“Filters Make the Synth”

Why do people say the filter is the heart of an analog synth, when it’s only one of several components that make up a synth’s sound? People genuinely think that if you put an MS-20 filter in a Minimoog, it’ll sound like an MS-20, despite synths like Diva that can do this and actively show how different parts change the overall sound significantly. Even with a synth like Bass Station 2 which has two 24db lowpass filters of different types, sure it changes the sound, but it still sounds like a BS2.

But… why exactly?

Well, while basic waveforms (saw, square, triangle) are fundamental, the actual circuits that generate these waves vary greatly between synths. These variations mean you get variable slope shapes, drift, and modulation/ saturation behaviours. The “warmth” or “power” of the source (remember, GIRATS – get it right at the source) determines the base sound pre-filter. Synths like the 2600 are more heavy and rich in character from the start, whereas synths like multi/poly (imo) give a thinner, lighter sound.

Synths have a mixer before the filter to sum or average oscillators, noise, etc. noise. The design of this mixer can also introduce subtle saturation as signals are combined. Things like the Moog CP3 are renown for their characteristic drive at higher levels, whereas the Prophet series tends to have a cleaner, non-driven mixer.

The VCA isn’t just a volume knob. The specific VCA circuit used can introduce its own subtle colour in the form of saturation and compression, especially as input volume increases. The way the filter output interacts with the VCA input can significantly impact the final sound, including high resonance levels creating peaks. Transistor VCAs are more likely to take on such behaviour than op-amp or OTA VCAs.

Envelope generators can vary significantly too. While ADSR is a standard setup, the speed and curve of the attack, decay, and release stages can vary considerably between different envelope generator circuits. Some are snappy and percussive like the 555 Thomas Henry design, others are slower and more organic like the Arp 4020. These differences greatly influence how a sound can change over time, impacting everything from sharp plucks to evolving pads.

Crucially, it’s not just the individual modules, but how they interact with each other. The types and routing capabilities of modulation (including FM) play a huge role in what a sound can be. This is why things like the Prophet Poly-Mod is touted as being a huge factor in that synth’s sound – though i feel this is more of an interface thing nowadays and less the powerful new feature it was when it was new.

Don’t discount components too. The specific models of transistors, capacitors, and diodes used, the power supply design, even the overall layout of the circuit board contribute to the nuances of a synth’s sound – this includes digital models of said circuits. Analog components have tolerances, meaning their exact values can vary slightly. These tiny variations across numerous components in the signal path add up to a unique sonic fingerprint for each synth.

So, while the filter is a powerful tool, the oscillators and VCAs matter too, the cumulative effect of the entire design contributes to the overall sonic identity that makes a Minimoog sound like a Minimoog and an MS-20 sound like an MS-20, even if they swapped filters.