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Deep Dive: Vintage Moog Modular Synthesizer Filters

Defining the Moog Sound

Ryan Gaston · 09/07/21

So, if you're even vaguely familiar with the history of electronic music, you most likely already know about Moog. Robert Moog was one of the first commercial synthesizer manufacturers, developing his first synthesizer systems in the mid 1960s. With the help of many close collaborators and the influence of designers like Harald Bode and Raymond Scott, Moog created an instrument that gave musicians unprecedented control over all aspects of sound—timbre, pitch, rhythm, and so on—and gave them the ability to manipulate these in real time in a countless number of ways.

This was a tremendous contrast to other workflows in electronic music at the time, which most commonly relied on the use of appropriated test equipment, recorded sounds, and tape manipulation, which typically led to a process with only limited realtime control and little opportunity for "musical performance" in a traditional sense. Moog took inspiration from the use of test equipment—which often employed oscillators, filters, amplifiers and the like—and through collaboration with musician Herb Deutsch and through his own familiarity with musical electronics in the form of the theremin, Moog was able to devise a system in which multiple devices could be housed in a single chassis and interconnected to influence one another's sound in a variety of ways, and even to alter the way that one another behaved over time. That is to say, the early Moog systems are modular (or comprised of individual, distinct devices), and that by coming together (through synthesis), they can create a more meaningful whole. By patching these modules together, you create an audio signal path AND craft the way that sound evolves...through the use of control voltage.

So Moog's instruments quickly gained popularity in the hands of countless gifted musicians—Wendy Carlos, Paul Beaver and Bernie Krause, and Keith Emerson, to name only a couple—to the point that the name "Moog," for many, became SYNONYMOUS with the word "synthesizer." And in fact, many of Moog's design concepts quickly became standards that OTHER designers looked to when developing their own instruments...and to this day, the workflow of Moog instruments is still an obvious influence present in almost every commercially available synthesizer.

We recently had the opportunity to spend time with a particularly special early 1970s Moog modular system, and opted to take the time to dig into one of the Moog system's best-known features: its filters. Of course, we should first take time to understand the system as a whole, and to point out what exactly makes this particular instrument so interesting.

The Moog Model 12

The system used in our video is a modified version of the Model 12, one of Moog's early small systems, produced in the early 1970s (around the same time as many larger studio systems and the smaller, self-contained Minimoog). Now, despite the fact that this synth is modular and can be configured to act in a wide number of roles, this system really feels like it wants to behave like a prototypical monosynth, and contains many of the same types of things you'd expect to see in a modern keyboard monosynth. So, let's take a quick look at what this system contains to get a better sense of how things work altogether.

So this system contains three oscillators—a 921 and two 921Bs. These each have individual outputs for multiple wave shapes, as well as inputs for sync and frequency modulation. Each oscillator has its own fine frequency control and octave selection switch. Now, the 921 functions more or less the way that we'd expect an oscillator to in this day and age, but the 921Bs are a little bit different. In the Moog ecosystem, groups of 921B oscillators are always paired with a 921A Oscillator Driver module, which provides a master pitch control and pulse width control for all attached 921Bs. So, these are connected behind the panel in such a way that the 921A's frequency and pulse width knobs and CV inputs affect both of the connected 921B oscillators. This keeps the oscillators tracking together, and makes the common technique of using multiple oscillators for a single synth voice logistically simpler to execute. The 921 is independent of these—and while it is certainly a more than capable audio oscillator, in the context of a system like this, it is commonly used as an LFO, to provide cyclical modulation of other parameters. We'll explore that more in a bit.

Original Moog Model 12 sales flyer Original Moog Model 12 sales flyer

This system also contains a 903A Random Signal Generator, which produces white and pink noise, two 911 Envelope Generators (the original ADSR envelopes, despite the fact they aren't labeled as such), a single 902 Voltage Controlled Amplifier, a dual multiple module (used to split signals to send to multiple destinations), and the Moog Model 12 Control Panel, which contains the interface connections for external controllers (like a keyboard), as well as a reversible attenuator (now sometimes called an attenuverter) and the CP-3 mixer, which is actually itself a fairly significant part of the instrument's sound. Again, don't worry—we'll take a look at how to use all of these modules as we go along.

