The history of rock is synonymous with the history of analogue distortion.
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.
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
Tube sound – an analysis
The characteristics of tubes for amplifying audio signals can be outlined with several key words.
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.
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
But we found nothing to fit the bill. It seemed that there was merely the choice between an analogue stomp box with only one good sound configurable via potentiometers or a digital modelling device with intuitive setup but poor sound.
- 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
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.
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.