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ARP Quadra: Part 2

Keyboard Synthesizer (Retrozone)
Published May 2002
By Gordon Reid

In Part 1 we finished with the question: 'What is the Quadra? Is it a turkey of the first order, or a misunderstood and overlooked masterpiece?' This month, we'll conclude with a closer look at the instrument itself to find the answer.

Before looking at the sound-generating sections themselves, let's start by examining the brain of the ARP Quadra: a primitive Intel device called the 8048. More than just a microprocessor, we should perhaps call this an integrated microcomputer because it incorporates an on-board ROM containing the programming needed to control the Quadra. At first sight, this seems like a good idea, but it has a big drawback. If the processor in a Z80-based synth goes 'phffzzzt!' you can always remove the fried chip and insert a replacement, so long as there's no other damage. However, if the processor in a Quadra dies, you can only replace it with an 8048 programmed for a Quadra — and how many of those do you think exist today?

ARP Quadra.Photo: Richard Ecclestone

Setting aside this difficulty, I should also point out that in today's world the 8048 is significantly less intelligent than a washing machine. The most obvious consequence of this is that, despite ARP's claims to the contrary, the Quadra is not a programmable synthesizer. Sure, it has 16 memories, but its 64 bytes of memory (yes, 64 bytes!) can't store the parameter values that determine each patch. Let me explain...

The processors used in hybrid analogue/digital synthesizers can store and respond in real time to dozens of parameters, while simultaneously scanning the keyboard and generating real-time LFOs and envelopes. Ignoring other factors, the power of the central microprocessor determines how well a given synth does this, so the more recent the model, the quicker and smoother its digital facilities should be. However, there is a threshold below which a processor is incapable of carrying out these tasks: and the 8048 falls way short of the mark. This means the Quadra is an idiot, capable only of scanning its keyboard to see what you're playing, controlling a handful of performance facilities, and storing the on/off status of the 40 switches on the control panel.

To understand how limiting the last of these is, let's consider parameters such as attack rate, sustain level, or filter cutoff frequency. On an analogue synth without memories, each of these has a theoretically infinite number of possible values; but when you add digital circuitry to scan and record them in memory, the number of possible values is determined by the number of 'bits' used in the storage process. If, for example, the cutoff frequency is stored in five bits it has two to the power of five possible values, which is 32. Considering that a single bit can only be on or off, and nearly all the Quadra's parameter memories are 1-bit, this means that instead of storing a value the memories store only an on/off status associated with each parameter. In other words, the memory determines whether a given control is active or not. And, to make matters worse, not all of the Quadra's controls have these switches, so an already primitive system isn't even comprehensive.

You may ask, 'what happens when a parameter is off?', and to answer this, take a look at the Quadra's control panel. You'll find that 21 of the on/off switches are linked graphically to 21 of the faders, and, looking more closely, you'll see that 17 of these faders have small red dots drawn alongside them. These dots tell you the preset value used when the fader is off — which is to say, when it's disconnected from the parameter it otherwise controls.

You might think this system is ridiculous and, in my opinion, you'd be right. However, it's surprising how much mileage you can get from these memories, switching one or more of the Quadra's four synthesizer sections in and out of the patch. The trick is to find a combination of fader settings that allows you to jump between dissimilar but still usable patches when you change memories. This isn't trivial, but it's entirely possible if you're prepared to compromise a little.

Having got that major obstacle out of the way, let's take a look at each of the Quadra's four synthesizers in turn. We'll start with the Strings, which could hardly be more basic...

More Reliability Issues

In Part 1 I noted some of the reliability problems encountered on the Quadra, but here's another: the membrane switches in the control panel have a finite lifespan, and when they die there's no way to resurrect them. Cannibalisation is your best bet, but that requires that you find another Quadra with a functional panel, which isn't very likely. Furthermore, I've heard rumours that there were two versions of the panel and the board that drives it, making it possible that, even if you find a potential replacement, it will be incompatible.

Apparently, one fabled Quadra owner has built a substitute panel using sheet metal and modern micro-switches, which must have been a labour of love and resulted in Frankenstein's Quadra. I'd love to see a photo of this — are you out there, Dr F?