Now, this particular instrument is a bit different than the standard Model 12—because at the request of the original owner, Moog expanded its filtering section. So while a typical Model 12 would only have the 904A Lowpass Filter and the 907 Fixed Filter Bank, this one also contains an 904B Highpass Filter and 904C Filter Coupler—which have physically displaced the 907, but it all still works just as well when connected to the courtesy power connection on the rear of the unit.

So, without further delay, let's start digging into this instrument's filters, starting with the 904A Low Pass filter.

904A Low Pass Filter and The Moog Sound

Moog's 904A well may be one of the most significant developments in the Moog system altogether. Despite countless other revolutionary concepts in the Moog ecosystems, the 904A is the source of what many consider to be the "Moog" sound: a huge, growling, undulating character that goes from smooth swells to punchy articulations and everywhere in between. The 904A Voltage Controlled Low Pass Filter is the original transistor ladder filter, and the only design for which Moog acquired a patent. With a unique internal construction and sound unlike anything else, the Moog system simply wouldn't be the Moog system without it.

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On the surface, the 904A seems straightforward: it's a 24dB/oct (four-pole) lowpass filter with a single audio input and single audio output. The panel controls are simple: the "fixed control voltage" knob acts as a filter cutoff frequency control, and the regeneration knob controls what we now usually call filter resonance. The Frequency Range control offsets the cutoff frequency by two octave steps. When set to 1, the cutoff range is roughly 1Hz to 5kHz; in setting 2, the range is roughly 4Hz to 20kHz; and in setting 3, the range is roughly 16Hz to 80kHz (well above the range of human hearing). In many musical applications, people stick with range 2, due to the fact it neatly covers the range of human pitch perception...but if you're looking for darker or brighter sounds, ranges 1 and 3 are still perfectly musically useful. (Scope out the accompanying audio example for an idea of how even just simple filter sweeps and modulation can sound.)

The module also features three CV inputs, each used to control the filter's cutoff frequency—so you can combine CV sources to control the filter without even needing an external mixer. While this might seem mundane, it plays a huge part in interacting with the filter...as you can combine LFOs, envelopes, keyboard CV, or even audio-rate oscillations in order to impart articulation and dynamic timbral control at the level of the filter itself. In a system that truly relies on subtractive synthesis and has little in the way of novel audio processing, being able to take control of the behavior of the filter is crucial: in the Moog system, it's where so much of the nuance in every sound comes from.

Aside from listing specifications, it's really difficult to explain what it's like to interact with this filter, and I strongly suggest checking out the video above to hear what it can do. It's no secret that many musicians consider the 904A the source of the Moog system's magic—and admittedly, I now find myself among them. For ages I've been something of a filter skeptic (I know, I know...), feeling like many players and many instruments rely on filters too heavily for sonic interest. As such, I've always looked at the 904A with a touch of skepticism. Could it really be that good? Is it really that special?

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I think so. Now that I've had the opportunity to explore it for a considerable amount of time, there's something uncanny about its sonic response. Even just running a simple saw wave through it, you get a sense of breadth and wooliness that is a bit hard to describe. Even when used with this simple a sound source, it feels thick, tactile, and warm. Once you start to mix different oscillators together via the CP-3, even more magic emerges: the subtle phase cancellation from two slightly detuned oscillators causes constant dynamic fluctuation at the filter's input. The more you push this input, the more it breaks up, and the more these dynamic fluctuations begin to manifest as changes in character at the level of the filter. Even without tweaking the cutoff frequency or resonance, this sound is simply engrossing. By modulating the cutoff frequency with another audio-rate oscillator at a related pitch (perhaps the same pitch, or perhaps some harmonic interval above or below the other oscillators), you unearth an absolutely ear-bending world of continuously undulating, woolen electrical peculiarities. (Check out the accompanying audio example to hear how that can play out.)

The resonance character of the 904A is worth note. Some filters thrive on highly resonant squelch, and frankly, there's so much to explore on a 904A without ever even touching the resonance knob. While I'd say that the most iconic form of the "Moog" sound doesn't require extreme settings of the regeneration control, this feature does provide the potential to push this filter into territory vastly different from the forceful, thumping bass lines we know and love. When increasing resonance, you do lose some low end in favor of ever-more-nasal accentuation of specific harmonics. Performing slow cutoff sweeps with relatively high resonance, it becomes easy to use the filter as a means of reinforcing particular overtones...and of course, because the filter can be made to track incoming CV, you can animate these spectral effects in any number of ways. Adjusting resonance while performing audio-rate modulation tricks can be particularly rewarding, turning the aforementioned undulating woolen sounds into something that snarls, hisses, and sounds like it might just rip straight through the fabric of time.

