Few electronic components have shaped the sound of electronic music in the 21st century more than the seemingly ubiquitous PT2399 echo chip. Quietly released in the mid-1990s by Princeton Technology Corporation—a Taiwanese semiconductor firm specializing in inexpensive consumer electronics—the chip was initially just one small part of the company's diverse catalog of integrated circuits. At the time, PTC was better known for designing remote-control decoders, voice synthesizers for talking toys, and inexpensive chips for home appliances—hardly the sort of company you'd expect to spark a cult following in the music world.
The PT2399 was designed to provide a rudimentary reverb effect for karaoke machines, intended to make amateur singers sound a little more polished and professional without increasing production costs. Yet, its ingenious design and simplicity—a single resistor controlling delay time—and its unique sonic quirks quickly caught the attention of musicians and DIY enthusiasts. Almost by accident, PTC had created a chip that perfectly embodied the emerging lo-fi aesthetic. Its technical limitations, rather than hindering its adoption, gave the PT2399 a characteristic sound, transforming what might have been considered performance deficits into intriguing musical character that musicians and DIY electronics hobbyists embraced.
× Neither an analog bucket-brigade delay nor a modern DSP-based delay, the PT2399 integrates an analog-to-digital converter (ADC), digital-to-analog converter (DAC), and approximately 44 kilobits of internal SRAM memory for audio sample storage, operating at a fixed sampling rate of around 44 kHz—all contained on a tiny chip the size of a Tic Tac. These technical constraints—limited memory capacity, fixed sampling rate, and relatively low bit depth—introduce characteristic sonic artifacts such as subtle distortion, aliasing, and frequency roll-off at longer delay times. This "imperfect" sound profile, rather than being a limitation, has become highly sought after as an effect that tends to blur and distort sounds when used with longer delay times. The PT2399 chip is now used in almost 200 documented circuits, finding its way into numerous Eurorack modules, guitar pedals, and other popular synthesizer formats, including 4U and 5U/MOTM systems. The chip is reputed to be an ingredient in numerous "lo-fi" reverb and delay effects, like the Caroline Météore and Earthquaker Devices Space Spiral, which caught on with a new ambient-interested audience via the popular Knobs YouTube channel in the mid 2010s. BugBrand, the marque of Tom Bugs—who began experimenting with PT2399 chips in the early 2000s—has also developed a series of distinctive desktop synthesizers based around this versatile delay chip. New designs using the chip continue to appear each year, and it remains a common choice for DIY audio projects.
The Météore from Caroline Guitar Company
History of the PT2399 Chip
Princeton Technology Corporation (PTC, 普誠科技) was founded in 1986 in Taipei, Taiwan and initially focused on resale of chips and computer peripherals, later adding an integrated circuit design division. Today the company—whose low-key corporate headquarters occupy a nondescript grey-tiled building by the Xindian River near the corporate headquarters of legendary diode manufacturer Vishay General Semiconductor—is mostly known for its consumer electronics integrated circuits used in everything from smart locks and massage chairs to automotive lighting. The enduring success of a single archaic and inexpensive CMOS chip, originally designed as an echo effect for cheap karaoke machines, seems to have been as much a surprise to PTC as it was to anyone. Famously sparse in its documentation (the exact purpose of pin 7 on the chip has apparently never been explained by PTC, although some designers circuit-bend this pin), the PT2399 is mentioned only once on the corporate website in a two-page PDF last updated in 2022, which optimistically lists possible applications for the chip as “KARAOKE Mixer, CD/DVD Player/Recorder, Multimedia TV, Car Entertainment System, Music Instrument effecter, Electronics Toy.”
