides instant Hendrix. In contrast, being an even older hippy, I'd place something like a Eventide Harmoniser across the String section to create huge, lush swirls of sound, a Hughes and Kettner Rotosphere across the Poly section, and, oh all right, a Line 6 Pod across the Lead section!

So although you need to invest in a range of external pedals and rackmounts, the Quadra may well be the earliest keyboard to offer a true polyphonic effects structure you can remix internally and then output through a stereo pair, which is pretty amazing.

In Use

Having read all the pros and cons in the above descriptions, you may be asking yourself why anyone would bother with a Quadra. As Peter Forrest wrote in the A‑M Of Analogue Synthesizers, "It can all be done (probably much better) with a good MIDI controller and three modules," but that only tells half the story. Sure, you could almost do more with an Omni II and a couple of Axxes, and possibly save yourself a few pennies into the bargain, but the Quadra is one of those rare instruments that's more than the sum of its parts. While it will never be a source of instant gratification for players who want to switch on, select a patch, and play, it does reward perseverance.

So let's assume you want to use the Quadra as a five-octave monosynth. Your sonic needs may or may not be satisfied by the Lead synthesizer section alone, but have you considered what happens when you add the Polyphonic section to the Lead? You now have three freely tunable oscillators with two independent waveforms and LFOs, two rates of portamento, pressure sensitivity on two oscillators, two of ARP's superb filters, a stereo phaser, multiple outputs, and more foot controllers than you can control simultaneously without enlisting the help of two friends. Is that enough? It should be. And if somebody offered you such an instrument, what would it be worth in these analogue-crazy times?

Alternatively, let's assume that you want to create lush pads. Layering the Strings and Polysynth is the way to go: simply add 8-foot and 4-foot Strings to an 8-foot and 4-foot polyphonic pad that passes through the phaser. There's nothing thicker or more lush and, since the Quadra is fully polyphonic, you'll never run out of voices. Now reprogram the Poly section to produce a percussive sound and use just the thinner, more ethereal 4-foot strings. If you like, you can select single triggering, using your playing technique to control the presence of the percussive patch.

But why stop there? Add the Lead synth again and set it to two-voice. You now have monosynth voices in addition to Strings and Poly on the highest and lowest notes played. For good measure, you can then add the Strings Bass to thicken the bottom end, and the Electric Bass on full resonance to give it a nice 'twang'. The lowest Lead voice and the Poly section now sit above the Basses (because they're mutually exclusive), whereas the Strings and Basses overlap in the second octave. This is weird, but it offers yet more possibilities.

You can also use the foot pedals to latch an arpeggio, and trill the lead synth section while you continue to play the Poly, Strings, and Bass sections. And make sure that you're making full use of the phaser and the individual outputs, assigning them to appropriate EQs and external effects. The results can be nothing short of glorious. I mean it — despite its limited palette the Quadra can be massive, and it sounds like no other single instrument in the vintage synthesizer world. It's a true classic, and had it appeared in 1975 rather than 1978 it would have been hailed as a miracle of music technology.

So, to be fair to Peter Forrest, I'd better add the second half of his sentence, which I so deviously left out a few paragraphs ago. He concluded, "but there's still a character that the Quadra has which is quite hard to duplicate." He wasn't wrong.


Despite its flaws, limitations, and the continual worry that it may go 'phffzzzt!' and die, I remain very fond of my Quadra and have never regretted buying it. Other people must feel the same way — a working Quadra, let alone a near-mint one, is rapidly becoming a prized possession. However, you won't be seeing mine for sale in the SOS Readers' Ads, no matter how lucrative this might be. And that, as far as I'm concerned, is the true test of its worth.

ARP Quadra: Part 1 of 2.

ARP Quadra: Part 2 of 2.