Of course, there's something to be said simply for its no-frills design and its cult legacy: when you use a 904A, you set yourself up with a certain group of expectations. But, in my experience, these expectations were far surpassed—the combination of the spacious layout, the deep, full tone, and its surprisingly dynamic sonic response to changes in incoming signal level make it feel like there's something more at play than a cutoff control, resonance control, and a few CV inputs. By hitting the filter with dynamic signals, you can start to see how it truly imparts its own sonic character (filtering aside)—and in that way, it always feels as if it truly responds to the way that you play it. When you push it, it pushes back...and speaking as a performer, that's a simply fantastic and engaging thing for an instrument to do.

Exploring the 904B High Pass Filter

Despite all the power and character of the 904A, there's a lot more to the filtering capabilities of the Moog modular system. Thus, we turn to the 904A's trusted companions: the 904B Voltage Controlled High Pass Filter, and the 904C Filter Coupler. While the 904A is perhaps the best-loved of the trio, the others add a considerable amount of sonic flexibility to the system—and in my explorations with the three together, I was able to uncover sounds I've simply never heard from a Moog system.

The Moog 904B High Pass Filter The Moog 904B High Pass Filter

Like the 904A, the 904B high pass filter seems at a glance to be quite simple—perhaps even more so. It features a single signal input and output, three CV inputs (for cutoff frequency), a fixed control voltage knob for adjusting the cutoff frequency, and a frequency range switch with two settings: low and high. Happily, as with its low pass counterpart, there's much more to the sound of this module than the front panel alone reveals. Featuring a discrete design similar to the 904A, the 904B's coloration responds dynamically to fluctuations in the incoming signal level...but where the 904A growls and groans, the 904B hisses and fizzes, turning incoming sounds into a buzzy, crackling stew of high-frequency energy. Even if set to a static cutoff frequency, running multiple oscillators into it via a mixer yields ever-evolving sonic spaces that evoke a surprising combination of dryness and emptiness.

Simple frequency sweeps on the 904B can have the energy of a laserbeam blasting your face off, or a spaceship taking flight. And of course, by adding CV control of the cutoff, you open up a huge range of other possibilities: helicopter-like chops, slow, dry sweeps, and, with audio-rate modulation, a strange universe of stammering, frothy snarls.

In the video above, we decided to experiment with a classic trick for adding resonance to seemingly non-resonant filters: feedback. In several of our audio examples, we achieve feedback by routing our audio signals into the control panel mixer, the control panel mixer's output into the 904B's input, and the 904B's output to a multiple. The multiple is used to send the 904B out to our speakers and to route its output back into the control panel mixer—completing our feedback path. By changing the level of the 904B output in the mixer, you can dial in different levels of resonance—and by first routing through the inverting attenuator, you can alter the phase of the feedback for even more tonal control.

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Sonic experiments with the Moog 904B High Pass Filter

How does that sound? Well, check out the accompanying audio examples for an idea of how simple modulation, feedback, and audio-rate modulation can play out with the 904B. At low levels of feedback, it's relatively subtle, and as you turn up the feedback amount, it quickly devolves into a screeching, distorted, self-oscillating howl. You have to really dial this in to get a sound that most would call "pleasant," but it's a testament that these systems are capable of way more than '70s cape-wearing prog rocker leads or disco basslines. And like with the 904A, the 904B responds beautifully to audio-rate modulation—creating similarly complex, rolling textures, but with an airier fizz, whereas the 904A creates a fuller, rounder filter FM sound. These synths can really have edge to them. And while it might seem absurd to some to use an instrument like this to create grinding, clangorous shrieks...well, what can I say? I like seeing how far things can be pushed, especially when there's such a firmly-established sense of how a particular instrument "should" sound.

