About

 

The history of rock is synonymous with the history of analogue distortion.

 

Historical background

As one of the most versatile instruments ever, the electric guitar is inseparable from the evolution of modern rock and pop music. Together with the distortion of an overdriven tube amp it is capable of creating a wide range of sounds, some of them highly recognisable in our listening perception.

Historically it all started in the fifties and sixties with the typical “Fender-style clean sound”. As well as surf and country music this sound is an essential part of many pop music productions.

A further milestone was set by Jimi Hendrix. With brilliant virtuosity and the distorted sound of his Marshall amps he is undoubtedly the forefather of classic rock sounds, inspiring generations of rock musicians who followed.

At the beginning of the eighties Marshall launched the JCM800, a tube amp with higher gain possibilities than the former “Plexi” or “Super Lead”. This amp was the ancestor of modern British high gain sounds and was inextricably linked to Heavy Metal. The characteristic sound of the JCM800 formed the basis for many modern British high gain amps created by HiWatt, Orange, Soldano and others.

With the punchy, dark-aggressive sound of the legendary “Black Album” Metallica set another impressive milestone for the overdriven guitar sound. Mesa Boogie’s “Dual Rectifier” amp used by Metallica became the benchmark for a fat, aggressive US high gain sound.

Developing your personal guitar sound

In learning to play the instrument, a guitarist will need to master the feedback between his head and hands. A lot of discipline is required as well as analysis of the top players. So what if you ask a guitarist about his idea of an ideal sound? The answer will characterise sounds which defined his favourite genre over the time. In most cases they can be roughly categorised into the four styles stated previously. Numerous forums and internet communities are populated with guitarists still searching for Pink Floyd’s clean sound or the overdriven tone of Hendrix or Metallica. As a matter of fact – as well as the virtuosity and technique of these artists – their sounds are highly dependent on their musical equipment.

Finding your favourite amp tone

Buying a special guitar and a specific tube amp is what almost every guitarist would do in order to get his favourite tone. But a good sounding tube amp is expensive, heavy weighted and not easy to handle. Additionally the adjustment of a true analogue tube amp isn’t particularly intuitive. Due to the high voltages inside the unit only passive components like switches, relays or potentiometers are suitable. The versatility of sounds is limited to the number of channels available.
But is there an alternative? The market for musical equipment offers a wide range of digital amps and sound processors with a high level of convenience, low weight and cost. All of them convert the analogue audio signal to a digital stream and back again after processing. Despite the numerous adjustment options available, this software based sound generation (aka “modelling”) stands in contrast to a true analogue signal path. Even the most powerful signal processors can’t be “analogue” as modelling is a mathematical simulation that shapes the sound using algorithms. Consequently you have to deal with latency and a lack of dynamic response, both of which are rather uninspiring.
Tube sound – an analysis

The characteristics of tubes for amplifying audio signals can be outlined with several key words.

Vacuum tubes have a very fast slew rate. Along with high signal voltages this results in a high dynamic response to the guitarist’s fingers. Changes between soft and loud playing are presented powerfully and differentiated. In addition to the nonlinear tube characteristic, which creates an expansion of the signal, an increasing gain at higher frequencies (presence) leads to a brilliant and detailed sound.

Overdriving a vacuum tube results in cutting off the signal peaks. This effect is called clipping, which is nothing more than signal distortion. Because of its square characteristic curve, tube distortion will be dominated by even harmonics, which is recognised as sounding pleasant and warm. Clipping an audio signal with semiconductors results in a distortion sound with more odd harmonics. The human ear perceives this as a dissonant and cold sounding tone.

To get a heavily distorted signal (high gain sound) commonly several clipping stages will be cascaded. Overdriving the output stage of a tube amp affects the sound with another distinctive effect: compression. When hitting the strings dynamically, compression compensates for the resulting volume differences. This behaviour is typical for vintage amps. Technically it is caused by short voltage drops at the anode voltage supply. Undersized filter caps, valve rectifiers instead of silicon diodes and poorly dimensioned transformers are some reasons for this phenomenon often called “sag”. A good example is AC/DC using a Marshall JTM45 on their album “High Voltage” in 1976.
Some thoughts on tone stacks

The tone stack plays a key role in forming the special sound of an amp. Consisting mostly of passive high and low pass filters, these parts of the circuit decisively shape the tonal “voicing” of the unit. The mid frequency band between 200 and 1000 Hz is especially important for an individual sound. This is where most tube amps have a unique frequency drop which is called “mid scoop”. Essentially, the adjustment of this mid scoop determines the characteristics of the whole guitar sound.

