Q: As an engineer and hi-fi enthusiast, what led you to design a DI box?
A: When I was working on hi-fi sound systems, I noticed that if you run an audio signal through an isolation transformer, it sounds different. I wouldn't say better or worse, just different, because it cannot be objectively measured, but subjectively, the difference in sound is usually perceived as a positive improvement.
I remember when me and my colleagues got our hands on a vintage Harrison mixing console. It was very old, extremely worn, but it sounded beautiful. It had big, bulky transformers on each of the channels, which was definitely a big part of the sound. A lot of people like to talk about the warm, rich sound of tube amplifiers, but what is not so well known is that all the good sounding tube amplifiers also have high-quality input and output transformers, which also are a huge part of their sound. These days, a lot of manufacturers cheap out on the transformers, or even develop transformerless designs, probably because good transformers can get very expensive and are not that easily marketable, but I am convinced that the end user's experience suffers for that.
At the heart of every passive DI box, there is a transformer, so I wanted to ensure that we get the best transformers that are available on the market, to let musicians and listeners experience the best possible sound.
Q: I always hear that the DI box is able to clean up the sound, can you explain that?
A: The original purpose of the DI box is to prevent electrical hum caused by ground loops. Basically it means that when two devices in the same signal chain are connected to outlets with different ground, we can experience a loud 50hz or 60hz hum, which can be solved by disconnecting the ground on one of the devices. This is what the lift ground button is for. So that is one thing.
Electric guitars and basses output an unbalanced signal, which means that the ground is basically one half of the signal path. The ground picks up all kinds of interference from appliances connected to the circuit, phase regulators, dimmers, you name it. That could cause serious noise issues, and the DI box helps with that by converting the unbalanced signal to a balanced signal, which means that the signal can travel isolated from the ground.
Then, we can talk about galvanic isolation. If you look at the ground through an oscilloscope, you will probably see an impenetrable jungle of stray electricity that is usually outside of the hearing spectrum, but it can be still detrimental to the signal; you can experience loss of detail and dynamics in the sound. If you put an isolation transformer before the input, any of the unwanted signal outside of the useful spectrum is negated; infraacoustic signal is effectively shorted, and ultraacoustic signal is filtered out.
Q: What exactly happens with the audio signal itself when it passes through a transformer?
A: As I said, the transformer does not allow a lot of unnecessary frequencies outside of the listening spectrum to pass through. There are a lot of very high frequencies, like radio waves, wi-fi and other interference, that can travel along with the signal. The transformer does not process frequencies upwards from a certain point in the frequency spectrum. We are talking about frequencies that are not audible, but can be detrimental to the overall sound, if they travel along with the audio signal.
The lowest frequencies, let's say around 10hz or bellow, are also not transmitted, which is a good thing because these cannot be properly processed by our ears and can only muddy up the reproduction of the sound. Also, when we talk about bass frequencies, it is usually bellow 80hz or less, but these frequencies are more felt than heard. For example, even though that the fundamental frequencies of the bass guitar are in the lowest parts of the spectrum, the human ear hears them mostly through their higher harmonic orders.
The core of the sound of the bass guitar is in the lower mids, somewhere around 200-300hz, this is what makes the bass audible in the mix. If there is a huge peak in the signal amplitude, like slapping the bass or hitting the bass drum really hard, some of the extremely low frequencies (under and around 40-60Hz) cannot be processed by the transformer as they are, but with a good quality transformer, they energy is converted and these frequencies are transmitted in higher harmonic orders, sometimes 2nd, but mostly 3rd and 5th harmonics, which makes them much more audible to the human ear, resulting in a clearer sound that still has plenty of low end information. This coloration of the sound is subjectively perceived as more pleasant - "fat" or "compact" sound – you could say that this is a sort of "audio alchemy". Of course – everything in moderation.
(Karel Stouda, DIVerse designer)