The Best of Both Worlds: the 904C Filter Coupler

So, many of us may be used to synths with state-variable filters—single filters that provide us access to several different filter types: usually including lowpass, highpass, bandpass, and sometimes notch responses. But on these classic Moog systems, the filter modules serve a fixed function: 904A is just a lowpass filter, and 904B is just a highpass filter. If we think about these other types of filter responses, though, an interesting trick might become apparent: if a bandpass filter is just a filter with upper and lower frequency range boundaries, surely we could create one by combining a highpass filter and a lowpass filter...right?

Moog's 904C Filter Coupler Moog's 904C Filter Coupler

Yes—and that's exactly what the 904C filter coupler does. The 904C uses a series of behind-the-panel connections and circuitry to reroute the 904A and 904B to be able to act collectively as a bandpass or notch filter, and for their cutoff frequencies to be controlled by a common set of macro controls. One particularly interesting facet of this setup is that, unlike state-variable filters, the bandwidth of the 904A/B/C combo's bandpass and notch modes is continuously variable. So, for instance, you can create a bandpass response that passes only a narrow band of frequencies, or a very wide band...or, you can create a very subtle notch to remove specific frequencies, or carve out a broad range in the middle of a sound's overall spectrum.

When using the 904C, your audio is routed through the 904C's input and output rather than the individual filters' I/O. Generally speaking, you'd also use the 904C's center frequency and bandwidth panel controls and CV inputs for manual adjustment and modulation of these parameters. Hypothetically, using clever signal routing (and an additional signal mixer), you could entirely replicate the 904C's sonic behavior with just the lowpass and highpass filters alone. However, using the center frequency and bandwidth controls to more predictably/uniformly control each filter's settings relative to one another makes this a much simpler and more practical task, giving you access to a huge range of sounds that these filters cannot achieve on their own. That said...the 904A and 904B's panel controls do still function when using the 904C, so you can refine the response in a number of ways, and can even add resonance to the half of the pass/reject band that uses the 904A, creating a host of curious and interesting filter responses.

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Various audio experiments with 904C filter coupler, from straight sweeps to audio-rate FM and feedback

So how does a vintage Moog variable-width bandpass/notch filter sound? Amazing. With the 904A's resonance turned down, the bandpass response it has all of the strength, clarity, and dryness of the filters that individually comprise it, allowing for considerably more focused sonic control. When swept, the notch/band-reject response has a phaser-like quality, great for sculpting noise or drones. And of course, using the same types of audio-rate modulation and feedback tricks I discussed above, we can get to considerably weirder, edgier, and richer sounds.

Fixed Filter Banks in the Moog Modular System

So as we said before, when people talk about the "classic Moog filter sound," they're mostly talking about the 904A lowpass filter. But as we've already seen, Moog made several different filter designs as well. But beyond the 904A/B/C, things get a bit weird. The last filter in our Model 12 is the 907 Fixed Filter Bank: the odd one of the bunch. We discussed fixed filter banks extensively in this article—and the 907 (and the larger 914) is one of the earliest implementations of such a design in a modular synthesizer. In my opinion, it's also one of the best-sounding filter banks out there.

The 907 is a ten-band fixed filter bank comprised of a low pass filter, eight bandpass filters, and one highpass filter, each with a distinct fixed cutoff frequency and a dedicated manual level control. You might be wondering...why would you want to have a bunch of filters if you can't change their cutoff frequencies? The answer, as with basically everything in modular synthesis, is that it sounds rad.

Running noise through a filter bank like this is a classic technique. It turns an otherwise huge, character-less wash into something with a number of distinct accentuated spectral regions...something like the inner workings of a retro-futuristic space ship, or an eerie Lynchian soundscape. Running harmonically rich oscillators through the filter bank results in a nasal sound somewhat like running a sound through a resonator. Regrettably, it has no CV inputs (dang, wouldn't it be rad to be able to modulate all of these filters at once???)—but as with everything else, it is not immune to the destructive wake of feedback.

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Sonic experiments with the Moog 907, noise, oscillators, and feedback

Using our old feedback tricks with the 907 really brings out the resonator-like response. Sending low-frequency pings from a saw or square LFO while utilizing feedback allows the module to produce a glassy ringing sound, almost as if you're striking a clay pot. While this still makes me wish it were modulate-able, you can get to an astonishing number of distinct sounds simply by manually adjusting the level of each frequency band.