Creating the poets One


So what constitutes a “convenient device with a perfect guitar sound”? How can all the advantages of analogue and digital worlds can be combined? That was the point at which we decided to create the “poets One”. Here are the main features we put on our wish list:
 

  • Stompbox sized preamp for maximum mobility and acceptable cost
  • Four channels with different basic amp sounds from Tweed to Rectifier
  • True analogue signal path with tube-sounding distortion and high dynamic response
  • Every sound must be adjustable in its voicing, tone color, gain and volume
  • Battery or power supply unit operation with voltage monitoring
  • “Two button” design – true bypass and preset retrieval (momentary switch)
  • Convenient smartphone based real-time adjustment via Bluetooth
  • Possibility to save and retrieve sounds to several presets
  • Visualisation of presets with high luminance LED
  • Stage friendly standalone operation (Bluetooth disable function)
  • Input stage suitable for all kind of guitar pickups
  • Output impedance suitable for a connection to any (clean) amp

 

We started with the development of the analogue audio section of the circuit. Because our device should be battery powered we decided to use semiconductors instead of valves to emulate tube sound as close as possible. Another point is that the settings-adjustment circuitry should be digital to allow smartphone based control. In addition, digital control components can’t withstand the high voltage common to tube circuits.

It was a stony road. We tested hundreds of diodes, op amps, capacitors and other components which would be responsible for a perfect sound and added several weeks of LTspice simulation.
The goal was to emulate tube distortion without compromising dynamic response and to avoid the noisy and jarring sound typical when using semiconductors for signal clipping.

Another major challenge was the tone stack design. In particular the mid filter should be able to manipulate the preamp’s voicing range from “classic tweed” to “modern metal”. We achieved this by designing a filter where the amplitude, centre frequency and Q-ratio can be adjusted with just one control.

Upon completion of the sound stages the first prototype had been created – albeit a breadboard design with mechanical switches and potentiometers.

The next challenge would be to develop a digital control that doesn’t affect the audio quality. Checking out a lot of digital switches and potentiometers, it turned out that only a few components on the market would be suitable for our purposes. We envisaged that the link between control circuit and smartphone would be an Arduino equipped with a Bluetooth low energy (BLE) module. But we soon realised that the open source Arduino software is not efficient enough for our needs. Additionally the Bluetooth module had a big issue with latency.

So we developed a completely new control unit, coded our own operational system for the underlying Atmel microcontroller and chose a professional Bluetooth module with the possibility to implement a private BLE stack.

While we where at it, we also decided to create a complete new housing. Almost every FX pedal uses Hammond aluminium cases. But using Bluetooth inside a metal case isn’t ideal and it also seemed a good idea to have a modern design with a direct view into the inner PCB and several LEDs. Eventually we designed a 3D-printed housing with a massive transparent cover out of hardened plexiglass XT.

Having a functional breadboard design is one thing – creating a professional mixed signal SMD unit on a multilayer PCB another. Parallel to the programming of an attractive iOS app with intuitive handling we had to suffer disappointment on our first PCB. Due to the high gain of the audio circuit and the pulses on the digital part of the power supply caused by the BLE module we had several EMC issues. Furthermore, numerous SMD film capacitors downgraded the audio quality and therefore we had to take a step back and replace them with through hole parts. At that time we began to work with professional German manufacturing companies in order to ensure the best manufacturing quality possible for PCB and SMD soldering.

During the entire development process our guiding principle was to make no compromises in respect of sound. After nearly one year of work, two PCB redesigns, a new ground concept, a special power supply circuit and different revisions of the case we finally had a fully functional “golden master”.
And after half a year of testing the poets One on stage, in the studio and practice rooms with professional guitarists we are proud to present it to you on Kickstarter:

The poets One.