The Strings

The underlying sound generation of the string section is provided by the top octave divide-down technology used in cheap organs, electric pianos, and ensemble keyboards throughout the 1960s and '70s. This means that a single master oscillator is divided down into 12 frequencies (one for each note in the octave), and then further divided in factors of two providing the frequencies for each of the notes on the keyboard. However, instead of having 48 (or even 96) oscillators covering the four octaves over which the Strings are playable, there's just one! Moreover, only a square wave is produced by the oscillator, so the Quadra requires a wave-shaper to provide the sawtooth waves used for conventional string ensemble sounds.

From the player's point of view, the Strings section covers the top four octaves of the Quadra's five-octave keyboard, offering two pitches (4-foot and 8-foot, which can be played individually or in unison) with variable attack and release. And that's it — just two octave settings and an AR envelope!

Unfortunately, that really is it because there's only a single envelope and VCA for the entire string section, and unlike the Logan String Melody II, for example, the Quadra is unable to articulate notes individually. In fact, this problem is so severe that if you program a sound with a fast attack and a long release and then play two chords in quick succession, the first chord swells up slightly with the second before dying away. Not too distressing if you play two inversions of C major, but it's a different story if you play F# major after Cmaj7. On the positive side, though, many ensemble keyboards produce a continuous 'swishing' noise whether you're playing them or not. The Quadra doesn't suffer from this because when you stop playing, the VCA gain drops to its minimum, closing the signal path.

However, to concentrate on these strengths and weaknesses is to miss the point. The Quadra's lush string sound, created by ARP's revered chorus ensemble (called a "phasor"), is a classic — not surprising, given its pedigree. The Quadra's predecessors (the Solina and ARP String Ensemble, both built by Eminent and imported from Holland, and the Omni and Omni II) dominated the mid-'70s and remain objects of desire to this day.

The Quadra's Filter & The Continuing Mystery Of ARP Filters

The Quadra incorporates two ARP 4075 filters: one for the Lead synthesizer, and one for the Polysynth section. The 4075 is a self-oscillating 24dB/octave device introduced by ARP in 1976, which became the company's standard filter for most of its instruments thereafter. These include Odysseys with model numbers from 2810 to 2823, Axxes from 2310 to 2323, the Omni and Omni II, the ProDGX and the Avatar, in addition to the Quadra itself.

The advantages of the 4075 are that it offers low noise and distortion, sounding much like a Moog filter at low resonance settings, and, unlike many filters, it passes an input signal at high resonance. The disadvantage is that, due to a design miscalculation, it has a maximum cutoff frequency of around 12 kHz, but fortunately all is not lost. If you're a bit of a whiz with a soldering iron, you can increase the cutoff frequency by replacing four resistors. If you're prepared to take the risk on such a fragile machine, I suspect that you'll improve the sound of the Quadra considerably by doing so.

The Electric Bass & Strings Bass

To the left of the Strings is the Bass synth, which can be played using the bottom two octaves of the Quadra's keyboard. There are two sub-sections: the first is the Electric Bass and, as the name suggests, this produces something akin to an electric or synthesized bass sound. It's based on a single oscillator but, thanks to another frequency divider, offers 8-foot and 16-foot octave settings. A 'fine tune' fader is provided, together with a 'Down One Octave' switch that allows you to play deep bass at 16-foot and 32-foot pitches. The VCO itself produces a simple sawtooth wave, and this is used unmodified for the 8-foot setting. However, a wave-shaper converts the sawtooth to a pulse wave for the 16-foot setting, although ARP's documentation is inconsistent on this point. Sometimes it suggests both the 8-foot and 16-foot pitches are pulse waves, but elsewhere it states the 8-foot waveform is the unmodified sawtooth.

The mixed 8-foot and 16-foot waves pass down a simple VCF-VCA signal path, and an AD envelope (with variable Decay) controls the pulse width of the waves, the filter cut-off frequency, and the VCA gain. There's also a slider that allows you to control the filter resonance, which is slightly weird since the documentation says the Bass section uses a single-pole (6dB/octave) filter. If I remember my electronic theory correctly, you can't have a resonant single-pole filter — there's something odd going on here.

