Darkglass Microtubes B7K Ultra with Aux In

The Darkglass Microtubes B7K Ultra with Aux In is a bass preamp and overdrive pedal. Darkglass describes it as delivering "defined and powerful saturation in an intuitive format," and says it allows tone tailoring "from crisp, punchy clean preamp to obliterating high gain distortion, and everything in between"¹. In other words, one box can serve as your clean preamp, your grind, and your DI on the way to the desk.

The pedal grew out of the Microtubes B7K line. Premier Guitar called the Ultra "an enhancement of the company's revered Microtubes B7K," and noted one change from the original: the parallel output of the earlier B7K was not carried over to the Ultra². Bass Musician Magazine's reviewer Kilian Duarte described the original B7K as Darkglass's "crown jewel," revised with new, more user-friendly options based on customer input³.

From clean preamp to high gain

The core of the pedal is a blend-based overdrive sitting alongside a clean preamp path. The clean signal stays at unity gain while the overdriven signal's volume is set separately, so you can dial in dirt without losing your low end¹. On top of that sit a four-band equalizer with six selectable mid frequencies, and low-end shaping before the clipping stage¹.

Bass Musician Magazine's Ty Campbell, reviewing the B7K preamp, found the low-end controls did their job under gain⁴:

For players comparing circuits, the Ultra with Aux In sits in a wider Darkglass range that includes the Microtubes B7K Ultra, the Microtubes B7K V2, and the limited-edition "The Squid" Microtubes B7K Ultra V2⁵².

The controls

Darkglass's product page and manual describe the main controls as follows.

The EQ section is a four-band equalizer with six selectable mid frequencies¹. On the limited-edition "The Squid" version of the B7K Ultra V2, Guitar World reported that a three-way toggle offers additional control for each mid-range section⁶, and guitar.com's Daniel Seah noted that version also carries a master volume control⁷.

Power and connections

The pedal runs on a regulated 9V DC adapter with a center-negative plug only, drawing 120mA¹.

Connections on the back panel:

• Input jack: 1/4" mono jack for your instrument or the output of your pedalboard¹
• Output jack: 1/4" output to connect to the input of your amplifier or another effects pedal¹
• Direct Output: balanced XLR output for connecting to a microphone preamp, PA system, etc.¹
• Aux In: 3.5mm stereo input for backing tracks, usable only with the headphone output, with the level set from the audio source¹
• Headphone output: the internal stereo headphone amplifier can drive a minimum load of 16 ohms from the 3.5mm stereo jack¹
• USB port: a Micro USB B connector to connect to PC/Mac to load cabinet simulation impulse responses and control various other settings¹

One quirk worth knowing: when the pedal is on bypass, the master volume does not affect the headphone volume. In that case the total volume is adjustable through the Darkglass Suite software⁸.

The Aux In and headphone practice setup

The Aux In is what separates this version from the standard B7K Ultra. It is a 3.5mm stereo input that lets you practice with backing tracks from, for example, a smartphone or laptop, monitored through the pedal's internal stereo headphone amplifier¹. The auxiliary level is set from your audio source, not on the pedal¹.

A typical practice chain looks like this:

1. Plug your bass into the 1/4" input jack¹
2. Connect your phone or laptop to the 3.5mm Aux In¹
3. Plug headphones into the 3.5mm headphone output¹
4. Adjust the backing track level from the audio source¹

Cab sim and the Direct Output

The Direct Output carries a switchable cabinet simulation. The Cab Sim switch toggles it on and off, and when switched off it completely bypasses all digital circuits, allowing a zero latency signal¹. Through the Micro USB B port you can load different cabinet simulation impulse responses and control other settings from a PC or Mac using the Darkglass Suite¹. Guitar World described this as a switchable IR cabinet simulation on the balanced XLR output⁶.

As a DI, Bass Musician Magazine's Kilian Duarte called the Direct Output "Perfect for use as a straight DI"³, and the maker positions the balanced output for connecting to a microphone preamp or PA system¹.

Where it came from and what reviewers said

The B7K Ultra is part of the Microtubes B7K family from Finland's Darkglass Electronics². The earlier B7K was described by Bass Musician Magazine as an analog bass preamp with 1/4" input and output, a parallel output, a direct output, and a ground lift for the direct output⁴; Premier Guitar noted the parallel output was dropped on the Ultra². The "The Squid" limited edition, announced in August 2020, was an upgraded version of the Microtubes B7K Ultra with Aux In, adding a Micro-USB B port for loading Virtual Cabinets through the Darkglass Suite and a 3.5mm headphone output with cabinet simulation⁵.

Premier Guitar's reviewer highlighted the master control for balancing the overall signal, the balanced direct out, and the ground lift as additional features of the B7K Ultra².

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