PAiA TubeHead Audio Preamplifier
The PAiA TubeHead Preamplifier is a stereo audio preamplifier that provides sonic modification for various audio devices (CD players, electric guitars etc.) with a wide gain range and controlled and adjustable tube distortion in a modest package, available in kit form.
The innovative design of the TubeHead involves operating the twin-triode valves at a point very low on the plate I-V characteristic curve allowing for
characteristic tube distortion making use of the smoother amplitude limiting feature of vacuum tube characteristics (compared with more "abrupt" clipping behaviour characteristics of overdriven solid-state electronic devices). This note shows some results from a kit assembled by the author.
Both sides of the assembled PC board are shown below. Assembly is fairly tight with very little clearance in the case, particularly for the
potentiometer lugs. Also, very carful attention must be given to the coax shield wires and this is particularly so since the shield of the coax
cable connections are not ground points! PAiA provides some useful guidance on contruction techniques to help prevent inadvertent shorts:
The scope trace below shows input and output signal forms with the Blend control adjusted to maximum tube-sound and the level/drive controls
adjusted to provide about unity gain, with an input signal of about 900mVpeak. The output signal (blue) shows the characteristic soft "squashing"
tube distortion. With the Blend control adjusted CCW to complete solid-state sound, the output signal is amost identical to the sine-wave input signal (red):
The frequency response (for Blend control set completely CCW with no tube sound) is shown below for gain settings of 6 dB (Av=2) and 20dB (Av=10) with potentiometer positions shown for typical settings of the first (Drive) and final (Output) stages. At the 6dB gain setting, the -3dB bandwidth is 200 kHz and at 20dB gain, the bandwidth drops to 100 kHz. The maximum output signal available is about 14Vp (~ 10Vrms):
The TubeHead provides a fairly pleasing tube distortion guitar sound when used with a buffer input device (e.g. another effects pedal) or with the input circuit modification suggested in the assembly manual to raise the input impedance from the default value of about 24 kohm to several hundred kohms, providing a suitable load for high-impedance guitar pickups.