The second sub-section is the Strings Bass, and this too offers 16-foot and 8-foot settings, incorporating wave-shaping and PWM of its 16-foot output, fine-tuning fader and a Down One Octave switch. However, the Decay and Resonance controls do not affect it. Instead, the output from the Strings Bass is controlled by a dedicated volume fader before passing to the String section's chorus generator, and then through the Strings' VCA, which is, of course, subject to the Strings' AR contour generator. This means that you can extend the attack time of the Strings Bass, but not its preset release.

The Quadra's designers may have hoped this arrangement would allow you to detune and mix the Strings Bass with the Strings themselves to create fat, synthesized bass sounds, but, if so, they'd have been disappointed. Low stringy sounds, yes. Fat basses — not a chance. And, while you can use the Electric Bass to create a wide range of plucked bass sounds, the use of a single oscillator for all four of the footages in the String and Electric Bass sections ensures the classic sounds of, for example, Moog Taurus pedals are totally out of reach.

The Lead Synth

Next we come to the Lead synthesizer, which covers all five octaves of the keyboard, or just the top three if the Bass section is active. The manual proclaims that, "if not contained in the Quadra... its features would rival those of other lead synths in its class", but I'm not sure I agree. To be fair, the sparse controls hide a wealth of clever ideas, but many important facilities are missing, such as a noise generator, adequate modulation, and truly independent oscillators. Still, rather than concentrate on what's lacking, let's see what survived.

There are two detunable oscillators, and you can further offset these in semitones using a novel approach requiring you to 'write' an interval using the keyboard. Each oscillator produces sawtooth, square, and 10 percent pulse waves, with PWM for the last two of these. Unfortunately, the pulse width is modulated only by the envelope, so there's no cyclic PWM for 'chorus' waves. Furthermore, there's only one waveform selector for both oscillators, so you can't mix sawtooth and pulse waveforms.

The keyboard generates two monosynth pitch CVs, so, like the ARP Odyssey, the Quadra offers a two-voice mode allowing one oscillator to track the highest note played, while the other tracks the lowest. This is not true duophony because there's only one signal path, but it can still be very useful.

A dedicated LFO provides vibrato, but this is always delayed by a preset amount. I tend to use delayed vibrato, or control its amplitude using pressure sensitivity, so this doesn't bother me; but I expect that there are some players who will find it very annoying. The other pitch control is Trill, and this does what you would expect: it trills between two user-defined pitches. However, setting this up is arcane, and you'll never stumble across the method by accident.

The output from the oscillators passes through a self-oscillating 24dB/octave filter (see box) that tracks the keyboard accurately at 1V/octave. There are cutoff frequency, resonance, and envelope 'amount' controls, but no variable keyboard tracking or LFO filter modulation. And this brings us to the envelope generator itself — one of the weirdest parts of the Quadra.

The Attack is simple with its conventional attack rate. The Decay isn't quite so easy. If the 'Sustain = Low' switch is on, the Decay slider acts conventionally, and the Sustain Level is preset to approximately 20 percent of the Attack level. But if the 'Sustain = Low' switch is off, the decay time is preset, and the Decay slider sets the Sustain Level between 0 and about 75 percent of the Attack level. The envelope generator holds the Sustain level (set by the Decay slider) for a preset time of around 1.5 seconds, and then enters a second attack to climb back to 100 percent of the peak level, where it stays until you release the keys, before falling at the rate set by the Release slider. ARP described this as an ADSASR generator, and it turns out that its shape (known classically as sforzando) is ideal for brass patches.

The Lead synth has a number of other goodies tucked away. For example, the VCOs have independent portamento speeds, and played monophonically these provide a range of interesting effects. When played duophonically, they allow you, for example, to slew the melody while jumping between notes in the counterpoint. The keyboard also offers single and multi-triggering, plus pressure sensitivity that affects either the pitch or the brightness and loudness. It's a shame you can't control the LFO depth this way and, since the Quadra lacks pitch and modulation wheels, a joystick, or a ribbon, I reckon ARP should have been more generous with its aftertouch.

Finally, as far as the Lead synth section goes, there's a basic arpeggiator with 'up' and 'up & down' modes. There's no way to select the number of octaves over which the arpeggiator plays (it's just the notes you hold across the keyboard), but you can combine the arpeggiator with the Trill feature to generate some interesting effects.

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.

Published May 2002