× Exactly when the earliest version of the PT2399 was released seems to be uncertain, but in 2015 the company confirmed via email that it has been “under mass production for at least 20 years,” suggesting that it was likely released as early as 1995, although the earliest known version of the datasheet is dated 1998. Regardless of the chip’s provenance, it seems to have been discovered by the DIY audio hardware community around the turn of the millennium. The earliest online mentions of the PT2399 chip used in a synthesizer context date from forum posts and schematics published around the year 2001 or 2002. DIY synthesis luminaries René Schmitz, Scott Stites, and Tom Bugs of BugBrand seem to have been some of the earliest designers experimenting with the chip in the early 2000s. The PT delay first found a commercial musical application in DIY guitar pedal kits. One of the first—if not the first—kits based on the PT2399, was the PT-80 Delay by General Guitar Gadgets which was released in 2002 and is still available as of March 2025 (23 years later!). 
The humble chip itself
Soon after the PT-80 Delay’s appearance in 2002, the PT2399’s popularity surged among boutique guitar pedal makers. Between 2003 and 2005, DIY forums such as DIYstompboxes and Tonepad promoted PT2399-based projects like Francisco Peña’s Rebote Delay series, which established the chip’s potential for analog-style echoes at very low cost.
In the mid-2000s, mass-market pedal brands also embraced the PT2399’s appeal. Danelectro’s release of the affordable FAB 600ms Delay pedal brought the chip into mainstream awareness, though notably without explicitly advertising its digital nature. Boutique builders took a different approach: rather than obscuring the chip’s quirks, they celebrated them. In 2007, Skreddy Pedals introduced the Skreddy Echo. The pedal gained immediate acclaim for its tape-like echos and self-oscillation capability and cemented the PT2399’s status as an essential element in the boutique pedal-builder’s palette. Similarly, Björn Juhl’s acclaimed Mad Professor Deep Blue Delay (2008) gained fame for its musical transparency, helping redefine expectations of what could be achieved with such an inexpensive IC.
The PT2399 also began appearing beyond guitar effects pedals, crossing into synthesizer territory. Flight of Harmony’s Sound of Shadows Eurorack module, released in 2009, offered voltage-controlled delays and embraced the chip’s noisy, unpredictable character. Around the same time, Tom Bugs’ explorations culminated in BugBrand’s standalone PT Delay desktop unit in 2011, featuring CV control, saturation circuits, and an aggressive, hands-on aesthetic that turned the PT2399’s glitches into desirable artistic features.
[Above: a few examples of PT2399 excellence.]
The PT2399’s versatility was again highlighted when Korg employed the chip in its highly successful Monotron Delay synthesizer, released in 2011. Positioned as a compact and accessible analog synthesizer, the Monotron Delay effectively brought the chip’s distinctive lo-fi echoes to a mainstream, global audience, significantly expanding the PT2399’s user base.
Through the 2010s and into the 2020s, the chip’s presence continued to expand, permeating the designs of an array of Asian brands like Joyo and Donner, alongside ever-more sophisticated boutique offerings. Even as Princeton Technology Corporation remained relatively quiet about their modest IC, a robust global community of musicians, circuit designers, and hobbyists kept pushing its creative boundaries. Ironically, the same simplicity and imperfection that originally made the PT2399 obscure and cheap had by then transformed it into a beloved staple of modern audio hardware design.
An Accidental Analog Emulator: The Prior Art of the BBD
× Although designed as a low-cost solution to add echo and reverb effects to karaoke machines, in the early 2000s the DIY audio community found that the PT2399 chip could effectively emulate an earlier analog technology that was becoming increasingly scarce. Before digital delay chips like the PT2399 became widely available, delay effects were primarily created using either magnetic tape delays or bucket brigade chips (BBDs)—a technology first introduced in the 1970s. Bucket brigade delays operate by passing an analog signal through a chain of capacitors—electronic components that can temporarily store an electric charge, similar to a tiny rechargeable battery. Each capacitor holds a voltage level that represents the instantaneous amplitude of the audio signal at a given moment in time. These capacitors are arranged in sequence, forming a delay line where the signal is passed from one capacitor to the next.
The unlimited coolness of Panasonic's BBD manual
The movement of the signal through this chain is controlled by a high-frequency clock, which in this context refers not to a timekeeping device but to an electronic timing circuit that produces a steady stream of electrical pulses, similar to a square wave. Each clock pulse acts like a handoff signal, instructing each capacitor to transfer its stored voltage to the next one in line. This step-by-step progression is analogous to a human bucket brigade, an early method of firefighting where each person passes a bucket of water to the next at regular intervals.
