Korg KR-55 Cymbal, Hi-Hat, & Cowbell Synthesis

PercussionKR-55Korg
Written By Peter Barata

The Korg KR-55 was a preset drum machine released in 1979. Korg designed it to compete with Roland’s similar drum machines, particularly the CR-78.

I don’t own one, though Cherry audio does have a VST emulation of it, and you can find samples online. Soft Cell, Cabaret Voltaire, Jean-Michel Jarre, and others all used the KR-55 at some point. Notably, the its snare was featured throughout Depeche Mode’s debut album ‘Speak & Spell.’

While the KR-55 lacks individual outs, it does have level control pots for some of the sounds. Extensive documentation can be found online for hardware modifications including clock input/sync, MIDI, and individual outputs. However, I’m interested in understanding how the sound generation circuits work.

Electric Druid’s website appears to be the only source that analyzes the KR-55’s sound circuits in detail. The information below is mostly cribbed from his excellent analysis, which I highly recommend reading. The block diagrams I’ve created are based largely on his work.


VCA and Envelope Circuit

The cymbal, hit, and cowbell share common VCA and envelope designs, with minor variations in envelope timing and highpass filtering.


Envelope

The metal-noise based voices use a simple two-stage, decay-only envelope.

The circuit operates as follows: A trigger pulse charges a capacitor. When the trigger goes low, the capacitor begins discharging through one of two paths, depending on its voltage. Above 3.5V, it follows a fast discharge path. Below 3.5V, it switches to a slow discharge path. This dual-path design creates a ’Tch-Tsss’ envelope with a sharp  transient attack followed by a sustained release. Different resistor and capacitor values across the drum voices produce varying decay times.


VCA

The VCA is Korg’s implementation of the ’swing-type’ VCA that Roland used for the metal voices throughout its analog drum machines.

The VCA’s design inherently allows some low-frequency noise from the envelope to bleed through.

A highpass filter follows the VCA to eliminate this noise.

Rather than just cleaning up the signal, the highpass filter also becomes part of the sound design by stripping away bass and midrange frequencies to create the bright, metallic character of a cymbal and hi-hats.


Metal Noise Swarm

Metal noise sources require a complex signal that’s almost, but not quite, random noise. Unlike drum sounds that use white noise (for a snare’s snap) or simple oscillators (for a kick or tom’s body), metallic sounds need an harmonically-rich signal generated by ring-modulating a swarm of inharmonically-tuned square waves.

The KR-55’s cymbals, hi-hats, and cowbell all use metal noise as their sound source. It has two distinct metal noise generators: ring1 is for cymbals and hi-hats, and ring2 is for the cowbell.


Ring1

Ring1 uses six square wave oscillators, each built around a Texas Instruments LM339 comparator. They are arranged in three pairs, with each pair’s output combined through an XOR gate. The three XOR outputs are then mixed together, creating a rich, untuned metallic hum. The result is not complex enough to sound like straight random white noise, but is still highly noise-like.

XORing square waves produces the same effect as ring modulation. Since the oscillators are detuned to create a beating effect, the XORing creates a chaotic, stepped random waveform. This same technique is also found in synthesizers like the Korg MS-20 and ARP Odyssey.

Two key differences distinguish the KR-55’s metal noise from Roland’s approach. First, the KR-55 XORs pairs of oscillators. Second, while Roland employs filtering to create different flavours with more focused timbres, the KR-55 produces a broader, less tonal spectrum.

Ring1 Oscillator tunings:

Pair 1

272 Hz = C#4 -33c

8529 Hz = C9 +32c

Pair 2

633 Hz = D#5 +30c

9073 Hz = C#9 +32c

Pair 3

1458 Hz = F#6 -26c

13,500 Hz = G#9 +27c


Ring2

Ring2 is used exclusively for the cowbell. It takes a simpler approach, using just one pair of square waves combined through an XOR gate. However, some of the original signal is mixed back with the XOR output. This produces a more pitched, tonal result. Where ring1 is a metallic hum, ring2 has a more clangorous, bell-like quality.

Ring2 Oscillator tuning:

731 Hz = B7 - 2c

3946 Hz = F#5 -21c

Cymbal

The cymbal blends a brief burst of bright, highpassed ring1 with a darker, sustained ringing. The cymbal takes the ring1 signal and sends it through two parallel paths that are mixed at the output. Both path use a common trigger that fires two separate envelopes (using the same envelope design described above).

Path 1: Cymbal Strike

The first path creates the snap of the stick hitting the cymbal.

Ring1 passes through a VCA, modulated by a very short transient envelope, then through a highpass filter.

This produces the brief, bright hissing “chk” of the initial strike.

Path 2: Sustain Ringing

The second path creates the sustained ringing body. Using a longer envelope, a VCA feeds into a bridged-T filter with a centre frequency of approximately 156 Hz (D#3 +7 cents) with moderate resonance. The filter frequency can be calculated from the component values using the formula on page 5 of the 808 service manual. This filter sculpts the broad ring1 noise into a more focussed metallic hum.

The two paths combine to form the complete cymbal sound. This mimicking a real cymbal’s initial crash followed by sustained ringing.

Hi-Hat

Both the open and closed hi-hats share a common signal path, using ring1 as their source and the KR-55 ENV/VCA described above. However, they have independent triggers, and the circuit’s complexity arises from how these triggers interact to create a choke group.

The hi-hat circuit uses a comparator-based latch that switches between two states. An open trigger switches the latch to an high voltage state, and holds the envelope open, creating a longer decay. A closed trigger switches the latch back down to a low voltage state, and chokes the envelope, setting a shorter decay of approximately 130 ms.

Cowbell

The cowbell is the simplest of the three voices. Ring2 goes through the same VCA/ENV design as the cymbal and hi-hat, though with a slightly simplified envelope. That’s it.

You could probably create a pretty great cowbell by combining the KR-55 ring2 source with the 808 cowbell bandpass filter …


Further Reading

Peter Barata
Next

808 Cowbell Synthesis