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X-Load LB-2 Reactive Load Box

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About the LB-2

The LB-2 is a load box, that allows using a traditional tube amp without having to connect a speaker cabinet, therefore enabling silent playing. It passes and processes the signal from the amp's speaker jack, to be amplified further down the chain or to be used with cabinet modeling.


The product name LB-2 suggests that an LB-1 also exists. The LB-1 has been mentioned by Fractal Audio in the past, but it was never been released and perhaps has never been created.

Dimensions are: 10.15 W x 8.26 D x 3.6 H inches (including feet which are 0.53"). [[1]]


For more information read the Owner's Manual.

The Forum discussions have more information. See:


Official product information:

The LB-2 is a passive load box that replicates the complex impedance of a guitar loudspeaker. It features our Dynamic Impedance Technology, which delivers unprecedented realism by dynamically changing the impedence of the load depending on the power levels of the connected amplifier.

The X-Load LB-2 Load Box represents the pinnacle of speaker emulation technology for live use or recording. It allows you to use a traditional tube amp without connecting a speaker cabinet. The X-Load LB-2 provides an 8Ω load (100W maximum) while outputting a balanced or unbalanced line level signal.

Unlike other load boxes and attenuators the LB-2 uses Fractal Audio Systems' Dynamic Impedance technology to create a level of realism never achievable before. Record your amp in its sweet spot without deafening volume, or get great tone consistently without the hassles of a speaker cab and microphone setup.

It is important to note that the LB-2 does NOT simulate the characteristic sound of a loudspeaker. This is done by routing your DI Amp signal through speaker simulation hardware—like that found in our award-winning Axe-Fx or AX8 products—or software, like our Cab-Lab application/plugin.

The LB-2 features a Speaker Thru jack so you can connect a real speaker while also feeding line signal to a recorder or speaker simulator.

The Voicing switch shifts the resonance of the load. The “UK” setting is characteristic of the typical British guitar speaker, while the “US” position replicate the lower resonance of a typical American guitar speakers.

A LOT of current flows out of a tube amp. Under normal conditions that current goes into the speaker and returns to the amp since the speaker isn't grounded. A poorly designed loadbox will allow some of that current to return on a different path.

[2] The idea is that you use this with any of our products that have cab modeling (Axe-Fx, AX-8, Cab-Lab). Some people refuse to use modelers so this is a product for them.

[3] The load itself uses proprietary, custom-designed magnetics that mimic the dynamic impedance of an actual loudspeaker. I've been testing the passive version and it sounds much better than any other loadbox I've tried. No high end loss and nice, chunky bass response.

[4] The impedance of our load actually resembles a real speaker. The same can't be said about some of the other so-called "reactive loads". Furthermore I've developed proprietary magnetics technology so the impedance changes with applied voltage, like a real speaker does. Most importantly though, it sounds really good. I'd go so far to say it sounds better than when I use my iso cab as a load.

[5] A real speaker resembles a 4th-order network, at a minimum. Our load is 4th-order.

[6] It's just a load box. There are several others on the market: Suhr, Torpedo Captor, etc. Ours sounds really good. I haven't tried the others so I can't comment but Suhr makes good stuff so I don't doubt their's sounds good. You can use the load box with any product that does speaker emulation including our Axe-Fx, AX-8 and Cab-Lab products or other products (like plug-ins) allowing you to record your favorite amp without the noise and hassles of micing a speaker. Furthermore this allows you to record a DI that you can reprocess later using different IRs.

While the frequency response of a speaker to a voltage excitation is very linear and can be accurately captured by an IR, the various parameters of a speaker change with applied power. First of all the voice coil heats up and as a result it's resistance increases [7].

Secondly the equivalent inductance of the voice coil and the resonance frequency of the speaker change with excursion (see Dodd, Klippel, et. al.).

Our load recreates these parameter changes that occur with excitation level using proprietary magnetic components (not magnets) which are made in USA.

[8] The LB-2 is about 50/50 foreign and domestic content.

[9] There's no point in using a load box with a SS amp.

[10] Load boxes can do all sorts of strange things due to the creation of feedback loops and such. I struggled with this with the LB-2. It took six iterations to get it right.

[11] […] You should use a Suhr RL or an LB-2 though. Much more accurate impedance curves. The Two Notes (and the UA OX) don't use inductors for the low-frequency response so there's no low-frequency resonance. Instead the impedance curve has a "bathtub" shape. With the inductor you get a true low-frequency resonance but that inductor is big, heavy and expensive.

[12] The X-Load is a far superior load as it has an inductor.

[13] It's a far superior load therefore you will get results more similar to a real cab. The frequency response of a tube amp is dependent upon the load impedance. The Captor is a poor approximation. The X-Load and Suhr are far more accurate therefore the frequency response of the tube amp will more closely resemble using a real cab.

[14] The LB-1 will be a load box with integrated IR player.

[15] There are two models. The LB-1 is a full-featured product that includes stereo IR processing plus effects and a whole lot more.


The LB-2 is an 8 ohm device. Be sure to connect its Amp In jack to the proper output on your amplifier to avoid damage. Mismatching impedance can result in damage to your amp!

The maximum power rating of the LB-2 is 100W.


[16] It's only 120W if you turn the master volume up high. If you keep the MV at reasonable settings then it would be fine.