The total time it takes for the signal to travel from the first capacitor to the last (the delay) is determined by both the number of stages (capacitors) and the frequency of the clock (how many times per second the transfer happens). For example, a BBD with 1024 stages and a clock running at 10 kHz (10,000 pulses per second) produces a delay of about 102.4 milliseconds—an extremely short delay useful for reverb-like effects but not what most people consider a “delay effect” in a familiar sense.
Although a BBD is an analog technology, it still samples the signal at discrete time intervals defined by the clock frequency. If the input signal contains frequency components above half the clock frequency (the Nyquist frequency), these components will fold back into the audio range as unwanted artifacts—essentially, aliasing in the analog domain. A poorly designed or omitted input filter will therefore result in harsh, distorted delays with non-harmonic content. The high-frequency switching noise generated by the internal clocking of the BBD stages can also cause issues. As each capacitor in the chain is charged and discharged, the fast transitions of the clock signal introduce switching artifacts that ride along with the delayed audio. To suppress this, a low-pass filter is required after the BBD. This filter attenuates frequencies near and above the clock rate, cleaning up the output and making the delayed signal usable in a musical context.
While BBDs are a noisy and imperfect technology, in time some of the ways in which they distort audio became sought-after characteristics of analog delay effects. For example, at longer delay times—achieved by lowering the clock frequency—the signal-to-noise ratio decreases and high-frequency loss becomes more pronounced, often resulting in dark and murky echoes, which can add an interesting timbre to analog bucket-brigade delayed audio.
With few uses outside of audio processing and growing industry interest in digital technologies, manufacturers like Philips and Panasonic had phased out production of BBD chips by the late 1990s. This left designers and musicians increasingly reliant on dwindling stockpiles of BBD chips, causing prices to rise and availability to drop. Although Visual Sound (now Truetone) resumed limited production of select models like the MN3102 and MN3207 in 2009, the early 2000s were marked by a noticeable scarcity of reliable, affordable analog delay chips, which left a gap in the market for an affordable, analog-style delay solution. The PT2399, despite being technically a digital chip with on-chip ADC and DAC stages, became a popular substitute due to its simplicity, low cost, and surprisingly analog-sounding response—offering a compelling middle ground between true analog delay and digital emulation.
The Design of the PT2399
While the PT2399 is by far the most prominent, it is part of a small family of audio delay chips developed by Princeton Technology Corporation. Its siblings—the PT2395 and PT2396—offered alternative approaches to digital delay. The PT2395 allowed for much longer delay times by offloading memory to external DRAM, making it more complex to implement but potentially higher fidelity. The PT2396, a pin-compatible variant, introduced subtle internal differences but has never been available in large quantities. Neither chip achieved the cult status of the PT2399, whose compact, self-contained architecture and uniquely degraded sonic signature have earned it a dedicated following among musicians, pedal builders, and DIY synth designers.
The PT2399 chip emulates key characteristics of analog BBD delays using clever hybrid between analog and digital circuitry, all packed into a chip the size of a Tic Tac. At the same time, the design compromises that make the PT2399 so compact and affordable introduce their own distinctive forms of distortion—artifacts that have, in turn, become iconic to the PT delay’s unique sonic character.
×
The PT2399’s strange architecture relies on a clever combination of analog and digital processing. First, the incoming analog audio voltage is passed through a low-pass filter and then converted into a digital signal using an analog-to-digital converter (ADC). But unlike most modern ADCs that use 8, 16, or even 24 bits to represent each sample, the PT2399 uses a much simpler method called 1-bit delta-sigma modulation: Instead of recording the precise level of the incoming voltage, the chip tracks only how the incoming audio voltage is changing—essentially asking, “Is the voltage rising or falling?” Each time it samples, it outputs just a single bit: a 1 if the signal is going up, a 0 if it’s going down. This process happens at a very high speed—tens of thousands of times per second. Over time, the pattern of 1s and 0s represents the shape of the original audio wave, like a fast series of yes/no decisions that capture the general motion of the sound.