Of course, thinking of a fixed filter bank as part of a studio setup brings even more potential uses to light: process external sounds, use it before an oscillator's FM input to limit the spectral regions in which modulation takes effect, use it in the feedback loop of a delay line to focus the timbre of your repeats—devices like this fixed filter bank remind us that before "the synth lead" or "synth bass" were solidified concepts, synthesizers were simply a tool for sonic experimentation and exploration. Though now we tend to approach them to achieve very specific sounds and specific tasks, using an old system like this really makes you realize that in the early days, everything was much more open-ended. What does a fixed filter bank do, for instance? Well, in so many words, that's entirely up to you.

The Ladder Filter's Legacy: Modern Moog Semi-Modular Synths

So at this juncture of the article, we have to face an unfortunate fact: vintage Moog systems are rare and valuable, and most of us won't have the chance to spend a significant amount of time with one. So, sadly, the experience of turning that giant cutoff frequency knob on the 904A isn't something that most of us are likely to experience anytime soon (alas, I miss it already). Of course, Moog modular systems have been reissued in recent years, but even these handmade new editions are prohibitively expensive for most working musicians.

But even in the company's earliest days, it became clear that these filters—especially the 904A—were something truly special, and luckily, they became staple parts of many Moog designs that followed. The legacy of the transistor ladder filter is extensive, its spirit having found its way into countless Moog products thereafter, from the reimagined Minimoog filter to the recently-discontinued Moogerfooger MF-101 Lowpass filter and, more recently, into their line of modern semi-modular instruments. In fact, each present-day Moog instrument features a transistor ladder filter descended from the 904A—and as such, each present-day Moog synthesizer retains a ton of the sonic character of these vintage systems, from the Subsequent 37 to the One, from the Mother-32 to the Matriarch.

If you're aiming to discover the heart of the Moog sound, perhaps one of the best entry points is the Mother-32, a standalone semi-modular synth designed to combine an excellent-sounding synth voice + sequencer combo with the open-ended nature of modular synthesis. With its switchable highpass/lowpass 24dB/octave filter response, the Mother-32 can process both an internal oscillator and noise source or even external sounds—making it a great addition to any synthesist's arsenal, and dare we say it, one of the most interesting and excellent-sounding ways to get into modular synthesis altogether.

Of course, since the Mother-32's release several years ago, it has been joined by two equally capable semi-modular siblings, the DFAM and Subharmonicon. Whereas the Mother-32 focuses on the classic "synth voice" architecture for creating leads, bass lines, and all manner of monosynth sounds, DFAM and Subharmonicon each take distinct approaches. DFAM (Drummer from Another Mother), is a percussion synthesizer designed to leverage an internal sequencer to create modulations that transform its voice architecture into everything from hard-hitting FM kicks to disco toms and snappy hats and snares all at a moment's notice. With two oscillators (compared to M32's single oscillator) and an identical switchable highpass/lowpass transistor ladder filter, DFAM can make everything from classic analog drum and monosynth sounds all the way to edgy, aggressive modern percussion.

Subharmonicon offers yet another spin on the Moog sound—using a network of suboctave generators and clock dividers to create an experimental tapestry of evolving harmonies and rhythms. It features two internal oscillators with two definable suboscillators each, as well as multiple internal sequencers and rhythm generators that can address these oscillators and suboscillators in any combination. Drawing inspiration from the Mixtur-Trautonium and Rhythmicon, Subharmonicon is a surprisingly peculiar concept coming from Moog, and quite a welcome one, designed to encourage exploratory play rather than a more deterministic approach to music-making. But despite its idiosyncratic approach to generating harmony and rhythm, it still has the fundamental soul of a Moog instrument, with the 24dB/octave transistor ladder lowpass filter at its core.

Of course, all three of these semi-modular instruments are fully Eurorack-compatible, and can all play quite nicely together. While it's beyond the scope of this article to offer an in-depth assessment of each one, scope out this article, in which we dive deeper into explaining what makes each one tick.

