Why am I hearing distortion with my OX Amp Top Box?

OX is a reactive load box that allows you to drive the power section of your amplifier while listening at reasonable volume levels, letting you hear your tube amplifier in a way that was not previously possible. Because of this, you may hear tube crackles, phase inverter distortion, crossover distortion, or other unexpected sounds that were not perceptible when using your amp at a normal volume without OX. 

While it can be a bit surprising at first, this is completely normal in most cases - these artifacts are typically part of the sound of your amplifier already, but you may simply be noticing them for the first time when using your amplifier with OX. That said, there are some additional things to be aware of and a few steps that can be taken to avoid ending up with additional unwanted distortion or noise on your rig. 

Sonic Characteristics of Tube Amplifiers

When pushed to their upper limits, higher wattage amplifiers will often exhibit tube “crackles” or harmonic distortion that you may be hearing for the first time due to the attenuated monitoring levels made possible with OX. You may find that you are hearing harmonics and distortion even on lower gain or “clean” settings that were not previously perceptible. These are all components that lend themselves to the overall tonality of your amplifier that you are now able to hear distinctively due to the unique monitoring possibilities that OX provides.

It is important to take into account the intended application and design of your specific amplifier. Not all tube amplifiers are created equal and many are designed to achieve very specific tonal goals. This means that certain tube amplifiers are simply built and internally tweaked to sound their best at a volumes of 4-6, where as another amplifiers “sweet spot” may be in the 6-8 range. Many amplifiers (for example, silver face Twin Reverbs and vintage tweed amps) are very linear in their power stages which can cause them to be a bit harsh or fuzzy when driven too hard.  Since OX gives you the opportunity to turn up and really drive your amplifier for the first time without it being too loud, you are going to hear artifacts from the power section of the amp that you weren’t able to hear before - and you may like or dislike them.  

Most people are used to playing and listening to their amplifiers at bedroom volumes, off-axis (meaning not directly in front of the speaker) and in an open room environment. This is very different than plugging into OX’s headphone output and listening to close mic’d cabinet emulations, on-axis and without any room tone or reflections, which can make your amplifier sound thinner or otherwise different than you’re used to - especially if you’re driving your amp more than you’re able to in a normal situation. 

To be clear, this is not a result of OX changing the way the power section of your amp sounds or changing how it’s reacting to the speaker load; it is simply that you’re able to listen to the same source material in a more isolated and sterile way. It is for this reason that we have included features like off axis mic positioning options and room tone in the OX application to help smooth things out and replicate the way we are used to hearing our amps in a “natural” setting. The amplifier’s power section reacts to the load presented by OX in the same exact way it would react to a load provided by a speaker cabinet.  

To help drive this home, here are two recordings of a Tweed Deluxe amplifier with identical amplifier and guitar settings - the first example was recorded via a Celestion speaker cab close mic’d with a Shure SM57, and the second example was recorded directly into OX. 

Tweed deluxe into 8Ω 4x12 Celestion Cab with an SM57

Tweed deluxe direct 8Ω into OX

If you listen to the way that the power section of the amplifier breaks up in each of these recordings, you can hear that the amp is reacting to the load of both the cab and OX in the same way that it would a speaker cabinet. The color or artifacts added by the power section are going to be just as subjectively “good” or “bad” as if you were utilizing a cabinet, underlining the importance of having realistic expectations for the particular amplifier you are using.

Crossover and Phase Inverter Distortion

Some other very common types of distortion that you may hear much more prevalently with the use of OX are crossover and phase inverter distortion. 

Class B and AB or “push pull” amplifiers have two main components (usually tubes, or transistors) that are responsible for amplification. These amplifiers switch between these two components as part of the amplification process, and if these components are worn or not biased correctly distortion can occur between the time when one of these components turns off and the other turns on. This type of distortion is known as crossover distortion, and it can be very sonically similar to digital distortion. 

Crossover distortion is going to be present in most push pull tube amplifiers of this kind to some extent, but when running an amp and cab without OX, the signal is typically loud enough and there are enough reflections in the room that this distortion is masked and not recognizable. The audible effects of this can be minimized or eliminated by making sure that the amp has been properly biased.

With some class A/B amps, you may hear a subtle raspy distortion as certain notes ring out - this type of distortion is typically caused by the phase inverter tube. The phase inverter tube on a class A/B  amplifier takes the signal from the input stage of your amp, splits it into two identical signals that are 180 degrees out of phase with each other, and sends each signal to the output stage of the amp. If the phase inverter tube is worn, additional distortion can occur. Under normal usage, phase inverter distortion isn’t very obvious, but when using a reactive load box like OX and listening critically through headphones it is much easier to hear.