What results from this process is a waveform that looks a bit like stacked LEGO bricks. This 1-bit stream is stored in the PT2399’s 44-kilobit internal memory (RAM), where it can be delayed by a programmable amount of time. After the delay, the blocky bitstream is smoothed out a bit by the filter, where the associated loss of information is considered an acceptable tradeoff for the ultra-compact form factor. These imperfections, along with the chip’s built-in limitations, are part of what gives the PT2399 its signature warm, lo-fi sound that musicians have come to love.
Another critical component of the PT2399’s sonic identity lies in its internal memory: a mere 44 kilobits of SRAM (Static Random-Access Memory—a type of volatile memory that stores data using flip-flop circuits made from six transistors per bit), used to store digitized audio data before playback. This seemingly meager capacity is central to the chip’s charm and limitations. At a sampling rate of roughly 44.1 kHz, each 1-bit sample of audio takes up very little space. However, storing even a few hundred milliseconds of audio at this rate quickly consumes the available memory. The maximum delay time the chip can achieve—typically quoted as between 300 ms and 600 ms, depending on the external resistor setting—represents the absolute limit of what this tiny RAM bank can hold before wraparound and data degradation become significant.
To store longer delay times within the confines of this limited memory, the PT2399 lowers its internal sampling clock frequency—essentially reducing the number of audio samples it records per second. This clever tradeoff allows the same memory to stretch over a longer period, but at the cost of audio fidelity. As the sampling rate drops, the chip captures less detail from the original signal, resulting in audible aliasing, reduced bandwidth, and a loss of high-frequency content. The delayed audio becomes increasingly grainy, dark, and blurred. These effects intensify at the longest delay settings, where the combination of sparse sampling and analog filtering produces a smeared, tape-like echo that masks transients and smooths out the signal in a musically pleasing way.
While some builders have used the PT2399 to build relatively clean-sounding delays, the chip’s limitations have also become a source of inspiration. Far from masking the chip’s imperfections, many pedal makers and synth designers built entire product identities around the PT2399’s aliasing, frequency roll-off, and memory artifacts. Devices like the Kilobyte (Caroline Guitar Company), T-120 Videotape Echo (Demedash Effects), and Echo Degrader (Industrialectric) are styled to evoke dead media formats and deterioration artifacts of nostalgia with names that reference retro digital effects, VHS degradation, and glitch. The T-120 in particular is a masterclass in cultural branding: named after the standard runtime of a VHS tape and housed in a case design that mimics old-school videocassettes, it directly taps into the visual and sonic language of vaporwave, glitch, and hauntological aesthetics. Its PT2399-powered repeats are intentionally smeared, detuned, and unstable.
Recently, Eurorack has been rapidly catching up to the guitar pedal world with a growing number of modules that explore the PT2399 delay chip in both classic and unconventional ways. Befaco’s Crush Delay v3 introduces three new patch points that intervene in the delay circuit via hardwired bend locations. Each is controlled by a toggle switch or gate input, and routes through a CD4053 analog switch to affect pins 6, 7, and 8 of the PT2399—key points in its timing and feedback network. These controlled "bends" destabilize the delay clock, introducing pitch glitches, noise bursts, and gritty textures that can be sequenced or performed live.
Another manufacturer who has made an extensive study of the PT2399 chip is Perth-based DIY synth designer Andrew Fitch, who runs Nonlinearcircuits. He has designed at least four Eurorack modules based on the chip, including the popular Delay No More module, as well as I was Sitting in a Room, a module that likely holds the record for the most PT2399 chips used in a single circuit by a wide margin, with nine PT2399 chips squeezed into just 10hp of rack space!
Beyond Delay: Hacking the PT2399
Despite its minimal documentation and modest origins, the PT2399 has remained a staple in both DIY and commercial audio hardware for over two decades. Its internal architecture—simple enough to understand, yet flexible enough to manipulate—has made it a frequent subject of experimentation. Through years of trial and adaptation, designers have discovered a range of modifications that extend the chip’s functionality beyond what the manufacturer intended, improving stability or deliberately exploiting its quirks.