The Moog Modular + Modern Moog Keyboards

Perhaps some of the most interesting iterations of Moog's modern semi-modular lineup, though, come in the form of two keyboards: the Grandmother and Matriarch. One look at these instruments' front panels quickly reveals the nature of their inspiration: vintage Moog modular systems. Each one is comprised of a series of clearly-delineated submodules, each full of 3.5mm patch points. These instruments have a semi-modular signal path, meaning that they function more or less as you'd expect a typical synthesizer to without the need for patch cables...but once you start using patch cables to re-route signals, you are able to reach an insane degree of personalization, turning them into the synths that you want them to be. And while it might seem at a glance like saying they're "inspired by vintage Moog systems" is just a marketing talking point, there's a lot of truth to this—now having spent time with each, I can say that taking the time to explore these new instruments can get you into a similar sound world and headspace, taking you surprising directions with every use.

Grandmother is a relatively straightforward monosynth, featuring a non-modulate-able high pass filter and the classic Moog lowpass filter. With its built-in spring reverb, all-analog audio path, and classic synth architecture paired with a powerful sequencer and Eurorack-compatible connectivity, Grandmother is an amazing pairing of classic and modern synth concepts—anyone looking for an instrument to cover vintage-style leads and bass lines with the authentic power of a Moog would do well to play one.

The Matriarch takes the solid groundwork of the Grandmother and develops it into something much more expansive—a four-note paraphonic structure with plenty of internal modulation options, a signal path designed to run in stereo, a beautiful analog delay, and perhaps one of the finest points of the lot...a stereo Moog ladder filter, capable of Series HP/LP, Stereo LP/LP, and Parallel HP/LP operation. Featuring independent resonance controls, a single master cutoff control, and a filter spacing control for generating offset between the two filters, I can't help but be reminded of our old friend the 904C filter coupler. In fact, given that each filter does have dedicated input and output patch points, these filters can be used fully independently...though truthfully, one of my favorite overall effects involves modulating each filter's cutoff frequency inversely from one another, creating a Leslie-like binaural spectral panning effect (featured in the first patch in the video above). As it turns out, a stereo ladder filter is pretty profoundly mind-blowing.

For me, Matriarch especially embodies the spirit of classic Moog instruments, taking the best of a keyboard-based and modular approach to sound design, making profoundly inspirational, vast opportunities for sonic exploration. It will do lush pads, it will do sequenced rhythms, it'll do huge stereo bass sounds, it'll do all-out heavily-modulated generative music—it does all the things that a great analog synth should. If you want to read more of my ramblings about this amazing synth, check out this article.

An Enduring Sound

So this article has covered a lot of ground—we looked at early Moog history, described a rad system that we had the good fortune to house at our shop for a while, we discussed the classic early Moog filter designs, talked about lots of ways to use them and misuse them, and talked about how these ideas and designs are still at work in Moog products even to this day. So where does that leave us?

Bob Moog left a huge impact on the world of music-making...basically everyone in the world of synths and electronic music knows this. For me, one of the most interesting things about getting to know a vintage Moog system, though, is that so much of its magic is about more than we typically discuss. Having played tons of Minimoogs and modern Moog synths, it's easy for me to forget that Moog's chief innovation wasn't just a filter—it was an entire way of working, an open-ended approach to working with sound that was all about exploring, recombining ideas, and making something new out of simple parts.

As a modular synth enthusiast, I must admit that I've spent much more of my time thinking about Buchla, Serge, and other left-field angles of sound design than I have about Moog. Beyond the context of things like the Minimoog and more recent developments, I've spent fairly little time with these instruments. But spending time with the Model 12 made it much clearer to me why Moog is so dear to so many musicians. On the surface, something like the Model 12 or the Minimoog might seem simple...but when you sit down and take the time to really listen to them, you'll notice so many details that can be lost if you're not in the right headspace. A filter isn't just a filter—it's a place where sounds go to commingle and produce a rolling, ever-evolving earthy froth and fizz. Resonance isn't just resonance—it's where you reach to focus a sound's center, to highlight hidden overtones. That giant cutoff knob isn't just a cutoff knob—it's the place you reach to give an entire patch a sense of breath, breadth, power, and motion.

And that's not just true of a vintage Moog. Sit down with a Matriarch or Grandmother, sit down with a Mother-32 or DFAM or Subharmonicon, gradually adjust the filter and listen. There's more hiding in that sound than you might first realize.

(Image credit: Moog 904A, 904B, and 904C images via Google Arts + Culture / The Bob Moog Foundation)