Use the LB-2 as a silent load box and DI


  1. Connect your amplifier’s 8-ohm output to the Amp In jack at the rear of the LB-2.
  2. Connect either the Unbalanced or Balanced DI output to your audio interface or hardware cabinet emulator.


[17] If you use the load box as a load box then you would set the amp's impedance to 8 ohms to match the impedance of the load box.

[18] The LB-2 has become an indispensable tool in our modeling process. It's impedance accuracy is unmatched and it it exhibits very similar dynamic behavior to a real speaker. It also works extremely well as a DI, better than most, if not all, dedicated DI boxes I've tried.

[19] One setup I use is Guitar -> Axe-Fx III -> Amp (via Output 4) -> X-Load -> Axe-Fx III (via Input 4). This allows you to put Drive blocks before the amp and effects after.

Use the LB-2 as a DI with a guitar speaker


  1. Connect your amplifier’s speaker output to the Amp In jack at the rear of the LB-2.
  2. Connect a speaker to the rear panel Speaker Thru jack.
  3. Connect either the Unbalanced or Balanced DI output to your audio interface or hardware cabinet emulator.


[20] The DI quality of the LB-2 is better than any of the speaker level DI's I've tested. Most of them have poor low frequency response.

Use the LB-2 as an attenuator

The LB-2 can be used to reduce the volume of your amp by diverting half the power into its load.


  1. Connect one 4 ohm output of your amp to an 8-ohm speaker.
  2. Connect the other 4 ohm output to the LB-2 Amp In jack.

This will present two 8-ohm loads in parallel to your amp, which is equivalent to a 4-ohm load. Half of your amp’s output power will go the speaker, and the other half into the LB-2 reducing the volume of your amp by 6 dB. You can also use the DI features of the LB-2 in this configuration.

Don't forget to select the appropriate impedance curve in the Amp block (Speaker page) when using the LB-2.


[21] You can use our reactive load as an attenuator by running your amp at 4 ohms. Plug your speaker into one jack and the load into the other. Half the amp's power will go to the speaker and the other half to the load. Not a huge reduction in volume but might tame things enough so you don't piss of your neighbor/wife/GF/mother-in-law/etc. The LB-2 can be used with IR Capture.

[22] You also need to make sure your Speaker Impedance Curve matches that of the load box.

Front panel controls


This controls the DI output volume of the LB-2 at both the unbalanced 1/4” and balanced XLR jacks. Set this knob as required so as not to overdrive the input of a device connected to the DI Output of the LB-2.
A tube amp interacts differently with different types of speakers. The voicing switch of the LB-2 changes the resonance of its load to replicate this.
UK — Replicates the voicing of a classic British guitar speaker: Marshall 4x12 with greenbacks.
US — Lowers the resonance frequency to replicate the experience of playing through a classic American guitar speaker: Fender combo with a 12" Jensen speaker.

These voicings are also available as speaker impedance curves in current firmware for the Axe-Fx III, FM9 and FM3.


[23] The X-Load is equivalent to setting the LF Resonance to about 4. With the Voicing switch set to UK the LF Resonant Frequency is about 100, with it set to US it's around 70.

[24] The UK voicing is based on a 4x12 with greenbacks. The US voicing is based on a 1x12 with a Jensen.

[25] The LB-2 is roughly equivalent to a LF Res. Frequency of 100 Hz, Q of 1.6 and LF Resonance of 4.0. With the voicing switch in the US position the frequency changes to about 70 and the resonance increases to about 5. Due to tolerances the frequency can vary +/- 10%.

Rear panel controls

LB-2 rear.jpg

Connect the speaker output of your tube amp to this jack. The LB-2 is an 8-ohm (8Ω) device with a maximum power rating of 100W.
Connecting a speaker to this output jack disconnects the internal load and connects the Amp In jack to the speaker instead. The DI output will still be active, so you can simultaneously record or process the DI signal while using your amp with a real speaker.
This passive DI output provides a line-level signal.
This active DI output provides a line level signal. For long cable lengths and maximum delity the XLR output is preferred as it provides a low- impedance output and immunity to noise conduction. This output requires that the LB-2 be powered in one of two ways:
  1. Connect an external power supply to the power input jack of the LB-2. Use a 9–24 VDC, center negative power supply (as typically used with “stomp boxes”)
  2. Supply phantom power via the connected XLR cable
This switch disconnects the ground of the LB-2 from the ground of the XLR output. This may help in breaking noisy ground loops.

Watch out! Never connect Speaker Thru to any mic, instrument or line-level inputs!


[26] If your XLR has phantom power you don't need a wall-wart. The active outputs provide a low-impedance buffered output without the coloration (and cost) of a transformer.

[27] The ground lift also lifts the ground on the 1/4". I find I usually need the ground lift engaged.

Speaker connection

Because it's a load box, the LB-2 can be used WITH or WITHOUT a connected speaker. This allows you to use a speaker simulator for recording or front-of-house, while operating either silently or at normal levels.

The LB-2 does NOT produce the sound of a loudspeaker. It outputs a line-level version of the signal your amplifier normally sends to a connected speaker. This must be processed to sound as if it has been through a speaker and microphone, typically using software like Cab-Lab or a hardware solution like the Axe-Fx II or III, AX8, or 3rd-party “speaker simulator” products.