× Individual PT2399 chips are often modified at the pin level—that is, by directly altering the voltages, resistances, or signals applied to specific pins of the chip. One such modification involves connecting a resistor between pin 6 (delay time control) and pin 8 (CC0). For example, adding a 1MOhm resistor (or resistor in series with a capacitor) between these pins introduces a gentle warble to the delayed signal and enhances its spatial quality without complex circuitry. Another innovative modification to the PT2399’s standard usage is found in the Little Angel Chorus, designed by Rick Holt of Frequency Central. Traditional PT2399-based modulation effects attempt to create a chorus by modulating pin 6—the delay time control—via a low-frequency oscillator (LFO). However, this approach is limited: at extremely short delay times (necessary for chorus), the LFO's influence becomes minimal, and the signal can become stagnant. To sidestep this limitation, Holt took an unconventional approach: instead of modulating pin 6, he grounded it to force the delay line as short as possible—essentially removing it from modulation altogether. He then redirected the LFO to modulate pin 2, the internal 2.5V voltage reference used throughout the chip’s analog signal path. This effectively “wobbles” the entire chip’s internal bias, resulting in a rich, warbly modulation reminiscent of tape flutter or vibrato. Because the delay time remains fixed and minimal, the effect retains the tight, zero-latency feel of a chorus, while the modulation depth is no longer limited by the chip's minimum delay threshold. A more complex hack centers on pin 5, which outputs a clock signal from the PT2399’s internal voltage-controlled oscillator which represents the internal delay time. While this pin is rarely used in commercial designs due to the complexity of synchronizing high-frequency signals (up to 22 MHz), it opens up compelling possibilities for experimental control schemes. Notably, some developers have successfully leveraged pin 5 to monitor the real-time delay time of the chip, enabling closed-loop systems for external synchronization. For instance, in DIY projects that aim to synchronize PT2399 delay times with MIDI clocks, pin 5 can be used as a feedback point in a frequency-locked loop. An Arduino or similar microcontroller can be used to measure the frequency of this output and adjust the effective control voltage—via a digital potentiometer—to lock the chip’s delay time to musically quantized delay times. This approach circumvents the PT2399’s nonlinear and unstable resistance-to-delay curve, which is otherwise impractical for precise control with MIDI or CV. × Some designers have also explored the potential of combining multiple PT2399 chips. One popular approach is to cascade two or more PT2399s in series to extend the maximum delay time beyond the typical 300–600 ms range. For example a schematic for a “King Dubby Delay,” posted on freestompboxes.org by user Nocentelli in 2015, shows a method of chaining multiple PT2399 chips in series to achieve longer delay times up to 2.2 seconds while incorporating tone-shaping feedback loops and filtering stages to tame the resulting noise and bandwidth loss. An especially influential application of the PT2399 emerged in the Belton Brick, a compact reverberation module developed by electrical engineer and DSP architect Brian Neunaber and later commercialized under the Accutronics brand. As described in Neunaber’s 2007 patent (US8204240B2), the ping-pong-ball-sized “brick” implements a feedback delay network (FDN) using three cascaded PT2399 digital delay lines—each operating at short, fixed delay times between 30-40 milliseconds—and an analog summing and filtering network. Unlike fully digital reverberation systems, which typically use digital signal processors (DSPs) and flexible algorithms, the Belton Brick uses fixed hardware topology: the PT2399s provide the core delay paths, while surrounding analog circuitry manages signal injection, feedback weighting, and frequency-dependent filtering, typically via low-pass filters. The reverb decay is generated by feeding delayed signals back into the input through weighted summing nodes, and in some designs, additional external feedback loops are used to extend or modify decay behavior. This hybrid approach allows the Brick to emulate the dense, nonlinear reflections of spring tanks while maintaining a low component count and compact footprint. The Belton Brick has become a popular drop-in solution for many reverb pedals such as the EarthQuaker Devices Ghost Echo, Caroline Guitar Company’s Météore, and Death By Audio Reverberation Machine—each of which builds on the module’s characteristic filtered, metallic decay with additional controls for tone shaping or modulation.
