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Tube bass preamplifier
The A+ Flex is a tube bass preamplifier aimed at players who prefer the classic East Coast sound.

Here at Shift Line, we've always been particularly fascinated with bass-focused devices. Our product lineup includes the Olympic preamp with its "California sound", so the introduction of the East Coast-style Flex preamp was just a matter of time. We're extremely happy to introduce the new device, as the Flex can significantly expand your sound palette.

The A+ Flex is a fully analog preamp based on the 12AX7 tube. The device allows for soft compression and characterful harmonic saturation while also giving you control over your instrument's tonal balance and volume. The Flex has three outputs (main, balanced and THRU) allowing you to set up multiple independent signal chains. Additionally, you can use the device as a drive pedal or toneshaper due to its bypass switch.

Key Features

  • Fully fledged tube preamp with high-voltage power.
  • Fully analog signal path.
  • Wide-range input volume control.
  • Control over saturation character within tube stages.
  • Active EQ section with detents in neutral positions.
  • Harmonic saturation for any broadband signal.
  • Drive/toneshaper pedal capability.
  • Active tube DI Box with high-quality balanced output and Ground Lift function.
  • Small, lightweight and pedalboard-friendly aluminium enclosure with top-mounted jacks.
IN is a high-impedance input for monophonic signals. You can plug in an active or passive bass, as well as any other instrument or signal source.
THRU is a buffered and unprocessed output which duplicates the input signal.
OUT is the main monophonic output. It can be used to connect the Flex to a power amp or to another amp's input. If the device is plugged into a power amp's INPUT or a bass amp's RETURN / POWER AMP IN, it will act as a preamplifier (the Flex's bypass will send the buffered original signal down the signal chain). When plugging the Flex into the instrument input of a bass head or combo amp, make sure to set the MASTER knob low at first and tweak it to taste later; this way, you will avoid unwanted input distortion.
BALANCED OUT duplicates the OUT signal and optimizes it for balanced signal chains.
POWER IN is a power supply input (12V DC 500mA).
PAD is a switch controlling the amplification ratio of the first stage. In the DOWN position, the sensitivity is -15dB; in the UP position, it is 0dB. The Flex is designed to work equally well with active and passive basses. With the PAD switch, you can adapt the device to your instrument's volume and shift the GAIN knob's range to the desired area.
The GAIN knob controls the amplification, compression and harmonic distortion of the incoming signal. With this knob, you can go from a perfectly clean sound to classic East Coast-style overdriven tones. The gain range depends on the incoming signal volume and the PAD switch position.
The TOP switch controls high frequencies in the signal. Set this switch to "+" to boost the highs and highlight the attack. Set it to "-" for a softer tone. In the neutral position, the TOP switch has no effect on the high frequencies. The filter has a mild effect on the signal between the preamp's tube stages, allowing you to control the character and intensity of harmonic distortion.
The DEEP switch shapes the initial frequency response in the preamp's tube stages. In the UP and DOWN positions, low and high frequencies are boosted while mids are scooped at 600Hz (UP) or 400Hz (DOWN). The filter has a strong effect on the distortion and can significantly alter the instrument's tone.
The BASS and TREBLE knobs constitute the 2-band EQ section. The EQ is set after the preamp section, allowing for frequency control without altering the character of the harmonic distortion. The BASS knob has a ±12dB range at 40Hz. The TREBLE knob has a ±16dB range at 5kHz.
The MASTER knob controls the OUT and BALANCED OUT volume.
The GND switch allows you to override "ground loops" that can negatively affect the signal when using balanced connections. In the lower position of the switch, the preamp's "ground" is connected to the signal receiver's "ground". In the upper position, the "grounds" are disconnected (a.k.a. Ground Lift).

The optimal setting of the GND switch is selected by ear: there should be a minimum amount of 50/60Hz hum and interference.
The BYPASS footswitch is used to switch the device on and off. When the preamp is engaged, the LED glows white.


Signal Routing

Recommended Initial Settings

Before tweaking any settings on the preamp, make sure it has been properly connected to the rest of the signal chain.

When connecting the Flex to a power amp (RETURN / POWER AMP IN), set the GAIN, TREBLE and BASS knobs to the neutral position (12 o'clock), the DEEP and TOP switches to the middle position, the PAD switch to the up position, and the MASTER knob fully CCW. Engage the preamp by pressing the BYPASS footswitch (the LED should light up). Set the output volume via the MASTER knob and set the other controls to taste. When switching the signal chain off, make sure to disengage the power amp first.

When connecting the Flex to an instrument input, set the GAIN, TREBLE, BASS and MASTER knobs to the neutral position (12 o'clock), the DEEP and TOP switches to the middle position, and the PAD switch to the up position. Engage the preamp by pressing the BYPASS footswitch (the LED should light up). If you can hear a significant jump in volume when engaging the preamp, tweak the input and output levels via the PAD switch, GAIN knob and MASTER knob until the volume is the same regardless of whether the preamp is on or off. After that, you can proceed to tweaking the parameters to taste.


  • Tube type: 12AX7.
  • IN: 6.3mm, mono, unbalanced, 1MOhm input impedance
  • THRU: 6.3mm, mono, unbalanced, 1kOhm output impedance
  • OUT: 6.3mm, mono, unbalanced, 1kOhm output impedance
  • BALANCED OUT: XLR, mono, balanced, 1kOhm output impedance
  • POWER: 12V DC 500mA, negative tip, 2.1/5.5mm diameter. Phantom power of the balanced line (+48V) or battery power are insufficient for the preamp to operate on. If phantom power is turned on, it will not damage the device. Use a regulated PSU only.
  • Dimensions (LxWxH): 101x125x55mm (3.9x4.9x2.2")
  • Unit weight: 430g (0.95lb).
  • PSU weight: 120g (0.26lb).

Delivery Set

  • A+ Flex preamplifier;
  • 12V DC 1000mA PSU with 4 changeable plugs.