Tube VCA

Tube-based VCA and distortion

Introducing the NT Tube VCA, a voltage-controlled distortion, gate and amplifier module for Eurorack synthesizers, handcrafted in Denmark.

Unlike a discreet VCA, this module features a vacuum pentode of Soviet manufacture as the variable gain element, which can produce unique and interesting timbres and overtones when overdriven.

The idea of using the 1Ж24Б tube as the heart of the amplifier originates from Ken Stone's CGS65 guide, while the design and looks were heavily influenced by Erica Synths' Fusion modules, notably by the Plasma Drive. The circuit includes a revised CV conditioning and output stage, variable bias between ground and the negative rail, a dedicated feedback path and dynamic backlighting, which can be set to follow the level of the output, stay constantly on, or off.

• IN1, IN2 - are standard audio inputs. Their gains can be set individually with their respective knobs, IN 1 Gain and IN 2 Gain. The inputs have a high maximum gain, therefore they can be used to bring line level signals up to synth levels, or introduce further clipping to the sound.
• CV - this input drives the CV Bias knob and its gain can be adjusted with the CV Amt. knob. Higher settings create soft-clipping distortion and, depending on how Amp Bias is set, negative wave-folding. Generally, a lower Amp Bias results in a cleaner signal.
• OUT - this is the output of the VCA, which is also fed back to the input stage. The amount of feedback can be set with the Fdbk knob. A high feedback can trigger self-oscillation, even when there's no input. Note that the output of this module is AC coupled, making it suitable only for audio signals.