Pin configuration from the PT2399 datasheet
The Belton Brick
Conclusion
While audiophile culture most often valorizes the warmth of vintage analog gear, in contradistinction to the assumed sterility of digital effects, the PT2399 complicates that binary. It’s a digital chip, but one whose unique and characteristic sound arises precisely from its hackability and limitations of its hybrid design: low-resolution 1-bit delta-sigma conversion, minimal onboard memory, and a fixed sampling rate that degrades as delay times increase. These aren’t features engineered for musicality—they’re cost-cutting compromises, baked into a chip meant to make karaoke vocals sound just a little more polished.
The PT2399’s unlikely rise reflects the odd commensalism of boutique music hardware within the technology industry. In the semiconductor industry, the goal is optimization: more functionality, lower cost, smaller footprints, and greater reproducibility across massive product runs. Companies like Princeton Technology Corporation are in the business of designing components that quietly disappear into the background of everyday consumer devices. These are parts meant to be produced by the millions, installed at the factory, and never noticed again. The PT2399 was just another such part: a self-contained echo effect for karaoke machines, engineered not for sonic beauty or creative potential, but for minimal footprint and maximum margin. Its famously limited documentation speaks to those innominate industrial origins. It was designed to make a cheap product feel slightly less cheap—and nothing more.
That a chip like the PT2399 could cross over into boutique pedals and modular synths speaks to the permeability of genre in electronic music, where high fidelity exists alongside experimental practices that often filter up to the mainstream in surprising ways. In the early 2000s, circuit bending was a fringe practice made possible by a particular moment in electronics history. Consumer devices were still both cheap and hackable; their chips—still large enough to solder—could be isolated, analyzed, repurposed, and pushed into interesting failure modes that produced surprising results.
× Today, that landscape has changed. Circuit benders now speak of the “black blob of death”—a blob of epoxy covering a miniaturized digital chip, often the only component left visible on otherwise featureless PCBs. Miniaturization and integration have made devices smaller, cheaper, and more opaque. Fewer parts means fewer points of entry. The kind of hands-on experimentation that once drove circuit bending has grown more difficult, and in many cases, impossible. The PT2399 is a quirky holdover from a brief period of time when consumerism made the PT2399 chip available but before miniaturization made experimentation impossible. It’s a leftover part from a more hackable moment within the historical arc of globalization. In this, it recalls another unconventional use of CMOS chips: the WASP filter. Designed in the late 1970s for Electronic Dream Plant’s (EDP’s) budget-friendly WASP synthesizer, the WASP filter has become famous (and infamous) for using a 4069 logic chip in an unorthodox way, which contributes to its saturated and characteristically nonlinear sound. Like the PT2399, the WASP was built out of necessity and constraint. It was a cost-cutting move that bypassed conventional analog op-amps, resulting in a filter that screams, distorts, and sometimes literally causes its CMOS chip to explode. Like the CMOS chip in the WASP filter, the PT2399’s continuous reinvention is not merely nostalgic but exemplifies a dynamic described by philosopher Andrew Feenberg, a leading figure in the critical theory of technology. Feenberg argues that modern technologies are often shaped by operational autonomy—the power of designers and institutions to dictate how technologies function and who controls them. Yet, this autonomy can be reversed when users creatively repurpose tools in ways their designers never intended. The lack of documentation around the PT2399 creates an experimental gap that must be bridged through empirical tinkering, turning passive consumption into active reinterpretation. In modifying, stacking, or reprogramming the chip, artists are continuously recontextualizing a mass-produced component within new social and aesthetic practices. This shift reflects what Feenberg calls secondary instrumentalization: the reintegration of technical artifacts into richer human contexts that exceed their original design. In doing so, artists collapse the one-way relation from designer to user into a reciprocal, participatory process of meaning-making.
The "Black Blob of Death"