Setup Basics - Connections and Gain Staging

When using OX, you’ll want to make sure that you have everything physically hooked up correctly to avoid unwanted audio issues. For help with the physical connections between OX and your amplifier, please refer to the OX Quick Start Guide. If you are using your OX with a combo amp, refer to the Setting up OX with Combo Amps article. 

Gain staging issues are the most common reason users experience unwanted distortion - when setting your tone, it is important to make sure that you do not see clipping on the meters in the OX application, especially on the master output fader. 

Even with a 150W amp fully cranked you won’t clip the input stage of OX, however it is possible to push the outputs of the unit a bit too hard and clip OX’s digital outputs or the input of the audio interface, headphones, or device that you are listening through. Clipping is indicated by the red clip lights at the top of the master output meters. The moment you see the red clip lights, this indicates that OX’s digital outputs are clipped and that you are likely clipping the inputs of the receiving interface or recording device you are using.


With so many variables to consider when attempting to troubleshoot audio issues with a tube amplifier, it always helps to start by simplifying your setup. The tips below will help you identify the source of your distortion and determine if it’s indicative of a larger problem:

    • Be sure that you’re using a high quality speaker cable. Cheap speaker cables with plastic connectors can cause audio issues. Also, be sure that you’re not using an unbalanced TS guitar cable by accident. 
    • When using the S/PDIF output, avoid the possibility of crackles, clicks, or pops by ensuring that the clocking settings on the receiving device are correct. OX’s S/PDIF output is fixed at a 44.1 kHz sample rate. If you’re not sure if what you’re hearing is caused by a clocking issue, try switching to any of OX’s analog outputs (headphone or line outputs) to see if the problem is present in the analog path - if only present in the S/PDIF output, the issue may be related to clocking. It is also very important to be sure that you are not clipping the master output meters in the application. Any time you see the red clip lights it is possible that you are overloading the input of your recording interface or device:


    • Disable any plug-ins that are active in the master section of the OX application to simplify the gain staging by tapping or clicking on the LED light to the right of the listed effects:


    • Ensure that your Cab and mic emulations are not being bypassed by checking that the DIRECT Mic option is not selected on any channels in the OX application (DIRECT means full range, which can mean full frequency distortion that is typically not very pleasing to listen to).


    • It is important to be aware that the “Speaker Output” on OX that runs directly to your speaker cab is a purely analog path and digital clipping is simply not possible here because there are no digital components involved. As mentioned before, under normal circumstances it is not possible to clip the analog input of OX. There is not any modeling or processing of any kind being done on the speaker output, just reactive attenuation. If the noise you are experiencing is present there, odds are it is coming from the amplifier, your effects pedals, cables or your speakers in one way or another. From there you can remove or replace cables, pedals or anything else in your chain one-by-one to zero in on the source or problem. 
    • Ensure that the impedance selector on the rear of OX is set to match the impedance rating of your amplifier. Some amplifiers will have multiple speaker outputs so you will want to make sure that OX is set to match the impedance of the specific speaker output you are using on the amp. While some amplifiers are capable of being run at different impedance settings without causing damage or audio issues, others are designed to be run at a very specific load. We always suggest you reach out to the amp manufacturer to see what they recommend for safe use. 
    • Make sure that your amplifiers components are properly calibrated by having the bias on your tube amplifier checked by a qualified technician, especially if tubes have been recently replaced. The purpose of biasing your amp is to make sure that it is set up for optimum flow of current throughout the various amplification stages. 
    • Remove any effects pedals or external effects units to simplify your setup and help zero in on any unwanted artifacts, distortion or other sonic anomalies. Compressors especially can amplify and bring out certain crackles or natural distortion that may be natural to your amplifier. Certain gear can also automatically compress your audio on their input, causing the natural sound of the amp to be heard in a way you are not used to. 

It is important to note that the various distortion types listed in this article are just some of the more common sources of tonal confusion specific to tube amplifiers. With so many different variables at play, there is always the possibility that a sonic anomaly not talked about here may be causing your problem, but these steps should help you zero in on the source of the issue. 

There are many different sonic components that lend themselves to the overall tone you have always heard from your amp. When using OX, some of these things are going to be heard more clearly because you are monitoring at lower levels while still providing a proper load to the amp. Being able to clinically and critically listen to your driven tube amp in this way, in many cases, is a brand new experience, so it is good to keep these things in mind as you play and experiment with your OX.

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